FUR~BEARING MAMMALS OF CALIFORNIA Their Natural History, Systematic Status, and Relations to Man





GOLDEN BEAVER BEAVERS GOLDEN BEAVER . Castor canadensis subauratus Taylor

OTHER NAMEs.-Castor fiber~ part; Castor canadensis, part; Castor americanus; Castor pacificus; Castor subaU1'atus; Castor subauratus subauratus; Castor fiber var. canaden­ sh; Castor fiber americanus; Castor canadensis pacificus, part; American Beaver, part; California Beaver; Pacific Beaver, part; Beaver, part. General characters.-Size, largest of all the rodents (in adults, length of body about 30 in., of tail alone, 16 in., weight upwards of 40 lb.); strong incisor teeth, orange brown in color, protruding from mouth; pelage dense, consisting of long, glossy over­ hair and thick underfur; eyes and ears small; tail flattened horizontally, broadly paddle­ shaped, scaly; hindfeet large, toes completely webbed. (See PI. VIII, frontispiece.) Descrt'ption.-Adults: General tone of color, golden brown. More exactly: Under­ fur, very nearly unifor~ all over, varying around bister, warm sepia, and cinnamon­ brown, paling slightly on hinder part of belly and around muzzle; overhair on back, dose to ochraceous-tawny, on lower surface where much shorter and scantier, pinkish buff; on base of tail brightening to orange-cinnamon and even vinaceous-rufous. Be­ cause of notable gloss of overhair, the color tone varies greatly with the angle at which it is viewed. As a pelt is turned this way and that in good light, an impression of metal­ lic gold is given. Length of longest overhair on middoqum, 2 in. (50 mm.), on belly, IYs in. (28 mm.); depth of underfur on back, %-in. (I9 nun.), on middle of belly, )':i-in. (I3 mm.).Whiskers relatively short (longest up to 2% in. [70 mm.]), pale yellowish brown in color. Soles of both hind and fore feet and nose pad, naked, with skin rough, blackish brown in color; tops of feet devoid of underfur, covered with short overhair, russet in tone on hind feet and dull cinnamon-buff on forefeet. Front foot relatively small, but all five toes well developed and each armed with a heavy claw, the longest (middle one), Va-in. (22 mm.) long and %6-in. (5 mm.) wide. Toes';f hind foot each have a claw which projects beyond margin of webbing; claws of outermost three toes, stout, wider than high, the longest, I in. (25 mm.) long and Ya-in. (IO mm.) wide; claws of inner'two toes much smaller, second with horny blade underneath its claw which leaves a narrow slit; this' second-toe structure constitutes the so-called louse comb. Claws dark brown at bases to horn color at tips. Tail hairy at base continuously with body, terminal flat part (about % total length) scaly, meagerly beset with short hairs; "scales" wide and short, tending to form transverse and diagoI!-al rows; but these are not perfectly regular in arrangement a~ on a carp's body. Young, taken from house, small (length, including tail, 15 in. [380 mm.], weight 2IYz oz. [610 gm.]): LIke adult, but pelage "woolly" in texture and general color darker. Underfur on back deep clove brown; on lower surface, fuscous; overhair luster­ less, snuff brown on ·back, pale buff below. Feet like adult's; tail similar to adult's but flat part relatively narrower, and scaly surface shows fine hairs, of same dark brown color as rest of tail. The general features of the beaver's skull are shown "in figure 258. Detailed descrip­ tions and illustrations have' already been published, by Taylor (1916); and need not be repeated here. Our measurements from skull~ of the Californian races, as available to us, are given on pages 630 and 722. Fur-bearing Mammals of California

Color vart'ation.-As· far as can be seen from external examination, the sexes are alike with regard to size, color, and texture of pelage. Only one molt takes place each year, occurring very slowly through summer and fall. The fur does not become prime before the first of December. Winter-taken pelts show but slight variation in color. The tone of the underfur is wonderfully uniform. Such variation as is shown in the overhair generally seems to be caused by the amount of "tipping"; whereas most· of this is individual, it may also be modified by abrasion and, perhaps, fading. The "golden brown" cast of color in Castor c. subauratus is so constant as to be diagnostic when com- parison is made with C. c. frondatol'. .


'S "8 :1i ;'l 'S to ~ ;'l eli, 'E.~ ~ ;'l 'il§ j M No. "Ii, •• • 1 ""~~ :Be 0 0 ~~ Sex Locality ~ jj M.V.Z. .!! .p =-6 a ~~ ~~ ~" 'S ,,0 • ",' P ,,8 8 .~ ~ ·:di " 1 • ~- lie ~] 8" l ~ ~ :>: r:5~ ~ ~o ~~ -->'" ---- 16383 Grayson •••...•••.•••...••..• 120.6 96.0 25.6 65.2 49.3 26.1 11.1 19.0 31.7 31081 ,,' Mendota ....•. , ...•.• , ... , .. 127.0 105.6 28.3 68.8 56.8 27.9 10.1 19.1 33.9 31082 '" Mendota .•..•.. , .... , •...••. 121.0 100.3 26,9 6S .8 S3.7 24.7 11.9 19.7 31.8 31083 '" Mendota ...... •...• , ...•. 121.5 102.8 26.7 66.5 53.4 24.8 12.7 .... 32.0 31084 '" Mendota ...••...••..•.•....• 125.0 99.3 27.1 65.9 53.1 25.8 10.4- 18.4 32.5 31108 ,,' Fair Oaks ..•.•....•.•...•..• 126.8 99.3 29.6 67.8 52.1 26,S 11.9 19.6 33.9 31255 " Hopeton .. , ...... ,. 123.6 95.9 25.6 68.0 52.9 26.6 12.:3 19.2 34.6 31276 . " Snelling .... , ...•...... 126.8 98.3 26.7 70.1 57.7 28.2 13.3 19.2 33.2 31285 " Hopeton •.••..... , ..... , •... 124.5 94.7 26.5 66.8 52.8 25.2 11-.9 20.3 33.7- 31302 " Snelling •. , ... , ...... 122.9 93.7 25.5 63,6 5:3 .8 26,4 12.8 19.:3 33.0 8987 ~' Grayson ..•..••. , •.•.. " ... -.' 113,1 88.8 23.3 63.5 50.2 24.0 10.8 18.4 29.8 12654 ~' Grayson., .. , .....• , ....• , ... 126.4 103.1 28.1 70.2 54.2 28.0 10.0 18.3 33.2 12669 ~, Grayson ...... , ...... , .... 115.2 94.0 25.4 66.4 51.0 25.7 12.4 18.8 30.9 31079 ~' Mendota." .. , .....•..••.• ,. 115.0 92.1 24.0 60.0 48.0 23.5 12.8 18.6 30.8 31273 ~ Snelling ...... " •. ,." .... , 127.3 97.3 27.1 69.7 57.3 27.9 13.6 21.2 34.2 31278 ~ Snelling .... " .. , ...... 130.5 97.5 27.3 68.4 56.6 27.5 11.9 19.5 34.2 i 31290 ~ Snelling, ...... " ..•.. , ... 123.3 92.8 24.5 66.1 52.7 24.8 11.2 18.5 31.3 31293 ~ Snelling ...... ,., .. , .. , ...... 116.0 90.8 26.0 64.8 51.6 24.3 12.0 19.2 29.7 31294 Snelling .. , ...• , .... , .••... ,. 123.2 93.9 26.5 67.7 51.1 26.S 11.4 19.7 32.3 ..ill' ~ 31296 ~ Snelling ...... •.•.... , .... , 126.1 99.1 28.6 69.3 54.2 26.5 13,4 19.5 33.6 I ------! General average, both sexes ... 122.7 96,7 26.5 67.7 53.1 26.0 11.9 19.2 32.5

* Tne sex as here recorded is as determined and written on the labels by the trappers, But, because of the difficulty known to have been experienced in sexing beavers, the accuracy of the records in those specimens starred in the table is, in our minds, open to some doubt; for this reason, separate averages for the sexes are not given.

Measurements.-The following are average and extreme external measurements, in millimeters: Of 9 adult males: Total length, 1091 (1025-II75); tau vertebrae, 371 (35(}--405); scaly part of tail, 12ox28r; ratio,' per cent, width of tail to length of tail, 42.8 (37.9-48.3); hind foot, 188 (180-195); ear from crown, 26 (23-28). Of 15 adult fe­ males: Totallength, II07 (103o-r200); tail vertebrae,383 (345-420); scaly part of tail, r20 X 295; ratio, width of tail to length of tail, 40.5 (31.5-46.8); hind foot, 188 (175- 205); ear from crown, 28 (27-29). Of 24 adult examples (9 males and 15 females): Total length, I rOI (1025-1200); tail vertebrae, 379 (345-420); scaly part of tail, 120X289; ratio, width of tail to length of tail, 41.4 (31.5-48.3); hind foo1; 188 ('75- 205); ear from crown, 27 (23-29)'

, -'~ --,--_.

Fig. 258. Skull of golden beaver, side view, male (no. 3I085, Mus. Vert. Zo01.). trapped on February I7, I920, near Men~ dota, Fresno County. X 1. Noteworthy features include, as correlated with the beaver's special mode of existence: the huge incisor teeth for gouging and grasping, their .housing in the massiv~ rostrum and mandible, the great development of ridges for attachment of powerful muscles, and the corresponding relatively small size of the brain case. Fur-bearing Mammals of California


II ~"O;;::'" > ~i1 ~ .g• ] 'il.~ d No, Total 8~t! Date Ago Sox Weight § M.V.z. length "e ~• ~~~ ] a 1 .9~B ~ J! ~ ~ ~ 0 '" ~ --- " 1920 '" --'" 31269 Feb. 25 adult d' 39 1080 350 13SX290 46.5 185 23 31271 Feb. 26 adult d' 37 1030 405 12SX305 40.9 195 27 31274 Feb. 28 adult d' 34 1025 350 115 X275 41,8 185 27 31276 Feb, 29 adult d' 42M 1150 370 HOX290 48.3 195 28 31255 Mar. , adult d' 43 1175 370 llOX275 40.0 190 28 31285 Mar. , l).dult d' 38 1170 370 llOX285 38.6 190 27 31302 ,,"e, 6 adult d' 40 1035 395 115X250 46,0 185 26 31291 Mar. 8 adult d' 42Ylj lOSS 365 125 X275 45.4 180 ., 31299 Mar. 14 adult d' 45 1100 365 llOX290 37.9 190 " I 1922 33597 Dec. 6 adult d' 37)1 1045 350 115X295 39,0 177 26 33601 . Dec. 12 adult d' 35 1027 347 llBX290 40,7 180 27 .... Dec. 17 adult d' 36» 1050 355 119X295 40.3 180 27 I 33607 Dec. 20 adult d' 50 1130 415 135 X325 41.5 195 29 1923 33616 Jan. 8 adult ,d' 47 1080 370 150X295 50.8 190 28

1922 33596 Dec. 5 adult 0 36 1045 380 120X290 41.4 180 25 ... " Dec. 7 adult 0 35 1015 342 118X282 41.8 178 25 ,\1 33598 Dec. 8 adult 0 34 1012 360 118X287 41.1 180 27 33602 Dec. 14 adult 0 41 1055 375 118X290 40.7 185 28 33605 Dec. 17 adult 0 48 1153 370 123 X300 41.0 187 29 33608 Dec. 20 adu,lt 0 38» 103u 375 115X285 40.4 180 28

1923 33612 Jan. , adult 0 4' 1095 370 117X300 39.0 187 28 33614 Jan. 7 adult 0 46 1120 400 130X320 40.6 184 27 1920 ..... Feb. 24' adult 50 400 0 1140 142X310 45.8 195 " 31270 Feb. 24 adult 0 .... 1100 345 125 )(305 40.9 180 ,. 31301 Feb. 27 adult 0 35 1030 365 117X290 40.3 185 27 31273 Feb. 28 adult 0 35 1090 420 115 X310 37.0 205 28 31277 Mar. 1 adult 0 41 1135 380 118X300 39.3 205 29 31278 Mar. 2 adult 0 45 1180 400 125 X305 40.9 195 27 31283 Mar. 5 adult 0 45 1105 385 125 X285 43.8 190 28 31254 Mar. 5 adult 0 42% 1100 385 110X295 37.3 185 28 31290 Mar. 8 adult 44 1200 0 395 130X295 44.0 195 " 31293 Mar. 10 adult 0 38» 1050 365 131X279 46.8 175 " 31294 Mar. 10 adult 0 37Y.i 1097 375 110X284 38.0 179 " 31296 Mar. 11 adult 0 50 1105 390 125X290 43.1 185 " 31297 Mar. 11 adult 0 36 1050 367 90X285 31.5 180 " 31298 Mar. 11 adult 0 45 1107 395 120X295 40.6 187 " 31300 Mar. 15 adult 45 0 1115 380 112X295 37.9 187 "

(Continued on next paul Golden Beaver


~ ~'O~ "~ ~ . No. e 'B ~~~ d Total .g 8...!::t: M.V.Z. Date Ago Sox Wcight length § ); uS 11 t " c:>,.-;:: ~ ! r jj o,s .. .g j ..!;" ~" .~'S :.:: ~ d ~ ~ ro " iii 1922 , "' 33595 Dec. 5 2-year-old 0' 31 950 320 lO7X260 41.2 177 25

}923 33613 Jan. 6 2_year-old 0' J1 980 "0 110X270 40.7 170 26 33615 Jan. 7 2-year-old 0' 35 (very ...... fat) 1920 31275 Feb. 28 2-year-old 0' 32 1030 350 105X280 37.5 175 26 31288 Mar. 7 2-year-old 0' 28 1000 340 95X265 35.4 180 25 1922 33604 Dec. 15 2-year-old 9 25 930 JJ7 }OOX265 37.7 165 28

1923 33611 Jan. 5 2-year-old 9 22 940 345 96X270 35,6 160 28 (poor) 1920 31284 Mar. 5 2-year-

1923 33610 Jan. 3 yearling 0' 20 865 300 93X2SS' 36.5 160 25

1920 31286 Mar. 6 yearling 0' 19 850 280 70X220 31.8 165 25 31289 Mar. 8 yearling 0' 18" 960 340 90X240 37.5 175 .. 31279 Mar. 2 yearling 0' 18 "880 320 87X245 35.5 175 24 31282 Mar. 4 yearling 9 22 925 345 10X270 38.8 170 24 31281 Ma,. 4 yearling 9 20 820 270 75X210 35.7 150 23 31287 Mar. 6 yearling ~ 19 865 280 72X220 32.7 162 24 31280 Mar. 2 y~ar1ing 9 18 860 300 80X230 34.7 170 24

1922 33599 Dec. 8 y~arling 9 19 880 320 92X2S2 36.5 168 2J 33606 Dec. 17 yearling 9. 13 730 255 62X200 31.0 140 2J 33609 Dec. 23 yearling 9 20 900 300 95X260 36.5 160 25

1921 32187 April 25 about 17 days 0' 1% 380 115 30X 90 33.3 74 12

Note.-The table shown above gives the age of specimens as determined from body measurements and weights. In this grouping) age of specimens also accords with size and general development of the skulls; but if age be determined alone by the degree of emergence of the permanent upper premolar teeth, then nos. 31289 and 31280 would be classed as two-year-olds, and no. 31272 as a yearling. "Adult"=thirty-two months old or older, 341bs. or more in weighti "two­ year-old"=about twenty to twenty-two months old, 28 to 33 lb.; "yearling"=about eight to ten months old, hs to 22 lb. Fur-bearing Mammals of California

Average and extreme measurements-in millimeters of 20 adult skulls, both sexes supposedly in equal numbers, are as follows: Basilar" length, 122.7 (II3.1-I30.5); zygo­ matic breadth, 96.7(88.8-105.6); interorbital width, 26,5 (23.3-29.6); mastoid b,,;adth, 67.7 (60.0-70.2); greatest length of nasals, 53.1 (48.0-57.7); width of nasals, 26.0 (23.5-28.2); vertical diameter of foramen magnum, 11'9 (10.0-13.6); transverse diam­ eter of foramen magnum, 18.7 (18.3-21.2); ratio vertical to -transverse diameter of foramen magnum, 62.0 (52.9-69.2) j alveolar length of upper molar series,,' 32.5 (29.7-34.6).


.S 1: ~ ~ .. Locality Date .S tOe • Development ::95" ~E ~ ~~ , z" ~~ !i.e --- Netherlands Cut, Yolo Co ..... Jan. 9, 1921 5 25 1.0 Body naked, feet and tail not fully formed Netherlands Cut, Yolo Co .... Jan. 16, 1921 2 32 1.8 Body naked, tail round Netherlands Cut, Yolo Co ..... Jan. 16,1921 5 53 7.5' : Body naked. tail slightly flattened Snelling, Merced Co...... Mar. 2,1920 2 65 13.0 'Body naked, "tail fl.at"tened , I Body naked, tail flattened, hind feet pa.r~ Netherlands Cut, Yolo Co ..•.. Feb. 3,1921 3 67 22.2 :1 ,tially webbed Snelling, Merced Co ...... Mar. 5,1920 4 110 30.0 Body naked, "louse comb" distinct Netherlands Cut, Yolo Co ..... Feb. 3,1921 2 130 35.8 Body naked. feet fully webbed II Snelling, Merced Co .... , . ... Mar, 15, 1920 1 140 80.0 Body naked, tail scaly but hairless Snelling, Merced Co."., ...... Mar. 11, 1920 3 150 88,0 Body naked, tail scaly but hairless i Mendota, Fresno Co ... " . , .. Feb, 20,1920 2 200 ..... , ...... Netherlands Cut, Yolo Co ... , , Feb. 27,1921' 1 227 . 302.2 Body well haired, vibrissae well developed

Snelling, Merced Co, ..... ".' Mar. 2,1920 1 240 315.0 Ove'rhair on body 3 mm. long, hairs ap~ :.·.·.I ' . pearing on tail ;,1'. Snelling, Merced Co. .. " .. ... Mar, 8,1920 3 285 354,0 ; Overhair on body 7 mm. long; under£ur I scant Snelling, Merced Co., ..•. , "., ~r, 11, 1920 4 325 485,0 Overhair 10-13 mm. long, underfur thick, 8~lQ mm. long; body fully deVeloped; i.' ready for .birth.

Avetage number in litter ••.•..••..... ,' ...... 2.7 I

.. Note.-Besldes the above mentIOned, two old females WIth enlarged utenwere taken February 27, 1920, and another female, believed to be a two-year-old, in similar condition, March 5, 1920. •

~or details, see tables (pp .. 632, 633). For discussion of variation in proportiqns of tail, see text, p. 642. Weigh~.-Average of 9 adult males, all taken in February and March, weighed before skinning, 40 lb.; extremes, 34 lb. and 45 lb. Average of -14 adult females, of same season, 42 lb.; extremes, 35 lb. and, 50 lb. for det·ails.see t~.ble (p. 632) and for discussion see text (p. 637). Type iocality.~Grayson (San Joaquin, River near its confluence with the Tuolumm;: River), Stanislaus County, California (Taylor, r9<2, p. 167). Distribution area.-The lower courses of the San Joaquin and Sacramento rivers and the larger tributaries Df these, from Tulare Lake (formerly), Kings County, and Kings River (formerly), near Sanger, Fresno County, north to Sacramento River and Butte Creek, in vicinity of Marysville Buttes, and (formerly) to the McCloud and upper Sacramento rivers; in Shasta County, and eVen (in 1883) to Sisson (now Shasta City), ------

Golden Beatier in Siskiyou County. (Se~ fig. 259.) Altitudinal range, from sea level up to 1000 ft. Life zone, Lower Sonoran and, locally, Upper Sonoran. Specimens examined.-Skulls, skeletons, skins or embryos, contained in the Museum of Vertebrate Zoology: Am~rican River near Fair Oaks, Sacramento County, I; Cache Slough, 10 mi. riorth of Rio Vista, Solano County,_~; San__ Joaquin River near confluence with Tuolumne River (vicinity of Grayson and Westley), Stanislaus County, 10; Mer­ ced River, near Snelling and Hopeton; Merced County, 48; San Joaquin River near Mendota, Fresno County, 15. Total, 75. Besides the material mentioned above, there is contained in the Museum a skull bearing the following data: Sespe River, Ventura County, California; :May 19, 1906; r! adult. As suggested by Taylor (1916, pp. 446, 449), corroboration of this record is needed before it can be credited as an indication of a point of occurrence of beavers in a natural state. The fact that beavers exist in California at the present writing (1931), though in smaller numbers than formerly, is unknown to most people liv­ ing in this State. Doubtless this is because the beaver, despite its relatively large size, is aquatic in habits, mainly nocturnal, and, in California, l~aves comparatively little sign of its activities. The more northern and eastern races are famous for the construction of houses and dams, and for the ulti­ mate formation of meadows resulting from the dainming of streams. Our beaver, a bank dweller, in adapting itself to life in and along sluggish low­ land streams that are subject to great seasonal fluctuation in volume, seems to have lost in some measure its constructive proclivities. The beaver, the largest rodent in California (see fig. 260), equals in size a setter dog and averages 40 pounds or more in weight. Its body is stout, squat, and short-legged; the head joins the body directly, without any con­ striction at the neck; the eyes and ears are small; the hind feet are broad and webbed; and, most curious specialization of all, the tail is horizontally flattened, paddle-shaped, and scaly, not hairy. The coat, of dense underfur and long overhair, is rich both in color and in texture; hence the pelt has been in great demand from the beginning of Western history. As already intimated, everywhere it occurs, or did occur, in North Amer­ ica; the beaver is variable geographically. Some twelve races or subspecies have been distinguished under separate \lames, three of which are credited to this State, Of these three, the golden beaver occupies the great ~entral valley of California, which comprises the lower drainages of the San Joa­ quin and Sacramento rivers. Curiously, in our State, at the present time, no beavers (save by introduction) live in the high mountains, although here mountain streams and bordering growths of deciduous trees abound, like those which constitute ideal abodes of beaver in Colorado and else­ where. Indeed, the golden beaver, so far as we know, never existed at an Fur-bearing Mammals of California altitude higher than about 1000 feet, and all the existing colonies live below the 300-foot level. The most persistent are near sea level. Of the other two subspecies of beaver inhabiting California, the Shasta beaver is limited to the drainage basins of the Klamath and Pit rivers, in

1.. Castor' c. shastensis

2.. Ca.stor c. 5uba.uratu5

.3.... Castor c. frondator


UN'VE"S'TY OF eAUFO ....' ...

Fig. -259. Assumed former general range of beavers in California outlined; Spots indicate stated places of occurrence, chiefly prior to 1924. Ranges of the three races shown: I, Shasta beaver; 2, golden beaver; 3, Sonora beaver. Three record stations outside these ranges are shown with a question mark, since in each of them uncertainty exists with respect to the occurrence or to the raCe represented. the extreme northern part of the State, the Sonora bea:ver to the lower Colo­ rado River and its distributaries. With regard to California, of the three races named, the golden beaver is, and probably always was within history, the most important to human interests. (See map, fig. 259.)

- Golden Beaver , There is really slight difference between the golden beaver and· other races in the gross structural characters. A person familiar with eastern or northern beavers must not expect to find any conspicuous peculiarities about the Californian animals. The color of the coat, more brighdy golden brown than in more northern races, is distinctive only for skins subjected to close comparison; the two chief differentiating features of the skull, broadness of nasals and flattened oudine of the foramen magnum, are found in the great majority of examples but require careful study of series of specimens for their appreciation. Because of the relative slightness and variability of these diagnostic characters, we prefer to consider the form subauratus as a subspecies of the North American species Castor canadensis, rather than as a full species by itself. The closeness of the relationships of all the North American beavers indicates common ancestry at no very remote period of time, phylogenetically and geologically speaking, or else an exceedingly slow and incomplete process of differentiation. We have heard it said that certain sections of the country produce extra- . large beavers. The golden beaver has been referred to.as of maximum size am~ng the North American races. The fact is, according to the evidence now accumulated, that all beavers continue to grow at a greater or lesser rate throughout their lifetimes; So that old and hence large individuals are likely to be found anywhere. The largest California beaver of which we have authentic record was trapped by Edgar Standiford in the fall of 1895 on the Merced River about three miles above Snelling. Mr. Standiford testifies that this animal, a fat male, alive in the trap when found (not drowned and hence not full of water), weighed 82 pounds on standard platform scales which had been tested and found accurate. The weighi1;lg of this Goliath beaver was watched by several citizens of Snelling, and in March, 1920, two of these men ver­ bally corroborated Standiford's record. Of 104 beavers caught in the vicinity of Snelling between January 15 and March 15, 1920, 40, including the largest, were weighed and the heaviest was found to be a female which tipped the scales at 62 pounds. Early in January, 1920, 2 large beavers, collected for the Oakland Public Museum, were trapped near Mendota, Fresno County. These 2 animals weighed 53 and 58 pounds, respectively, and were said to be the largest of the 35 beavers trapped in that locality in January and February, 1920. Of a mated pair of middle-aged adult beavers, taken March 5 and 6, 1920, in an isolated slough near Hopeton, Merced County, the female, containing two small embryos, weighed 42 liz pounds, and the male 43 pounds. Fur-bearing Mammals of California

All the data available to us tend to show that the two sexes are of prac­ tically the same dimensions and weight. See tables on pages 632 and 633. Judging from a series of seventeen embryos now at hand, a beaver at

Fig. 260. Photograph showing relative sizes of a beaver and an average man. In this golden beaver, just trapped, as held up, alive, the distance from the end of the nose to the tip of the tail was 44 Yz inches. The animal weighed 37 pounds. Mus. Vert. Zoo1., no. 3238.

birth weighs close to I pound. We have record of a week-old youngster which weighed 25 ounces. The average weight of seven "yearling" beavers was 20 pounds; five "two-year-olds" averaged 30 pounds. From weights obtained at the close of the rutting season, at which time the adult males , Golden Beaver were not so fat as the pregnant females, the average weight of nine adult males was 40 pounds, whereas fourteen adult females averaged 42 pounds. (See table, p. 632.) However, even among the yearlings and two-year-olds, the females were found to be heavier than the males of corresponding age, so that with regard to weight the females equal or surpass the males. Because of its great size and special structure, we may infer that the bea­ ver's tail serves important uses. And, indeed, the functions of this member proy.~ to be important and varied. In the water the beaver uses its tail as a

Fig. 261. Photograph of a golden beaver. "galloping," taken on February 26,1920, near Snell­ ing, Merced County. 'The beaver is not a speedy animal on land; a man can easily outrun one. However, it is not easy to keep pace with a beaver and operate a camera at the same time. The humped posture of the beaver here shown is' characteristic .. Mus. Vert. Zool., no. 3239. rudder by means of which to steer the body upward, downward, or side­ ways; it is also used to prevent the animal from swimming in circles when towing branches of trees. In directing the body in a laterally changing course, the tail is tilted sidewise so that when viewed in cross section, instead of appearing horizontal as it does when the animal is swimming straight ahead, it would appear more nearly vertical in position. Beavers custom­ arily communicate a signal of alarm by violently splashing or "popping" the water with their broad tails, applied flatways. Furthermore, on land the tail serves regularly as a prop or "third leg" when the beaver sits up on its hind feet to reconnoiter or when at work cutting trees. And finally, when the beaver runs, it gallops and thuds the ground forcefully with its tail. This then serves to support the body momentarily at a certain point in the sequence of locomotory movements and to help propel it forward. (See fig. 261.) . Fur-bearing Mammals of California

The tail is also said to be used, in times of stress, as a scull to assist in propelling the body rapidly through the water; but no observation of ours confirms this idea. The tail of an adUlt beaver is shaped like the blade of a canoe paddle and is about 16 inches in length (see fig. 262). Some writers believe that certain North American Indians, in designing canoe paddles, took the tail of the beaver as a pattern. The large muscular basal part is covered with coarse

Fig. 262. A beaver's tail is flattened horizontally, has a scaly surface, and is shapeq something like the blade of a canoe paddle. In an average adult the scaly part of the tail is 12 inches long, 5 inches wide, and I inch thick in the center. The tail is used as a rudder; also to "whack" the water, thereby producing a warning sound. From freshly trapped male, no. 16383. Mus. Vert. Zool., taken in San Joaquin River, near Grayson, Stanislaus County, December 2, 1911. overhair and soft dense underfur like that on the rest of the body, but the terminal two-thirds is Battened and scaly. In a full-grown golden beaver this scaly part is roughly 12 inches long, 5 inches wide, and I inch thick in the center, tapering to thin margins. The outside covering of this part con­ sists of a layer of tough blackish brown skin which appears to be covered with scales like those on a fish. Actually this is only a superficial resem­ blance, for the epidermis composing the beaver's scales is continuous and merely folded, not imbricate like the shingles on a roof (see fig. 263)' The largest "scales" are in the center of the tail above and below the vertebrae; their width is greater than their length. Their usual form is ·shown in fig­ ure 264, but there is much variation in form and size. A selected average Golden Beaver - " scale for ,an adult is 5 millimeters wide, 3 millimeters long, and supports 7 hairs. The scales are arranged more or less regularly in rows that extend diagonally across the tail. The number of these scale rows across the middle of the tail averages twenty for both young and adults; that is to say, a young beaver is born with its full quota of scale rows. Rooted beneath each scale and protruding backward and slightly upward through its posterior margin, seemingly from between the scales, may be seen a number of stiff brownish hairs, usually as long as the scale itself (see fig. 263). These are best seen in embryos or in young individuals, since in later life they are kept worn down through abrasion by external objects. In adults they are most easily observed along the edges of the tail, where they are subjected to a mini­ Fig, 263. Longitudinal section of the dermis of a beaver's tail, x 2; to right mum of wear. In a beaver embryo no milli­ is anteriorward. A beaver's tail appears meters long, the scaly part of the tail is 26 to be covered with scales, but this is be~ cause the tough black epidermis or millimeters long and IO millimeters wide; outer skin, a, is folded. The so-called there are no hairs on the body or tail, al­ scales are not separate like the scales on a fish. The short coarse "scale hairs," b, though the scaly pattern of the tail is plainly are rooted in the underlying fatty tis­ visible. In a larger embryo, millimeters sue of the tail, c, and protrude back­ 285 waI:d through the rear margins of the in length, the scaly part of the tail measures "scales." Drawing diagrammatic. 60 by 25 millimeters. There is a thin gela­ tinous coating over this part through which appear numerous scale hairs 2 millimeters long. A third embryo, V.5 millimeters long, fully developed and ready for birth, has a well-scaled tail measuring 70 by 27 millimeters. On a typical scale of the tail of this embryo are as many as three stiff brown hairs from 5 to 6 millimeters long. A juvenile golden beaver (no. 32I87) about seventeen days old, has a scaly tail 30 by 90 millimeters, over which are numerous, long, and unbroken scale hairs. The proportional dimensions of the scaly part of the tail for many years have been used by mammalogists as characters in differentiating between the various geographical races of the American beaver (Castor canadensis). Perlpps this is because the unusual structure and function of this highly specialized member has attracted the interest of naturalist and trapper alike. From a critic's point of view, however, the diagnostic value of any charac­ ter must be inversely proportional to its individual variability. Let us, there­ fore, inquire into the individual variation in size, proportions, and shape of beavers' tails. Between February 24 and March I5, I920, one of us (D.) was able to obtain measurements, in-the-flesh, of 36 specimens of Castor canadensis Fur-bearing Mammals of California subauratus, all from one locality. Hitherto, lacking field measurements, the dimensions of the scaly parts of beavers' tails have been obtained only from dried museum specimens, and this, admittedly, has not been satisfactory. The tails of the 36 specimens, with but two exceptions, were all measured by one person. All the tails were "in-the-flesh," not dried, when measured. By eliminating all the yearling and two-year -old beavers, a series of 24 adults is left for consideration. In these adults, the scaly part of the tail is more slen­ der, on the average, in females than in males, a difference not great enough, however, to use as a sole criterion for determining sex. In I5 adult females the breadth of the scaly part of the tail averages 40.4 per cent of its length

Fig. 264_ Part of golden beaver's tail (same as in fig. 262) near middle, X2, to show form and size of the "scales." These are largest near, the axis of the tail, at right, and· decrease in size toward the margin, at left. From Mus. Vert. ZooL, no. 656. (extremes, 31.5 per cent and 46.8 per cent); in 9 adult males the average is 42.8 percent (range, 37.9 per cent to 48.3 per cent). The largest tail of the' series, measuring I42 by 3IO millimeters, belongs to a female. The range shown is greater among females (I5.3 per cent) than among males (IO.4 per cent). Our examination of the 24 adult beavers shows that shape and size of the scaly part of the tail are exceedingly variable (see table, p. 632). The out­ line of the tail otherwise than as shown by ratio of width to length is also notably variable. For the two sexes together, the total range of variation in the proportions of the tail is I6.8 per cent. Among all the seven described races of beaver inhabiting western North America from to Mexico, as listed by Taylor (I9I6), the extreme total variation of this character is . only I6.5 per cent. Extremely broad-tailed individuals (usually old males) occur in distinct and widely separated forms. The type of Castor c. phaeus, from Admiralty Island, Alaska, has a tail the width of which is 54-I per cent of its length (Taylor, op. cit., p. 438). Mearns (I907, p. 353) indicates the maximum for Castor c. frondator from Arizona and Sonora as 54.3 per cent. From the figures given above, it is believed that the importance. of the size, shape, and proportions of beavers' tails has been greatly overestimated.

, , Golden Beaver This member is subject to such great variation (16.8 per cent in a series of 24 adults of one subspecies taken from one locality at one time) and the overlap between subspecies is so great that its value as a diagnostic charac­ ter, even when averaged, seems doubtful. The following experience (D.) is given as typical of what an earnest observer may expect to see of the behavior of beavers under favorable con­ ditions. A beaver pond near Hopeton, Merced County, was visited at7 o'clock in the evening of April 15, 1921. By this hour several beavers were out, swimming back and forth across the pond. Only the small dark head of each animal was to be seen above the water, and this might readily have passed for a floating chunk of wood had it not been for the V-shaped silvery ripple that broke thesmooth surface of the dark water. One of the animals swam swiftly past within ten feet of the observer, who had hidden himself behind a bush. Even at that short distance, he heard no sound of splashing or rippling made by the swimmer. The pond was visited again at daybreak the next morning. At 5 A.M., when daylight was growing fast, a beaver was sighted swimming boldly upstream below the dam, toward a well-worn beaver trail which led over the middle of the dam from the stream to the pond. The beaver, a huge fellow and apparently the patriarch of the pond, crawled quietly out of the stream and clambered up onto the dam, the water dripping from the long glistening overhairs. 1ltanding erect on the crest of the dam, braced by his tail and two large broadly webbed hind feet, helooked like a gigantic prairie dog. After standing motionless, listening intently for several seconds, he re­ sumed his customary position on all four feet, waddled down the upstream declivity of the dam 'and disappeared under the water without making the least splash or noise. In a few seconds he appeared about eight feet offshore and then turned around and swam back along the face of the dam, stopping here and there to inspect some leaks near the crest. T~ice he scooped up mud between his forepaws and his chin aJid plastered it over the breaks. Mter making sure, as it seemed to the observer, that the entire dam was in good order, he turned and, passing close to the watcher, who remained hid­ den, swam away upstream. When the beaver was sixty yards distant, the observer suddenly turned his head to get a better view; apparently the beaver had been looking back over his shoulder, for he lifted his broad tail high in the air, brought it down on the surface of the water with a "pop," signifying danger, and dis­ appeared beneath the water in the direction of his den. This was a burrow in a high bank under a valley oak (see fig. 265). Fur-bearing Mammals of California

Fig. 265. Entrance to burrow of golden beaver in a slough bank under an oak. This entrance was 12 inches wide and 14 inches high. Ordinarily such an entrance is entirely covered by water, but the one shown here is exposed as the result of a break in the dam below. Photographed on August 7. 1920, one mile west of Hopeton, Merced County. Mus. Vert. Zool., no. 3284.

-. Golden Beaver At 5:<>7 another beaver, of middle size, came up the channel, crossed the dam, slid into the water noiselessly, dived 10 feet, and then swam, top of head above water, straight for its home. At 5: I5, a third individual, also medium sized, came up the slide, stood motionless upright on the crest of the dam and then, entering the water as the others had done, swam thence straight on home. At 5: 20, a fourth, large beaver forged up the channel to the dam, and stood on it for a moment until it caught sight of the observer partly hidden 75 feet away. Bolting into the water, "popping" its tail as it .dived, it went off under water toward its home burrow for a distance of ~oo feet before reap?earing on the surface. At 5: 25 the fifth and last bea­ ver went over the dam without taking alarm, dived quietly, soon came up again, and swam on toward its home. ·During these performances, because the air was moving gently from the direction of the dam toward him, the observer kept as silent and motion­ less as possible behind the partial screen of a tobacco bush, giving the bea­ vers little chance to detect his presence except by sight. The fact that two of the five animals did catch sight of him testifies to a certain sharpness of vision despite the relatively small size of the eyes. It was further apparent that beavers see much better by daylight than they do in the dark. An adult male beaver, caught at Snelling, Merced County, ·was held by one hind toe in a No.2 Victor trap. The toe was little injured and the bea­ ver was released in an open pasture and later turned loose in a pond to give the observer opportunity for close watching of its manner of walking, run­ ning, swimming, and diving. This the observer (D.) proceeded to do with notebook in hand. On land the beaver usually maintained a humped-up position in which the back was strongly arched (see fig. 26I). When stand­ ing, more or less erect, the hind feet were planted well apart and forward under the middle of the body. The entire hind foot from tip of toes to heel was placed firmly on the ground and the fully webbed toes were widely spread (see fig. 266). Most of the time, the beaver's hind legs and feet sup­ ported approximately three-fourths of the load. When the beaver attempted to run, it ~ssumed a clumsy lope or gallop, first raising the head and neck and quickly thrusting both front feet for­ ward in unison. The moment the weight of the forward part of the ani­ mal came to bear upon the forefeet, the back was arched and the hind feet quickly slid forward under the middle of the body. At the same moment, or an instant before, the tail, which had been carried slightly recurved along the ground, was pressed forcibly against it (see fig. 26I). In this way the tail was used as a lever to help support the heavy posterior part of the body while l Fur-bearing Mammals of California the hind feet were being moved forward. The beaver's body is thus provided with three places of support, the front legs together, the hind legs, and the tail. A galloping beaver is like a large measuring worm In action, though much more rapid in its movement. When walking, the beaver ambles along somewhat like a slowly moving rat, swaying from side to side as a result of the diagonal instead of parallel

Fig. 266. When a beaver sits up on its hind legs, the tail may extend forward under­ neath, as in this photograph, taken February 26, I920, near Snelling, Merce.d County, or straight out behind. Either way, it se~ves as a balance or prop. Mus. Vert. -Zoo1., no. 3240. This animal had been injured slightly. use of thefore and hind legs; that is, the legs of each pair operate alternately; The broad scaly tail then simply drags behind the body, without giving any apparent service. Good, clear-cut beaver tracks are hard to find. These animals rarely leave tracks, since they spend most of their time either in the water or in their dens. Moreover, when they do go ashore their routes, which are often well defined, frequently pass over sand or mud bars where the tracks are usually brushed out when the body and tail drag over them. Those that are to be found are naturally the ones left by the hind feet rather than by the forefeet. Golden Beaver

If the bank of the river or pond is abrupt, claw marks and prints of both fore and hind feet will sometimes be found at the "slides" where the beavers "haul out." (See fig. 267.) Claw marks and tracks of the forefeet are usually

Fig. 267. A well~traveled beaver trail. This path was 9 inches wide, 4 inches. deep and 36 feet long, It'led from the main slough over the top of a reclamation levee to a tule pond that had been cut off by the dike. Photographed on January 4, 19:'U, 01) Cache Slough, near Dozier, Solano County. Mus. Vert. Zoo!., no. 3458. in evidence where beavers rak~ up mud and decayed vegetation to form their musk mounds. (See fig. 268.) On the night of March 2,1920, a beaver crossing a mud bank near Snelling made ~ series of tracks in the soft mud. All except one made by a right hind foot were smeared over or obliterated Fur-bearing Mammals of California by the action of the animal's body and tail in dragging over them. The one track that escaped obliteration measured, in soft mud, 6Yz inches in length and 4% inches in width. The beaver was traveling uphill (see fig. 269); hence, the track, found on a sloping bank, was imperfect because not quite all the heel pad touched the mud. Later the beaver was caught and proved to be a large female that measured 43Yz inches from nose to tip of tail and tipped the scales at 45 pounds.

Fig. 268. Tracks and claw marks made by a beaver's front feet may be seen in the foreground. These were macie-when the animal scratched up sand to ~orm a scent mound. After depositing scent the beaver had circled about, dragging the broad tail over the mound. Photographed on March 7. 1920, near Snelling, Merced County. Mus. Vert. Z061., no. 3276.

The beaver is as graceful in the water as it is awkward on land. It is sur­ prising that such a large animal can move through the water with so little commotion. Indeed, it causes as little disturbance as a fish a foot long. When the beaver is swimming under normal conditions, more than nine-tenths of the body is submerged. Once, by actual measurement, 94 per cent of the body was under water (see fig. 270). Ordinarily, the upper part of the ani­ mal's head is all that is visible, though sometimes the rump or the top of the back will show also. When the animal's suspicions are aroused, only an inch or two of the top of the head, including the eyes, ears, and nose, is exposed. These most important organs of sight, smell, and hearing are thus kept above the water where they will function up to the last moment possible. The animal under observation did not use the front legs in swimming. , Goldetz Beaver They were relaxed and "streamed" back against the breast, almost touch­ ing each other. The body was propelled forward by the broadly webbed hind feet, operated alternately. In turning, the broad flat tail that ordinarily trailed along just under the surface of the water, almost in line with the top of the back, was tilted edgewise at an angle of about thirty degrees. This beaver often tried to elude observation by submerging quietly, leav-

Fig. 269. Track, in soft mud, of hind foot of a beaver. This track, of an adult individual, was 6 Yz inches long and 4 * inches wide. The imprint left by the tail is to be seen just in front of the track. Photographed on March 7, I920, near Snelling, Merced County. Mus. Vert. Zool., no. 3244. ing scarcely a ripple as it disappeared. In these movements the tail was used as a horizontal rudder to steer the body downward. On the bottom of the pool in 3Yz feet of water the beaver would lie stretched out motionless, and when wrapped about with delicate wreaths of green algae, accumulated in­ cidentally in diving, it resembled to a surprising degree an old waterlogged chunk of wood. As far as could be seen, the beaver did not hold onto the bottom. By actual count of two observers whci stood by, watch in hand, this beaver remained under the water, on three successive dives, 2y.f, 2y.f, and 4Yz minutes, respectively. The extremes of many observations on this beaver were 2 and 5 minutes. Another beaver, alive in a trap, remained under water for II minutes. The first-cited beaver, upon coming to the surface after div­ ing, breathed rapidly and deeply at a rate of 6 respirations iIi IO seconds. At such times the nostrils were distended, nearly circular in outline and almost or quite as large as the eye (see fig. 27I). The beaver spent from I to 2 min­ utes above water between dives. In diving, upon alarm, the beaver's back is arched and the tail raised clear and then brought down. horizontally onto 650 Fur-bearing Mammals of California the surface with a resounding splash just before the animal disappears un­ der the water. Thus it gives warning to its neighbors. When injured or irritated the beaver under observation at Snelling ex­ pressed its displeasure by blowing through its nose, thereby making a hiss­ ing sound similar to that made by an angry goose. On. two occasions, at night, calls undoubtedly given by beavers were heard from different direc­ tions. This call note of one member of the family to another, possibly mother to young, is similar to the soft whimpering of a young puppy . . On June I, 1918, in the middle of the forenoon, at Belldsland, near· Men­ d?ta, Fresno County, one of us (D.) investigated a large beaver house. As



Fig. 270. Diagrams of a beaver swimming. Left, side view; right, rear view.Whe.Q. swimming, under normal circumstances, nine-tenths of the beaver's body is under water. The forelegs are not used in swimming but are relaxed and "stream" back against the breast.

the observer crept noiselessly up onto the house, he heard a subdued gut­ turar or rumbling noise issuing from within the lodge every five to ten sec­ onds. By lying down on the house and listening he was able to hear it more distinctly. It might have been caused by beavers snoring intermittently; or, more likely, as Mr. Ayres puts it, he could "hear their guts working." After listening to this strange rhythmic sound on· a number of occasions, often with ears pressed against the wall of the lodge within three or four feet of the beaver, the observer concluded that Ayres was correct and that the sound was the "rumbling" of the intestines. The animals at this time were feeding almost entirely on the succulent underwater stalks of the cattail and on the roots of a species of water lily, and it is possible that such material produces much intestinal gas during the process of digestion. The "plopping" of the tail has already been described, and may be con­ sidered a manifestation of "voice" because it is a means of audible commu­ nication. This device employed by a beaver may serve to transmit vibrations through water to .individuals submerged at some distance away. Golden Beatler

. The function of the ordinary beaver dam in California is to create a per­ manent pond (see fig. 272) which will: (I) afford a ready refuge from enemies, (2) protect the beavers' house from invasion by keeping the en­ trance under water, (3) serve as a highway of travel,and (4) afford a route for easy transportation of saplings and trees cut for food or for construction purposes. In the northern part of North America two other functions of the beaver pond operate importantly. In these cold regions, by means of the special construction of the dam, the home pond is made deep enough

Fig. 271. Ordinarily a beaver when swimming keeps just his "conning tower" out of water. His eyes, ear~, and nose' are thus able to function, while the rest of the body is under water, safe and out of sight from animals in the air or on the surface. Photographed on February 26, I920, near Snelling, Merced County. Mus. Vert. Zoo1., no. 3.243. to keep it from freezing solid in winter; and sufficient depth of water is provided for the storage of food materials beneath the ice for winter use. Neither of these two uses of ponds applies to our golden beaver, which lives in a warm region at low altitude. After the main dam has been built, the beavers sometimes construct a series of smaller dams across the stream below it. These smaller "support­ ing" dams, often extending nearly straight across the channel, back up the water, and partly serve to reenforce the main dam. The principal function of such a series of supporting dams is, however, to provide a chain of con­ nected ponds, which together constitute a through highway for the beavers, making accessible a greater area. As compared with the northern races, the golden beaver builds relatively few dams. Those that are constructed in California are low and of comparatively small size, since it is not necessary

- Fur-bearing Mammals of California

Fig. 272. By damming streams at strategic points, beavers are able to form ponds. These;': ponds afford protection for the beavers and their nurseries; they make possible the growth of aquatic vegetation, thus increasing the beavers' food supply; and they provide avenues of transportation to and from willow and other trees which the beavers cut down for food. Photographed on November I, 1921, near Hopeton, Merced County. Mus. Vert. Zool., no. 3790.

- Golden Beaver , to make the pond deep enough to prevent its freezing solid during the win­ ter. The presence of permanent deep-water streams in our lowlands tends also to make dams unnecessary. Beavers rarely attempt to dam the main river channels, but devote their energies to building dams across the more quiet old river channels, sloughs, and irrigation ditches. Their repeated damming of irrigation canals in the dry season, when water is scarce and essential to growing crops, has caused trouble and has given the beaver a bad name in several localities in the State. Measurements have proved that beaver dams are not so high as they ap­ pear to be. To our knowledge, the average beaver dam in California is 30 inches high and 50 feet long. A few dams are of small size, apparently be­ cause of a real scarcity of building materials; but this is not the usual cause. The chief factors that determine the ultimate height of a dam seem to us to be height of adjacent ground and kind as well. as amount of available building material. The largest dam out of more than thirty, measured 4 feet in height and ISO feet in length (fig. 273). The following will give a good idea of the location and distribution of beaver dams in California, as existing in I92I. On the north side of the Merced River near Snelling an old river channel leads off from the present main stream and runs through a .series of pastures and fields of corn and alfalfa, uniting with it again six miles downstream. All new growth of willow and cottonwood is nipped off and kept down by the cattle, sheep, and hogs which crowd the pastures, so that the timber bordering the stream now consists almost entirely of large trees, chiefly willows and valley oaks. Although building material is rather scarce, seven beaver dams and three beaver houses or dens were found in one mile along ithis stream. The bea­ ver had long been protected in this territory and had become so accustomed to the presence of pigs, horses, cattle, and even man, that they built their largest dam beside the county road. Tracks made by pigs and beavers were found intermingled in cow trails within a short distance of human habita­ tions (see fig. 274). At another place, just across the main river channel from the town of Snelling, there is a tract of some 400 acres of river bottom where recent dredging for gold has removed the soil and left only huge heaps of smooth water-worn cobblestones. This rock-pile area, of very uneven contour, has numerous depressions, many of which are deep enough to fill up with seep­ age water and form small ponds. When this bottom land was dredged, three straight channels were left through the middle to car;y off the flood­ waters which, when the river was up, threatened to engnlf the town. The Fur-bearing Mammals of California

beavers soon built a series of dams across each of these channels and con­ verted them into rows of long, narrow, beaver ponds. Each dam consisted of one uninterrupted span across the channel, which varied in width from 30 to 100 feet. Some of them were nearly straight, bowing but slightly down­ stream. Others, where the current was swift, were decidedly crescent-shaped with the curve downstream (see fig. 275). None of the dams was arched against the stream. The dams stopped the current, and the silt held in sus­ pension in the muddy waters was deposited over the bottom and sides of the ponds. Many willow seeds, carried along and deposited with the silt

Fig. 273. Beaver dams in California are of relatively small 5iz~. The dam here 'Shown was 4 feet high and ISO feet long, the-largest that we have found within this State-, Photographed I on December 23. 1919. near Hopeton, Merced County. Mus. Vert. Zoo1., no. 3253.

around the margins of the ponds, had sprouted and flourished. The result­ ing thrifty growth of smooth-barked, slender willows furnished the bea­ vers with an excellent and abundant food supply. The high banks about the ponds afforded burrow sites. Beavers respond quickly to conditions favorable to dam building. This was strikingly illustrated by the behavior of a family of beavers near Hope­ ton. In the winter of '92(}-2' a large dead cottonwood tree, standing in the middle of a grain field, was blown down in a storm. The farmer owning I the field chopped up the tree for firewood and dumped the dead brush in :1 several good-sized heaps at the edge of the field, which was sixty to seventy ,I feet distant from the stream. In a few nights the beavers discovered this ,I "windfall" of ready-cut and delivered building material and proceeded at I once to start 'building a dam across the stream. Instead of constructing it

I:, i[ ------~.--~------.-.----~--.. --,--"------

Golden Beaver close to the brush piles;" they carried the brush to the creek and then swam upstream, towing the brush some hundred yards, where they ultimately built a dam 50 feet in length and 2 feet in height across the stream just above a shoal. The naturalist, seeking an explanation for this unusual behavior, waded up and down the stream, noting the width and depth of the channel as well as the character of the bottom, and conCluded that for the beavers' welfare the dam might just as well or better have been built opposite the brush pile or slightly downstream and thus have saved the extra work of towing the brush a huudred yards upstream against the current.

Fig. 274. Beaver "lodge" in a pasture. In the region ahout Hopeton, Merced County, beavers were (when this photograph was taken, in 1921) valued highly by the fanners because dams built by these rodents seryed to maintain the water level during the dry summer months. The natural and inexpensive subirrigation thus furnished, through maintenance of a high water~ table, kept the pastures green and made possible good summer crops on the sandy soil of the near-by cultivated fields: Mus. Vert. Zoo1 ... no. 36°5.

As far as could be discovered (D.), there was no pressing need for the building of a dam by beavers at this particular time and place. Ordinarily, beavers do most of their dam building in the late summer or fall when the water in the streams is low. In this instance the work was done between April I and April 20, when the water was high, and hence with extraordi-

c nary difficulty. It is believed by us that these beavers responded to the stimu­ lus 9f the brush pile for much the same reason that a boy with a sharp-bladed. pocket knife, finding a piece of soft wood, picks it up and starts whittling­ not because he needs to carve out anything, but simply because the stimulus

- Fur-bearing Mammals of California and opportunity are afforded. Why the beavers chose this particular site, so far above the source of materials, is not explainable. Dams built bybeavers differ in form, composition, and constiuction from those built by man. Ordinarily, a man-made dam on the Merced River is

Fig. 275. 1£ the current is swift when beavers build a dam, the structure will usually bow noticeably downstrea~. Photographed on February 28, 1920, near Snelling, Merced County. Mus. Vert. Z081., no. 3251.

composed of concrete and built on a foundation of solid rock across the main channel at a point where the stream is narrow. Beaver-made dams near by are built of saplings, sticks, mud, roots, and rocks (see fig. 276), usually across broad lateral channels, and have for a foundation firm earth or clay. Thus it will be seen that the two types of dams require different sorts of location and that a site which might be best from man's point of view would not be at all.adapted to the beaver's needs, or to its special building powers.

- Golden Beaver Much has been said ~nd written about the wonderful sagacity shown by the beaver in building dams"arched against" the stream. An examination of a number of beaver dams in process of construction throws a great deal of light on this question and leads us to the conclusion that the facts about beaver dam building have been subject to much misinterpretation. We do !lot believe that, as at one time supposed, the curve of the dam is thought out or planned by the beaver. Indeed, the form of the dam and its curve up or down stream appearto be largely a matter of circumstance, as will presently be explained. Beavers do seem to exercise some choice in deciding where their dams shall be built; but our observations have shown that even in this regard they sometimes make mistakes. Having fixed. upon a suitable site where the stream bed is firm and will not readily wash out, and where a suitable supply of building material is available, the beaver family begins to build its dam. If a beaver dam is to be successful, the Initial building unit must be of small size, and elastic, so that strains and pressures are widely distributed and easily absorbed. Construction of a dam often begins at the same time on both banks of a channel and progresses toward the center of the stream, where the two wings finally meet. The beavers start the structure by laying a foundation of 'saplings and brush in the water at the margin of the stream. Foundation saplings are usually laid with their trunks parallel with the cur­ rent and are anchored and held in position by mud and rocks which the beavers pile on them. When thus weighted down, the branches are forced against the bottom of the stream and the dam is to some degree anchored. As the building progresses, the channel becomes more and more constricted. Be~ause the water has to pass through an opening that grows narrower each night, the velocity of the current at this point increases rapidly; and the material toward the center of the dam is gradually forced downstream be­ fore the increased current (see fig. 277). The amount or degree of the curve of the dam downstream is determined in large part by the velocity of the current at the time it is built. If the stream is swift, the curve will be great; if the stream is small or the current sluggish, it will be slight. Occasionally, a beaver dam will be found arched against the stream; but we do not believe that this should be credited to any special "intelligence" of the beaver. Beavers are quick to take advantage of any natural obstruc­ tion lying in the channel of a stream. Obstructions such as rocks, stumps, trees, clumps of bushes, grassy islets, or gravel bars, anyone of which will afford a natural support, are often used as starting points in building dams. As the dam, which may then be built from near the center of the stream Fur-bearing Mammals of California

toward both banks, increases in length, the channels on either side become gradually narrower and the current swifter, with the result that the ends of the dam curve more and more downstream. In new dams which are arched against the stream, the starting point or obstruction can often be found near the middle. As time goes on, the stump may rot or the tree die and decay, and then someone comes along and finds a beaver dam which he supposes was "knowingly" arched against the stream by the beavers. Where the water is shallow or where there is little or no current, it is built

Fig. 276. Beaver dams are built of peeled sticks, saplings, and roots. Sometimes the mass is weighted down with rocks, as here shown. The smaller openings in a dam afe often chinked with mud and matted roots. Dead leaves and floating -debris drifting downstream from above the dam automatically tend to keep it watertight. Photographed on February :28, 1920, neaf Snelling, Merced County. Mus.Vert. Z061., no. 3254.

from one natural anchorage or support to the next. The resulting dam under such circumstances may, when complete, form a letter S and be a hundred i or more yards in length. It has been our observation in California that the , majority bow downstream. Many bow both up and down stream in differ­ ent parts of their courses, whereas orily a very few are arched symmetrically against the current. As the dam increases in height, the beavers often obtain mud and roots by digging a trench across the bottom of the stream parallel to and just above the dam. Material from this source is worked into and solidifies the upper face of the dam. Silt, waterlogged sticks, and decaying leaves also float downstream and lodge naturally against the upper face of the dam,

:~ ;;1,

------.------""_.. _------_ .. _-----

Golden Beaver

where they tend to chink up the smaller openings and thus help to keep the water level near the crest of the dam. Furthermore, fallen leaves are most abundant in the streams in the late summer and fall months, at which time the water in the stream is lowest and the beavers' need of conserving water in the ponds greatest. Because many of the larger sticks which float down­ stream are deadwood, they are light and hence float on the surface. They balance on the crest of the dam, tip over, and lodge in a slanting position on the downstream face of the dam, where they break the force of the water which at flood time pours over the top. The lower ends of the sticks often become bedded in the mud and then become incorporated in the dam, where they serve as supports or braces (see fig. 278). It is thus seen that some of the features of the dams are purely fortuitous so tar as the beavers' own efforts are concerned. Beaver dams need constant repairing. They continually fall into decay because the constituent material is for the most part soft, short-lived wood. Beavers are ever on the watch for breaks in the dam and these are promptly repaired, at any cost. In some dams the crest is skillfully carried along on the same level so that a little water trickles over all along. When the flow of a stream is small, all the escaping water may seep through the dam. In others well-defined spill­ ways are left for the water. Such spillways are often situated in clumps of growing willows, with the result that the flowing water does not wash out a section of the dam. The wood and leaves which form a large part of the dam decay in a short time and form a ready-made, well-watered bed for willow and cottonwood seed. Bushes and even trees soon spring up along the crest of the dam and their roots penetrate and strengthen the structure, which in time becomes a solid earthen embankment bound together with interlacing fibrous root­ lets (see fig. 279). One such dam, on the ranch of W. L. Means near Hope­ ton, is definitely known to have been built by beavers before I860. It has been and still is (in I923) added to and kept in repair by them. This dam is now used by man as a diverting dam for two irrigation canals which carry water to several hundred acres of alfalfa near by (see fig. 280). In general appearance the lodges of the golden beaver resemble large, crudely made haycocks. They are rough in appearance because they consist m~inly of coarse materials-slender poles, sticks, roots, and dead branches. In Galifornia, beaver houses or lodges are few, relative to the numbers of beavers. Conditions favorable to so-called bank burrows-permanent deep water and high dirt banks-abound in the habitat of the golden beaver, ---- -~------

660 Fur-bearing Mammals of California

and burrows in banks rather than lodges usually suffice as homes. Houses are built, for the most part, where the beavers have made dams in sloughs and where the banks of the streams are not high enough to leave room for dens in them above the water level. Four houses have been found that were built in the middle of shallow tule ponds (see fig. 281). Seven large houses, well hidden in dense willow thickets and subject to inundation, were found on slight elevations of low ground bordering streams. Many of those exam­ ined have been in more or less open situations, on the margins of the beaver­ made ponds, from 3 to 20 feet from the water's edge.

1 \ /~ e / \ \b oj

A B c Fig. 277. Diagram demonstrating why beaver dams curve up or down stream. If the dam is started from both banks and extended toward the center of the stream, the current at a is increased and tends to force the material gradually downstream; the completed dam then bows downstream, as at A. If the dam is started at some stump Of rock, d, in midstream and advanced toward the shore, both ends, at band c, tend to curve downstream a:ld when finished the dam will bow upstream, as at B. In water where. there is little or no current, the dam may curve both up and down stream, as at C, according to the contour of the ground Of to the location of willow dumps, as at e, which serve to anchor the dam and keep it in place.

Willow poles from I to 5 inches in diameter and from 6 inches to 14 feet in length were found to form more than half of the material used in lodge construction. Most of the willows are freshly cut by the beavers themselves, although sometimes a great many loose dead limbs have been gathered up and worked into the houses. Also, a great deal of alder and cottonwood is cut and used. Dry redwood planks, fence posts, oars, barrel staves, and even hoe handles have been appropriated by the beavers and worked into their houses. One lodge, situated near the center of a cattail swamp, was composed in large part of cattails and tules. Rocks, fibrous willow roots, bark, and dead or decaying vegetation are often worked in among the interlaced sticks of a house. Mud is used sparingly, and only a few well-plastered houses (see fig. 282) have been found among all that have been examined. Inthe habitat of the golden beaver, the mud coating of a lodge would never freeze solid and would therefore be of litde value as protective armor in winter, as is true in colder countries. ------

, Golden Beaver 661 No two beaver houses are exactly-alike with regard· to size, composition, or site, In illustration of this variation, four houses representing the com­ moner types have been chosen for description. House A, situated near Hopeton, Merced County, represents a transition stage between the burrow in a bank and the aboveground house or lodge. It is of particular interest because it affords a reasonable explanation of how the lodge-building habit may have originated in the far distant ancestral history of the beaver. The beavers apparently had begun by constructing an ordinary burrow in a steep, 8-£oot, north-facing bank of the slough. The

Fig. 278. Diagram showing sectional view of a beaver dam. The foundation of the dam consists of saplings, c, which are cut by the ,beaver, dragged to the stream, and arranged usually parallel with the current. Brush is added; and then rocks, 'e, are piled on top, pressing the foundation saplings ~gainst the bottom and thus helping to keep them in place. As the structure ipcreases in height, mud is dug from the bottom of the pond~ at tI, and used to chink the upstream surface of the dam, at b. Sticks which fioat -downstream are often swept over the crest of the dam, as at f, and later become embedded in the soil, as at d. Such sticks, with the foundation saplings, c, form a double ,set of braces for the dam. original den or nest cavity, situated about 6 feet from the water's edge, was merely an enlargement at the end of the burrow. The den was reached by two passageways under the water; one was short and direct and the other, the back door as it were, p.aralleled the bank for several feet, then turned and entered the slough three feet beneath the surface of the water. A well­ traveled cow trail extended across and above this den, and appearances in­ dicated that following a spell of wet weather the entire roof of the den had caved in. The beavers removed the dirt that had fallen and clogged their burrow, but the roof was gone; the only covering now consisted of a few straggling roots from a near-by valley oak. The resourceful mammals then set to work cutting willow saplings, which they placed crosswise over the top of the open den. Soon driftwood, peeled sticks, and dead fallen limbs were piled on top of this framework, until a substantial stick roof was com­ pleted (see fig. 283). Thus the den which had for its beginning a mere bur­ row in a bank, having caved in, was roofed over with sticks and transformed into a house or lodge .. Fur-bearing Mammals of California

The thatched roof of this house covered an area 12 feet long and 10 feet wide. The inside cavity was 5 feet long, 4 feet wide, and 2 feet high. The house was visited several times in March, 1920, but was found to be unin­ habited. On August 7, 1920, it was again visited (D.), and the following notes were written after a short period of observation.

Fig. :279. The soft wood used in heaver dams soon decays. Then willow and cottonwood trees spring up and cover the darn. In time this becomes a solid earthen embankment which is kept' from washing away by the roots of the trees that grow upon it. Photographed on November I, 192I, near Hopeton, Merced County. Mus.Vert. Zoo1., no. 3791. "As I approached the house at'ten o'clock in the forenoon of an exceed­ ingly hot day, I could detect the peculiar odor of the beavers when I was still six feet away. I crawled cautiously out on the willow poles and sticks Golden Beaver 7 that roofed the den and found that I could look down through the inter­ stices. Just as my eyes were becoming accustomed to the shadows and gen_, eral dimness of the place 'a splash and a gurgle announced the departure of one of the old beavers. Peering about as best I could, I was eventually able to make out another beaver curled up asleep on a bed of freshly cut cattail stalks near the back entrance of the den. A slight move for a better peep­ hole resulted in a twig snapping under my knee. This was immediately fol-

Fig. 280. The dam (in right background) was built by beavers more than sixty years pre­ viously, and had been kept in repair ever since by the animals. Man for years used it to divert \vater into two canals which served to irrigate several hundred acres of alfalfa near by. One of these canals is shown in the foreground. Photographed on February 22, 1920, three miles west of Snelling"Merced County. Mus. Vert. Zoo1., no. 3249. lowed9Y two diminutive splashes and then a final big splash and gurgle, as the baby beavers and another old one became alarmed and dived out of the side entrance into the deep water of the slough. "By removing seven of the larger sticks I opened the roof of the den suf­ ficiently to stick my head and arms through and was able to examine and measure the interior of the house. The floor was level and firmly packed, so hard that I had difficulty ih making an impression in it with my thumb. It stood about six inches above the general water level of the slough, which was low at this season of the year. There were four separate beds, all made of freshly cut and shredded green cattail stalks and blades. The central bed was the largest and measured thirty inches in length, twenty-four inches in width, and two inches in depth. Two small beds were located beside a larger bed near the side entrance of the den [see diagram, fig. 284]. These three Fur-bearing Mammals of California closely grouped beds had been occupied by the other large beaver and the two young ones. The latter were about the size of cotton-tail rabbits. There were no traces of droppings or urine in this den, which was thus sanitary, as it was also damp, cool, and well ventilated. Such houses as this, that are thinly roofed, may well be compared to a screened summer-house, and in­ vestigation has shown that they are regularly resorted toby the beaver fami­ lies during the heat of late summer when the thermometer in the general range of this race frequently registers over 100 degrees Fahrenheit." House B, the largest one noted, was near Mendota, Fresno County. The structure, measuring 30 feet long, 16 feet wide, and 6 feet high (see fig. 285),

Fig. 281. Four beaver houses out of twenty~five, in the , were built (at the time this photograph was taken) on islets in·the centers of ponds,-as here illustrated. Ordinarily. the houses in California are built on the banks of ponds. Photographed November I, 1921, at Hopeton, Merced County. Mus. Vert. Z081., no. 3789. was built around the base of a clump of large spreading willow trees that grew on an island which is completely submerged at high water. In 1918 this house was regularly inhabited by eight beavers, which have since been trapped in irrigation canals more than a mile distant from it. House C, the smallest one that we have measured, was on an exposed bank of an irrigation canal ten miles north of Firebaugh, Fresno Couuty (see fig. 286). This little hut was believed to be inhabited by a single, small­ ish, presumably two-year-old beaver. It was 10 feet long, 9 feet wide, and 28 inches high, measured at the surface of the ground. The floor was 6 inches above the water level of the canal, which was then low. Appearances indicated that this beaver had tried to dig a burrow in the bank of the canal and, finding the bank too low for this, had piled willow boughs and mud over th~ open end of the projected burrow. Then going inside the pile as far as it could, the beaver, beginning under the center and turning round and ------

Golden Beaver round, had gnawed off e~ery projecting limb and twig until it had com­ pleted a suitable cavity with smooth, close-cropped walls, like the inside of a large oval basket. The thickness of the roof above the cavity was 14 inches, the upper 8 inches consisting almost entirely of mud which had been scooped out of the irrigation canal six feet distant and shoved up gopher-, fashion onto the house. The house was visited at the time (November II, 1920) when the mud coat was being applied. Tracks indicated that the soft semiliquid mud had been gathered up and held between the front paws and the chin, and that the beaver had then slid forward on its belly, propelling itself up the bank by its strong hind legs and feet. From the points where the

Fig. 282. We have found only a few mud-coated beaver hOllse·~ in Cali£~rnia. Ra~ely or neve1- would the mud coating freeze here in winter; it would not afford the protection from enemies that it would in a cold country. Photographed on November II, 19:W; ten miles northwest of Firebaugh, Fresno County. Mus. Vert. Z081., no. 3459.

mud had been obtained well-worn trails led up onto the roof of the lodge. (See fig. 282.) The cavity. within was 42 inches long, 36 inches wide, and 14 inches high. A small but well-made bed 14 inches long, 12 inches wide, and 3 inches deep, placed near the back of the cavity; consisted of weed stems, twigs, . ,\nd finely split willow wood. A typical willow sliver used in the bedding measured 75 millimeters in length, 5 millimeters in width, and 3 milli­ meters in thickness, . Although the mud-plastered roof had no openings in it for ventilation, ------

666 Fur-bearingMammals of California there was a loophole between the back wall of the hut and the ground. This opening, just behind the beaver's bed, was 3 inches wide and I inch. high. It was concealed on the outside of the hut by dead weed stalks and growing grass, enabling a beaver to he.ar and probably even to smell a stealthily approaching enemy. A dead and partly devoured mudhen, found in the center of the hard­ packed floor of this house, had tooth marks on it which indicated that per-

Fig. 283. The transition from burrow to "lodge" shown in this beaver hOllse (A <;If the text) flC'l! Hopdon, Merced County. At first there was merely the usual burrow in a bank. Wet weather came and the earthen roof of the den fell in. The beavers at once set to work and made a roof of willow poles over the exposed den. One year after rCp'airs had thus been made, this l?-ouse was used as a breeding den. ~is example gives a reasonable clue to the possible origina­ tion of beaver houses from mere burrows in high banks. haps a mink had been at work, and not a beaver. The house had not been visited by any beaver for several days before the time it was dug out. .. In tlie neighborhood of Snelling in December, I922, all the larger beaver houses examined had been heavily plastered on the outside with mud. The preceding fall had been an unusually mild one. One house, an exceptionally 'old one, was in an exposed situation on a low bank at the edge of a slough. On its sides were three runways up which the beavers had shoved mud to plaster the house. . Apparently; rain, even it small amount of it,will keep beavers inactive and within their houses. There was rain for part of the night on December 6, I922, in theSndlingregion. The following morning, there was very little

- Golden Beaver

.~~------..------... Entrance ------



Fig. 284. Diagrams of interior of the beaver house shown in figure 283. A, sectional view; B, plan. In August. 1920, the bed~ (at h, d, and a, c) were used by two large and two small beavers. Broken line, I, shows reduced size of this house on April 15, I92I, when it was occupied by one adult: female beaver and her week-old kitten. The baby beaver was found in a large freshly made bed of cattails at e, after the mother beaver had become frightened and escaped by the back-door exit, g. 668 Fur-bearing Mammals of California evidence that beavers had been active during the night. Breaks made in the dams on the previous day had not been repaired. Only one of seven traps had been sprung. The weather cleared on December 7, and then the beavers were again active, repairing breaks in dams and moving about. There is no apparent reason for this seeming trait of behavior. House D, built in the fall of 1919 near Snelling, Merced County, was in the middle of a willow clump that had grown up on the bank of a channel

Fig. 285. House B-(o£ the text), the largest beaver lodge found in the State, was near Mendota, Fresno County. This house, photographed o~ June 2,1918, proved to measure at the water level 30,by 16 feet, with a height of 6 feet. In December, 1919, this house was inhabited by eight beavers: This is the largest number of beavers that we have found in anyone house, and it prob~ ably included members of tWo seasons' litters besides the parents. Mus. Vert. Zool., no. 2757. left by a dredger, and was made almost entirely of slender willow poles. It had been inhabited by the builders, a single pair of beavers. In February, 1920, the dried foot of one beaver was found in a steel trap near this lodge, and the decaying carcass of another was found in a burrow within a few feet of the house. This evidence showed that the inhabitants of this home had 6eefi destroyed as a result of illegal trapping. The observer likewise noted that the dam below the house had been cut by the trapper and the water had drained from the pond, leaving the abandoned lodge high and dry so that the main entrance was exposed above the water (see fig. 287). This house measured 22 feet in length, 14 feet in breadth, and 6 feet in height. The slender willow poles had been cut and trimmed of branches by the beavers, then piled crisscross upon the lodge. These poles, which had not been peeled, Golden Beaver were from I to 2 inches in diameter and from 6 inches to 14 feet in length. With them were also several pieces of an old 3-inch plank which the ani­ l' mals had found and dragged up onto the house. In taking this house to '"i: pieces to examine and photograph.the interior, the observer (D.) counted nearly 2000 sticks greater than liz-inch in diameter and more than I foot in length. The largest was 4 inches in diameter and 4 feet in length; many were 2 inches in diameter and from 8 to 12 feet in length; one was more than 14 feet in length. A quantity of matted will0w roots .and small rocks had been used to chink up the openings between the poles, but there was not more than one cubic foot of mud in the entire structure.


8 Feet '.

Fig. 286. Diagram showing a small~sized beaver house (C of the text). This lodge was on the bank of an irrigation canal paralleling the San Joaquin River not far from Firebaugh. It was 10 feet long, 9 feet wide, and 2 Yz feet high, and accommodated but one beaver. Burrow, b, opening into canal; bed at n. A clump of willow saplings. protected the egg-shaped nest cavity (see fig. 288). It was 42 inches l{)ng, 30 inches wide, and 18 inches high. The walls were tightly chinked with short pieces of wood, matted roots, and rocks. The bottom had been excavated in the bank and was thickly lined with soft, shredded, reddish bark of the incense cedar. This bark, from cedar saw logs which had been cut many miles up the Merced River near the boundary of , had floated down the river from the sawmill at Merced Falls. The roof above the nest was 2 feet thick, and there was ample ventilation through the interlaced twigs and poles. Two entrances led from the pond into the house, uniting at the mouth of the bedchamber (see figs. 289, 290). One was short and direct; the other widened out into a chamber 42 inches wide, 30 inches long, and 30 inches ------

Fur-bearing Mammals of California

high which formed a sort of hallway right at the water level, Marks of muddy droplets on the walls indicated that the beavers used this as a place in which to shake their coats free of water before entering the nest proper. The relation of the beds in the lodge to the water level of the surrounding pond is apparently important to beavers. Beds are always built a few inches, usually from two to six, above the mean water level, and the water usually just fills the entranceways. The beavers thus have a constant and reliable water gauge inside their houses and within a few inches of their beds. Upon

Fig.'287. Beaver house D (of the text), which was abandoned afte'r a poacher had trapped two of its occupants, broken their dam, and drained the pond. The entrance, originally under water, is here shown exposed. Photographed on February 28, 1920, near Snelling, Merced County. Mus. Vert. Zoo1., no. 3271. entering or leaving a house they quickly become aware of any change in the level of their home pond, such as would be caused by a break in the dam. Water sufficient to cover the entrances of their houses is necessary to protect the beavers from predatory enemies. This is one reason why they are so alert to keep their dams in repair and their ponds full of water. In early December, '922, a whole family of beavers trapped near Snelling Was thought to be from one house. This family was made up of the 2 adult parents, 3 young approximately twenty months old, and 3 young about eight months old. Seven of this group were caught within one week. These Were all taken within a stretch of one-half mile along a slough where there were four fair-sized dams, 50 to IDa feet long and 24 to 30 inches high. Only two seasons before, in '920, this neighborhood had been thoroughly trapped Golden Beaver over. The appearance of the sexual organs in the two adults indicated that the mating season was not far off. The golden beaver is characteristically a bank beaver. High dirt or clay. banks are common along many of the permanent water courses inhabited by this race. In such streams there are holes or pools deep enough to afford a ready refuge for the beaver and to cover the entrances to their burrows. Our observations in California lead us to believe that here beavers' breeding . dens are usuaily secluded burrows in banks densely overgrown with brush

Fig. 288. House D (of the text), cut away to show interior. This house was composed of nearly 2000 willow sticks, by actual connt, from Yz -inch to 4 inches in diameter and from Yz -foot to 14 feet in length. The nest chamber was situated behind the clump 'of growing wil­ low stems near"the upper center of the photograph; the picture was taken on February 29. 1920, near Snelling, Merced County. Mus. Vert. Zool., no. 3273.

rather than lodges, even though lodges in the vicinity may at the same time be used for ordinary shelter' purposes. In some places a series of burrows has been found, some of which are situated nearer the top of the bank than others. This arrangement had been developed, apparently, to meet the fluc­ tuating level of the water in the stream. By having a number of burrows at different levels the beavers are enabled at any time to occupy a den that is protected by having its entrance under water. , On several occasions beavers have invaded a large irrigation canal near , Mendota and have made numerous burrows in the high banks left on either side of the ditch by the canal dredger. A number of these burrows, examined on March 25, 1920, were found to be from 20 to 24 feet in length and from Fur-bearing Mammals of CalifOrnia

18 to 24 inches in diameter. They led from beds of tules at the margin of the canal to the highest parts of the adjoining levee or bank (see fig. 291), ex­ tending beneath the surface of the water until within two or three feet of the den. One of the burrows which was dug out measured 24 feet from the entrance to the nest cavity or den. The den was 6 feet in length, 4 feet in width, and 2 feet in height. The top came within 14 inches of the ground surface. Ventilation was afforded, at least during dry weather, through the cracks in the adobe soil that formed the roof. Dry, shredded willow bark and leaves carpeted the floor. Numerous short, peeled willow sticks, meas­ uring from liz-inch to 2 inches in diameter and from 12 to 18 inches in

a b d '

Fig. 289. Diagram to scate, showing plan o~ house D (of text). The main entrance, a, connects with a spacious hallway, b, which leads to the nest chamber, c, A second entranCe is shown at f. The nest is protected by growing willow stems at hand i. A large alder, g, also protects the house. A wood rat had built its nest in the wall of the beaver house among wild fose bushes at j.

length, were strewn about on the floor of the passageway and den; the sticks had probably been taken into the den and the bark eaten off at leisure, the peeled remnants being then discarded. Sanitary conditions appeared to be ideal; there was no evidence that feces or urine were ever voided in the den. The construction of canals appears to be nearly a lost art among golden beavers. Canals that have been found in California have been few and of small size. Here, we have not found, nor would we expect to find, any com­ plicated system of locks and canals such as has been described of beavers elsewhere (Mills, 1913, p. 88; Dugmore, 1914, p. II6). All the canals which we have examined have been dug on the same general level as the adjacent ------,------

Golden Beaver , streams or ponds with which they connected. The levelness of the beaver's habitat in California, together with the prevailingly alluvial soil, would seem to be favorable to the construction of beaver canals. However, the primary purpose of the canal is to make the obtaining of food easier and safer, and in this State beavers have no need of transporting and storing large quanti­ ties of heavy green wood for winter use; hence canals are not necessary here

I \ I

Fig. 290. Diagram to scale showing sectional view of house D (of text). Underwater entrances, a and f, lead tei nest c. A burrow·, e, used by the beavers to duck into in emergency, extends under and behind the nest c, ending blindly in the bank at d, high above the former watedeve1 indicated by dotted line w. .

. 6 Feet ,

Fig. 291. Diagram of beaver burrow in canal bank. About IS feet of the burrow, b, is under water. The nest or den, at u, is above the water level in the higher part of the levee. A burrow such as this may cause the levee to give way and result in damage through the flooding or "drowning out" of the growing crops near by. and are rarely built. Another important reason why they are not built may be because man destroys many of the predatory natural enemies of the beaver. Beavers are quick to learn the advantages of sanctuary, and do not now hesitate to make long portages or to wander about overland in search of food such as acorns, going even as far as one hundred feet from streams and ponds in open country. Illustrating how canals are sometimes built, the following account, which we believe to be typical, may be given. In December, '9'9, a canal near Fur-bearing Mammals of California

Hopeton had its beginning as a mere slide or landing place where a well­ traveled beaver trail dropped into a pond. There was nothing at this time to indicate the beginning of a canal, but as time passed the indentation in the bank grew deeper and wider. In the summer of 1920 this trail was aban.­ doned by the beavers, but the following March it was again in use. When examined on April 15, 1921, it was found that the beavers had recently con­ verted a part of the trail into a ditch filled with water, thus forming a canal 10 feet long, 12 inches wide, and 10 inches deep. It was gradually extended until at last it almost entirely re­ placed the 40-foot trail that con­ nected the two ponds. In this instance as well as in others we have noted, the canal had its be­ ginning at the water's edge and was dug landward. As the mud and roots were removed, the water, always seeking its own level, flowed back into the newly dug canal. Thus it will be seen that the level or grade of the canal is automatically determined by the water, and all that the beaver Fig.' 292. Feces of beaver in one foot of water. The constituent material resembles coarse sawdust. has to do is to follow the water. When the mucous envelope decays, the droppings Examination of numerous beaver go tq _pieces and the grains of wood float into the interstices of the dam,-helping to make it still more canals in Alaska and British Co- impervious to water. The white peeled stick in the lumbia, as well as in California, photograph was exactly 12 inches long. Photo· graphed on PcbJ,"Uary 28, 1920, near Snelling, Mer- leads us to believe that fully 90 per ced County. Mus. Vert. Zool., no. 3277. cent of the canals dug by.beavers anywhere are built in this manner: the beavers let the water flow outward from some body of water, stream, or spring which they use as a base. With this fact in mind we have no ground for "marveling at the beavers' engi­ neering skill in running levels." Only a few beaver "forms," or beds outside of dens, have been noted by us. Those found were on steep brush-covered banks near deep water, some at the bank tops. This setup exists on the San Joaquin River near Mendota. Here, slight surface beds have been found in secluded localities. In one place, judging from the tracks, a lone beaver had crawled out of the stream, climbed the bank, shaken his fur free of water, and, after digging or wallow­ ing out a slight depression, had lain down to rest. Such retreats are obviously Golden Beaver insecure, and the occupant is always ready to escape by a short scramble down the bank, ending with a plunge into the stream. The beaver is a good-sized animal and a single individual consumes a relatively large amount of food. Much of the wood eaten passes through the body with but little change. Despite these facts, it is difficult to find drop­ pings or feces of this animal. All that we have been able to discover had been deposited in water. Once, feces were found in a foot of water in a small pond just above a dam. Some of these had been deposited singly and others in . bunches of six or eight (fig. 292), or adhering to each other end to end,

Fig. 293. Materials before and after passage through the beaver's "mill" (m.olar teeth in action). Young beavers do not chew their food as well as old beavers do. 'this has been shown by examination of many stomachs. In this photograph, the fine material, at upper right, was taken from the stomach of an old beaver, and the pile of coarser material, at center, came from the stomach of a young beaver. In both piles were represented the wood and bark of willow. Mus. Vert. Zoo1., no. 3797. sausage-fashion. An average individual dropping was cylindrical in shape, slightly longer than thick, and measured I inch in length and Ys-inch in diameter. A very thin gelatinous (mucous) covering wa.s noted on many of the droppings. This covering disintegrated when the feces were lifted from the water, thereby revealing that the droppings consisted almost entirely of finely chewed pieces of clear white wood, chiefly willow and cottonwood, which resembled clean coarse sawdust (see fig. 293). Many of the pieces of wood were from I to 3 millimeters in diameter and from 3 to IO millimeters in length. In this material there were numerous finer particles of wood, but the bark had almost all disappeared. Several pieces of wood liz-inch in

- Fur-bearing Mammals of California length were noted in one large dropping. Droppings found in the large in­ testine of beavers, when examined proved to coincide closely with the above_ mentioned findings with regard to size, shape, structure, and composition. It was noted that the feces deposited in the water by beavers float down and collect just above the first beaver dam downstream, where in time they disintegrate. Observation in some places showed that the fine white particles of wood had lodged in the smallest openings in the upper face of the dam and chinked them up, just as any other buoyant sediment would do. It appears that male and female beavers, with the exception of females nursing young, cannot dependably be distinguished by external examina-

Fig. 294. Photograph of female beaver fetus, from Snelling, Merced County, M~rch 16, 1920, showing situation of nipples, here indicated by the four white spots. Both pairs of mammae are on the breast. These spots are absent in males. Mus. Ver.t. Zool., no. 3248. tion. There is a common canal (cloaca) for receiving and discharging the products of the genital, urinary, and digestive organs. The reproductive organs lie within the body cavity and cannot be seen except by dissection. Both the two pairs of mammae are pectoral. The positions of the mammae are best seen in female embryos of advanced age (see fig. 294). The first pair is directly between the forelegs, and the second, in adults, is about four inches farther back. All the mammae, in the adult, are concealed in the dense underfur of the breast and have proved difficult to locate except in females that are pregnant or are suckling young. Females are as large as males of corresponding ages. No differences in color or in quality or quantity of fur have been noted between the sexes. Males are likely to be scarred as a result of combats occurring in the m'lting season, and hence their pelt value, other things being equal, is frequently less than that of females. The genus Castor differs from all other genera of Californian rodents in .its possession of two pairs of highly developed scent glands. The beaver does not use the products of these organs as a means of defens~, as do the skunks (Mephitis and Spilogale). Indeed, their function is of quite another sort, as

, Golden Beaver

will be seen from the following account. The scent glands of the beaver are situated on the under side of the body near the cloacal orifice (vent). The larger pair of glands constitutes the so-called "bark bags," which secrete the castoreum. These castor bags are flattened and oval in form and of a light yellow color. In an average breeding adult they are about 5 inches in length and 2 Yz inches in width, but are subject to noticeable variation with season. The walls are thin, almost membranous, and have a creased or folded appearance. Numerous pockets inside the bags contain the casto­ reum, which, when fresh, is a granular, sticky, yellow substance of a rather pleasant odor. When dried, it is much used in the preparation of certain "animal scents" used by trappers in luring various other fur-bearing mam­ mals, as well as beaver, to their traps. It is well known that formerly, and for a long time, it was used in perfumery and, in certain countries, as medicine. The oil sacs are to be found behind the "bark bags," and like them they also open into the cloaca. These oil sacs are thick-walled and about one-half the length of the bark bags. Because they are similar in size and form to the testicles of the male, they are sometimes confused with them. Both pairs of scent glands are as well developed in the female as they are in the male. In both sexes, they attain their maximum size and development in the mating seasoh. An adult male beaver, weighing 44 pounds, taken on the American River near the town of Fair Oaks, Februaty 17, 1920, was found upon dissection to be at the height of sexual development; in this animal the testes were 3 inches in length and I inch in diameter. Outside of the breed­ ing season, their length would be scarcely more than 1Yz"inches. The vasa deferentia were greatly enlarged and convoluted. The bark bags of this breeding male were 6 inches long, 3 inches wide, and contained a rather large accumulation of castoreum. The thick-walled oil sacs were about the size of the testicles and when pressed exuded a thick, pungent, yellow oil. The combined weight of the oil sacs and castor bags, when removed from the animal, was 250 grams (more than 8 ounces), and 100 grams (3Yz ounces) when thoroughly dry. At this rate it would take only 4Yz sets of large-sized scent glands dried for marketing to weigh a pound. Ordinarily, r 5 or 6 sets would be required to make a pound. r As previously intimated, these scent glands are close! y associated with I the reproductive system in function as well as in location. Their main func­ tion seems to be the distribution of sex "information." Both sexes deposit the secretions of these glands, and are attracted by them. At Snelling, dur­ ing the rutting season, traps set beside scent mounds-that is, placed where castoreum and oil are regularly deposited by breeding beavers-caught Fur-bearing Mammals of California

both males and females, in about equal numbers. The following. incident shows that besides preventing the unduly rapid loss of scent by evaporation, the oil from the oil sacs also apparently carries some special, subtle message. Several old "trap-wise" male beavers were being trapped for, without any success, for several weeks at Snelling. Castoreum alone was being used, in renewed quantities, as a lure at the traps. Then, oil from the oil sacs of a female was substituted and the old male beavers were promptly caught in the very traps which they previously had shunned. During the mating season bea­ vers of both sexes nightly deposit both oil and castoreum at certain points that are prominent in their nightly itinerary. These deposits apparently provide a means of in­ tercommunication among the adult beavers of the general neigh­ borhood. They probably give in­ formation about the stage reached on the part of each individual in the season's reproductive cycle. During the mating season this record is read through the sense of smell, by the members of the colony as well as by any visiting beaver that may happen along. Scent mounds are composed of Fig. 295· Scent mounds are at the water's edge mud, sand, dead leaves, dried on certain bars and points. During the mating sea~ d lk d son the mounds are, it i~ indicated, regularly visited wee sta s, an sometimes green by beavers of both sexes. vegetation, -mixed with secretions of both the oil sacs and the bark bags. They are situated on sand bars, islets, promontories, or other points regularly visited by the beavers. (See fig. 295.) Judging from the tracks left in the soft ground in the vicinity, each beaver, after taking olfactory note of the sign left by his or her predecessor, deposits oil or castoreum, or both, on top of the mound and then covers it up by scratching together damp refuse or mud with the forefeet, so that event­ ually a conical mound, sometimes a foot in height and cover,ing nearly a .square yard of ground, is accumulated. No deposition of feces or urine is made in or about the mound; at least, none has been found in more than ninety scent mounds examined by us. On March 5, 192o,ten scent mounds Golden Beaver • were counted within a distance of 100 feet along the margin of a cattail swamp. This row of mounds was about 200 yards from a large inhabited lodge. One of the largest mounds, composed mainly of decayed cattails and tules, measured 4Yz feet in length, 3 feet in breadth, and I foot in height. On a warm damp morning a person could detect the odor of the castoreum in this mound at a distance of 50 feet; beavers, believed to have a much keener sense of smell than man, could probably detect the site of such a 'mound at a much greater distance. '. Occasionally a small scent mound may be found in any month of the 'year, by careful se';"ch. It has been our observation, however, that scent mounds are primarily associated with the mating season and reach their highest development within that period. In the fall of '9'9, and again in '920, a search made for scent mounds in several localities where beavers were common resulted in the finding of only one small one, barely 3 inches in diameter. This was discovered on the bank of the San Joaquin River near Mendota, Fresno County, on November 10, '920. It consisted of a bunch of dry grass that had been twisted up into a ball, upon which had been deposited a small amount of dark-colored castoreum. In February, '920, scent mounds were found to be numerous in the vicin­ ity of Hopeton. Two weeks later (March 5) the mounds had attained what was estimated to be their maximum size, although some were being added to slightly. When the sites of these mounds were again visited five months later (August 7) they were overgrown with a dense growth of weeds, and the mounds had almost disappeared. At this time, no trace of recently built scent mounds could be found, although beavers were inhabiting a lodge close by. On February 28, '92', the locality was again visited and many freshly made mounds were found; about half of them were situated just where sirriilar ones had been seen a year previously, the rest from two to ten feet from their former sites, in the same general vicinity. At this time, seven fresh scent mounds were found on an islet 20 feet wide and 30 feet long which had grown up to willows and cattails, in water somewhat over knee deep. A medium-sized mound on this islet was composed of decayed roots, dead leaves, and freshly pulled cattail stalks, all twisted up together; it measured 18 inches in length, '4 inches in width, and 5 inches in height (see fig. 296). The larger of two fair-sized scent mounds found at this time measured 42 inches in length, 32 inches in width; and 10 inches in height (see fig. 297). The odor of the castoreum in this one could be readily de­ tected, in calm atmosphere, by a person ten feet distant. On April '5, '92', these scent mounds were again visited and the two largest ones that had 680 Fur-bearing Mammals of California been so prominent six weeks earlier had almost disappeared. Nearly all showed signs of decay; only a few had been kept up and even these consisted of but a few handfuls of material. . From the foregoing account it will be seen that scent mounds are regu­ larly constructed by the adult beavers at appointed places. The period dur-

Fig. 296. A scent mound at an early stage of accumulation, as here shown, is composed ot matted foots, green cattail stalks, and mud, upon which scent (castoreum and "oil") is deposited. The function of such mounds is to con~ vey information having to do with the annual reproductive program. Photo~ graphed on February 28, 1921, two miles southwest of Hopeton, Merced County. Mus. Vert. Zool.,- no. 3612 ..

ing which they are under construction coincides closely with the mating season. When the mating season is over the scent mounds are no longer repaired or visited by the beavers. It has been said (for example, Seton, 1909, p. 471) that the beaver is "a strict monogainist." This statement seems not to accord with the facts. If beavers are monogamous it is in marked contrast to the promiscuous mating of rodents in general. The following data bear on this question. Golden Beaver 681

In 1920, 80 per cent of the beavers trapped at Snelling in January, in the fore part of the breeding season, were males. Probably at that season of the year the males were traveling about more than the females. Traps set at scent mounds were notably successful, indicating that the males were alert to the purpose of the mounds. Many of the males trapped in the mating season had recent incisions intheir rumps and tails, and the shape, slant, and depth of these wounds showed that they had been made by the incisor teeth of other beavers. Female beavers caught at the same time did not show such scars. If the 'males are monogamous and already mated for the season, why, throughout the mating season, should all the breeding males chase about over miles of territory, fight viciously among themselves,. make numerous scent mounds, and otherwise spend a great amount of energy, unless they were seeking further opportunities for breeding? If the males are strictly monogamous, why didn't these stay at home, each with his chosen female? Yet many of the females trapped were found to be pregnant, and at the same tirne almost all the males were traveling about as if seeking still other females. In 1920, a certain male beaver, identified by the absence 'of two toes of a , , hind foot previously lost in some trap, was noted living in an isolatedpopd where also was a pregnant feinale. Both beavers were subsequently ttapped and their identity, as well as the pregnancy of the female; established. The tracks of the male showed that at that time, which wa";n the mating sea­ son, he made nightly excursions, heaping up scent mounds as far distant as two miles from his headquarters. The actions of this male, which were characteristic, indicate that although he already had.one mate. he was look­ ing for another. In 1920 and 1921, the monogamy theory of beavers was giveu'a crucial . test, and was found wanting. Between JanuarY1sand March't5, 1920, II4 beavers were trapped undet permit in a region near Snelling 6 miles long and 2Yz miles wide. Careful checking before and after trapping showed that the uumber caught equaled about'one-half of the estunated total beaver population (228). All the beavers caught at this time were skinned by one man, who, when fresh specimens were available, repeatedly proved that he was able correctly to tell the sex of a beaver by dissection after the animal had been skinned. of the II4 beavers trapped, 77 individuals (70 per cent) were males and 37 (30 per cent) were females. 'i The follo~ing data and consequent reasoning have a bearing lipon the monogamy theory with regard to beavers .. We have found, from examina­ tions of embryos, that at birth the sexes are about equally represented. Adult Fur-bearing Mammals of California males might number somewhat less than females in a given locality, since in the mating season the males are known sometimes to fight to the death among themselves. Granting an equal representation of the two sexes be­ fore trapping began, there were 'some II4 pairs of beavers at Snelling pre­ vious to the trapping period. Since 77 males and 37 females were known to have been trapped in '920, 37 males were left to mate with 77 females in '921. Under such circumstances, either one of two things might happen.

Fig. 297. As the rutting season advances, the scent mounds increase in size. When the mating season wanes they are neglected by the beavers and soon de.cay, or the materials composing them become scattered. Photographed on February 28, 1921, two miles southwest of Hopeton, Mer­ ced County. Mus. Vert. Zool., no. 36rr.

If beavers were strictly monogamous, then there would be 40 beaver widows left unable to secure 'mates and to breed. As a result, the crop of young bea- . vers for that season (1921) should have been greatly reduced. However, if mating were promiscuous, then almost all the 77 females should have bred. During '92', the year following the trapping, a special watch was kept to see if there existed any large number of nonbreeding females; in vain. Most of the old dens and houses revisited were found to be occupied. The females had all bred as usual, as was shown by the litters of young known to be in the dens. This would not have been possible if the relict males had each been constant to one female. There is little doubt that a female is far more constant to a given male than a male to a given female. The female beaver is much the more closely identified with the home locality, because there fall to her lot the care and Golden Beaver

safety of the young. Having once settled upon a favorable homesite, she is likely to stay in that place for life; whereas, the male is not closely tied down to one locality by duty to offspring and can wander about without jeopar­ ,dizing the interests of the race. The chances are, then, that a female, once established, will mate year after year with one male. He adheres to her established homesite as a sort of headquarters; at the same time there is a strong probability, supported by the data just given, that he continually. casts about in the mating season for chances to mate with any other females agreeable to his advances. From the facts given above we conclude that in one sense the females of the species may in many instances be monogamous, the males at the same time polygamous. The species, as a whole, is not in any strict sense "monogamous." We do not believe, as has been suggested (Morgan, 1868, p. 137), that "beaver outcasts" are individuals that have lost their mates and refused to mate again. We think that such outcasts are simply old males that have bee~ "licked" or "driven out of the herd" by the younger, more vigorous males and that they obtain new mates when opportunity arises. Beavers are generally believed to reach maturity and to breed in the third year of their lives. In two instances known to us, beavers which, judged by . skull characters, were barely two years old, were found mating. Judging from the reproductive organs of twenty-eight females examined between January 9 and March 15, the mating season for some individuals begins the' first of January and for others continues to as late as March 5. For the entire population it thus extends over a period of two months. Beavers in the Sac­ ramento Valley appear to breed slightly earlier than those of the San Joaquin (see table, p. 634). It has been our observation that the early breeders are old females and that most of the late breeders are small young females. We believe that the period of gestation in the beaver lasts about three . months. This is figured from the time of the beginning of the mating season to the time of birth of the earliest litters, It is difficult to determine exactly when young beavers are born, since the females retire to the secluded breeding dens for this event. Our most trust­ worthy information on this point is furnished by embryos found in trapped females and by small young beavers found in breeding dens. A set of four well-furred and fully developed fetuses, 13 inches in length, weighing a pound (more exactly, 485 gm.) each, was taken from a beaver trapped " March I I, 1920. This female would probably have given birth to her young in less than a week. Judging from the stages of development in the various sets of embryos examined (see table, p. 634), the time of birth varies from


-. ------

Fur-bearing Mammals of California the middle of March to the last of May. The majority of young are probably born in April. Our information on this topic is meager. On April 15, 1921, a week-old baby beaver was found in a lodge near Hopeton, Merced County. The den (see fig. 283), which had previously been visited, was originally a burrow in the bank. It had caved in, and had then been roofed over with willow poles by the beaver and made into a' lodge or house. When visited, at midday on March I; 1921, it was found to be untenanted. On April 15, as the observer (D.) approachedthe lodge at dusk, he saw that the outside of the structure had recently been repaired.

Fig. 298. At birth, a baby golden beaver weighs about a pound. The individual here shown, when one week old on April 15, was 14 inches long and weighed 25 ounces. The average num­ ber of young in a-litter is 2.7, with I and 5 as extremes. These figures are given on the basis of 14 litters exa11?-ined. Mus. Vert. Zool., no. 3615 .. Upon a closer approach, when he was within six feet of the lodge, he heard a loud whimpering coming from near the back of the den. The cry re­ minded the listener very much of the softer whimperings of a very small puppy dog. After listening for several minutes to this cry, he became fairly certain that all the noise was made by one baby beaver. When the observer crawled right up onto tl,e house, the mother beaver, which until then had remained quietly in the nest with her lone young, nursing it, as was shown by its milk-wet muzzle, plunged noisily out through the main underwater exit of the lodge. The baby beaver at once ceased whimpering. When the visitor made an opening through the roof of the house he found the youngster squatting motionless near the back of the bed that so recently had been occupied by its mother. The interior of this lodge, which Golden Beaver 685 had been examined previously on August 7, '920, was now found to have been recently remodeled, as was the exterior, so that it was decidedly smaller than it had been before. There was but one small bed, used by the one adult beaver and her baby. There was no indication that the male parent or any other adult occupied the den or shared in any way the care of the young. The bed, which measured 24 inches in length, 18 inches in width, and 3Yz inches in depth, was too small to be shared by two adult beavers. The lining, unusually deep and soft, was composed of clean, slightly damp, shredded tule stalks. The baby beaver showed no fear of man, and instead

Fig. 299. When sought out in its home, the baby beaver made no effort to escape by taking to the water, though given repeated opportunity to do so. However, a week later (April 23, 1921), when placed near a stream, it proceeded to swim and dive. In the picture it is shown drinking. Mus. Vert. Zool., no. 3618. of fleeing, snuggled up to the hand that was thrust into the interior of the lodge. When captured, the young beaver was judged to be not more than a week old. It measured 14 inches from end of nose to tip of tail and tipped the scales at 25 ounces (see fig. 298). It was well protected from cold and dampness by the thick, soft pelage, the underfur and overhair of which were well developed. The overhair on the middle of the back was 23 milli­ meters long; the adjacent underfur was 15 millimeters long. When the young beaver was first found, its fur was perfectly dry.· The youngster, though given repeated opportunities, made no effort to escape by entering the water. This young beaver was brought to Berkeley and an attempt was made to rear it. When it was about two weeks old it was left at the edge of a shallow 686 Fur-bearing Mammals of California pool (see fig. 299)' At first the beaver did nat seem to know what the water was, but after taking the first smell it plunged boldly in and swam about and dived with evident enjoyment. When first obtained, it had just cut its upper incisor teeth. The lower incisors were long and sharp, yet the young­ ster made no effort to bite one's finger even when this member was placed between its teeth. The eyes were dark brown and small, the part showing between the lids measuring 3 millimeters in diameter; yet the animal was able in dim light to distinguish readily objects (children) in motion across a 20-foot room. Whenever possible, the beaver avoided direct sunlight and other strong light. The scaly, fully developed tail was much used as a prop when the animal sat up. Even at this early age both of the scent glands, the bark bags and the oil sacs, functioned whenever the animal was irritated, at which times a musky fluid was emitted. . The young beaver was sociable and appeared to enjoy being held in a per­ son's lap. It responded readily to the human voice and soon learned to come· when called. Its whimpering or crying was so similar to that of a very small child that the two were frequently mnfused.. When the baby beaver and a human baby were crying at the same time, it was difficult for the child's father to distinguish between them when he was in an adjoining room. rt was quite difficult at first to getthe young beaver to nurse from a bottle. Various sizes and forms of rubber nipples were tried and finally a short stubby one was obtained which was similar in shape to the nipple of a feinale beaver. The baby beaver took readily to this nipple and consumed an ounce of warmed cow's milk fout times a day. When nursing, the dimin­ utive beaver sat up on its hind feet and used its scaly tail as a brace behind. The nipple was held habitually in one side or the other of the mouth so that it was not injured as it would have been if it had been held in the middle, between the sharp incisor teeth. While the beaver was nursing, the forepaws were used continually as hands to work and massage the rubber base of the nipple. When the animal ambled slowly along over a smooth surface, the flat tail, held slightly curved upward just off the ground, swayed from side to side. The young beaver never, even when in a hurry, galloped, or hit the ground with the tail, as adult beavers do. Many full-grown beaver parasites were found on this young beaver, doubtless immigrants from its mother's coat. Under favorable circum­ stances, as many as five or six of these aberrant beetles (Platypsylla castoris) were seen at one time about the head of the animal. These parasitic insects

- Golde1t Beaver , are 2 to 3 millimeters in length, yellowish brown in color, oval in form, and flattened like a louse. We have found them to abandon the beaver host within a few hours after the animal dies. This probably explains why we have only once been able to find them on trapped and drowned beavers. After birth, baby beavers remain in the house or burrow about three , weeks before they begin to venture out much. Their tiny tracks indicate that at first they do not wander far from home. It has been our observation that . for many subsequent weeks the mother and her small young go about to­ gether, unaccompanied by the male parent or by any other adult beaver. From personal observation, W. L. Ayres, of Mendota, describes the actions . of a family of beavers adollows. On an island, one morning late in June, the observer saw an old beaver accompanied by three little ones, about the size of large cottontail rabbits. The young were romping about in the rank .grass twenty feet back from the water's edge. He saw the mother pick up the youngsters one after another and turn them over and over, meanwhile combing out their fur with the claws of her fore paws and apparently trying to find and dislodge the parasites that infested her young ones. By late summer or early fall, baby beavers are weaned and the old and young of both sexes are again united under one roof. Of many houses exam­ , ined at various seasons of the year, we have never found any in which more than eight beavers were living together at one time~ The young and old 6ccupy the lodge or den together during the winter, but the following spring the yearling beavers start to shift for themselves while the parents begin to center their interests in the activities of a new breeding season. Fol­ lowing the mating season, the pregnant females assume exclusive rights to the old breeding dens or else they retire to new dens to give birth to their young, while the adult males travel restlessly about, often far from the home pond. By this time, the second summer of their lives, the young are able to look out for themselves with entire independence; and they usually hunt up burrows of their own. In the East, muskrats have been commonly known to live in beaver dams and in the outer parts of beaver houses. In California, there are no muskrats native to the San Joaquin and Sacramento valleys and these smaller rodents are therefore unknown in the habitat of the golden beaver. The only mam­ mal that we have found associating itself closely with the beaver has been a species of wood rat (Neotoma fuscipes streatori). At Snelling, these rats were found utilizing the stick house of the beaver for the outer covering of "their own nests, instead of build~ng a separate stick house .. As would be expected, the formation of ponds and sloughs by the beavers 688 Fur-bearing Mammals of California very intimately affects the vital interests of quite a number of other animals. Among these are frogs, fishes, turtles, various insects, and other essentially aquatic types. Among birds, favoring conditions are afforded for kingfish_ ers, mudhens, herons, and several kinds of fresh-water ducks. Among the latter, of especial interest, is the wood duck. . . In the bottom lands around Snelling, wood ducks are restricted almost entirely to beaver ponds. On November I, 1921,51 wood ducks were counted as they flushed from one beaver pond; 28 flew by overhead in one flock at one time. W. L. Means, of Hopeton, counted 65 of these ducks in one flock on another beaver pond. On the ranch of Mr. Will Ferrell there is a third beaver pond full of dead trees and brush that have been drowned out by the rising water, where wood ducks are (or were) to be found the year round. At least 150 were present on these three beaver ponds on November I, 1921. The species was entirely absent from other open ponds and sloughs in the region. . However, the spreading of the water areas and the rising of the water table beneath the adjacent lands undoubtedly drive or drown out a number of nonaquatic animals. Pocket gophers and ground squirrels, for example, disappear from s\)ch territory. Beavers in California are now just about free from any natural enemy. Bears, mountain lions, and river otters may have disturbed them formerly. A valley coyote near Snelling was seen in February, 1920, "snooping" about a beaver lodge, thereby suggesting the possibility that these carnivores may occasionally capture beavers, especially 'Young ones. But we really have no definite occurrence of this sort to report. Man, at the present time, is the chief enemy of the beaver. For food and building materials, beavers levy upon certain kinds of plants. At the same time, these same plants-willows and cattails, for instance­ thrive because beaver ponds are present, where otherwise they would not be able to grow at all. There is thus in evidence a certain compensation by which the coexistence of the beavers and plants is mutually beneficial. Of course, the "drowning out" of nonaquatic vegetation, such as oak trees, at the same time often does occur. " The beaver is strictly a vegetarian (see fig. 293). Of 20 beaver stomachs examined (D.) not one contained even a trace of animal matter. Castor as a group exhibits maximum activity in a northern or boreal habitat. The form subauratus is a southern, indolent race of beaver. Because the climate is mild, the growing season of aquatic vegetation is relatively long in the habi­ tat of the golden beaver; and having a co~stant supply of green vegetation Golde" Beaver 689 ready at hand for a large part of the year (see fig. 300), this beaver leads a relatively carefree existence. The ponds about his house are rarely covered by even a skim of ice, so that all winter long he is able to come and go at will in search of food. Here, there is no great need for storage of food such· as exists among beavers that inhabit colder regions. Only occasionally are willow saplings cut and stored under the water, where they form a wood­ pile near the den or lodge. Although storage proclivities are thus sometimes apparent, their manifestation is very weak. This mild manifestation is the

Fig. 300. Bulrushes and a yellow-flowered composite formed' the principal food of the golden beaver at one place during the summer season, The tips of such vegetation had not yet been touched by frost when this photograph was taken, November 2, 1921, near Hopeton, Merced County. Mus. Vert. Zoo!', no. 4064. result of a habit inherent in the parent stock, name! y, storage of food for winter use, modified to meet changed conditions in a special habitat. California beavers cut comparatively few trees in summer. Occasionally an observer will find a place where a thrifty green willow sprout Yz-inch in diameter has been smoothly sliced off by a single nip of the beaver's pow­ erful incisors. At other places the larger limbs of willow trees that overhang or dip into the water will show white patches where the bark has been chis­ eled off. Beaver cuttings can, of course, always be recognized by the peculiar contour of the tooth marks left in the wood: each incisor tooth of the beaver leaves a shallow groove, about 14 -inch in width, and since the incisor teeth are slightly separated, but in pairs, the grooves left by them will likewise be found in pairs, with a little ridge of wood, which has escaped the slightly Fur-bearing Mammals of California separated teeth, sticking up between each pair. (See fig. 293.) A certain amount of gnawing seems to be necessary to keep the rapidly growing in­ cisor teeth sharp and worn down toa proper level of emergence. Apparently

Fig. 301. In 1919, a colony of heavers ncar Hopeton, Merced County, cut down a number of valley oaks from 6 to 12 inches in diameter. The largest oak attacked (here illustrated) was 18 inches in diameter, but this one was not cut through. The other oak trees were left where they fell and were not cut up and used in building dams Of houses. Neither was oak bark or twigs used for food. Photographed on February 22, 1920, Mus. Vert. Z06l., no. 3260. for this reason, dry fence posts, bridge timbers, and even oak trees are some­ times extensively gnawed by beavers that happen to feed largely on roots and stems of tule, cattails, and other soft aquatic vegetation, and to live in a locality where there is a scarcity of young cottonwood, willow, or other trees which are suitable for food. Golden Beaver

The golden beaver does the greatest amount of tree felling in November and December. Willow, cottonwood, and ash are the principal trees cut by beaver in California, here named in order of estimated importance as food trees. Quite a bit of alder is cut, but this wood is used chiefly or solely in building dams and houses. The golden beaver is the only race of beaver, to our knowledge, that cuts down oak trees. (See fig. 301.) This is done reg­ ularly along the lower Merced River, though for no obvious purpose other than to "exercise" the gnawing mechanism. Such trees are left where they fall until some person comes along and cuts them up for wood. It has been noted that trees growing along the banks of streams, when cut by beavers, usually fall toward the water. It is to the beavers' advantage that the trees fall streamward, for once the tree is in the water the branches and bark are easily cut off by these chisel-toothed rodents; the beaver has therefore been credited with the ability to fell trees in any desired direction. In felling trees, however, the beaver is merely profiting by favorable natural conditions, and is not to be credited with any special engineering skill. Plants and trees naturally grow toward the light. Trees growing nearest a stream often come into keen competition for light with other trees grow­ ing to the landward side of them; the stream or pond side of the tree receives an unusually large amount of light, which falls unobstructedly on that side; the leaves and branches are most numerous and heaviest on the side of the tree which gets the most light. Therefore trees which grow on the margins of streams or ponds are heaviest on the side toward the water. Irrespective of how they are notched when cut off at the base, in calm weather such trees fall inevitably toward the water. Streamside trees also, and for the same reason, nearly always fall clear, rarely lodging in thickets, as sometimes do trees elsewhere. (See fig. 302.) Stumps of former streamside trees often show that the trees fell into the water when gnawed only one-half or two-thirds through, which proves con­ clusively that the tree leaned strongly. In densely wooded areas, trees are often so evenly balanced that they do not fall until completely severed, and then they often lodge against neighboring trees and fail to come down even when the trunk is cut through at two or three points. In cutting streamside trees, beavers at Snelling usually made the first deep cut or notch on the landward side of the tree (see fig. 303). A woodsman would have made the first and deepest cut on the opposite side, that is, on the side on which he wished the tree to fall. Ten cottonwood and oak trees from ro to 28 inches in diameter, all growing beside streams or ponds, were examined closely on different days while they were being cut down by beavers. In eight of the --_.__ .. ------

Fur-bearing Mammals of California

ten trees, the first and deepest gnawing was made on the side of the tree away from the water. It is difficult to see what advantage might arise from cutting on the landward side of the tree first; if safety were the factor con­ sidered by the beaver, then the first cutting should be done on the stream­ ward side (namely, on the side toward which the tree was bound to fall, anyway) and the last cutting on the opposite side so that the beaver would be well out of the way of the falling tree.

Fig. 302. Beavers have been credited ~ith the ability to fell a tree in any desired direction. Stream­ side trees that when cut by beavers have fallen into the water are often cited as proof that they were so "intended" to fall. It is true that trees growing beside streams or ponds usually do fall into the water when cut by beavers; but this direction is the result of gravity and not of any planning on the part of the animals. As will be seen from the diagram, s,treamside trees such as c, d, and e have their largest and heaviest branches on the side that receives the most light. They often lean toward the lighted open space above the stream or pond, and when cut they naturally fall in that direction. Experience has shown that trees, such as a, that lean away from the water fall away from the water when cut. A perfectly balanced tree; like b, may fall in any direction.

Perhaps, after all, the direction in which the tree WIll fall is a circumstance not conditioned by any feature of the beavers' mental make-up. When cutting trees, the beaver stimds upright on his hind feet, braced behind by his broad tail, which serves as a third leg or prop. The animal then reaches up as high as is convenient, from 16 to 32 inches in ascertained instances, and starts cutting. The direction of the tooth marks left on the wood shows that, as a rule, when the cut was made, the beaver's head was held at nearly right angles to the fiber of the tree trunk. The beaver bites above and below the center of the cut and then forces out the chip by driv­ ing his teeth (lower ones chiefly) behind the piece cut off (see fig. 304). The size of the chips is dependent upon the hardness and toughness of the tree that is being cut. In soft, straight-grained woods, such as willow and cottonwood, the chips are ordinarily from 1 to 3 inches long, liz-inch to 2 inches wide, and )Is-inch to \r.4-inch thick. Oak-tree chips are much smaller,

- Golden Beaver' averaging ry' inches in length, %-inch in breadth, and \i-inch in thickness. Chips cut by the beaver for bedding are usually cut from straight-grained willow shoots. These slivers are long and slender, measuring in ascertained

-;-. ,\

Fig. 303. A woodsman in felling a tree makes the first cut Of notch in the side on which he wishes the tree to fall. In 8 out of 10 con~ secutive cuttings by beavers, examined by liS, the first and deepest cut was made by beavers on the side away from the stream toward which the tree leaned and finally- fell. Photographed on De6:niber 23. 1919, three miles east of Snelling, Merced County. Mus. Vert. Zoo1., no. 3267. instances 9 inches in length, %-inch in width, and \i-inch in thickness. Such splinters should not be confused with ordinary chips (see fig. 305). It has been our observation that usually a small tree is felled by one beaver, working alone. After a tree is down, several beavers may join in trimming Fur-bearing Mammals of California up the branches andremoving the bark from the larger limbs. After the tree has been stripped of its branches and bark, the main trunk is rarely cut up, but is usually left just where it fell (see fig. 306). Beavers sometimes "show poor judgment" and attempt to cut down trees that prove too much for them. This tendency has been noted many times by us, and three of the most striking examples are here given. On March 4, 1920, near Snelling, marks indicated that several beavers (the tooth marks were of different sizes) had been working off and on for weeks gnawing into the trunk of a large cottonwood tree that measured 62 inches in diam-

Fig. 304. Willow chips, showing how they are cut off at each end and then pried out by the chisel~like front teeth of the beaver, X X. eter at the point where the beavers were working (see fig. 307). This cut was never finished. Although plenty of small cottonwoods were available, this particular colony of beavers had notched several cottonwoods more than 36 inches in diameter. They had not actually cut down any tree more than 30 inches through; therefore much of their work had been wasted. In December, 1919, a valley oak that measured IS inches in diameter at the cut was gnawed one-third of the way through (see fig. 301), and this cut was never finished, though none of the beavers was trapped or molested in any way to prevent the finishing, and other trees within a few feet of the oak tree were subsequently cut down. On the Tuolumne River near Lagrange, on March I, i:92I, a cottonwood tree was found which had recently been double-cut by beavers. This tree Golden Beaver was 13 inches in diameter and 40 feet high. The center of the first or upper . cut was 35 inches above the ground. This first gnawing had seemingly been done while the beaver was standing on a pile of driftwood that had lodged about the trunk of the tree in a time of flood. For some reason not apparent to us, the beaver had given up the gnawing and quit before it was half fin­ ished. Some weeks later the same beaver, or another with teeth of exactly the same width, had trimmed off all the driftwood within 16 inches of the tree trunk and had then started a new cut below the first one (see fig. 308). This new cut, the center of which was 21 .inches above the ground, was

Fig. 305" The beaver makes a clean, sanitary bed-out of shredded, straight~grained wood of willow or cottonwood. Average slivers used for bedding are here shown, three-fourths natural size, in comparison with an ordinary small, 2-inch-long chip. ~us. Vert. Zoo!', no. 3796.

never completed. On April 12 the tree blew down in a windstorm. In the three nights following, the beavers stripped the upper part of the tree of its smaller branches and bark. Also, one of the largest branches had been stripped of bark for more than 20 feet. Although the beavers mentioned above showed poor responses, other bea­ vers in the same general region were found carrying on their work effi­ ciently and consistently. One example of such work is cited in the following account. Several willow and cottonwood trees grew in a row within twenty feet of a slough inhabited by beavers (see fig. 309). These trees were from II to 14 inches in diameter and from 30 to 50 feet high. In a period covering about two months, beavers had cut seven out of ten of these trees. The trees (see fig. 310) had been cut off from 16 to IS inches above the ground. The Fur-bearing Mammals of California broken-topped stumps showed that most of the trees had leaned strongly and that five of the seven cut would have fallen into the water, no matter how they were notched or cut. The two trees that fell away from the water were stripped by the beavers of their limbs and bark and then left where they fell (see fig. 306). Trees which would fall landward were tackled and cut clear down and trimmed. Other trees which leaned strongly toward the water Were left standing The beavers apparently exercised no selective choice in this particular matter, since the trees were cut in the approximate order in which they stood along the slough.

Fig. 306. This 4o~foot cottonwood tree when cut down by beavers feIt away from the water. Even so, the animals at once cut off and carried away the branches and some of the smooth bark.

At Hopeton, in the fall of 1920, beavers systematically cut off nearly all the willows that grew in an isolated pond (see fig. 3II). The willows were cut at an average height of 24 inches above the ground or shallow water, extreme heights of the remaining stubs being 12 and 32 inches, respectively .. This severe pruning did not kill the stumps but merely caused them to send out sucker sprouts which eventually served as further food for the beavers. A typical cutting of alder, used chiefly in dam building, is shown in figure 312. These alders were 6 inches in diameter and were cut 16 inches above the ground. Such cut stumps usually rot .off at the ground and disappear within a feW years, leaving few or no traces of beaver activity. As a result of our observations upon beaver-cutting we are led to conclude ------~-"----~------

Golden Beaver that there is great individual variation among beaver workers. Mistakes and apparent miscalculations are often made by certain individuals, whereas others do not attempt tasks that are beyond their powers to fulfill. Some

Fig. 307:"In I919.:i beaver started to gnaW dO-.yll this large ~otton'Yo6d:. tree, which was 5 feet, :i inches in diameter, even though smaller arid bettcr­ situated cottonwoods 'Yete plentiful in the-vicinity. The 'cut was n~ver CO~~ , . pleted; the tree was still standing in 1922", Photographed on March 4; f920~'>: . three miles wes~ of Snelling, Merced Count?_ 'Mus,.Vert.·Zool., tio. 3264_ .

beavers work spasmodically;· others carryon their work· to its c~mpletion in a businesslike way. At least they keep continuously at it. The beaver's main purpose in gnawing down trees is obviously, as a rule, to procure the bark for food. Sometimes a good deal of the wood just be- ~~~~~~~--~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.-~~~~~~~~--.--,-.------~~-----

Fur-bearing Mammals of California

Fig. 308. On this double~cut cottonwood a beaver did quite a bit of wo~k need· le5s1y. The upper cut at its cente! is 35 inches above the groun~. The direction of the tooth marks, across the tree trunk, indicates that the beaver must have stood on a pile,of driftwood that had lodged against the base of the tree. When this cut was half finished, the work was abandoned. Several months later the same beaver. judg­ ing from the width of the tooth marks, returned, gnawed off the drift sticks about • the base of the tree, and, then standing on the ground, began the second cut 25 inches above the ground. Before the beaver finished this second cut, a hard wind blew the tree over. Photographed on 'March I, 1921, on the TUolumne River, four miles southwest of Lagrange, Stanislaus County. Mus. Vert. ZoOl., no. 3619. ------~~----

Golden. Beaver neath the bark is eaten, but this may be done more or less accidentally, just as a hungry man is likely to take in some of the cob along with the corn in eating a roasting ear. Bark that is old, with the surface checkered or scaly, is rarely taken, a decided preference being shown for smooth bark of recent growth. Although bark may be said to be the beavers' staff of life, many other food materials are eaten at various seasons and in different localities. In December, 1919, beavers were foraging about for acorns that littered the ground under valley oaks that grew near sloughs at Snelling. Beaver tracks

Fig. 309. A typical habitat of the golden beaver on a slough paralleling the Merced River is here shown. The cottonwood in the foreground had 'recently been felled by beavers au9. stripped of its upper l,imbs and b?rk. found at this time and stomach contents examined in January proved con­ clusively that acorns form a staple article of food for the beavers of that locality during the late fall and winter months. At Cache Slough, Solano County, on January 4,1921, beavers were digging up and eating the under­ ground root stalks of the tule. On March I, 1921, at Snelling, beavers were scratching up the ground under clumps of budding willows in search of sprouting willow roots, which at that time were about the size of toothpicks, white and tender, "and seemingly much relished. In the summer months the white underwater st~lks of the cattail are eaten extensively, as are also the bulbous roots of pond lilies and the stems of other aquatic plants such as the yellow waterweed (Jussiaea). Feeding places are frequently found at the water's edge where the beavers "haul out," to eat leisurely the white

- --~----~,------~ ------~ --.~--- ~ 700 Fur-bearing Mammals of California

II II ! I,

Fig. 310. Out of ten cottonwood and willow trees that stood in a row beside a slough near the railroad bridge over the Merced River below Snelling, seven were cut down by beavers in the fall and winter of 1919. These trees ranged from II to 14 inches in diameter and -were cut off at a height of 16 to 18 inches above the ground. The stumps show that the trees fell when slightly more than half severed. This fact indicates that they leaned strongly, some toward and others away from the water. Investigation showed that the trees feU in the direction in -which they leaned, irre~ spective of how they were notched by the beaver. No selectiv.e choice was shown for trees leaning toward the water. The next to the last tree in the row had been deeply notched on the landward side when this photograph was taken, March 3, 1920. Mus. Vert. Zoo!., no. 3265. Golden Beaver

basal parts of cattail stalks which they obtain in the water near by (see. fig . . 3r3). Once, in August, beavers were found eating green stems of alfalfa. The bark of apple, pear, and cherry trees is relished, and such fruit trees growing near beaver ponds are subject to being cutfdown. We have no record showing that beavers have bothered vegetable gardens, although in some places known to us, they have had excellent opportunities'to do so, and surely would have, if their food preferences had so pres~r1bed. The beaver is a hearty eater. Because the food eaten is relatively low in I I I j I I I I !

Fig. 3II. Beavers regularly pruned this w.il1ow patch near Hopeton. The willows had been cut off from 12 to 32 inches above the ground and responded to this rigorous pruning by sending out a new growth of sprouts which were harvested by the beavers the following year. Photo­ graphed on February 27. 1921. Mus. Vert. Zool., no. 3608.

nutritive value, a large amount has to be taken. Data obtained by us from . the examination of beavers' stomachs, on the quantity and kinds of food eaten, are shown in the table on page 705. The stomach of an average adult beaver is, roughly speaking, bean-shaped and measures 6 by 6 by 3 inches. Exclusive of contents, three stomachs ex­ amined weighed 4, 4'15, and 5 ounces, respectively. One quart of finely chewed material has been found to constitute a square meal for the average beaver. The weight of the stomach and contents in ascertained instances constituted Ys2 to '124 of the total body weight. If arSo-pound man should eat asheartily as a beaver he would consume six pounds of food at a meal. ~~~~---""------.

702 Fur-bearing Mammals of California The food particles found in the stomachs of young beavers are not as finely chewed as is the food found in the stomachs of adults. By the same token, the young beavers also eat more wood with their bark than do the old ones (see fig. 293). This may result from differing tooth structures.

Fig. 312, These alder stumps, which are 6 inches in diameter and 16 inches -high; show typically how the beaver operates. Alder is used primarily for building dams and 'houses, being rarely eaten. Photographed on March 3. 1920, near Snelling. Merced County. Mus. Vert. Zool., no. 3268.

We have found that people commonly overestimate the number of bea­ vers in a given locality. On January I, 1920, it was "conservatively estimated" by several persons well acquainted with the territory that there were rooo beavers inhabiting IS square miles of river bottom in the vicinity of Snell- . Golden Beaver ing. Subsequent trapping indicated that there were not more than 250 bea- vers in that area, and this was then admitted by the same people who had first believed that there were 1000. Furthermore, the estimate of a trained observer for the saine area proved 100 per cent too high. At least three­ fourths of the 15 square miles of river-bottom land at Snelling is cultivated fields and pastures, separated by old river channels and sloughs inhabited by beavers. If we include the total area, taking the fields and sloughs to­ gether, there is an average of '5 beavers per square mile or I beaver to each 42 acres. In one 400-acre section of dredged-over river channel, extensive trapping showed that there was an average of I beaver to ,each 10 acres. In '920, on the San Joaquin River near Mendota, trapping showed beavers to be present at the rate of 12 per square mile of river bottom. Figuring the density of the beaver population on the basis of average number per mile, we have the following data. In 1921, on a favorable old river channel near Snelling, there were present about '5 beavers on 5 miles of slough, or an average of 3 per mile. Along the San Joaquin River near Mendota, the beavers would average about 5 per linear mile, with 10 per mile as a maximum along the most favorable parts of the river. On January I, '92', the total population of golden beavers in the State was, in our estimation, about 1000. We have visited"most of the known colonies of beavers in the State, and some of our figures are substantiated by known numbers of beavers caught, so that our estimates are thought to be in truth conservative. The recuperative powers of the beaver, as a species, are well illustrated by the beavers whose habitat includes the Merced River bottom in the vicinity of Snelling. Before '9II, when the State law protecting beavers was enacted, the beavers of this region had been so reduced by continued trapping that only a few individuals, probably not more than five pairs, were left in an , area of IS square miles. Eight years later (1919) the beavers had become conspicuously numerous, and in that year they caused ,much trouble by their activities in irrigation canals. Beaver remains found in abandoned steel traps at this time showed that some beavers were being trapped illegally. In '920, out of a total estimated population of 228 in the territory, II4 bea­ vers were trapped under permit. In March, '92', beavers were found in small numbers in all places where they had been found in '920, before legal trapping began. This was true in one place where a determined effort had been made to remove every beaver from the vicinity of a certain dam. Nine­ teen had been caught in '920 on this one dam, and the locality was declared free from the animals. Yet, just one year later, fresh sign showed that an

I 1 Fur-bearing Mammals of California

Fig. 313. A beaver's feeding station is usually near the water's edge on some point or islet. The remains of food, in this figure, show that the beaver had been eating the white underwater parts of cattail stalks. In summer this sort -of food forms a large part of the golden heaver's diet, Photographed on June 4, 19X8, near Mendota, Fresno County. Mus. Vert. Zo01., no. 2764. ------

Golden Beaver attempt had been made by a surviving beaver to repair the break that the rancher had made in the dam in efforts to drain the "waterlogged" land above it. Beavers come persistently to favorable places.


Total Weight of weight of stomach Condition Date beaver, a!ld c

1920 Feb. 28 32 16 % full Many small willow roots 1 mm. in diameter and 25 rom. long; wood coarsely chewed Mar. 4 22 16 % full Willow wood and bark coarsely chewed, chips aVeraging h3xl mm.; many bud scales Mar. 5 33 20 % full Chiefly willow bark and wood; many pieces coarse,S mm. long and 2 mm. thick; willow roots Feb. 28 34 22 %fu11 Hundreds of pieces of cattail, 10 mm. long, 2 mm. wide; some willow bark, and one willow root 20 mm.long Feb, 29 42;; 25;; % full Cattail; Bermuda grass root stalk 3 incncs long; willow hark and dark-colored roots 1922 Dec. 5 31 16;; ...... Acorns 10 per cent; cattai120 per cent; willow 70 per Cent Dec. 5 36 28;; ., ... Cattail 10 per cent; willow 90 per cent Dec. 6 37;; 21 ... .. - Cattail IS per cent; willow 85 per cent Dec. 7 35 15% ~ full Acorns 10 per cent; cattail IS per cent; willow 75 per cent Dec. , 34 5M ~full CattailS per cent; willow 9S per cent Dec. 8 19 8;; Va full Cattai1S0 per cent; willow 20 per cent D,~ 8 18 5 % full Cattail 50 per cent; willow SO per cent D~. 12 35 20 ...... Acorns 5 per cent; wiJIow 95 per ceut Dec. 14 41 12% ...... Willow 100 per cent Dec. 14 20 6j{ ...... Cattail 50 per cent; willow 50 per cent Dec. 15 25 7j{ ...... Cattail 10 per cent; willow 90 per cent D,~ 17 48 42 ...... Cottonwood bark and wood 100 per cent Dec. 17 36;; 15 ...... Willow SO per cent; roots 10 per cent; cattail 10 per cent Dec. 17 13 7 ...... Cattail 10 per cent; cottonwood 90 per cent Dec. 20 50 8 ..... Cattail 50 per cent; willow 50 per cent Dec. 20 38Va lOVa ...... Cattail 100 p~r cent Dec. 23 20 4)4 ...... Cattail 100 per cent

1923 Jan. 3 20 7 ...... Cattail 100 per cent Jan. 5 22 13 Va ...... Willow 100 per cent Jan. 6 48 17% ...... Cattail 25 per cent; willow 75 per cent Jan. 6 31 17 ... .. Ca ttail40 per cent; willow.60 per Cent Jan. 7 46 19 _. .. . . Cattail 40 per cent; willow 60 per cent Jan. 7 35 21% ...... Cattail 90 per cent; willow 10 per cent Jan. 8 47 7;; ...... Cattail 25 per cent; willow 75 per cent

Observation of these rodents elsewhere in the United States and Canada indicates that the beaver has the capacity to increase in numbers rapidly when it has adequate protection. . From December, 1919, to sometime in 1925 one of us (D.) was called upon several times each year to investigate reports of alleged depredations of bea­ ver, and, if the damage proved real, to suggest ways and means of prevent­ ingfurther impairment. In the course of this work it has been a common dis~overy that prospective trappers have been the first and sometimes the Fur-bearing Mammals of California only persons to report that harm was being done. Reports of such depreda­ tions thus fall readily into two classes. One class of report is made by persons who seek an excuse to obtain the valuable pelt of the fur bearer, the other by persons who are not interested in the pelt but seek relief from real dam­ age. The second class has the better claim to attention. The damage done by beavers in California is varied and, at times, of rather large extent. The following are the commoner kinds of depredation done by beavers in this State: (r) Burrowing into levees or dikes of reclamation works; (2) burrowing into banks of irrigation canals; (3) obstruction of

Fig. '314. Beaver burrows in this reclamation levee caused a 4o~foot section to collapse, thereby endangering 1000 acres of growing grain. Photographed on January 4, 1921, on Cache Slough, near Dozier, Solano County. Mus. Vert. Zo81., no. 3454. .

drainage canals by dams; (4) flooding and waterlogging of land; (5) gnaw­ ing at head gates of irrigation canals; (6) cutting down of fence posts and fruit trees. The. first two kinds of depredation are attributable to the bea­ ver's burrowing habits, the third and fourth to his propensity for building dams and creating ponds, and the fifth and sixth to his proneness to gnaw. A large part ·of the former habitat of the golden beaver lay in the delta region at the mouths of the San Joaquin and Sacramento rivers, and much of this territory has recently been reclaimed. Thousands of acres which for­ merly were flooded at high tide have been surrounded by levees and changed from tidal marshes into productive farm lands. But here the safety of the crops depends primarily upon the dikes which keep back the floods. Golden Beaver 70']

At certain points along the banks of Cache Slough in Solano County the levees are 100 feet wide at their bases and rise abruptly 20 feet above the low-water mark in the adjacent slough. In such places the-tops of the levees are from 12 to 14 feet above the level fields of growing grain. Ten thousand acres are often included in a single reclamation project. At the points men­ tioned above, Cache Slough has a width of from 200 to 300 feet and a depth of 20 feet. Its banks support a good growth of willows which furnish abun­ dant food for the beavers. The high levees, composed of fine silt and day dredged from the bottom of the slough, are easily burrowed into and afford ideal sites for beaver dens; in fact, they constitute the only land areas that remain above water in the event of general flood. The.water in the slough adjoining the levee is deep at all times, so that the combination of living conditions is ideal for the beaver. When the water in the Sacramento River is low, the danger from the beaver burrows in the levees is slight; but after heavy rains or when the snows in the Sierra Nevada melt rapidly, especially if there are extra-high tides at the same time, the water from the river is backed up at the river's mouth by the tidewaters o{ Suisun Bay until it rises many feet above the lowland fields that are protected only by dikes. At such critical times, beaver burrows in the levees become a real menace because they may cause widespread disaster. Cdntinued rain and rising waters soften the levees where they have been undermined by the burrows so that they collapse, and the fields are flooded through the breaks thus started. On January 4,1921, an incipient break in a levee was visited (D.). The first sign of trouble had been indicated by cracks in the earth where a sec­ tion of the levee began to sink in above a previously unnoticed beaver bur­ row. This burrow, similar in plan to the letter Y, forked so that a section of the levee 40 feet in length was affected. A dredger near by was- moved up to a place suitable for action, arid a section of the levee 40 by IDO feet in extent was torn up piecemeal by it, thus completely destroying the burrow (see fig. 314). However, the clay soil of the levee had been so softened by the water's penetrating along the burrow that it would not stand firmly under the additional weight of earth piled on top of it, but kept oozing out so that streams of mud ran like lava into the adjoining barley field. Several days passed before the levee became firm enough to permit the completion of the work of rebuilding. During this time the water stood within four feet of the top of the break, and a sudden rise in the river would have meant the destruction of 1000 acres of growing barley. If beaver burrows in levees are discovered in tiroe, the greatest danger may be averted, since they can then be uncovered and filled in before any I l.1 •.

-., Fur-bearing Mammals of California

dangerous stage of high water is reached; but because the entrances to such burrows are usually under water, it is difficult to locate them and they may therefore remain undiscovered until high water comes and the undermined levee gives way. The territory comprised in the delta region of California, where bea­ vers were originally abundant, includes some 268,000 acres of marshland, ·of which more than two-thirds had been reclaimed before 1921. As more and more of the marshland has been reclaimed, the beaver's habitat has become gradually restricted to the deeper sloughs and the adjacent high

Fig. 315. Beavers have invaded many irrigation canals in the San Joaquin Valley. Tn such places, as here illustrated, they cut off considerable aquatic vegetation, which "floats down the canal, dogs the head gates, and causes the.canal to overflow. They also cause breaks by burrow~ ing into the canal banks. Photographed on November II, 1920, ten miles northwest of Fire­ baugh, Fresno County. Mus. Vert. Zoo1., no. 3453.

levees. In 19II, a State law was passed protecting beaver at all times; pro­ vision was made, however, for the issuance of permits "to catch or kill any beavers that are endangering or destroying the levees or other protective works of any reclamation district, levee district, or swamp land district." In 19II relatively few beavers were left in the delta region, but eight years later, in 1919, they had bred up and become numerous enough to endanger the levees along Cache Slough, and in that year seventeen beavers were trapped there, under special permit. In the winter of 1919-20, noticeable damage was done to irrigation canals li ;i d II I 11 Golden Beaver in the vicinity of Mendota, Fresno County (see fig. 315). In one place, two large irrigation canals 50 feet wide and 6 feet deep ran parallel to each other for a short distance. They were only roo feet apart and the beavers, by bur­ rowing into the bank that separated the two canals, caused a 60-foot section of it to give way; a bad break resulted; and much valuable irrigation water was wasted because it went where it was not needed. At another point in the same vicinity the beavers dammed up one of the canal gates with freshly cut willow brush, cattails, and rubbish and then diverted the water from the canal back into the river through a series of burrows, thereby causing a serious washout or break in the irrigation canal at a season of the year when the water was greatly needed for growing crops. The winter of 1919-20 was extremely dry throughout northern and cen­ tral California. As a result of the lack of snow and rain, the flow of water even in the petmanent watercourses became unusually low. Where water was available, irrigation of alfalfa fields was resorted to until February, when the long-delayed rains arrived. In the Merced River bottom in the region about Snelling, beavers had increased greatly in numbers, having been protected by certain ranchers even before the State protective law went into force-in 1911. As the season advanced, the drying up of certain beaver ponds and the low stage of water in the irrigation canals as well as in the river itself caused evident uneasiness throughout the beaver tribe. Every effort was made by_ the rodents to conserve as much of the water as was possible within their ponds. Dams were tightly chinked to reduce seepage. Irrigation ditches that diverted water from these ponds were effectively closed by temporary dams of brush, stones, decayed tule stalks, and mud. The ranchers of the region were likewise anxious to obtain all available water for their parched alfalfa fields, since the chief industry of this region is dairying. As the season progressed, the riparian water rights became ob­ jects of contention between the