The Reproductive Biology of the Mountain Cottontail Rabbit in Oregon

AN ABSTRACT OF THE THESIS OF

Roger Alan Powers for the Master of Science (Name) (Degree) in Wildlife presented on (Major) (Date) Title:THE REPRODUCTIVE BIOLOGY OF THE MOUNTAIN

COTTONTAIL RABBIT IN OREGON Redacted for privacy Abstract approved: B. J. Verts

The reproductive characteristics of themountain cottontail rabbit, Sylvilagus nuttalli (Bachman),were investigated from 269 specimens collected in central Oregon between28 May 1968 and 28 August 1969.

Male rabbits were in breeding condition betweenmid-February and 3 July 1969.Nearly 77% of the females bore four litters between 7 March and 28 June; 23% bore five littersbetween 22 February and 31 July 1969. Evidence of immediate postpartum estrus and synchronous breeding were observed.The mean size of litters was 4.3young per litter in 1969.There were no significant differences (P > 0.05)among the sizes of synchronous litters. Juvenile males became sexually mature in February when they were at least seven months old.Pregnancy was observed in one juvenile female two to three months old. Maximum annual productivity was 18.2 young per adult female, and 0.3 young per juvenile female in 1969. The 1969 breeding season may have been delayed due to severe weather in January and February 1969. The Reproductive Biology of the Mountain Cottontail Rabbit in Oregon

by Roger Alan Powers

A THESIS submitted to Oregon State University

in partial fulfillment of the requirements for the degree of Master of Science

June 1970 APPROVED:

Redacted for privacy

Assistant Professor of Wildlife Ecology in charge of major

Redacted for privacy He /d of Department of Fisheries and Wildlife

Redacted for privacy

Dean of Graduate School

Date thesis is presented Typed by Cheryl E. Curb for Roger Alan Powers ACKNOWLEDGEMENTS

I would like to express my appreciation tothe Theodore Roose- velt Memorial Fund Committee,American Museum of Natural History, for their generous grant whichfinanced the major portion of this investigation. I am indebted to many persons whovolunteered their aid when it was most needed, among them wereMr. John A. Gill and Mr. Donald E. Trethewey of the Department ofFisheries and Wildlife Oregon State University, and Mr. Richard L.Lantz of the Oregon State Game Commission. My special thanks must go to Mr. MurlG. Bright of Corvallis, whose aid, regardless of personal inconvenience ordiscomfort, greatly contributed to the success of thisinvestigation, and whose experience, knowledge of the outdoors,and good humor were invalu-

able. I feel fortunate to have worked withDr. B. J. Verts, Assistant Professor of Wildlife Ecology, OregonState University, whose efforts on my behalf wereappreciated. TABLE OF CONTENTS Page

INTRODUCTION 1

MATERIALS AND METHODS 5

Study Area 5 Collection 7 Necropsy 9

RESULTS AND DISCUSSION 14

Sex Ratios 14 Breeding Season 15 Breeding Synchrony 17 Number of Litters in 1969 20 Size of Litters 21 Male Reproductive Cycle 22 Juvenile Sexual Development 27 Males 27 Females 28 Annual Productivity 29

BIBLIOGRAPHY 30 LIST OF FIGURES Figure Page

1 Locations from which mountain cottontail rabbits 6 were collected, Deschutes and Jefferson counties, Oregon, June 1968-August 1969.

2 Mean numbers of man-hours required to collect a 8 mountain cottontail, by month and by collecting trip, Deschutes and Jefferson counties, Oregon, 8 June 1968-28 August 1969.

3 Frequency distribution of dates of breeding for 18 mountain cottontail rabbits collected, Deschutes and Jefferson counties, Oregon, 1969.

4 Average weights by month of testes of adult male 25 mountain cottontail rabbits.Deschutes and Jeffer- son counties, Oregon, June 1968-August 1969.

LIST OF TABLES

Table Page

1 Size of litters and duration of breeding season 4' for the mountain cottontail rabbit as reported by various authors.

2 Numbers of female mountain cottontail rabbits 16 collected in Deschutes and Jefferson counties, Oregon, each collecting trip,1 January-28 August 1969, and numbers of females considered to be pregnant through use of each of several criteria of pregnancy.

3 Mean sizes of litters and mean rates of ovulation, 23 implantation, and resorption for each litter among female mountain cottontail rabbits, Deschutes and Jefferson counties, Oregon, 1969. THE REPRODUCTIVE BIOLOGY OF THE MOUNTAIN COTTONTAIL RABBIT IN OREGON

INTRODUCTION

The mountain cottontail rabbit, Sylvilagus nuttalli (Bachman), is a species native to much of the western portion of the contiguous United States (Hall and Kelson1959:261, Map 185).In Oregon, these rabbits occur in varying abundance in the Upper Sonoran life zone east of the Cascade mountains(Bailey1936:107, Dice1926:17). They appear to be most common in the vicinity of lava buttes, low lava hummocks, and other rocky outcrops (Bailey1936:107-108, Orr 1940:105-106). The mountain cottontail is designated a game species inonly 5 of the 14 western states constituting its principal range(Kozicky

and Madson 1969).The species is not considered to be a game animal in Oregon, thus, its populations do not receive the benefitof restricted hunting seasons or bag limits.Nevertheless, mountain cottontails appear to possess many of the characteristicsof an excellent game animal. Some of these rabbits areknown to be killed for sport and food in Oregon, but most are probably killed by sportsmenhunting upland game birds.The number harvested annually in Oregon is

unknown. 2

Knowledge of the biology of the mountain cottontailis extremely limited compared with that of other membersof the genus. A search of the literature failed to reveal a singlepublished report devoted solely to the ecology and life history of thespecies.Other than brief notes on the life history of mountaincottontails published as part of regional works on mammals (Bailey1936:107-109, Ingles 1965:148- 149, Dalquest 1948:387-389, Hall1946:607-612, Davis 1939:360-363, among others), in unpublishedtheses (Janson 1946:51-52 63-65), and in monographs devoted principally tothe taxonomy of the Order Lago- morpha (Hall 1951:119-202, Orr1940:98-109, Nelson 1909:199-211), information about the species is restrictedprimarily to that concerned with distribution (Dalquest 1941:408-411)and parasites (Jellison 1945:958-961, Honess 1939:281-284). In order to more completely understandthe role of mountain cottontails in the ecology of arid and semiaridregions, it is impera- tive that knowledge of the basicbiology of the species be augmented. Knowledge of the reproductive characteristicsand potential productivity of mountain cottontailsshould contribute to that understanding, and hopefully should provide a basisfor certain decisions regarding the species when initiation of managementpractices are deemed necessary, Information concerning the reproductivebiology of mountain cottontails tends to be sketchy, and, in most cases,based on small 3 numbers of specimens.Size of litters was reported to range from one to eight (Table 1),The onset of the breeding season was reported to range from February to April, andthe reproductive season was reported to terminate from June to August(Table 1).The gestation period was listed as 28-30 days (Cowan andGuiguet 1956:107), but the basis for this estimate was not reported.Orr (1940:108) believed that two litters annually were probably maximumfor the species in California, Cowan and Guiguet (1956:107) reportedtwo to three litters annually in British Columbia, and Bailey(1936:108) was unable to determine the occurrence of more than a single annuallitter in Oregon. The primary objective of this research wasto describe the reproductive biology of the mountain cottontailin central Oregon. Specific objectives were:

1. To determine the ages at which male andfemale mountain cottontails attained sexual maturity.

2. To determine the chronology of thebreeding season.

3. To determine the size of litters andnumber of litters produced annually.

4. To determine if breeding amongfemales was synchronous. 4

Table 1.Size of litters and duration of breeding season for the mountain cottontail rabbit as reported by various authors.

State No. Litter Size or Females Aver- Authority Province Examined age Range Season Hall r) 8 6.1 4-8 ? (1951) Hall Nevada 6 5 4-6 March-July (1946)

Dalquest Wash. ? ? 1-4 Apr. -June (1948)

Davis Idaho 1 6 Apr. -June (1939) Bailey Oregon Apr. -July (1936)

Orr Calif. & 8 6.1 4-8 Apr. -July (1940) Nevada

Dice Wash. & 3 4.7 4-5 ? (1926) Oregon Janson Utah 13 6, 6 5-8 Late Feb. - (1946) March; Late July or Aug. Cowan and British ? 2 Late April- Guiguet Columbia Late June (1956) 5

MATERIALS AND METHODS

Study Area

Eight areas located in Deschutes and Jefferson counties of central Oregon were used in this study (Figure 1).All areas used were within the Deschutes river drainage.Broad valleys and prominant buttes were characteristic of the main river valley, but deep incised canyons were associated with the smaller tributary streams.Elevations ranged from 2100-5100 feet among the eight study areas. Eroded Columbian lava flows, dipping gently eastward from the Cascade mountains, formed extensive continuous escarpments. Locally, lava occurred in small tortuous hummocks 10-30 ft high partially buried by eolian sands.These hummocky areas were identified on local maps as "lava lakes or beds. " The climate of the region was semiarid; annual precipitation averaged approximately ten inches.The greatest precipitation occurred as rain or snow between October and May.Monthly precipitation during the remainder of the year averaged one inch or less. Absence of cloud cover and low relative humidity contributed to large diel fluctuations in temperature (U.S. Weather Bureau 1968). Agriculture, except for cattle grazing, was restricted to irri- gated valley and canyon floors due to the steep slopes and rocky soil DESCHUTES CO, N A Collection Site

Map Location

Figure 1.Locations from which mountaincottontail rabbits were collected, Deschutes and Jefferson counties,Oregon, June 1968-August 1969. 7 of the uplands.Extensive upland areas were publicly owned and under the management of the U. S. Forest Service orBureau of Land Manage- ment. Natural vegetation found in the areas consistedof species characteristic of the Upper Sonoran life zone (Bailey1936:15).Big sagebrush (Artemesia tridentata), rabbitbrush(Chrysothamnus spp. ), squaw currant (Ribes cereum),bitterbrush (Purshia tridentata), wild oats (Avena spp. ), and western juniper(Juniperus occidentalis) were present in various proportions from area to area.

Collection

Two hundred sixty-nine mountain cottontailrabbits were collected between 28 May 1968 and 28 August1969.All specimens were collected by shooting except two founddead along the road. Collections were made at two-week intervals during1968, but in order to obtain sufficient specimens weeklytrips became necessary during the winter of 1969. Hunting success varied from 3. 0 rabbits perman-hour during late spring to less than 0.1 rabbit perman-hour during winter (Figure 2). Specimens were labeled with tags containingaccession numbers, dates, and locations, refrigerated inportable styrofoam coolers, and transported to Oregon State University.In the laboratory, A Monthly mean Trip mean Date of first collection of juveniles of 1969 20 z a)

CL 1 0

0 5

I MI J IJIAI 1968 Month and Year 1969

Figure 2.Mean numbers of man-hours required to collect a mountain cottontail, by month and by collecting trip, Deschutes and Jefferson counties, Oregon, 8 June 1968- 28 August 1969.

CO 9 specimens were stored in a refrigerator until necropsies were completed.

Necropsy

Prior to dissection, specimens were weighed and standard mea- surements recorded angles 1965:449). Testes of males were stripped of the epididymides, weighed, and means of individual pairs calculated.Fluid extracted from the tubules of the cauda epididymides was examined under a compound microscope for content, condition, and relative abundance of spermatozoa.Males were considered to be in breeding condition if copious amounts of spermatozoa were present in the cauda epididymides, the testes were turgid, and either testis weighed at least 2 g. The reproductive tracts of female specimens were excised, and examined for uterine swellings, resorbing embryos, or pigmented sites of previous implantations.Physiological saline solution was used to flush the uteri of female specimens in which no gross signs of pregnancy were noted.This effluent was then examined under the microscope for the presence of spermatozoa.Ovaries were examined for the presence of ruptured follicles or corpora lutea. Tissues of specimens collected during the investigation were fixed and stored in either Bouins fluid or 10% buffered formalin. 10 Histological sections were made from the testes of 18 juvenile males selected from those collected between 1 June 1968, and 28 February 1969, which had unossified epiphyseal cartilage in their humeri.Testes were sectioned at 10 micra; randomly selected sections were mounted using Mayer's albumin fixative, and stained with standard alum hematoxylin (Galligher and Kozloff 1964:309, 342-343), and eosin.The sections were examined under the microscope to determine the stage of spermatogenesis among these juveniles (Ecke

1955:298). Histological sections were also made from the ovaries of all adult females collected between 15 February, and 28 August 1969. Ovaries were sectioned at 10 micra and each 10th section mounted with Mayer's albumin fixative.Tissues were stained differentially with standard alum hematoxylin and eosin stains.Ruptured follicles and corpora lutea were counted using a 35 mm slide projector and compound microscope to examine mounted sections. Females were considered to be lactating if milk could be ex- pressed from the nipples. Dates that females became pregnant were estimated in the following manner:

1. Females whose uteri contained spermatozoa but whose ovaries did not contain corpora lutea or ruptured follicles were considered to have bred within 10 hours prior to 11

collection.Ovulation was reported to occur about 10 hours after coitus among other species of the Family Leporidae (Hammond and Marshall 1925:56, Casteel 1967:196, Heape

1905:260).

2. Female mountain cottontails whose ovaries contained corpora lutea but which had no visible uterine swellings were considered to have bred within the previous seven or eight days, because uterine swellings were not visible in other species of rabbits and hares until the seventh or eighth day of pregnancy (Casteel 1967:196, Brambell 1944:7).The median date, four days prior to collection, was arbitrarily chosen to minimize errors of estimation.Rongstad (1969: 164-165) reported finding visible uterine swellings in a female eastern cottontail rabbit, Sylvilagus floridanus, thought to be 5.5 days pregnant.However, this female may have been pregnant for 6.5 days because she was not examined the day prior to finding her previous litter.

3. Dates of conception for females whose uteri contained visible embryos were estimated from data for the determination of the age of embryos for eastern cottontail rabbits (Rongstad 1969:164-168), because similar data were not available for mountain cottontails. 12 Dates of conception for two litters were calculated for females that were concurrently pregnant and lactating (Evans et al. 1965:178- 179, Wight and Conaway 1962:95).Dates on which litters in utero were conceived were estimated by one of the methods previously men- tioned.Because the gestation period in other rabbits of the genus Sylvilagus was reported to be about 26-28 days (Marsden and Conaway 1963:168, Wight and Conaway 1962:94, Ingles 1965:151), 27 days were subtracted from calculated dates of conception of litters in utero to obtain estimates of the dates on which nursing litters were conceived. Estimation of the latter dates of conception required the assumption that conception of the litter in utero occurred immediately after the birth of the previous litter.This assumption was supported by many reports of immediate postpartum estrusand conception among other rabbits and hares (Wight and Conaway 1962:94, Casteel 1966: 165, Marsden and Conaway 1963:164-169, Lechleitner 1959:72, Bram- bell 1944:15, 17). Ages of specimens were estimated on the basis of the degree of ossification between the epiphysis and the diaphysis of the humerus

(Hale 1949:220-221).Specimens were considered to be adults if ossification of the humerus was completed, and juveniles if ossification was not completed. The mean size of litters produced by females collected in 1969 was determined from counts of viable embryos in31 adult female 13 specimens. Rates of ovulation were determined from counts of corpora lutea made from the sectioned ovaries of 33 adult females collected during the 1969 breeding season that were pregnant. Embryos, more than 25 mm long from crown to rump collected during the investigation, were dissected to determine their sex (Bram- bell 1944:3). Common statistical tests (Chi-square, Student's t ,Poisson test of randomness, Sign test, F-test, and Mann-Whitney U test) were ap- plied to ascertain the probability of chance occurrence of phenomena suggested by the data collected. 14

RESULTS AND DISCUSSION

Sex Ratios

Sex was recorded for 266 mountain cottontails collected in central Oregon, and for 76 embryos longer than 25 mm removed from pregnant females.Sex ratios were 1:1.05 in favor of females among embryos( 3 5 de: 37 ??) and 1: 1. 18 in favor of females among specimens collected by shooting (122 dd :144 ??).Neither ratio was significantly different from 1:1 (P > 0.05). The sex ratio of 40 specimens shot between 15 October 1968, and 23 February 1969, was 1:0. 66 in favor of males (25 dcr:16 ??), while the sex ratio of 31 specimens collected between 1 March and 1 June 1969, was 1:1.20 in favor of females (14 crcr:17 ??).Sex ratios of eastern cottontail rabbits collected by hunting or trapping were reported to be biased in favor of males during winter; this effect was attributed to a tendency for females to remain in dens during cold weather (Gerstell 1937:6, Allen 1938:467).Although sex ratios of mountain cottontails collected during cooler months (fall and early winter) were not significantly different from those collected during warmer months (late winter and spring), the apparent change in the ratios of samples was similar to that reported by Gerstell (1937:6) and Allen (1938:467). Seasonal differences in the denning behavior of both sexes of 15 mountain cottontail rabbits were suggested by the increased effort required to collect specimens during fall and winter (Figure 2).

Breeding Season

Based on 56 adult female mountain cottontails collected during 1969, the breeding season from first conception to last parturition, extended from 22 February to 30 July 1969 (Table 2).Few females conceived in either February or July, however, pregnancy was preva- lent from the second week of March through the end of June 1969. None of the eight female mountain cottontails collected before 22 February during 1969 were pregnant. A single adult female collected on 22 February was pregnant and had conceived within 10 hours of collection.However, the only two adult females obtained during the next collection on 1 March were not pregnant.During the second week of March, breeding among mountain cottontails became wide- spread, and all 29 adult females collected between 25 March and 21 June 1969 were pregnant. Six pregnant females collected prior to 13 April 1969 were thought to be intheir first pregnancy of the 1969 breeding season. These animals were the first pregnant females collected during the year and they were not lactating.Except for the female which conceived on 22 February, dates of breeding for females undergoing their first pregnancy of 1969, occurred during the week of 8-15 16

Table 2,Numbers of female mountain cottontail rabbits collected in Deschutes and Jefferson counties, Oregon, each collecting trip,1 January - 28 Aguust 1969, and numbers of females considered to be pregnant through use of each of several criteria of pregnancy,

Collection Number Number Number Number Number Number of date collected pregnantwith visible with with corpora lutea uterine sperm rupturedbut no visible swellings in uterus folliclesuterine swellings

3 January 1 0 0 0 0 0

18 January 3 0 0 0 0 0

19 January 1 0 0 0 0 0

8 February 1 0 0 0 0 0

9 February 2 0 0 0 0 0

22 February 1 1 0 0 1 0

1 March 2 0 0 0 0 0

25 March 1 1 1 0 0 0

26 March 1 1 1 0 0 0

2 April 3 3 3 0 0 0

13 April 3 3 2 0 0 1

20 April 3 3 3 0 0 0

26 April 3 3 3 0 0 0

4 May 3 3 2 1 0 0

17 May 1 1 1 0 0 0

18 May 3 3 2 0 0 1

25 May 4 4 3 0 0 1

6 June 2 2 2 0 0 0

14 June 2 2 2 0 0 0

21 June 5 5 5 0 0 0

3 July 2 2 1 1 0 0 19 July 4 0 0 0 0 0

26 July 2 0 0 0 0 0

9 August 2 0 0 0 0 0

28 August 1 0 0 0 0 0

Totals 56 37 31 2 1 17 March (Figure 3). Twenty-eight (90. 3 %) of thirty-one pregnant adult females collected between 13 April and the termination of breeding in July, were concurrently lactating.These females were considered to be in at least their second pregnancy of the season. Although all nine adult females collected during June 1969 were pregnant, only two (25.0%) of the eight adult females collected during July were pregnant, and none of the three adult females collected during August were pregnant.The last litter conceived during the season was calculated to have occurred on 3 July 1969; parturition would have occurred on 30 July 1969.The period from first conception to last parturition was 158 days.

Breeding Synchrony

Synchronous breeding, a condition occurring among local animal populations in which the females conceive concurrently, was reported numerous times among other species of rabbits and hares (Brambell 1944:15, Conaway and Wight 1962:289, Casteel 1966:165, Meslow and Keith 1968:826).Synchronous breeding among other species of rabbits was observed at intervals of one gestation period throughout the breeding season because immediate postpartum estrus occurred; female eastern cottontails were reported to be almost continuously pregnant throughout the breeding season (Wight and n 25 10 15 February March

m 1 15 20 25 30 5 10 15 20 April May

171 F-1771..t.11. 25 304 5 10 15 20 25 301 5 June July Date of breedingfor specimen pregnant but not lactating Date of breedingfor specimen pregnant and lactating Date of breedingfor juvenile specimen pregnant but not lactating Date of breedingfor nursing litter, calculated from specimen pregnant and lactating. Mean date of breeding for synchronous litters 95% confidence interval of mean date of breedingfor synchronous litters

Figure 3.Frequency distribution of dates of breeding for mountain cottontail rabbits collected, Deschutes and Jefferson counties, Oregon, 1969. 19 Conaway 1962:94, Conaway, Wight, and Sadler 1963:174). Schwartz (1942:8) reported that the periodicity and regularity of synchronous breeding declined as the breeding season progressed. Female eastern cottontails which underwent pseudopregnancy, infer- tile mating, or resorption of their entire litter were reported to conceive 14-16 days out of phase with the cycle established among females which experienced normal births throughout the season (Conaway and Wight 1962:289-290).Because the cumulative number of females experiencing some form of reproductive failure seemingly would increase with each successive litter, the decline of the regularity of synchronous breeding observed as the breeding season progressed, was expected. Dates that female mountain cottontails conceived during 1969 occurred in five groups.The interval between the mean date of each successive group was approximately 25-29 days (Figure 3).For the following reasons these five groups were assumed to represent the first, second, third, fourth, and fifth litters produced by adult females that lived throughout the breeding season and underwent immediate postpartum estrus and fertilization: 1. All 34 adult females collected between 25 March and 28 June were pregnant and had conceived between 7 March and 10 June 1969.If immediate postpartum estrus and fertilization had occurred in the population the probability of collecting females which were pregnant would be rela- tively small. 2. None of the six females whose dates of breeding comprised 20 the first group (Figure 3) were lactating.These females were considered to be in their first pregnancy of 1969.

3. Twenty-eight (90. 3%) of the thirty-one adult females that formed the second, third, fourth, and fifth groups (Figure 3) were lactating.These females were considered to be in at least their second pregnancy of 1969.

4. The interval between the mean date of each successive group (Figure 3) was approximately equal to the period of gestation reported for several other members of the genus Sylvilagus (Marsden and Conaway 1963:168, Ingles 1965:151, Cowan and Guiguet 1956:107).

Number of Litters in 1969

Dates of breeding among 34 (91.9%) of the 37 pregnant adult females collected during 1969 occurred during either the first, second, third, or fourth synchronous litters.These females were considered representative of adult females which survived the breeding season and produced four consecutive littersbetween 7 March, and 28 June 1969. Adult females which became pregnant either before or after the period 7 March- 28 June, may have produced fivelitters during

1969.The proportion of adult females which may have produced five litters during 1969 was estimated from the numbers ofadult 21 females that were collected between 4 February6 March, and 29 June 29 July 1969.Three (23.0%) of these 13 females conceived litters before 7 March, or after 28 June 1969 and may have produced five litters. Because the interval between the first conception and last parturition among rabbits collected during 1969 was approximately 5.6 times the estimated period of gestation, the production of more than five litters by any adult female surviving the breeding season was considered improbable. Assuming that 23. 0% of the adult females in the population produced five litters, and the remainder (77. 0%) produced four litters, the mean number of litters produced by adult females surviving throughout the breeding season was 4.2.

Size of Litters

The mean size of mountain cottontail litters calculated from counts of viable embryos in 31 litters was 4.3. Among eastern cottontails, the sizes of litters produced at the beginning and end of the breeding season were reported to be less than the sizes of litters produced during the remainder of the breeding season (Lord 1961:29, Conaway et al. 1963:174, Trethewey 1970:29). Among mountain cottontails, mean sizes of litters increased from the first to second litter, but declined from the second through fourth 22 litters (Table 3).However, the apparent differences between the means of synchronous litters were not statistically significant (F > 0.05). No comparisons were made with the fifth litter because only one female in this group contained viable embryos. The rate of ovulation was calculated from counts of corpora lutea contained in the ovaries of 33 adult females. Among adult females, 5.1 ova were shed per pregnancy during 1969.Ovulation rates were also calculated for each of the synchronous breeding per- iods (Table 3). Complete information pertaining to the number of ovulations, visible uterine swellings, and viable embryos was available for 30 of the 37 pregnant adult females collected during 1969.Eleven (36.7%) of these females contained fewer embryos than corpora lutea. Of 153 ova shed, 14 apparently failed to become implanted.Resorp- tion of individual embryos within otherwise viable litters occurred in 5 (16.7%) of these 30 litters and affected 9 ( 6. 5 %) of the 139 visible uterine swellings.

Male Reproductive Cycle

During 1969, adult male mountain cottontails were considered capable of fertile mating between 1 February, and 3 July.Males were considered to be incapable of fertile mating during the remainder of the year. Table 3.Mean sizes of litters and mean rates of ovulation, implantation, and resorption for each litter among female mountain cottontail rabbits, Deschutes and Jefferson counties, Oregon, 1969. No. and Mean Number of Ova Shed Number of Number of Number of Embryos date of synchronous Implantations Viable Embryos Undergoing Resorption litters Means. e. n Range Mean s. e. n Range Means. e. n Range Mean s. e. n Range

10 March 1969 4.4 0.24 5 4-5 4.0 0.32 5 3-5 4.0 0.50 5 3-5

II 7 April 1969 5.9 0.4110 3-8 5.4 0.11 9 3-6 5.1 0.51 9 3-6 0.3 9 0-3

III 1 May 1969 5.4 0.24 7 4-6 5.0 0.44 7 3-6 4.6 0.48 7 3-6 0.4 0.33 7 0-2

IV 31 May 1969 4.2 0.33 10 3-6 3.7 0.31 9 2-5 3.4 0.44 9 1-5 0.2 9 0-1

3 July 1969 6.0 1 5.0 1 4.0 1 1.0 1

Totals of all litters 5.1 0.3233 3-8 4.6 0.22 31 2-6 4.3 0.0331 1-6 0.3 0.13 31 0-3 24 Among other rabbits and hares, seasonal enlargement of the testes of males was found to correspond to active production of spermatozoa preceding the onset of breeding, and seasonal reduction of the testes of males was found to correspond to cessation of spermatogenesis at the end of the breeding season (Ecke 1955:289-300, Bram- bell 1944:5, Schwartz 1942:2-3).The loss of the ability to breed among males was reported to be the factor controlling the cessation of the breeding season (Meslow and Keith 1968:828, Brambell 1944:

17). Enlargement of the testes of male eastern cottontails was reported to occur in December in New York (Hamilton 1940:8). Male eastern cottontails were considered to be capable of fertile mating 2-6 weeks before breeding occurred because copious amounts of sperm were present in the cauda epididymides (Ecke 1955:310, Trethewey 1970:11). The mean weights of the testes of mountain cottontail rabbits in central Oregon increased noticeably in December 1968 (Figure 4); a few spermatozoa were present in the cauda epididymides of a juvenile.Two (40. 0 %) of the five males collected in January, five (83.3%) of the six males collected in February and 26 (96. 3 %) of the 27 adult males collected between 1 March and 30 June 1969 possessed copious amounts of spermatozoa in their cauda epididymides, and were considered capable of fertile mating. Range Standard Deviation Sample Size .41r 5 3 Mean Standard Error 8L 8L

6 4 7

3 r-1

6 5

2 9

1 0 4 5 1.

J J AS 0 N D I J F MA M J J A 1968 1969 Month and Year Figure 4.Average weights by month of testes of adult male mountain cottontail rabbits, Deschutes and Jefferson counties, Oregon, June 1968-August 1969.Standard deviations and standard errors of weight of testes for rabbits collected in July, August, September and November 1968, and August 1969 could not be plotted because of their smallvalues. 26 The mean weights of the testes of adult male mountain cottontails was maximal during April and May 1969.By 15 June, a definite reduction of mean testes weight was observed, and on 15 July 1969 the mean weight of the testes of males was approximately half of the maximum mean weight observed earlier in the breeding season (Fig- ure 4). Spermatozoa were found in the cauda epididymides of adult male mountain cottontails until 28 August 1969.Lechleitner (1959:70), reported finding spermatozoa stored in the cauda epididymides of black-tailed hares, Lepus californicus ,long after spermatogenesis had ceased. Meslow and Keith (1968:828) suggested that the presence of spermatozoa in the cauda epididymides of male rabbits and hares was not an accurate criterion of male fertility,because the decreased production of androgenic hormones which accompanied cessation of spermatogenesis, would cause a loss of libido among male rabbits. Seven (87. 5%) of the eight adult females collected during June 1968, and six (75. 0%) of the eight adult females collected during July 1969 did not conceive during those months although the mean weights of the testes of adult males were in excess of 2 g and copious amounts of spermatozoa were present in the cauda epididymides. Two expla- nations were suggested for the failure of adult females to conceive during June 1968, and July 1969.Either male mountain cottontails did not breed once testicular regression had commenced, orfemales 27 were the sex controlling cessation of breeding, as reported by Hamilton (1940:9).

Juvenile Sexual Development

Males

The chronology of sexual development among juvenile male mountain cottontails was found to parallel the seasonal sexual development of adult males.Juvenile males did not become sexually active until late winter after their birth.Because the last pregnant female of 1968 underwent parturition on 9 June 1968, these juvenile males were at least seven months old when they attained breeding condition. Among eastern cottontails, juvenile males 3-6 months old, and poses- sing copious amounts of spermatozoa in the cauda epididymides, were reported to occur frequently in July, August, and September (Corr way and Wight 1963:426-427, Trethewey 1970:13). No spermatozoa were present in either the testes or cauda epididymides of juvenile male mountain cottontails collected before December 1968.Both juvenile males collected on 15 December 1968 possessed spermatozoa within the lumens of the seminiferous tubules of both testes; one of the males possessed a few spermatozoa in the cauda epididymis of one testis. All five juvenile males, from which testes were selected for 28 sectioning from the January and February samples, contained spermatozoa in the seminiferous tubules of the testes; four (80.0%) of the five juvenile males contained moderate to copious amounts of spermatozoa in their cauda epididymides. All male specimens possessed copious amounts of spermatozoa in the cauda epididymides from 9 February until breeding commenced in 1969.

Females

A single juvenile female mountain cottontail, collected on 21 June 1969, was found to be pregnant but not lactating.Conception of the litter in utero was estimated to have occurred on 13 June 1969. This female was considered to be pregnant with her first litter.The possibility that this female would have conceived a second litter if it had continued to live, was considered unlikely because parturition of the litter in utero would have occurred 10 days after the last conception that was observed during 1969.This juvenile female was estimated to be 12 weeks of age or less when conception occurred because parturition of the first pregnancy of 1969 would have occurred on 22 March 1969. Breeding among juvenile female eastern cottontails was reported to occur in 30%-50% of those females examined (Conaway et al. 1963:173, Trethewey 1970:36-38, Negus 1959:451-452).However, Z9 the breeding season was reported to extend from January-March until August-September (Barka low 1962:32-34), but the breeding season among mountain cottontails in centralOregon was considerably shorter, extending from 22 February to 3 July during1969. Because the majority of juvenile mountain cottontails were lessthan two months old when the breeding season ended, thelength of the breeding season in 1969 was not conducive to breeding by juveniles.

Annual Productivity

Based on a sample in which 77.0% of the adult female mountain cottontails produced four litters, and 23.0% produced five littersof 4.3 young each, the maximum annual productivity was 18.2 young per adult female during1969. Based on a sample in which one (6. 7 %) of 15 juvenile females collected between 6 June and Z7 July produced one litter, the annual production was 0.3 young per juvenile female during 1969. The annual productivity of eastern cottontails was reported tobe considerably higher than the productivity observed amongmountain cottontails.Within its natural range the eastern cottontail was reported to produce 22-30 young in five-seven litters(Conaway et al. 1963: 172, Evans et al. 1965:181).Eastern cottontails introduced into the Willamette valley of western Oregon were reported toproduce 39 young in eightlitters during 1968 (Trethewey 1970:38-40). 30

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