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5-1970
On the Population Ecology of Uinta Ground Squirrels
Spencer R. Amend Utah State University
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This Thesis is brought to you for free and open access by the Graduate Studies at DigitalCommons@USU. It has been accepted for inclusion in All Graduate Theses and Dissertations by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. ABSTRACT
On the Population Ecology of Uinta Ground Squirrels
by
Spencer R. Amend, Master of Science
Utah State University, 1970
Major Professor: Dr. David F. Balph Department: Wildlife Resources
A marked population of Spermophilus armatus averaging 529
individuals per year was studied for two and a half seasons on a
22-acre tract in northern Utah. The average date of emergence from hibernation of the first squirrel was 9 April. Mating occurred as soon as animals emerged from hibernation. The average date of appearance of the first litter was 31 May. Emergence sizes of litters were from 5 .2 for yearling females to 6.3 for adult females. Few instanc es of predation were observed . Age and sex ratios varied throughout the season and for different portions of the study area.
The population increased 22 percent from 1964-1965 and 15 percent from 1965-1966. Juveniles moved more than adults and males moved more than females. In 1964 and 1965, 90 percent of the squirrels had disappe ared into hibernation by 1 August. Active periods were from
61 days for juvenile males to 97 days for adult males.
(70 pages) ON THE POPULATION ECOLOGY OF UINTA GROUND SQUIRRELS
by
Spencer R. Amend
A thesis submitted in partial fulfillment of the requirements for the degree
of
MASTER OF SCIENCE
in
Wildlife Biology
UTAH STATE UNIVERSITY Logan, Utah
1970 ii
ACKNOWLEDGMENTS
I wish to acknowledge the contributions of a number of indivi duals who provided help at various stages during the study reported in this thesis. Fellow graduate students Bob Walker and Dick Burns and Dr. Allen W. Stokes assisted with field work as members of the
Ground Squirrel Study Team. Dr. David F. Balph provided help through
out the entire study.
I am grateful to the Directorate of the Migratory Bird Popu lations Station, Bureau of Sport Fisheries and Wildlife, and espe cially to Mr. Henry M. Reeves, without whose persuasive encourage ment during the later stages of preparation, this report might never have been completed.
Special thanks go to my wife, Marcene, who helped with field work and provided patient support at all times.
~penc er R. Adiend iii
TABLE OF CONTENTS
I NTRODUCTION
METHODS 3
Cap turi ng Groun d Squirrels 3
Observin g Ground Squirre l s 5
Scans •
Si ghtings 8
Collectin g Grou nd Squi rr el s 8
RESULTS 9
Seasona l Cycle 9
Emer ge nc e f r om hibe rn atio n 9
Reproduction 9
Onset of hibern ation 15
Populati on Structure 19
Density 20
Age compositi on 20
Se x ratios 31
Natality 40
Litter size 40
Production 42
Mortality 42
Life table analyses 42
Sourc es of mortality. 47 iv
Movement • 48
Chan ges in loc ation 49
Ingre ss and egress 49
Movements wi thin the Station 49
Excursions 51
DISCUSSION • 52
Methods of Life Table Construction 52
Juvenil e Di sappearance 53
Populat ion Turnover 54
Variation in Age and Sex Ratios 54
Density a nd Ground Squirrel Population Ecology 55
SUMMARY • 56
CONCLUSIONS 58
LITERATURE CITED 59
VITA. 62 v
LIST OF TABLES Table Page
1. Direct observations of marked ground squirrels by age and sex, 1965 • 10
2. Extensions of season lengths by direct obse rvation s, 1965 11
3. Success of ground squirrel capture program as indicated by sightings of marked squirrels, 1965 12
4. Dates of emergence from hibernation, disappearance dates and lengths of aboveground active seasons by age and sex 13
5. Weights of ground squirrels at emergence from hibernation, 1965 and 1966 14
6. Overt reproductive condition of male Uinta ground squirrels as indicated by enlargement of testes, 1965 16
7. Overt reproductive condition of female Uinta ground squir- rels as indicated by nipple condition, 1965 18
8 . Age and sex composition of th e ground squirrel population, 1964 and 1965 21
9. Observed density on representative areas throughout the season, 1965 • 23
10. Observed density on representative areas throughout the day, 1965 • 25
11. Observed age ratios (yearlings per adult female) on repre- sentative areas throughout the season , 1965 27
12. Observed age ratios (juveniles per yearling and adult female) on representative areas throughout the season, 1965 28
13. Observed age ratios (ye a rling s per adult female) on repre- sentative areas throu ghout the day , 1965 32
14. Observed age ratios (juveniles per adult and yearling female) on representative areas throu gho ut the day, 1965 33
15 . Obse rved sex ratios on r ep resent a tive a r eas, 1965 35
16. Actu a l and obse r ved sex ratios of adul t, yearling and juvenile ground squirrels, 1965 37 vi
Table Page
17. Litter sizes of some common ground squirrels of the ge nus Spermopl1ilus 41
18 . Dynamic life tables fo r gro un d squi rr el s born in 1964 43
19. Time-specific life tab l es for ground squir r els present in 196 5 44
20 . Composite lif e tables for ground s quirr e l s present durin g 1964-1 966 46
21. Changes in l ocat ion by ground sq uirrel s , 1964 and 1965 50 vii
LIST OF FIGURES Fi g ur e Page
1. Map of Ut ah State Unive r s ity Forestry Field Station, s how in g loc atio ns of gr ound s quirr el observation tower s •
2. Breedin g condit ion of Uinta grou nd squirrels throu gho ut the season, as indicated by e nl a r geme nt of t es t es and v i s ible nipples, 1965 17
3. Observ e d and actual density of Uinta ground squirrels throughout the season, 1965 22
4. Obse rv ed density thr ougho ut the day , 1965 26
5 . Obse rved and actual age ratios (yearlings per adult female), 1965 29
6. Observed and actu al age ratio s (juvenil es pe r yearlin g and adult female), 196 5 30
7. Observed age ratios throughout the day, 1965 34
B. Observed a nd actual sex ratios, 1965 . 39 viii
ABSTRACT
On the Population Ecology of Uinta Ground Squirrels
by
Spencer R. Amend, Master of Science
Utah State University , 1970
Major Professor: Dr. David F. Balph Department: Wildlife Resources
A marked population of Spermophilus armatus averaging 529 in dividuals per year was studied for two and a half seasons on a
22 - acre tract in northern Utah . The ave r age date of emerge nce from hib erna ti on of the first squirrel was 9 April. Mating occur r ed as soo n as animals emerged from hibernation . The ave r age date of appea r ance of the first litt er was 31 May. Emergence sizes of l itters were from 5.2 for yearli ng females to 6.3 for adult fema l es . Few instance s of predation were observed. Age and sex r atios varied throughout the seaso n and for different portion s of the study area.
The popul ati on increa sed 22 percent from 1964-1965 and 15 percent from 196 5-1966. Juv e niles moved more than adults and males move d more than females. In 1964 and 1965, 90 perc e nt of th e squirrels had disappeared into hibern ation by 1 August. Acti ve periods were from
61 days for juvenile males to 97 days for adult males.
(70 pages) ON THE POPULATIONECOLOGY OF UINTA GROUNDSQUIRRELS
INTRODUCTION
The purp ose of this pap er i s to describe the ecology of a popula tion of Uinta groun d squ irr e l s (Spermophilus ar mat us) . Of primary concern is the estab li shment of norms for those population paramete r s which ar e likely to be affected by changes in dens ity.
Debat e on th e rol e of density in r eg ul at i on of natur al popula tions has been vigo rou s over the past seve r al decades (Nicholson , 1933;
Andr ewarth a and Birch, 1954 ; a nd papers by Nicholson , Birch,
Andr ewarth a , and Miln e , and r es ultant discuss i ons in the Cold Spring
Harbor Symposium, 1957) . More r ece ntly, theo r etica l papers by
Hairst on, Smit h, and Slobodkin (1960), Murd och (1966), Ehrlich a nd
Birch (1967), and Slobodkin, Smith a nd Hairston (1967) have provided the bas i s fo r co ntinuin g th e discussio ns .
Milne (1957) point ed out the need for sufficient field data from which to draw conclusions concerning natur al con tr ol . Stok es and Balph
(1965) stated that the pr obl ems ass ociat ed with field stud ie s measuring and expl aining fluctuations in pop ul ati on density are sufficie ntly complex to warrant cooperative s tudy by a gr oup of specialists, includi ng population eco l og i sts, physiologists, and ethologists.
Accordingly, such a team effort--of which th e present study is a part- was undert ake n to investigat e natural regulation in Uinta ground squirrels. 2
Following the present study, gro und squirrel numbers will be experimentally reduced to study the means by which the population attempts nat ur al control. In order to study the effects of the density reduction on various parameters, it was necessary to Eirst establish norms at the population's initial density. The objective of the present study was to help establish these norms.
Specific objectives of the study report ed in this paper were:
(1) To provide quantitative information on the seasonal cycle
of the ground squirrel population.
(2) To describe the population structure, emphasizing variation
in age and sex ratios at different times of day and season
and for various portions of the population .
(3) To determine the natality rate of the population .
(4) To provide information on the mortality and survival rates
for various segments of the population .
(5) To determine the extent of movement. 3
METHODS
The study was conducted on an unconfin ed popu l ation of ground squirrels at the Utah State University Fore s try Field Station (here after referred to as the Station), 22 miles northeast of Logan, Utah.
Reports of previous studies on this population contain a description of the study area and a general account of ground squirrel behavior
(Balph and Stokes, 1963; Balph and Balph, 1966).
The methods were designed to identify, and subsequently follow, as many animals as possible, Methods included capturing, observing and collecting squirrels .
Data for the present study were collected nearly every day from
April 14 - August 19, 1964; from April 18 - August 18, 1965; and from
March 28 - May 11, 1966. These periods covered two and a half seasons of the squirrels' aboveground activity .
Capturing Ground Squirrels
The primary r eason for capturing ground squir rels was to identify, mark and keep marked for field identification all ground squirrels at the Station. For each capture, data were recorded on a number of parameters, including identity, time, location, weight and overt reproductive condition. On the initial capture, squirrels were toe clipped, and a number was dyed on their fur with "Nyanzol D" from
Nyanza Color and Chemical Co., Lawrence, Massachusetts. On subsequent 4
captures, dye marks were renewed as necessitated by molt. Dye marks
cou l d be read at distances up to 300 feet with the aid of binoculars .
On the basis of external morphology, it was possible to determine
o nl y two age groups: juveniles and adults . However, s i nce nearly a ll
adu l ts captured after 1964 had been identified during a previous year,
determination of the following age classes was possible: Young of the
yea r were classes as juveniles; squir r els in their second seaso n, year
lings; and squirrels in at l east their third season, adults . In 1964,
it was not possible to separate yearlings and adults, and all were
termed adults .
Obse r vations of overt r eproductive condition were designed to
ind icate which males came into breeding condition and which females
nursed young . The condition of males was recorded as (1) testes inside
the body (no swelling between the genital orifice and th e anus), (2)
te stes inguinal (swelling prese nt but not pendulous), or (3) testes
sc r otal (pendulous). In females, the condition r ecorded was (1)
nippl es not vi sible, (2) nipples visible and the sur r oundi ng ha ir
similar in appearance to other ventral hair, or (3) nipples vis ibl e
and th e s urr oundin g hair less den se or changed in color f rom other
ventral hair.
Ground squirrels were captured by tr app in g or s naring . Trapping was the more succe ss ful method and approximately 75 traps were in
continuous operation. Traps were moved so that the entire Station was covered every 7-10 days. At less frequent intervals trapping was done in peripheral areas to provide movement data. "Number 2" live traps from both the National Live Trap Corporation,
Tomahawk, Wisconsin, and Havahart Company, Ossining , New York, were used with rolled oats as bait. Results from a study of ground squir rel tr ap r esponse provided help in designing the trapping program (Balph,
1968).
Snaring was an effective method of capturing juv eniles during the first few days after they emerged from nest burrows. At that time, they congregated at burrow ent r ances and were particularly susceptible t o this technique.
Observing Ground Squirrels
Uinta ground squirrels are easily observed. They are diurnal , inhabit open areas and are not secretive in their habits (Balph and
Stokes, 1963). Squirrels were observed from seven towers erected at different sites within the Station (Figure 1). Towers were 12 feet high and provided excelle nt vantage points from which to observe the squi rr els without influencing their activities . Two types of data pertinent to the present study were recorded from th e towers: scans and s i ghtings .
Scans
An area of 80 by 120 feet near each tow er was scanned periodically throu gh day and season . A s can involved systematic r eco rdin g of the id entity, l oca ti on a nd ac tivity of each animal on th e defin ed area near each t ower. The se data were used to sample aboveground den s ity, activity and age and sex compos ition. Scans covered about 8 percent of the Station's area.
() - f) Tower locations
: '.: J Areas covered by scans
Areas cov ered by s i ghtings
Buildings 120'
: : : : : :fl ,no; . a ~-
::t . 8
Sightings
To follow ground squir rel s over a l arger area than was covered on scans, the id entity , activity and location of al l squirrels visible from the towers were recorded following each scan. These observations, termed sigh tin gs , covered about 40 percent of the Station .
Collectin g Ground Squirrels
Ground squirrels in most outlying areas were not sufficiently numerou s to make trapping worth-while . Therefore, squirrels were collected by shooting in areas more than 1500 feet from the Station .
Squirrels were collected following emergence in 1965 and 1966, and prior to hibernation in 1965. 9
RESULTS
Direct observations (sc ans and sightings) supplemented the capture
program in providing records of the locations of squirrels. Direct
observations of 61.3 percent of the marked squirrels were recorded in
196 5 (Table 1). These records extended the observed season lengths of
23.7 percent of the marked squirrels (Table 2). Further, these data
indicate the success of the trapping program, inasmuch as trapping data alone were sufficient to provide the best estimates of season lengths for more than 75 percent of the squirrels. Few unmarked ground squirrels were recorded on sightings (Table 3). Since trapping efforts were no more intensive in areas around towers than elsewhere, it was there- fore assumed that nearly all squirrels at the Station were marked.
Seasonal Cycle
Emergence from hibernation
Ground squi rr els began emerging from hibernation on 14 April
1964, 16 April 1965, and 28 March 1966. Adult males emerged first, followed by adult females, yearling females and yearling males
(Table 4). Ground squirrels in most age-sex classes were heavier at the time of emergence in 1966 than were corresponding classes in 1965 (Table 5).
Reproduction
Breeding began within a few days after emergence. Upon emer gence, adult males were aggressive toward each other and fought and 10
Table 1 . Direct obse rv ations of marked ground squirrels by age and sex, 1965
Percent of group obse r ved di r ect lya
Age Males Femal es Both sexes
Juvenil es 54 .7 58 . 0 56 .4 (139) (1 53) (2 92 )
Yea rling s 68.4 74.0 68.8 (29) (5 7) ( 86 )
Adults 70.3 84 . 2 78. 7 (26) (48) (74)
All ages 57 . 2 64. 8 61.3 (194) (258) (452)
"sample s i zes in parentheses; th ese are the numbers of marked s quirr el s observed. 11
Tabl e 2 . Ext ens ions of seaso n len gt hs by direct observa ti ons , 1965
Percentages of gro up observed after l ast capturea
Age Males Femal es Both sexes
Ju veniles 21. 3 20 . 5 20. 8 (54) (54) (10 8)
Yearlings 33.3 23.4 27 . 2 (16) (18) (34)
Adult s 35.1 35.1 35.1 (13) (20) (33)
All ages 24.5 23 .1 23.7 (83) (92) (175)
"sample siz es in parenth eses ; these are the number of s quirr el s for which th e observ ed season l eng ths were extended . Table 3. Success of ground squirrel capture pr ogr am as indic ated by sightings of marked squir r els, 1965
Percent ages of sightin~s during each 20- dar interval which were of mark ed sgui rr elsa
Age 1-19 20-39 40- 59 60- 79 80- 99 100- 119 Season -- Adultsb 92.9 97 . 1 97 .6 95 . 7 95 . 1 94 . 5 96.0 (2,465) (3,234) (3,044) (681) (693) (345) (10,462)
Juvenilesc 93 . 7 97 . 0 96 . 5 97 . 2 96 . 9 (16) (1, 064) (1,555) (1,936) (4,571)
Total 92.9 97.1 97 .6 96.5 96.0 96.8 96 . 2 (2,465) (3,234) (3,060) (1,745) (2,248) (2,281) (15,033) asample sizes in par entheses; thes e are th e numbers of sightings of marked squi rr els; scan data not in cluded . brncludes yearlings and adu lts. CJuvenile s did not be gi n to appe ar in the population until th e third 20-day interval.
.... "' Table 4. Dates of emergence from hibernation, disa ppea r ance dates and lengths of aboveg round active seasons by age and sex
Age and sex EmerE;ence datea Dis appe ar ance dateb Active periodc 1964- 1965 1966 1964 1965 1964 1965
Males: adult plus yea rli ngd May 7 May 27 Apr 28 Jul 25 Jul 89 79 Females: adult plus yearlingd 30 Apr 30 Apr 24 Apr 11 Jul 18 Jul 73 80
Adult males 22 Apr 13 Apr 27 Jul 97 Adult females 27 Apr 17 Apr 14 Jul 78 Yearling males 10 May 29 Apr 1 Aug 83 Yearling females 30 Apr 26 Apr 24 Jul 85 Juvenile males 3 June 9 Ju n 4 Aug 9 Aug 62 61 Juvenile females 3 June 9 Jun 5 Aug 11 Aug 63 63 aoa te by which 75 percent of the group had emerged . hoate which 90 percent of the group had disappeared. It was assumed that disappearance indicated the onset of hibernation. cThe number of days between emergence date and disappearance date. dsince it was impossible to distinguish between yearlings and ad ults in 1964, these groupings are shown to allow comparison of 1964 with other years. eEmergence dates for juveniles were the dates of emergence from nest burrows.
.... w
....~....:- '" 14
Table 5 . Weights of ground squirrels at emergence from hibernation, 1965 and 1966
Weight in grams a Age and sex 1965 1966 Percen t change
Adult males 298 . 6 (7 . 30) 345 . 7 (7 . 64) +15 . 8**
Adult females 252 . 0 ( 6 . 32) 273 . 2 (8.07) + 8 . 4*
Yearling males 231 . 6 (8 . 57) 227 . 7 (6 . 31) - 1. 7
Yearl i ng females 196 . 0 (3 . 30) 206. 8 (4 . 48) + 5 . 5*
**Si gni fi cant at 1 pe r cen t l eve l. *Significant at 5 percent level . asta nda rd err or s i n parentheses . 15
voc ali zed repeatedly (Balph and Ba lph, 1966) , Most adult mal es had
scrot al t es t es at this time (Tabl e 6 and Fi gur e 2) , The on set of
br eeding was r apid compared to thir t een-lined grou nd squirrels
(~. tridecemline atus), fo r which Mccarl ey (1966) reported that
en l arg ement of testes continued for 2- 3 weeks following emergence ,
Two factors s uggest th at most male br eede rs were ad ult s , First,
emergence dates indic at ed that many yearling male s did not emerge
until after breeding had occurred (Table 4 ), Second, most yearling
males did not develop scro t al testes (Table 6 and Figure 2) , su ggest i ng
impotence ,
Prior t o hibernation, testes began to en l arge (Table 6) and
th ere was a r esu r gence of sexual behavior (unpublished data ).
Sexual deve l opment prior t o hibernation has been reporte d for seve r al
rod e nts . Bakko and Brown (1967) found resumption of spermatogen i c
activity in th e t estes of whit e-t ailed prairie dogs, and Fitch ( 194 8)
observed enl ar ged t estes in California gro und s quirr el s (~ . beecheyi) .
In cont r as t to males, th er e appea r ed t o be no diff ere nc e bet ween the proportions of yearling and ad ult female breede r s . Emergence dates indic ated that most ye arling females were above ground during br eeding (Table 4), and similar proportions of yearling and adult females showed signs of nursing young (Table 7 and Figure 2).
Onset of hibernation
The onset of hibernation was marked by reduced activity. It was not characterized by sudden di sappearance of large groups of squirrels. A typic al ground squirrel about to enter hibernation Table 6. Overt reproductive condition of male Uinta gr ound sq uirre ls as indicated by enlargement of testes, 1965
Per ce nt ages of adult and yearling males exhibiting characteristics Days since emergence from Testes scrotala Te stes inguinalb Testes abdomin al hibern ati on Adult Yearling Adult Yearling Adult Yearling
1-9 (80)C 89.2 16.7 3.6 16.7 7 . 2 66 ,6
10-19 (128) 73.0 22.4 22.8 8.6 4.2 69.0
20-29 (78) 5.5 4.9 59 ,2 9 . 8 35,3 85 . 3
30-39 (46) 3.8 o.o 11.5 15.0 84 .7 85 , 0
40-99 (204)d o.o o.o o.o o.o 100.0 100 . 0
100-109 (20) o.o o.o 17,0 7.0 83 . 0 93 , 0
110-119 (14) o.o o.o ll . O o.o 89 . 0 100.0
~his was the maximum stage of enlargement and was probably necessary before males were capable of breeding. bsquirrels in this intermediat e stage may not have been capable of breeding. csample sizes for each 10-day interval are shown in parentheses; these are the numbers of squirrels examined. dsix 10-day intervals were combined, since no changes in condition were observed during the period .
,... 0-, 17
"'., 100 ..-< ..-""'- Yearlings 0. ,, 0. .... 80 ' "., I \ Males ..-< I \ .....0 60 Females - - - - ...."' I \ > ... 40 \ 0 ) ..,"'., 20 \ ., / ' ..,"' 0 0- 20 - 40 - 60- 80 - 100- ""., bD... ..-<"' "., ."'..,... :, 100 ,..--- 0. I Adults 0 ' ..." 80 bD I \ Male s CJ I ' "'"'., 60 \ Females - - - - ... I \ 0 40 .., \ ., J " 20 ...CJ \.. ., ' "" 0 0- 20- 40- 60- 80 - 100- Days since emerge nce from hibernation
Fi gur e 2. Br eed in g condition of Uinta ground squi rr els throu ghout the seas on, as indicated by enl ar gemeht of testes and visible nipples, 1965. Table 7. Overt reproductive condition of female Uinta ground squirrels as indicated by nipple condition, 1965
Percenta~es of adult and yearling females exhi biti ng characteristics Days since Nipples visible, Nip ples visible, emergence from ha ir changed a hair not ch ange d b Nipp les not visible hibernation Adult Yearling Adult Yearling Adult Yearli ng
1-9 (164)C o.o o.o o.o o.o 100 . 0 100 . 0
10-19 (180) o.o o.o 4 .7 1.0 95 . 3 99 .0 20-29 (81) o.o o.o 23 . 2 20.0 76 . 8 so.a
30-39 (87) 34.4 25.4 31.2 41.8 34 . 4 32 . 8
40-49 (117) 95.0 85.9 2 . 5 7.7 2 . 5 6.4
50-59 (59) 100.0 97.6 o.o o.o o.o 2 . 4
60-69 (76) 81.5 77 .6 18.5 14.2 o.o 8.2
70-79 (91) 17 .8 20.6 53.7 54.0 28.5 25.4
80-89 (56) o.o o.o 7 . 0 19.0 93.0 81.0
90-99 (26) o.o o.o o.o 4.0 100.0 96 . 0
100-109 (22) o.o o.o o.o 0,0 100.0 100.0
aDevelopment to this stage almost certainly indicat es nursing young ...... boevelopment to this stage may not indic ate a successful pregnancy . ex, cs ample sizes shown in parenthese s are the numbers of squirrels examined per 10-day interval. 19 would emerge fro m its burr ow later in the morning th an usual, sit or feed qui etly for a short time and then retire under gro und for the rest of the day .
The order of hibernation was adult females, yearling females,
adult males, yearling males, juve nile males and juvenil e females .
Of the adults, females hib ern at ed 13 days ear lier th an males; of the yearli ngs , females 8 day s ea rlier; and of th e juveniles, ma le s 1-2 days earlier (Tab le 4) . Apparently, th e or der of hib ern ati on is not the same for all species of ground s quirr els . Mccarl ey (1966) observed th at male thirt ee n-lin ed ground squirrels hib ernat ed earlier th an females, while Fi tch (194 8 ) observed th at female
Californi a gr ound squirrels hib ern ated ea rli er than males .
Popul ation Structure
Durin g th e seaso ns of stud y, ther e were 1,069 ground s quirrels in the populati on. There wer e 180 adults and yearlings in the spring of 1964, 219 in 1965 (a 22 percent incre ase ), and 252 in
1966 (a 15 percent increase). The "actual" density and age-sex compositi on described in th e following para gr aphs were determined by subtracting the number of squirrels last tr apped or r eco rd ed on scans or sightings during successive 10-day intervals from the total number in each gr oup. The "ob served" figures refer to data from scans in the seven repre sentat ive areas (Figure 1). 20
Density
The actual numbers of ground squirrels in the study area changed throughout the 1964 and 1965 seasons (Table 8). Observed density varied more than actual density (Figure 3) . Fluctuations in observed density were significantly correlated with changes in actua l density
( r = 0.620; P = 0.05; terminology and symbolism after Snedecor and
Cochran, 1967) .
The overall observed density during 1965 was 1 . 78 squirrels per scan; however, this varied considerably in the seven repre sentative areas throughout the season (Table 9) and throughout the day (Table 10) . The observed densi t y was greatest between 6- 9: 00 a .m., then decreased during midday with a lesser peak during the afternoon
(Figure 4) . Observed density throughout the day was significantly d i fferent from the overall observed density (P = 0 . 01).
Age composition
Two age ratios, based on the pr ocreative segment of the popu l at i on, were calculated: ( 1) yearlings per adu l t fema l e and (2) j uveni l es pe r adult fema l e . For ca l cul at io ns of juve ni l es per adult fe mal e , year l ings wer e i nc lu ded as adult s .
Obse r ved yearling -t o- adul t - fe mal e and j uve ni l e- to - adult- female r ati os va ri ed betwee n areas and t hr ougho ut t he season (Tab l e 11 and Tabl e 12) . Obse r ved age r ati os wer e ge ner ally l ower t ha n ac tual age r ati os . Bot h ob s er ved and ac tu al ~ge r ati os incr ease d nea r th e end of th e seas on (F i gur e 5 and Fi gure 6). Fl uctu ati ons in observ ed ye arli ng-t o- adult-f emal e and ju venil e -t o- adult- f ema l e r atios 21
Tabl e 8. Age and sex composition of the gro und sq uirr el populati o n, 1964 and 1965
Total numbers of ground sg uirr els 12:resenta
Days since Adult s (males :f emales )b Ju veni l es (males:fem a l es )c emergenced 1964 1965 1964 1965
1-9 70:99 72:124
10-1 9 65:94 64:121
20 - 29 53:89 60:111
30 - 39 50:8 8 58:101
40-49 46:88 48:92
50-59 40: 72 46: 84 156:163 226 : 244
60-69 32 :49 37:73 134:144 204: 211
70-7 9 27:35 31 :49 101:119 150:162
80 - 89 21:21 22:28 63:99 115: 134
90-99 15:6 16:18 41:69 89:106
100-10 9 12:2 8 : 5 30:51 49: 70
110-119 4:0 2:0 19:25 27:32
120-129 0:0 0:0 0:0 13:14 aTrapping dat a only were used for these comparisons, since the numbers of dir ec t observations differed considerably between years. Numbers shown are th e total present, minus the number last captured durin g each 10-day interval, and therefore refer t o the squirrels pres ent at the end of each interval. brncludes adults plus yearlings, si nce it was not possible to distin guish between the two age groups in 1964. cJuveniles did not appear in the population until the six th 10-day interval. ~he number of days since squirrels bega n emerging from hibernati on; the season was divided into 10-day intervals. 300 ::,...... (/)c: I Q) 'O " Obse r ved c: - -- - <11 Q) a Actual .... <11 ,,,I c: 0 (/) 200 I 0 I Q) I .....> I ..... I (.) Q) 100 ) (/) I "'Q) y, ... l_ I \ .... -~ 0
Q) bO so .....
Fi gure 3. Obser ve d and actual density of Uinta ground squirrels throughout the season , 1965 .
"' 23
Tabl e 9 . Obse rv ed de ns ity on r e pr ese nt ativ e ar eas thr ou ghout th e se as on, 196 5a
Numb e r of gr ound sq uirr e l s see n pe r s c anc Day s s ince emer ge nce b Ar ea 1 Ar ea Are a 3 Area 4 Are a 6 Are a 7
1-9 o.oo 1.38 0.91 0.29 0.79 0.92 (2 7) ( 37) (21) (17) (24) (39)
10-19 0.10 5.25 2.79 0.91 2.56 o. 75 (48) (51) (24) (33) (48) (57)
20-29 0.16 5 .16 2.14 1.15 1. 77 0.82 (31) (56) (22) (26) (35) (50)
30-39 0.33 3.59 1.33 o. 74 2.04 0.59 (24) (44) (2 7) (19) (26) (70)
40-49 0.37 2.94 1.19 0.73 0.93 0.57 (Lil) (10 3) (99) (85) (28) (6 7)
50-59 0. 8 5 2.65 1.67 0.79 2.25 1.07 (20) (66) (31) (33) (4) (68)
60-69 1.75 9.90 1.22 2.98 ----d 4.04 (16) (40) (9) (60) (47)
70-79 1.00 8.21 1.52 2.00 (3) (29) (44) (27)
80-89 1.14 1.37 (57) (41)
90-99 2.33 5.10 0.62 1.06 (3) (10) (74) (65)
100-109 a.so 3.24 0.71 1.26 (2) (50) (48) (46) 24
Table 9 . Continued
Number of ground squirrels seen per scanc Days s in ce emergenceb Area 1 Area 2 Area 3 Area 4 Area 6 Area 7
110-11 9 1. 87 0.93 1.70 ( 23) (87) (79)
120-1 29 0 . 27 (33)
130-139 0.84 o.oo (37) (22)
Overall 0 . 41 3 . 97 1. 50 1.02 1. 77 1.27
aLocations of these ar eas are shown in Fi gure 1. bThe number of days since squi rr els began emerging from hibernation; the season was divided into 10-day intervals . cs ample sizes are shown i n parentheses; these are th e numbers of scans . drndicates no squi rr els seen on sca ns during a 10-d ay interval . 25
Tabl e 10. Observe d density on r ep r ese nt ati ve ar ea s thr ougho ut t he day , 1965
Number of groun d sq uir re l s seen pe r s canb
Locationa 6-9C 9-12 12-3 3-6 6-9
Ar ea 1 0.73 0.37 0. 25 0.40 o.oo (45) (63) (56) (48) ( 3)
Area 2 5.62 2.76 3.03 3.72 2.39 (190) (124) (93) (106) ( 33)
Area 3 2.18 1.31 0.6 8 1. 30 1.86 (72) (77) (34) (43) (7)
Ar ea 4 1.41 1.06 0.62 0.76 0. 57 (204) (197) (81) (121) (35)
Area 6 2.24 1.46 1.60 1.60 3.00 (55) (89) (5) (15) (1)
Area 7 2.15 1.02 0.43 0.94 1.07 (214) (181) (122) (111) (28)
Overalld 2.73 1.35 1.10 1.55 1.36
"Refer to tower location s in Figure 1. bsample sizes are shown in parentheses; these are the numbers of scans. ~eginning at 6:00 a.m., the day was divided into 3-hour intervals. or a graph of these data, see Figure 4. 26
200
>, .....µ
Q)"' 150 'O"
Q) """'.... Q) ~ 100 .... 0
Q)
.µ"""' Q) so "C) .... Q) "'
3-6 6-9 9-12 12-3 3-6 6-9 9-12 Time of daya
Figure 4 . Observed density throughout the day, 1965.
aobservations were grouped by 3-hour intervals, beginning with 6:00 a . m. 27
Ta ble 11. Obse rv ed age r atio s (yearlings per adul t female) on repr ese ntativ e areas throughout th e seaso n, 1965
Yea rling s per ad ult fem a l e observed on sca nsb Days since emergence 8 Area Area 2 Area 3 Area 4 Area 5 Area 6 Area
1-9 ----c 1.56 0.57 o.oo 1.00 1.00 0.20 ( 9) (7) (2) (3) ( 7) (10)
10-19 Inf. d 1.54 6.75 0.08 0.49 0.96 0.13 (0) (82) (4) (26) (5 7) (56) (24)
20 -29 Inf. 1.43 1. 55 0.24 1.44 0.55 0.36 (0) (104) (11) (17) (52) (33) (25)
30-39 Inf. 1 . 79 15 . 00 0 . 18 4.64 1.09 0 . 16 (0) (53) (2) (11) (11) (22) (25)
40-49 12 . 00 1.91 3 . 28 0 . 32 1.9 8 0.92 0.26 (1) (103) (25) (38) (129) (12) (23)
50-59 4.00 1. 72 2.38 0.18 3.23 0.60 0.08 (2) (60) (13) (17) ( 22) (5) (26)
60-69 Inf. 2 . 25 Inf. 0.09 1.44 0.09 (0) (40) (0) (45) (16) (35)
70-79 2.33 1.20 0.00 (24) (5) (12)
80-89 Inf. o.oo (0) (13)
90-99 Inf. 7 .00 Inf. 10.71 3 .00 (0) (2) (0) (14) (1)
100-109 Inf. Inf. 25.67 o.oo (0) (0) (3) (5)
Overall 15.67 1.87 3.15 0.24 2 . 39 0 . 87 0 .17
8The number of days since sq uirr els began emerging from hibernation; the season was div i ded into 10-day intervals. bs ampl e sizes in pa rent heses; these are the numbers of adult females. clndicates no sq uirrel s seen on scans du ri ng a 10-day interval . drnfinit y; indicates division by ze r o. 28
Table 12. Obse rv ed age rati os (juv enil es per yearling and adult f emal e ) on r epresenta tiv e areas thr ougho ut t he season , 1965
Juvenil es 12er yearlin g and adult female seen on scans b Days since emergence a Area 1 Area 2 Are a 4 Area 5 Area 7
50-59 0.60 0.01 o.oo o.oo 0.89 (10) (152) (19) (75) (28)
60-69 2.2 5 2 . 33 2 . 35 3.39 3 . 71 (8) (111) (48) (31) (38) ____ d 70-79 In f . c 2 . 26 11.80 3.0 8 (0) (69) (5) (12)
80-89 Inf . 2.31 (0) (13)
90-99 1.00 3.7 8 Inf. 4.39 11.50 ( 3) (9) (0) (101) (4)
100-109 Inf. 13.20 Inf. 10.00 9.20 (0) (10) (0) (30) (5)
Overall 1.4 8 1. 74 5.32 5.01 4.34 arhe number of days since squirrels began emer g ing from hibernation; the season was divided into 10- da y intervals . Juveniles did not bappear in the population until the sixth 10-d.ay interval . Sample sizes in parentheses; these are the numbers of ad.ult and yearling females . crnfinity; in dicates division by ze r o . drndicates no squ irrel s see n on scans during a 10- day interval. ! 12 I I I 10 I ...."' I Obse r ved - - - - I " I ~ I Actual "" I .....µ 8 " I "'" "'" I "' 6 ~"' .... t " :><" "' 4 I I I
2 .....,,...... _.-----...._ ,} ---- __.. --- 1- 10- 20 - 30 - 40 - 50- 60 - 70 - 80 - 90- 100 - Days since emergence from hibernation
Fi gur e 5. Obs e r ved and actual age ratios (year ·l ings per adu l t female), 1965. '°"' 12 I I I ' OJ Observed - - - - .... I e 10 ...."'OJ Actual I .....µ I :, I "" "' I "" 8 " I 'g!'" .... /'-- I .... I / ' "'"OJ 6 ' I >, / ' ' OJ / "p.. / Figure 6. Observed and actual age ratios (juveniles per yearling and adult female), 1965. w 0 31 were significantly correlated with changes in the actual ratios (r = 0.785 and r = 0.998, respectively; P = 0,01). Observed age ratios varied throughout the day (Table 13 and Table 14). The juvenile-to-adult-female ratio was higher than the yearling-to-adult-female ratio and increased more throughout the day (Figure 7). Observed age ratios throughout the day were significantly different from the overall observed age ratios of I.SS yearlings and 3.79 juveniles per adult female (P = 0 . 01). Sex ratios Observed sex ratios (males:100 females) in the representative areas varied from 10.4 to 178.8, and fluctuated widely throughout the season (Table 15). Actual sex ratios were l ess variable and generally higher than observed sex r atios (Table 16 and Figure 8) . For a ll age groups considered together, fluctuations in the observed sex ratio were significantly correlated with changes in the actual sex ratio (r = 0 . 719; P = 0.01). For yearlings only, the correlati on betw ee n observed and ac tual sex r atios was a l so signifi cant (r = 0 . 977; P = 0 . 01) . Howeve r, for adults only and juveniles only , the correlations betw ee n observed and ac tual sex ratios were not significant (r = 0 . 578 and r = 0 . 595, respectively; P = 0.05). Two contingency tabl es (2 x 3) were constructed, using the total numbers of ground squirrels in each age-sex group . One table was cons truct ed utilizing on ly 1965 data; the other, all yea rs. Results of thes e two analyses indicated that sex r at i os varied 32 T. itd c 13. Obse r ve d age r atios (yearl i ng per adult f emal e ) o n r epr ese nt ati ve ar eas throu gho ut th e day , 1965 Number of yea rlin gs per adult femal e see n on scansb Location a 9-12 12- 3 3-6 6-9 ____ e Area 1 6.00 14.00 Inf .d 11.00 (2) (1) (0) (1) Area 2 1.91 2 . 07 1.59 2.00 1.62 (211) (70) (71) (104) (13) Area 3 4 .9 5 2 . 37 3 . 33 1.41 s.so (21) (19) (3) (17) (2) Area 4 0.19 0.32 0.54 0.09 o.oo (64) (47) (13) (34) (3) Area 5 2 . 96 3.47 1.41 1.92 7.60 (102) (47) (5 8 ) (99) (5) Area 6 0.85 0.82 a.so 1.27 Inf. (60) (60) (4) (11) (0) Area 7 0.21 0.07 o.oo 0.23 1.00 (101) (54) (20) (26) (1) Overall f 1.61 1.46 1.31 1.56 3.00 Bilefer to tower locations identified in Fi gure 1. bsample sizes are shown in parentheses; these are the numbers of adult females. cBeginning at 6:00 a . m., the day was divided into 3-hour intervals. dinfinity; indicates division by zero . elndicates no squi rr els seen on scans . fFor a graph of these data see Figure 7. 33 Table 14. Obse r ve d age r ati os (juvenil es per adult and yea rlin g f emal e ) on r epr ese nt ative ar eas thr oughout th e day , 1965 Number of juvenile s per ad ult femal e see n on scansb Locationa 6-9c 9-12 12- 3 3-6 6- 9 Area 1 12.00 0.67 1.50 1. 50 ----d (1) (9) (4) (4) Area 2 1. 70 1. 98 1. 86 1.7 9 4.56 (228) (47) (35) (34) (9) Area 3 0 .4 0 0.67 o.oo Inf.e o.oo (10) (6) (1) (0) (2) Area 4 4.07 6,20 7 .6 7 9.40 Inf. (44) (20) (3) (5) (0) Area 5 5.30 5.84 22.33 13 . 94 6.37 (147) (58) (3) (18) (19) Area 7 4.32 3.32 4.20 7.00 27 .oo (60) (28) (5) (8) (1) Overall f 3.25 3.92 3 . 57 6.12 7.10 "Refer t o tower locations identified in Figure 1. bsample s i zes are shown in parentheses; these are the numbers of adult and yea rlin g females. cBeginning at 6:00 a.m., th e day was divid ed into 3-hour intervals. drnd.icates no squ irrels seen on sca ns. ernfinity; indicates di visio n by ze r o . fFor a graph of these data see Figure 7. 34 10 Juveniles------ Yearlin gs - - - - .. / / "' 2 ...."""' .....__ ...______.,, / "'>" i; 6-9 9-12 12-3 3-6 6-9 Time of daya Figure 7. Observed age r atios throughout the day, 1965. "observations were group ed by 3-hour intervals, beginning with 6:00 a.m. 35 Tabl e 15 . Observed sex r atios o n r e presentativ e areas , 1965 Number of males pe r 100 females seen on scansa Days s inc e emer ge nc eb Area le Area 2 Ar ea 3 Area 4 Area 5 Area 6 Area 1- 9 ---- d 65 . 2 18 . 2 a.a 175.0 21.4 158 , 3 (23) (11) (2) (4) (14) (12) 10- 19 o.o 15 .7 127 . 8 11. 5 14.3 2.8 55 . 6 ( 3) (223) (1 8 ) (26) (84) (109) (27) 20 - 29 0.0 16.1 106. 3 58 . 8 21.2 20 . 0 20 . 8 (5) (24 8 ) (16) (17) (ll8) ( 50 ) ( 24 ) 30-39 a.a 8 .2 620 . 0 16.7 29 . 8 23 . 3 42 . 9 ( 8 ) (146) (5) (12) (57) (43) (29) 40-49 30,0 6.7 210. 8 40.5 30.7 a.a 31 . 0 (10) (282) (3 7) (42) (326) (23) ( 29 ) 50-59 a.a 12.5 19 3 . 8 36.8 44 . 0 o.o 51.5 (10) (152) (16) (19) (75) (8 ) (33) 60-69 a.a 35.6 400.0 86.0 32.8 90 . 2 (8) (177) (1) (57) (64) (92) 70-79 Inf. e 37 .1 242.9 82 .6 (0) (124) (14) (23) 80-89 153.8 53.6 (13) (28) 90-99 25.0 80 .0 17.1 45.1 117 .4 (4) (25) (35) (417) (23) 100-109 84 .0 13. 8 80.8 134. 8 (75) (29) (208) (23) 36 Table 15. Continued Number of males per 100 females seen on scansa Days since emergenceb Area 1 c Area 2 Area 3 Area 4 Area 5 Area 6 Area 7 110-11 9 88 . 9 18 . 4 81.1 128.6 (18) (49) (212) (49) 120-129 25.0 91.8 (4) (98) 130-139 Inf. 90 .3 (0) (93) Overall 10.4 24 . 8 178.8 50 . 8 52 .2 10.5 80.6 asample sizes are shown in parentheses ; these are the numbers of females . bThe number of days since sq uirrels began emer gin g from hibernation ; the season was divided into 10-day intervals . cRefer to t ower locations identified in Figure 1. drndicat es no sq uirrels seen on scans . ernfinity; indi cates division by ze ro. 37 Table 16, Actu a l and observed sex ratios of adult , yearling and juvenile ground s quirr e l s , 1965 Male s Ee r 100 femalesa Day s si nce Adults Yearlin g s Juvenilesc emer ge nceb Actual Obse rv e d Actual Obse rv ed Actual Observed 1-9 47 .1 115.8 65.8 7.1 (491) ( 82 ) (227) (30) 10-19 47.1 29 . 3 60. 8 ' 11.2 (1215) (322) (912) (268) 20-29 46.0 31. 8 63.6 12.2 (969) (319) (926) (276) 30-39 47.8 37 .1 68.3 21.0 (850) (170) (681) (213) 40-49 42.9 20.9 62.1 37 .7 (1127) (399) (1149) (573) 50-59 46.2 27 . 3 67.9 35.6 94 . 0 137 .5 (482) (182) (445) (221) (300) (19) 60-69 48.3 16 . 2 66.7 29.7 95.5 101. 8 (266) (158) (243) (131) (842) (335) 70-79 42.9 24 . 4 69.2 31.1 94 .4 101.3 (137) (51) (236) (59) (924) (151) 80-89 77 . 8 69 . 2 74.1 Inf.d 89 . 5 82 .1 (137) (22) (292) (3) (999) (51) 90-99 so.a 117 .6 85.0 71.0 89.5 39.0 (96) (37) (272) (171) (1157) (538) 100-109 100.0 75.0 125.0 175.7 78.5 67.2 (68) (14) (151) (102) (954) (485) 38 Table 16. Continued Males eer 100 femalesa Days since Adults Yearlings Juvenilesc emergenceb Actual Observed Actual Observed Actual Observed 110-119 100.0 500.0 Inf. 483.3 82.5 67. 7 (104) (18) (63) (35) (1378) (535) 120-129 o.o o.o Inf. Inf. 91. 7 87.8 (0) (4) (18) (5) (385) (184) asample sizes in parentheses; these are the numbers of records. bThe number of days since squirrels began emerging from hibernation; the season was divided into 10-day intervals. CJuveniles did not appear in the population until the sixth 10-day interval. dlnfinity; indicates division by zero. 120 llO I- I I 100 ~ / I 90 I- J ~/ U) I \ Q) 80 ,--.../ '"" \ I El I '"Q) ... 70 I \ I 0 0 ,) \ I 60 \ '"" / \ I "Q) ( so \ v ""U) I Q) I .... I ;;: 40 I '" Obser ved ---- I .J 30 .,,....--- Actual I .,,,,, \...-- :: f I 1- 20- 40- 60- 80- 100- 120- Days since emergence Figure 8 . Observed and actual sex ratios, 1965. w "' 40 significantly be tween juveniles , yearlings and adult s, and differed only in the l eve ls of si gni f icance (P = 0 . 05 a nd P = 0 . 01, r espectively ). Natality Litter size Litt er s i zes wer e obtained by counti ng juvenile groun d squirrels as t hey emer ged from nest burrows. Counts were obta in ed of 16 litt er s in 1964 and of 42 litters (including those of 15 year li ngs and 27 adult s) in 1965. Embryo counts of 11 pregnant squ i rrels collected in pe ripher al ar eas provided additional inf ormat ion. Typi ca l gro un d squirr el litter size appears to be in the 4-8 ra nge (Table 17). Rongstad (1965) s uggested the ex ist ence of an inv er se relationship between litte r size and latitude for thirteen lin ed ground squ irr e l s. The overall average litter size, for ye ar lings and ad ults combin ed, was 5. 8 in 1964 and 196 5 (std . errors= 0.144 and 0.246, resp ectively). In 1965, the ave r age li tter s iz e for yea rlin gs was 5.1 (std. error= 0 . 396) and for adult s , 6.3 (std. error= 0.284); these averages were sig nific antly different (P = 0.05) . The average number of embryos in the 11 collected squi rrels was 5.3 (std. error= 0.304). The average number of embryos in 10 squirr el s collected in 1965 was 5.2 (std. error 0.327). The average number of embryos was not significantly different from th e average litter size in 1964 or 1965 (P = 0.05). The average number of embryos in 1965 was not significantly different from the average 41 Tab l e 17 . Litter s iz es of some common ground sq u ir r el s of th e ge nus Sp er mophilus Specie s Average litt er s i ze Source s. armat us 5a Davis (1 939 ) s. beech eyi Fitch (1 948 ) s . be l dingi 8 Hall (1 946) s. columbi anus 4 Manvill e (19 59 ) 4 Davi s (1 939 ) s . elega ns 6.6 Burn ett (1 93 1) s. l ateralis 5 Gordon ( 1943) s. l eucu ru s Hall (1946) s. trid ecem lin eat us yearling s 4.9b Mccarl ey (1966) adult s 7.0b McCarley (1966) aEmbry o count. bc ounted at emer ge nce from nes t burrow s . 42 litter size of yearlings, or of all squirrels, but was significantly differ ent from the average litter s iz e of adults (P = 0.05) . Productiu[1 Calcu l ated estimates of production were based on the total numbers of females observed in each group , even though some individua l s disappeared prior to pa r turition . Females producing litters included 79 percent of 105 in 1964 and 80 pe r cent of 135 in 1965 . In 1965 , 89 percent of 57 ad ults and 74 percent of 77 yearlings produced litters . Of the 14 femal es which were collected near the end of t he br eeding season, only 11 (79 percent) were pregnant . Using the average litter sizes and the numbers of females pro ducing litters in each group , the following estimates of production by ground squi r rels at the Station were calculated: For 1964, the estimated number of juveniles appearing abovegrou nd was 477 and for 1965, 608 . Based on the total ad ult female populations , produc t i on was 4 . 5 juveniles per adult female in both yea r s . Mor tality Li f e t able analyses Sy stema ti zed gr oupi ngs of cert ain vita l statis ti cs of ani mal pop ul ati ons have bee n t er med "lif e tab l es . " The i r co lu mnar s tr uctur e makes lif e t abl es es pec i al ly use fu l for s ummari zing i nf orma ti on conc erning mort alit y and su rvival. Constru cti on of thr ee ty pes of l ife t abl es was pos sibl e , based o n ( a ) s hrink ag e of a s in gl e coho rt (Tabl e 18), (b) ob s er vati ons of all age cl ass es duri ng a s ingl e int erval (Tabl e 19), and (c) 43 Table 18. Dynamic life tables for ground squirre ls born in 1964 Sex 1 s x x dx qx ex x Females 0-1 171 94 0.550 1. 318 0.450 1-2 77 41 0.532 1.3 80 0.468 2-3+ 36 36 ----a Total 284 171 0.602 0.39 8 Males 0-1 169 121 0.716 0.897 0.284 1-2 48 23 0.479 1.588 o.521 2-3+ 25 25 Total 242 169 0.698 0.302 Both 0-1 340 215 o.632 1.082 0.368 sexes 1-2 125 64 o.512 1.453 0.488 2-3+ 61 61 Total 526 340 0.646 0.354 aDenotes an omitted calculation which would be extremely bi ase d. 44 Table 19 . Time- spe cific li fe tabl es for gr ound squirr el s pr es ent in 1965 Sex e s x lx dx qx x x Females 0-1 264 187 0.708 0 . 912 0 .2 92 1-2 77 27 0 . 351 2 . 349 0.649 2-3+ 50 50 ----a Total 391 264 0 . 675 0 . 325 Males 0-1 254 206 0.8ll 0.733 0.189 1-2 48 24 0.500 1.500 0.500 2- 3+ 24 24 Total 326 254 0.779 0.221 Both 0-1 518 393 0.759 0 . 818 0.241 sexes 1- 2 125 51 0.408 1.951 0.592 2-3+ 74 74 Total 717 518 0.722 0.278 anenotes an omitted calculation whic h would be extremely biased. 45 combin ation of two years' data for each age cl ass (Tabl e 20). These three presentations were call ed dynamic, time- s pecific and compos ite lif e tables, r espe ctively, after Hick ey's (1952) definitions. Table 18 fits Deevey's (1947) definition of an hori zontal life table, and Tables 19 and 20 fit his definition of ver tical lif e t ables . Hence, Hickey' s assertion that Deevey's definitions of "hori zontal " and "vertical" are syno nymous with his own, of "dynamic" and "time- specific," re spective ly, does not appear to be entirely correct. Symbols used in the present life tables are defin ed as follow : The ti me (age) interval in years appears under a column headed "x;" the number alive at the start of each interval, "lx;" the number dying during each interv al, "dx;" the mortality rate per year (dx divided by lx), "qx;" the mean expectation of further life for those alive at the start of the int erval, "ex;" and the survival rate 0-qx)' "sx •" Life tables for wild populations are most frequently bas ed on mortality series which are entered under the column "dx" (Deevey, 1947). However, since the present study was based on counts of ground squirrels alive at specific intervals, data were entered in the "lx" column. The life t able estimate of overall surv ival rate was 0.32. The survival rate for adults was 0.60; for juveniles, 0 . 30; fo r fem ale s, 0.37; and for males, 0.27. This general pattern of survival, i.e., higher survival rates for females th an males and for adults than juveniles, has been reported in thirte en -lin ed ground squirrels (Rongstad, 1965) and black-tailed pr airiedo gs 46 Table 20. Compos ite li fe t abl es f or ground squirrels pr esent during 1964-1 966 Sex 1 d x x x qx ex sx Females 0-1 435 285 0.655 1 . 027 0.345 1-2 150 34 0.227 3.905 o. 773 2-3+ 116 116 ----a Tot a l 701 435 0.621 0 . 379 Males 0-1 423 318 0.752 0 . 830 0.248 1-2 105 39 0.371 2 .1 95 0.629 2-3+ 66 66 Total 594 423 o. 712 0.288 Both 0-1 858 603 0.703 0.922 0.297 sexes 1-2 255 73 0.286 2.997 0.714 2-3+ 182 182 Tot al 1295 858 0.663 0.337 aDenotes an omitt ed calculation which would be extremely biased. 47 ( Cynomys lu dovici a nus ) (Kin g , 1955 ) . Magni t ude of th e s urv i va l r at e of Uin ta gr ound sq uirr el s was betwee n r ates of the aforemen ti o ned sp eci es . Ass umpti ons involved in t he use of l i fe ta bles t o estim at e mort a l i t y will be d isc usse d i n a l at er sec ti on . Sour ces of mort ali ty Pr eda ti on and acci de nt s wer e th e only so ur ces of obs e r ve d mort a lit y duri ng th e pr ese nt s tud y . At le as t one inci de nt of pr eda ti on by each of th e f ol l owin g was known: weas el (Mus t e la f r enat a ), badger (Tax id ea t axus ) and r ed- ta il ed hawk (~ j amaice ns i s ) . Fitch (194 8) calcu l at ed tha t pr edati on r emoved a l it t l e mor e t ha n half of th e f annua l in cr ement f r om a popul ati on of Cali fo rn i a gr oun d s quirre l s . Pred ati on was pr obably r es pons ibl e for much l es s r emoval dur i ng th e pr ese nt s tudy , but may have been deterr ed by the co nst ant pr esenc e of huma ns on th e ar ea . Tr ap mor t al it y ( s qu i r r el s di ed i n unatt ended t r ap s with i n 30 mi nut es i n s ununer he at) and r oad ki l l s el imin at ed 10 . 0 per cen t of th e adul ts and 4. 4 pe r ce nt of t he ju ve ni l es in 1964. These wer e r edu ce d to 2. 4 per ce nt and 2 . 5 per ce nt , r es pec ti ve ly, i n 1965 . Cer ta in i nf erences co nce rni ng the tim i ng of ju ve nil e mort ali ty wer e poss ibl e , bas ed on inf or mati on pr esented in pr ev i ous sec ti ons . Sinc e th e number of embr yo s in co ll ect ed sq uirre ls was th e sa me as th e number of you ng emer ging from ne s t burr ows, pre- emer ge nce mortal i ty doe~ not appea r t o have been great. However, s inc e 21 perc ent of the calculated number of youn g squirrel s emer gi ng fr om nest burr ows were not captured, mort ality was lik ely hea vy durin g th e ir first f ew weeks abovegr ound. 48 Movement Studies over large portions of a species' range can ofte n ignor e movement as a pot entia lly important factor in population fluctuations. However, in studies on relatively small ar eas --such as the present study- -movement must be carefully measured, as it may be one of th e most important factors in f lu encing population level s . McCarley (1966) found population size in thirt een -1 ined ground squirrels to be mostly re gula ted by movement. In the present study, the extent of movement by sq uirrels within the Station was determined, as well as the extent of move- ment away from and into the Station. Locations were determined utili zing all availab le data, including tr apping records, sca ns and sightings, to calculate "centers of ac tivity," similar to the method describ ed by Hayne (1949). Following the suggestion of Stickel (1954), single records at long distances from the center of activity were excluded from det erminations of locations . However, the appearance of such records was so wide-spread that this type of movement was considered separately (see later section on Excursions). A minimum of three tr apping records and/or four di r ect observations was arbi trarily established as the number required to determine a location. If any doubt existed concerning a location (as when the required number of records were available, but were questionably spaced) , the animal was e li minat ed from consideration. Movements of less than 80 feet were not considered. 49 Chan ges in l ocation More det ail ed informat i on was avai l abl e for sq ui rrels r emai ning within the Station th an for th ose in vo lv ed in i ngr ess or eg r ess . The refore , it seemed desi r abl e to consider these as separate t opics , al th ough there was no ev id ence to indicate th at the types of movement were not the same . In gress and eg r ess . Fifty-four ground s quirrel s wer e tr apped a nd marked within 1200 feet of Station bound ari es ; only one of th ese attempt ed in gress. One hundred and eighty-five squirrels wer e col l ected more than 1500 feet from the Station; 21 of th es e were marked . Of th e 21 squirrels l eav in g th e Station, 11 were males ; 10 , femal es ; 18, juv en ile s; and 3, adults. Squirrel s collected after egress includ ed appr ox i mate l y two percent of th e population in 1964 and 1965 . Simil arly, th e on e squ irr e l at tem pting ingr ess repre se nted app r ox i mat ely two perc ent of th ose marked outside the Station . Movements within the Station . Juveniles we re significantly more mobile than adults, and juvenil e males were significantly more mobile than juvenile fema l es (P = 0.0005); however the differenc e in mobility betw ee n adult mal es and adult females was not significant (P = 0.05), (Table 21). This is in contrast to Evans (1951), who found little movement of thirte en-lined ground squirrels during their first summer, but in agreement with Mccarley (1966), who found movement to be characteristic of the juvenile segme nt of the population. When squirrels changed locations, the distances between centers of activity were measured. Males moved significantly furthe r than females, but the difference between distances moved by juveniles and adults were not significant (P = 0.05), (Table 21). so Table 21. Changes in location by ground squirrels, 1964 and 1965a Age and sex b Percenta ge of group moving Mean distance movedc Juvenile males 66.0 383 . 6 (100) (32.4) Juvenile fema l es 32.0 238.0 '---- (126) (27.6) Adult male sd 26.0 354.4 (35) (66.8) Adult femalesd 18 .0 197.2 (49) (40.0) aExcludes ingress and egress . bsampl e sizes in parentheses; these are th e numbers of squir r els for which sufficient information was available to determine location. cDistances in feet; standard errors in parentheses. drnclud es yearlings. 51 Excursions Ground squirrels frequently made rather long trips away from thei r ·normal areas of act ivity . These includ ed the movements men tioned in the introductory statements of the Movement section . Such trips were termed "excursions . " Fi tch (1948) found that California ground squirrels made similar trips . Excursions wer e reco r ded for 80 percent of the males and 77 percent of the females within the study area . Seventy-five percent of the juveni l es and 77 percent of the adults made recorded excursions . Distances of excu r sions (one - way measurements) averaged 348 feet (std . error= 13 . 9) for males and 324 feet (std . error= 10 . 7) for females . Unlike changes in location , "' there were no statistically significant differences in excursio ns between age - sex groups (P = 0 . 05) . 52 DISCUSSION Methods of Life Table Constructio n Sin ce life tables in the present study wer e based on tot al population censuses , the assumptions concerning sampling were avoided . As Deevey (1947, p. 312) pointed ou t, the dynamic method of li fe table 11 construction ••• can be used without qua li fication .. provided only th at th e season of birth is sha rply def in ed •••• " Thi s si ngl e co nditi on was met. Dynamic li fe tables provide unbiased s ur vi va l esti mat es fo r a particular cohort in spec i f ic years; however, th ese est imates may not be r ep r esentative of mortality patterns in the populati on if conditions durin g the observa ti on period were unu sua l or e xtr eme . Result s of dyn amic and time-specific anal yses will be equ al, only if t here is no change in the env ir onment and the population is at equilib riu m. Such co nditions may be approached in natur al population s (Deev ey , 1947). In time-sp ec ific life tables, the age distribution must not change throu gh tim e if morta lity calculatio ns ar e to be valid (Hickey, 1952). Statistical compari son of th e r e l ative numbers of yearlin g and ad ult ground sq uirrels indicated th at th e age distributions did not chan ge significantly between 1965 and 1966 (P = a.as). Composite life tabl es hav e the advantage of larger samples which generally l ess en the influ e nces of vagaries in a single y ea r. 53 Nevertheless, changing age - specific mort ality invalidat es mortality estima t es in composite life tables (Hick ey , 1952) . In the present study the necessary conditions for constructing li fe tabl es by all thr ee methods appeared to have been met. Survival r ates obtained by the thr ee methods were generally similar (Tables 18 - 20). However , since slight varia tions occurred between methods , t he most reliable estimates of annua l surviva l rates of juvenile and adult Uinta ground squirrels were probably those obtained by averag in g the r esults . Ju venile Disapp earance Since movement of juveniles, both within and away from the St at i on, was gr ea ter than movement of adults, it is possible that movement accou nted for t he disappearance of a portion of the 21 per ce nt of th e juveniles which were not captu r ed . Dispersa l of young Uinta gr ou nd squirrels from nest burrows was obse rved f r om towers and was sim il ar t o described movements of young thirt een -l ined gr ound squi rr els (Rongs t ad , 1965 and Mccarley, 1966 ). Young squirrels r emain ed fairly close to nata l burr ows fo r 1-2 weeks after emer ge nce . Howard (1960) s tated that dispe rs a l movements in many species occur at the time of puberty, but that this is not the ca se with all spec i es . Exactly what happened to the juveniles which were not captur ed was unkno wn, but disappear anc e was probably due t o several factors, including movement, and occurred during their first few weeks aboveground. 54 Population Turnover The 19 percent per year increase in adult population during 1964 -1 966 was not great in comparison with ground squirrels' capacity for reproduction and the population was therefore considered to be at equilibrium . Mccarley (1966) considered fluctuations of 42 per cent and 18 percent to represent normal yearly variations in thirteen lin ed ground squirrels . The theoretical survival rate in a balanced population with 120 adult females, each producing 4.5 young per year was calculated as 0.27. This was nea r the average overa ll s urvival r ate of 0.32 based on life table analyses. A slight increase in th e theoretical survival ~rate t o compensate for the population incr eases in 196 5 and 1966 would make these su rvi val rates almost identical. Variation in Age and Sex Ratios Explanation of the factors responsible fo r variation in age and sex ratio s was bey ond the scope of the present study. However, a few points seem worthy of mention. Age and sex ratios were influenced by timing of emergence from hibernation, by whether or not a particular area was used by females for nesting and by timing of onset of hiber nation . During the early part of the season, adult males were actively seekin g females . Following breeding, males disappeared from nesting ar eas . Females generally hibernated before males . 55 Dens ity and Ground Squirrel Popul ation Ecology Operation of density - related factors in the gro und squ irr el population at the Station wil l undoubt ed ly be more clearly und er stood following the population reduction and further study . Present statements ar e therefor e tentative and will be confirmed or refuted by later papers . Perhaps the most serious shortcoming of the ground squirrel study--from th e standpoint of population study- - was the lack of reliable censuses just before hibernation . Observed disappearance rates were undoubtedly co rr elated with the onset of hibernation, but it was impossib l e to obtain accu r ate popu l atio n estimates in / late surrn:ner. Had s uch estimates been ava ilabl e , it would hav e been possible to partition mortality throughout the season . It would have also been possible to test for density-dependent natality a nd mortality by comparing spring and fall de ns ity fi gures and various derivatives thereof . Using the available data, density-dependent natality was not demonstrated, but density-dependent survival may have been indicated. Comparison of litter sizes of ground squirrels at the Stat ion with litter sizes of squirrels in peripheral ar eas failed to show hi ghe r litter sizes in areas with few er squirrels. If the number of yearl ings was representative of spring-fall gai n th e previou s year, a decrease in survival accompanied higher adult density. However, other factors may have been more important than density in both cases. 56 SUMMARY A fr ee -livin g population of Uinta grou nd sq uirr els ave r ag i ng 217 adults each spr ing was s tudi ed to establ i sh norms for pa r amete r s li kely to be affected by changes in density . Methods included capturing, markin g , observing and co ll ec ting sq uirr el s , and wer e desi gned t o id entify and follow as many squirr els as possible . Age classes were juvenile s , yearlings and adults. Approximately 96 percent of th e popu lation wer e marked . More than 60 percent of th e marked squirrels were directly observed . Adult mal es did most, i f not al l, the br ee ding. Simil ar proportions of the yearling and adult females produced litters. Observed de ns it y and age -se x composition were s i gnif ican tly corr elated with age . Litter sizes averaged 5 . 8 ; adults had larger litt er s tha n yearlings. Production was 4.5 youn g per adult female in th e overall popul ation. Life tabl e analys es by dynamic, tim e-sp ecif ic and composit e methods yielded an ove rall annual survival r ate of 0 . 32 . The assumptions r equired for analysis by ea ch type of lif e table appeared to have been met . Adults survived better th an juveniles an d fem al es survived better than males. Sources of observed mortality were predation and accidents. Only two percent of th e squirrels were observed to make extensive movements. Juvenile males moved twice as frequently as the average of other age-sex gr oups . Survival rate calculated by the life table analyses was similar to the survival rate in a theoretical, balanced population averaging 120 adult f emales each 57 producing 4.5 young . The conclusion th at density-depend e nt natality or mortality could not be clearly demonstrated was prefaced with a discussion of the lack of fall census data. / 58 CONCLUSIONS 1. Nearly all ground squir rel s at th e Station were captured and mark ed . 2. Most, i f not all, of the br eeding males were adult s , r ather th an yearlings . 3 . The female segment of the breeding population in cluded approxi mat ely equa l proportions of the adu lt and yearling females . 4. Fluctu ation s in observed density and age - sex composition wer e corr elated with actual changes . 5 . Sex ratios varied sig nific antly with age . 6. Litt er sizes ave r aged 5 . 8 and were s i gnific antly gr ea ter for adul ts than fo r year lin gs . 7. Not all of th e bre ed ing-a ge females produced young. 8 . The overal l sur v i val r ate was 0 . 32 . Adults s ur v i ved better th an juveniles, and fema l es su rviv ed better than males. 9. Ju ve nil e males wer e twice as mobil e as the ave r age of all oth er age -s ex gr oups . 10. The population was at equilibrium during th e years of study. 11. The exis tence of density-d ependent natality or mortality was not clearly demonstr able with existing data. 59 LITERATURECITED Andrewartha, H. G. 1957. The use of conceptual models in population ecology. Cold Spring Harbor Symp. Quant. Biol . 22:219-236 . ------, and L. C. Birch . 1954. The distribution and abundance of animals. Univ . Chicago Press , Chicago, Ill ., 782 p. Bakko, E. B., and L. N. Brown. 1967. Breeding biolo gy of Cynomys leucurus in Wyoming. J. Mammal. 48 : 100 - 112 . Balph, D. F. 1968 . Behavioral r esponses of unconfined Uinta ground squirrels to trapping . J . Wildl . Mgmt. 32 : 778-794 . ------, and A. W. Stokes. 1963. On the ethology of a population of Uinta gr ound squirrels . Am. Midl . Natur . 69:106 - 126 . Balph, D. M., and D. F. Balph . 1966 . Soun d communi cation of Uinta ground squirrels . J. Mammal. 47 : 440-450 . Birch, L. C. 1957 . The role of weather in determining the distri bution and abundance of animals . Cold Spring Harbor Symp. Quant. Biol . 22:203-218 . Burnett, W. L. 1931. Life-history studies of the Wyoming ground squirrel (Citellus elegans elegans) in Colorado. Colo . Agr . Coll. Bull. No. 373-. ~~- -~~~ Davis, w. B. 1939 . The recent mammals of Id aho . Caxton Printers, Ltd., Caldwell, I daho , 400 p. Deevey , E. S . , Jr. 1947. Life tables fo r natural populations of animals . Quart. Rev . Bi ol . 22:283-314. Ehrlich, P .R., and L. c. Birch. 1967. The "b al ance of nature" and "p opul at ion control." Amer. Natur. 101:97 . Evans, F . c. 1951. Notes on a population of th e striped gr ound squirrel (Cit ellus tridecemlineatus) in an abandoned field in sou th eastern Michiga n. J . Mammal. 32:437 - 449 . Fitch , H. S . 1948 . Ecol ogy of the California ground squi rr el on grazing l ands . Am. Midl. Natu r. 39 :51 3- 596 . 60 Gordon, K. 1943. The history and behavior of the western chipmunk and the mantled ground sq uirrel. Studies in Zoo 1., Ore . Sta t e Coll ., No. 5 . 104 p. Hair sto n, N. G., F. E. Smith, and L. B. Slobodkin . 1960. Community structure , popu l ation control, and competition . Amer . Nat ur . 94:421 - 425. Ha ll, E. R. 1946 . Mammals of Nevada . Univ. Calif. Press, Berkeley, 710 P• Hayne , D. W. 1949 . Calculation of s i ze of home range . J. Mammal. 30:1-1 8 . Hickey, J. J . 1952 . Surviv al studies of ban ded bird s . U.S. Fish and Wildl . Serv . , Spec . Sci . Rept., Wildl . No. 15 . 177 P• Howard, W. E. 1960. Inn ate and env i ronme nt al dispersal of indivi dual vertebrates . Am. Midl . Natur . 63:152-161 . King, J. A. 1955 . Social behavio r, soc i al organization , and pop u l ation dynamics in a b l ack - tailed pr ai ri edog t own in the Black Hills of South Dakota . Contrib. Lab . Vert . Biol., Univ. Mich. 67. 123 p . Manvill e , R. H. 1959 . The Columbi an gro und squirrel in north wes t ern Montana . J. Manunal. 40 : 26-45 . Mccarley, H. 1966. Population dynamics and adaptive beh av ior of Cit e llu s tridecemlineatus . J . Mammal. 47:294-317 . Miln e, A. 1957 . Theories of natural control of in sect populations . Cold Sprin g Harb or Symp. Quant . Bio l. 22:253 - 271 . Murdoch , W, W. 1966. Community s tr uctu r e , populati on control, and competition--a critiqu e . Amer. Natur. 100 : 219 - 226 . Nichol so n, A. J. 1933, The bal ance of animal population. J. Anim. Ecol. 2 :1 32-178. ------. 1957 . The self - adjustment of populati ons to change . Cold Spring Harbor Symp. Quant , Biol . 22 : 153-173 . Rongst ad, 0 , J , 1965, A life hist ory s tudy of thirt ee n-lined gr oun d squirrel s in so uthern Wisconsin . J, Mammal. 46:76- 8 7, Slobodkin, L.B., F. E, Smit h, and N, G. Hai rst on, 1967 . Regul ation in terr es tri al ecosys t ems, and the implied balanc e of nature. Amer. Natur. 101: 109 -124. 61 I Snedecor, G. w. , and W. G. Cochran. 1967. Statistical methods. I owa State Coll. Press, Ames, 593 p . Stickel, L. F . 1954 . A comparison of certain methods of measuring ranges of small mammals . J. Mammal. 35:1-15 . Stokes, A. W. , and D. F . Balph . 1965. The relation of animal behavior to wildlife management . Trans. N. Am. Wildl . Conf. 30 :40 1- 410 . 62 VITA Spencer R. Amend Candidate for the Degree of Master of Science Thes i s : On the Population Ecol ogy of Uinta Ground Squirrels Major Field: Wildlife Biology Bi ogr aphical Information: Personal Data : Born at Great Bend , Kansas, May 25 , 1942, son of Dr . V. F . and Natt a B. Amend; married Marce ne Moser September 1, 1963 ; one son - -David Allen . Educa t ion: Attended public schools in Gr eat Bend , Kansas ; gr aduated from Great Bend High School in 1960; did unde r graduate work at Phillips University , 1960-62 ; r ece i ved the Bachelor of Science degree from Utah State University , with a major in wildl i fe management, in 1964 ; di d gr adu ate wor k in wi ldl i fe ecology, spec i al izing in population dynamics and ani mal behav i or , at Ut ah State Univers i ty, 1964 - 66, int erru pt ed st udy and compl eted r equ i r ement s fo r th e Maste r of Sc i ence deg r ee i n 1970 . Pr of ess i ona l Expe r ie nce : 1968 t o pr esen t, r esea r ch bi ol og i st , U. S . Fis h and Wi ld li fe Se r v i ce , Mi gr at or y Bird Popul ati ons St ati on , conducti ng a f i el d st udy of mourni ng dove pop u la ti on leve l s in r e l at io n t o habitat ch ar acte ri st ic s; 1966 -6 8 , s t at e game bi ol og i st with multipl e r espo ns i bil iti es fo r manageme nt of res i de nt game pop ul atio ns ; 1963- 66, act i ve i n Ut ah Sta t e Univer s ity Student Chap t er of The Wild l ife Soci et y - organi zed th e fir s t Wes t ern St ude nt s Wildlif e Concla ve .