Demography of a population of Peromyscus leucopus

RICHARD M. HARLAND, PETER J. BLANCHER,' AND JOHN S. MILLAR Department ofZoology, University of Western Ontario, London, Ont., Canada N6A 5B7

Received March 2, 1978

HARLAND, R. M., P. J. BLANCHER, and J. S. MILLAR. 1979. Demography of a population of Perornyscus le~lcopus. Can. J. Zool. 57: 323-328.

Livetrapping and removal trapping were used to monitor a population ofPerornyscus leucopus. Breeding adult and young of the year females averaged 2.0 and 1. I litters per year, respectively. Nest mortality was 12-31%, while58% of thejuvenilesdisappeared within 2 weeks after weaning. The rate of disappearance ofjuvenile males (77%) was higher than that ofjuvenile females (44%); the greater loss of males was attributed to mortality. Loss of subadult and adult mice within 2 weeks after initial capture averaged 45 and 51%, respectively, and did not differ between the sexes. However, more males than females were recorded as entering the trapping grids, indicat- ing that males tend to explore new areas more than females. Among resident mice, adult males had the highest rate ofdisappearance (0.31Iweek) while adult females had the lowest (0.05lweek).

HARLAND, R. M., P. J. BLANCHER, et J. S. MILLAR. 1979. Demography of a population of Perornyscus leucopus. Can. J. Zool. 57: 323-328.

Les mouvements demographiques d'une population de Peromyscus leucopus ont ete suivis grice a des techniques de piegeage d'animaux vivants. Les femelles adultes en etat de se reproduire pruduisen~ en moyenne 2.0 pol-tics par annle et les Femelles de I'iinnle. I . I . La mortalirt des pztitt; au nirl e.;t d e 12-13% et 58% dcs jeunes disparaissenr moins de I semaines apres lc sewage. Le taux de dispztririun desjeunrr; rnhles (77%) dcpasse celui desjeunes fcmclles (44%): la disp~~rition des msles ~'expliquc dims Is pluparr des cas par la monalitt;. La Wrte des wuris sub-adultes liu-del21 de I sern:iine\ i~pri:\ la prcrnitrc capture est de 45% en moyenne et celle des adulres tle 51% et eltes ne rliffemnl pns d'un liexe ;I 1-autre. Cepend:int, plus Je milesque de femeller p5nltreni dans l'aire ~ l e pidge;~pe. ce qui inrlique que les rnilcs onr une tendance plus forte h e.r;plomr de nouvelTes rtginns. Pami le* ksidtnrs de la popularion. fes miles adulte~ ont le taux de disparition le plus eleve (0.3l/semaine), les femelles adultes, le moins eleve (0.05lsemaine).

[Traduit par le journal]

Introduction cate that reproductive output varies inversely with Mice of the genus peromyscus appear to be population density in P . leucopus. Similar evidence

unique among extensively studied small mammals for other species ofPeromyscus is cited by Terman in that their populations are of relatively low den- (1968). The other hypothesis is that males are ag- sity and annual fluctuations in numbers are gressive towards juveniles and suppress their sur- tivel y small (Terman 1968). These trends indicate vival (Sadleir 1965; Healey 1967; Petticrew and that population growth is controlled, and nume,-- Sadleir 1974). Evidence for this is seen in the ag- ous authors have suggested control mechanisms. In gressive nature of males and the tendency for general, although weather (e.g., Fuller 1969) and ~ ~ ~ u l a t i o n s to hcrease after the breeding season food resources (e.g., Bendell 1959; Fordham 1971) when males are less aggressive (Sadleir 1965). may influence numbers, behaviour of animals Here we examine the production and disappear- within populations appears to be the most popular ance of animals in a population of Perom~scus explanation for controlled population growth. TWO leucopus noveboracensis (Fischer) and consider hypotheses predominate. One is that resident factors that might be important in controlling females control the population by excluding other population growth. females from their home ranges. Evidence for this in P. leucopus is seen in the low overlap of home Methods ranges among breeding females and the apparent The study was conducted in a 40-ha maple (Acer saccharurn) exclusion of immigrating females by resident woodlot 24 km north of London, Ontario.

A livetrapping grid (trapping area 2.20ha, 55 pairs of 1941' 1971)' Burt Longworth traps in five rows, ZOm spacing. baited with peanut and Manville present data which indi- butter, rolled oats, almond extract, millet seed, carrot, and

cotton bedding) was used to monitor the population. Traps were 'Present address: Department of Biology, Queen's Univer- protected with wood shelters (38 x 19 x I1 cm) and operated 2

sity, Kingston, Ont., Canada K7L 3N6. nights per week May through August and 2 nights every 2nd

0008-4301 1791020323-06$01 .OO/O 01979 National Research Council of CanadalConseil national de recherches du Canada

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324 CAN. J. ZOOL. VOL. 57, 1979

week Septemkr through April. Livetrapped micc were rue clipped and ear punched for idenrificntion. weighed tnearcst gram), sexed. and aged by pelage characreristics. and date and location of capture were recorded. Pelage was clasqified as follows: gray dorsal pelage = juvenile; rncll~ing = subadult: brown dorsal pelage = adult. The% were subject~vecotcgorleP, hui a comparison between pelage chaaracterrbtlc~ and age e ~ t i - mates based on eye lens weigh[< (after Millw and Iverwn 1Y7h) indicated the juveniles were 2 6 4 0 days or age. most ruhadi~lts were 30-50days o f a g , and r~dultx were more than 50dayh rrld. These were simrlar to the age estimates made by Snyder ( 1956). Livetrapped females were recorded as pregnant (swollen abdo- men), lactating (worn h a ~ r around prominent nipples), or non- breed~ng. Animals were considered to be transients for the first 2 weeks after entering the grid and residents if they remained on the livetrapping area For 2 or more weeks.

A snap-trapping prid {rrapping area ].&ha, 36 pairs of museum special Imps in four rows, 20m a p r t . baited with peanut butter and oatmeal) was located I5Om rrnm the livetrap ping area. and separated from it by a pond and sw~lrnpy drainagea which may have served ns a p;lniaI harrier to di~persal. The snap-trapping grid was uprated on the saint schedule as the livetrapping grid. Snap-tmpped mice were weighed. uged by pel:tg chriracteri~tics, and xuropsied. Reproductive status was recorded: n u m k r anti length of embryo? and presence ur ab- sence of lactat~on for females, testes length for males. Testes > 7 mm were cons~dered to be functional, after Jameson (1950). All statist~cal analyses follow Sokal and Rohlf (1969).

Results The breeding season lasted for approximately 5

months each year. In 1975, males were capable of breeding (testes > 7mm) when trapping com- menced in late May and the last functional male was captured September 11. Mid-September was considered to be the end of the breeding season. In 1976, only one of seven males had testes > 7 mm March 7, while all overwintered males caught during April and later were functional. The last functional male was captured August 25, so that late March to early September was considered to represent the duration of the breeding season in 1976.

The population appeared to be in a centinuous state of flux (Fig. 1). Movement ofnew animals into the livetrapping and snap-trapping areas occurred throughout the year (Fig. 1). Biweekly entries into the snap-trapping area varied, with the greatest numbers entering during August and early Sep- tember 1975, early March 1976and June, lnte July, and late August 1976. Entries into the livetrapping area were greatest during and soon after the breeding seasons during both years. The number of entries inte the livetrapping and snap-trapping areas during biweekly period5 were not significant1 y cor- related. This was evident among aII animals (r = 0.07, N = 39; P > 0.05), and for adults and subadults combined (r = 0.01, N = 39; P > 0.05).

The numbers of resident animals were similar during the 2 years (Fig. 2). Spring breeding populations were low (two females and one male = 1.4lha

FIG. 1. Movements of animals as indicated by the arrival of new animals into the snap-trapping area (A) and livetrapping area (B)during biweekly intervals. All agesand sexes combined.

in 1975 and three females and three males = 2.7lha in 1976) in both years. Numbers remained fairly low during the breeding season and did not exceed 10 females and 12 males = lOlha in 1975 and 9 females and 10 males = 8.61ha in 1976. The breeding season was followed by a sharp increase in number of residents in 1975; this trend was not evident in 1976.

The sex ratio of animals moving into the livetrapping and snap-trapping areas varied with age and season (Table I ). Data from both breeding seasons indicated that juveniles and adult maIes and females enter the population in equal numbers, but subadult males outnumbered subadult females. During the nonbreeding season there were no

I - J M M J S N J M M J S N

1975 1976

FIG. 2. Seasonal changes in the number of resident males (A) and females (B) on the livetrapping area. Residents were considered to he animals (all ages) that were known to be present for 2 or more weeks. Numbers plotted as the total number present during biweekly intervals.

. J. Z

HARLAND ET AL.

TABLE 1. Sex ratios of new animals entering the livetrapping and snap-trapping areas. Yates correction applied to x Z test against 50:50 sex ratios (years combined)

Livetrapping grid Snap-trapping grid

No. No. No. No. Sample period 33 9? x dd 9 9 x2

Breeding seasons Juvenile 20 21 0.00; ns* 7 12 0.42; ns Subadult- 56 11 14.45; P < 0.05 36 7 9.11; P < 0.005 Adult 32 13 3.60; ns 26 9 3.66;ns

Nonbreeding seasons Juvenile 7 15 1.11; ns 0 0 ns Subadult 38 22 1.88; ns 4 6 0.05; ns Adult 22 21 0.0: ns 26 14 1.57:ns

'ns, not significant,

TABLE 2. Attainment of residency (2 weeks) by animals entering the livetrapping area, all seasons combined

- - - - - --

Sample Achieved residency No residency % residency x

Juveniles Male Female

Subadults Male 37 54 41 Female 19 13 59 2.63; ns*

Adults Male 26 19 58 Female 9 15 3 8 1.82; ns

"ns, not significant.

ferent between the sexes and averaged 51%. - FIG. 3. The rate of disappearance of resident mice from the Rate of disappearance after attaining residency livetrapping area, as shown by the proportion of animals that

status varied between sex and age groups (Fig. 3). attained residency (2 weeks) that were still present in sub- Adult females had a disappearance rate of 0.051 sequent weeks. Open circles represent adults, closed circles week (based on the of the least squares re- represent juvenile and subadult animals. Solid lines represent

gression of log number on weeks), significantly females and broken lines represent males. Initial samples were 43 and 38juvenile and subadult males and females, respectively,

less (F = 236.29; P < 0.001) than the rate of and 26 and 9 adult males and females, respectively. See text for 0.3llweek for adult males. Juvenile and subadult explanation.

significant differences in sex ratios within the three age groups.

The fate of animals entering the livetrapping area differed between the sexes (Table 2). Of 63 ,a juveniles entering the area, 3 died in traps before .7 the residency period of 2 weeks was attained. Among the remaining 60 animals, 23% of the males z.6

0 and 56% of the females attained residency. These ;.5 proportions are significantly different (Table 2). B,q Four of 127 subadults died in traps; among the

remaining 123 animals the proportion attaining re- .3

sidency was not significantly different between the , .2 sexes and averaged 45%. Nineteen of 88 adults

1 entering the area died in traps; most of this mortality coincided with freezing nights during autumn.

:'\ \ ::

;, \;L :,, ;,

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' $ , '.\ \

:I;. .-. ':. \. -. -

%;a. -'-\.-.---.-.-. -. .~.g:.a.*.*.*-* -.--.- *-*-*-.-*'--*

2 5 10 15 20 25 Among the remaining 69 animals the proportion which attained residencv was not siernificantlv dif- RESIDENCY (weeks )

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CAN. J. ZOOL. VOL. 57, 1979

TABLE 3. Observed and expected number of double captures at single trap sites, between sexes. Proportion expected based on the abundance of resident males and females

Sample No. observed Proportion expected No. expected x - - - - -- - - -

Breeding season Female-female 1 0.283 6.51 3.85; P < 0.05 Female-male 19 0.498 11.45 4.34; P < 0.05 Male-male 3 0.219 5.04 0.47; ns*

Nonbreeding season Female-female 4 0.372 7 . 8 1 1.40; ns Female-male 14 0.476 10.00 1.22; ns Male-male 2 0.152 3.19 0.15: ns 'ns, not significant.

females and males had disappearance rates of 0.15 and 0.17lweek, respectively. These rates were not significantly different (F = 0.37; P > 0.05). Adult males had a significantly higher (F = 4.95; P < 0.05) disappearance rate than juvenile and subadult males, while adult females had a significantly lower (F = 131.9; P < 0.001) disappearance rate than juvenile and subadult females.

Behavioural interactions between animals were not monitored and recaptures of individuals were, in most cases, too few to calculate home ranges. However, the use of two traps per station on the livetrapping grid permitted an evaluation of the association of animals by the number of times that both traps at a single site were occupied by Peromyscus. Double captures were recorded 23 times during the breeding season and 21 times during the nonbreeding season. If double captures were random, they should have been proportional to the abundance of males and females residing on the area. The average number of female and male residents per biweekly period during the breeding season were 4.42 and 3.89; their relative abundances were 0.532 and 0.468 respectively. The average numbers of female and male residents per biweekly period during the nonbreeding season were 4.85 and 3.10, so that the relative abundances were 0.610 and 0.390 respectively. The expected proportion of female-female, female-male, and male-male captures (where f = proportion females present and m = proportion of males present) would bef2,2f. m, and m2, respectively.

A comparison of observed and expected double captures (Table 3) indicates that female-female en- counters occurred less frequently than expected if the association was random, and female-male captures occurred more frequently than expected. This implies that females avoid females during the breeding season, while males and females were attracted to each other. Male-male captures were not different from random during the breeding season, and none of the associations were different from random during the nonbreeding season.

Breeding did not appear to be restricted to resident females. In 1975. lO females were recorded as pregnant on the livetrapping area. One disappeared before residency was attained. In 1976, 16 females were recorded as pregnant on the livetl-apping grid. Two females disappeared before residency was obtained, and nine l'ernale~ attained residency hut diqappeared soon after being recorded as pregnant. This apparent tendency for some pregnant females to move was substantiated by the records from the snap-trapping area. Excluding females which may have been resident on the snap-trapping grid during June 1975. a total of 97 females moved inro the area during the two breeding seasons: 1 I were pregnant.

Anlong breeding fern~lles that remained on the livetrapping area long enough to wean their offspring {approximately 1 month postpartum). most produced only one litter during the breeding sen- son. Of 14 females breeding on t he area over the 2 years, 10 produced only one litter per season, 3 produced two litters. and 1 produced four litters. Five overwintered females averaged 2.0 litters per season (mnge 1-41 and nine young of the year averaged I .I litters per season (range 1-21.

The nesting successof females giving birthon the livctrapping area was estimated by comparing the potential numher of weaned young on the livetrapping area with the actual number of juveniles caught. Time of birth was estimated to be hetween the time females were recorded as pregnant and when they were recorded as lactating: number of young born was considered to be five if the female was experienced or overwintered and four if the female was a young animal breeding for the first time (Burt 1940). and the expected time of emergence into the trappable population was I month postpartum. A comparison of the expected number uf young (53 in 1975 and 38 in 1976) and the actual num her ofjuvvniles caught (40 in 1 9 7 and 23 in 1976) indicates that a large proportion of those born to surviving females stsrvived to weaning. A total of 63 of 91 expected juveniles (6%) were caught over the two breeding seasons. This esti-

. J. Z

HARLAND ET AL. 327

mate may be an underestimate since a juvenile that was not caught immediately after weaning could be classified as a subadult when first caught. In such cases the animals would not be considered to have been born on the area. The extent to which juveniles weaned on the livetrapping area may have been missed was estimated from the trapping efficiency of juveniles. Among the 25 juveniles that attained residency status on the livetrapping area, there were a total of 88 trapping opportunities between their first and last capture. They were actually captured on 69 of these occasions, so that trapping efficiency was 78.4%. Assuming all animals not caught at weaning to be subadults at the next trapping period, then 6310.784 = 80 animals were successfully weaned over the two breeding seasons and nesting success was 88%. Some juveniles un- doubtedly dispersed before being trapped, but this number was small and emigration was likely ba- lanced by immigration.

Discussion Seasonal population growth was far below the

potential for the population. Assuming females to be capable of producing litters at a rate of one per month, then each overwintered female could produce five litters of five offspring each in a 5-month breeding season. Assuming young of the year to mature at about 2 months of age, first cohort females (N = 2.5 per overwintered female) could produce three litters of four offspring each and second cohort females (N = 2.5 per overwintered female) could produce two litters of four offspring each. Thus an overwintered female could have 25 + 30 + 20 = 75 descendants born into the population each breeding season. On this basis, two overwintered females in 1975 and three overwintered females in 1976 could have 150 and 225 descendants. respectively. by the end of the breeding season. The observed resident population levels werle 92 in Ocrabel- 1975 and 10 in September 1976. only 15 and 4%. respectively. of the potential production.

Much of the difference between observed and potential production can be attributed to the number of litters actually produced on the area. Observed production averaged 2 litters by overwintered females and 1.1 litters by young of the year, and estimated number of young born was 53 in 1975 and 38 in 1976. On this basis, observed resident population levels at the end of the breeding season averaged 35% of the production.

Further losses occurred prior to weaning. An estimated 69-88% of animals born on the livetrapping grid entered the trappable population. This estimate of nesting success is similar to the esti-

mates for P. maniculatus of 71 and 56% by Fair- bairn (1977) and Howard (1949), and intermediate to Sullivan's (1977) estimates of 5, 72, and 94%. Comparisons with other studies may not be valid because trapping intervals and assessment techniques were not identical, but the nesting success recorded in this study does not appear to be unusu- ally low. Losses prior to weaning appeared to be equal between the sexes since the sex ratio of juveniles was not significantly different from 5050 (Table 1).

Losses within the first 2 weeks after initial capture accounted for 58.3% of all juveniles, with males having a significant1 y higher disappearance rate than females. This difference between the sexes appeared to involve mortality of juvenile males rather than dispersal. Assuming that the snap-trapping grid monitored dispersal, then dispersal was not different between the sexes and the missing juvenile males must have died.

Loss of transient subadult mice from the livetrapping grid was not different between the sexes and averaged 55% (Table 2). However, the sex ratio of subadults entering both grids during the breeding season differed significantly from 5050, with males outnumbering females 5.1 to 1. These data indicate that transient males and females had an equal tendency to maintain contact with previ- ously visited areas (and were recorded as attaining residency status), while males had a stronger tendency to investigate new areas (and were recorded as new arrivals). As new arrivals, transient subadults did not favor one grid over the other during the breeding season. Subadult males entered the snap-trapping and livetrapping areas at rates of 25.0 and 25.5/ha, respectively, and subadult females entered at rates of 4.9 and 5.01ha, respectively. This pattern changed during the nonbreeding season. At that time subadult males and females entered the snap-trapping and livetrapping grids at rates of 2.7 and 17.3lha and 4.2 and IOIha, respectively. The unoccupied habitat was clearly avoided during the nonbreeding season.

Loss of transient adult mice from the livetrapping grid was similar to that of subadults and did not differ between the sexes. However, combined samples from both grids during the breeding season indicated that males entering new areas outnumbered females 2.6 to 1. Thus adult males, like subadult males, appeared to explore new areas more than females. Adult males and females also entered the snap-trapping and livetrapping areas with equal frequency during the breeding season (18.0 and 14.5lha and 6.2 and 5.9/ha, respectively).

Loss of resident animals from the livetrapping area was different than the pattern seen among

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328 CAN. J . ZOOL. VOL. 57, 1979

transient animals in that adult males had a relatively high rate of disappearance (0.3llweek) while adult females had a relatively low rate of disappearance. These data support Snyder's (1956) data which indicates that males have consistently higher rates of disappearance than females, although the rates were generally higher (0.05, 0.15, 0.17, and 0.3llweek, compared with 0.03-0.07lweek) in the present study.

Taken together, these data indicate that a number of factors may contribute to reduced population growth in P. leucopus. Metzgar's (1971) suggestion that established females may be instru- mental in restricting recruitment is indirectly supported by the: nonoverlap of female home ranges. dispersal by pregnant females. and the relatively low disappearance rate of resident females. The role of male aggression in limiting recruitment is uncertain. If resident males were iinstrumentirl in limiting recruitment, their rate of disappearance should not be higher than that of younger animals. In addition, if the mortality of juvenile males is attributed to aggression by resident males, one would also expect to observe a similar loss of transient subadults, and this did not occur.

Acknowledgements This study was supported by the National Re-

search Council of Canada and the University of Western Ontario. F. B. Wille, D. May, B. Vickery, and B. Seghers read preliminary drafts of the manuscript.

BENDELL, J . F. 1959. Food as a control of a population of white-footed mice, Peromyscus leucopus noveboracensis (Fischer). Can. J . Zool. 37: 173-209.

BURT, W. H. 1940. Territorial behaviour and populations of some small mammals in southern Michigan. Misc. Publ. Mus. Zool. Univ. Mich. 45: 1-58.

FAIRBAIRN, D. J . 1977. Why breed early? A study of reproductive tactics in Peromjscus. Can. J. Zool. 55: 862-871.

FORDHAM, R. H. 1971. Field populations of Peromyscus with supplemental food. Ecology, 52: 13&146.

FULLER, W. A. 1969. Changes in numbers of three species of small rodent near Great Slave Lake, N.W.T., Canada, 1964-1967, and their significance for general population theory. Ann. Zool. Fenn. 6: I 1.7- 144.

HEALY. M. C. 1967. Aggression and self-regulation of popula- t ion size in deer mice. Ecology. 48: 377-392.

HOWARD. W. E. 1949. Dispcl.sal, amount of inbreeding. and longevity in a Iocal populalion of prairie deer mice on the George Reserve. southern Michigan. Contrih. Lab. Vertebr. Biol. Univ. Mich. 43: 1-50.

JAMESON, E. W., JR. 1950. Determining fecundity in male small mammals. J . Mammal. 31: 433-436.

MANVILLE, R. H. 1949. A study of small mammal population in northern Michigan. Misc. Publ. Mus. Zool. Univ. Mich. 73: 1-83,

METZGAR, L. H. 1971. Behavioral population regulation in the wood mice, Peromyscus leucopus. Am. Midl. Nat. 86: 434-448.

MILLAR, J . S., and S. L. IVERSON. 1976. Weight of the eye lens as an indication of age in Peromyscus. Can. Field-Nat. 90: 37-41.

NICHOLSON, A. J . 1941. The homes and social habits of the wood mouse (Peromyscus leucopus noveboracensis) in southern Michigan. Am. Midl. Nat. 25: 196-223.

PETTICREW, B. G., and R. M. F. S. SADLEIR. 1974. The ecology of the deer mouse Peromyscus maniculatus in a coastal conif- erous forest. I. Population dynamics. Can. J. Zool. 52: 107-118.

SADLEJR, R. M. F. S . 1965. The relationship between agonistic behaviour and population changes in the deer mouse Peromyscus maniculatus (Wagner). J . Anim. Ecol. 34: 331-352.

SNYDER, D. P. 1956. Survival rates, longevity, and population fluctuations in the white-footed mouse, Peromyscus leucopus, in southeastern Michigan. Misc. Publ. Mus. Zool. Univ. Mich. 95: 1-33.

SOKAL, R. R., and F. J. ROHLF. 1969. Biometry. W. H. Freeman and Co., San Francisco.

SULLIVAN, T. P. 1977. Demography and dispersal in island and mainland populations of the deer mouse, Peromyscus man- icularus. Ecology, 58: 964-978.

TERMAN, L. R. 1968. Population dynamics. In Biology of Peromyscus (Rodentia). Edited by J. A. King. Am. Soc. Mammal. Publ. No. 2. pp. 412-445.

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Demography of a population of Peromyscus leucopus

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