Reproductive performance of a laboratory colony of the deermouse, Peromyscus maniculatus gambelii.

During a 23-mo period approximately 600 litters and 2,500 offspring were produced in a laboratory colony established with wild-trapped deermice (Peromyscus maniculatus gambelii). The incidence of pregnancy and size of litters increased with domestication. Average litter size was 4.3 for all litters and 4.1 and 4.7 for litters born to wild-trapped and first generation females, respectively. Litter size increased up to the seventh sequential pregnancy for both wild-caught and first-generation females and then tapered off. In all litters more males were born than females. The departure from a 1:1 ratio was significant for the combined data and for litters of first-generation females, but not for litters of wild-caught females.     

Increasing returns in the life history of Columbian ground squirrels

1. We examined positive associations and trade-offs of maternal and reproductive traits in a population of Columbian ground squirrels, Spermophilus columbianus .
2. Structural size, body condition, mother's personal allocation to body mass during reproduction, and timing of littering were estimated for live-trapped reproductive females that were observed during an 8-year period, and were compared to litter mass, litter size, and average pup mass using path analyses.
3. Mothers exhibited age-structured traits that influenced reproductive patterns. Yearling mothers were significantly smaller, bred later, and had smaller litters than older females. Mothers that gained more body mass during reproduction and older mothers in good body condition that were structurally large had larger litters.
4. Early seasonal timing of littering was an important positive influence on successful reproduction by older mothers only in early breeding seasons and in years when conditions for reproduction were good for all females.
5. The number of offspring that survived to 1 year of age was most strongly associated with litter mass and litter size; date of breeding was of secondary influence, with earlier litters exhibiting greater success.
6. In general, mothers that gained the most in body mass during reproduction were concurrently more successful in weaning large litters (perhaps due to better quality of foraging habitat).
7. In addition to expected reproductive trade-offs, reproduction by Columbian ground squirrels exhibited positive associations of life-history traits that may reflect evolutionary increasing returns.     

Experimental manipulation of breeding density and litter size: effects on reproductive success in the bank vole

1. Reproductive success of individual females may be determined by density-dependent effects, especially in species where territory provides the resources for a reproducing female and territory size is inversely density-dependent.
2. We manipulated simultaneously the reproductive effort (litter size manipulation: ± 0 and + 2 pups) and breeding density (low and high) of nursing female bank voles Clethrionomys glareolus in outdoor enclosures. We studied whether the reproductive success (number and quality of offspring) of individual females is density-dependent, and whether females can compensate for increased reproductive effort when not limited by saturated breeding density.
3. The females nursing their young in the low density weaned significantly more offspring than females in the high density, independent of litter manipulation.
4. Litter enlargements did not increase the number of weanlings per female, but offspring from enlarged litters had lower weight than control litters.
5. In the reduced density females increased the size of their home range, but litter manipulation had no significant effect on spacing behaviour of females. Increased home range size did not result in heavier weanlings.
6. Mother's failure to successfully wean any offspring was more common in the high density treatment, whereas litter manipulation or mother's weight did not affect weaning success.
7. We conclude that reproductive success of bank vole females is negatively density-dependent in terms of number, but not in the quality of weanlings.
8. The nursing effort of females (i.e. the ability to provide enough food for pups) seems not to be limited by density-dependent factors.     

Effect of indomethacin on reproduction under laboratory and field conditions in deermice (Peromyscus maniculatus)

The purpose of this investigation was to determine the long-term effect of enterically administered indomethacin (a prostaglandin synthetase inhibitor) on reproduction in Peromyscus maniculatus (deermice) under both laboratory and field conditions. Indomethacin-coated wheat (1 mg/kg wheat) was provided to the experimental animals in the laboratory and under field conditions ad libitum. Rat chow was also provided to the animals in the laboratory. The laboratory data indicate that the number and size of the litters from the control breeding pairs were larger (number of litters=22; average number of pups/litter=4.9) than the experimental breeding pairs (number of litters=2; average number of pups/litter=2.5) over a 100-day experimental period. Field data indicate that the number of pregnant animals, the total number of animals, and mean juvenile males and females live-trapped decreased in the experimental group with respect to the control group during the experiment. The data suggest that deermice will consume indomethacin under both laboratory and field conditions, and that it effectively inhibits reproduction on a long-term basis. It is, therefore, possible that prostaglandin synthetase inhibitors may be used to control populations of rodent pests. Additionally, the data raise questions about the effect of long-term use of nonsteroidal analgesic compounds similar to indomethacin on human reproduction.     

Establishment of a deer mouse (Peromyscus maniculatus rufinus) breeding colony from wild-caught founders: comparison of reproductive performance of wild-caught and laboratory-reared pairs

The deer mouse (Peromyscus maniculatus) is a natural reservoir for several human pathogens, but little is known about the mechanisms by which such pathogens are maintained in nature. As a first step toward developing a colony of deer mice that were permissive for infection with Sin Nombre (SN) hantavirus, we collected 68 wild P. maniculatus rufinus from central New Mexico. Mice from this cohort were used to establish 26 breeding pairs, of which 85% were fertile. In subsequent generations, fertility decreased slightly to 73% (N = 59) in laboratory-reared F1 and F2 pairs. Wild-caught females delivered 7.2 litters on average (range, 1 to 18), whereas laboratory-reared pairs delivered 5.5 (range, 1 to 13). The average time between pairing and first litter was 106 days for wild-caught animals, whereas that for laboratory-reared pairs was 71 days. None of the pairs displayed a seasonal breeding preference. Cannibalistic behavior increased from 5% in founders to 26% in laboratory-reared pairs. Mean litter size for wild-caught females was 4.3, whereas that for laboratory-reared dams was 4. Founding animals have been maintained in captivity for longer than 2 years, with only 2 deaths (4.8%). Our colony is competent for infection with SN virus. Thus, it should be useful for testing of models for maintenance of SN virus in wild rodents, and other aspects of the virus-host relationship.     

Breeding of captive bank voles (Clethrionomys glareolus) related to dynamics of source populations

Proportionately fewer female bank voles from cyclic or dense populations bred than did females from sparse or non-cyclic populations when brought into a laboratory and kept as monogamous pairs for at least 1 year. Similarly, fewer litters were produced by females from cyclic or dense populations. Breeding frequency and litter production did not differ in the second laboratory generation. Large variance in litter numbers indicated further changes in reproduction in response to the laboratory environment. Certain differences between origins still persisted after long-term laboratory breeding. The regional variations between field-born animals are most parsimoniously explained by differences in sensitivity to density.     

Effect of 6-methoxybenzoxazolinone on sex ratio and breeding performance in Microtus montanus

The plant derivative, 6-mathoxybenzoxazolinone (6-MBOA), which has been demonstrated to initiate reproduction in field populations of the montane meadow vole (Microtus montanus), was administered via feeding or Silastic capsule implants to mated pairs of laboratory bred M. montanus. The animals remained paired for 120 days, and the number, size, and sex ratios of the resulting litters were recorded. Both the size and frequency of litters were significantly greater in 6-MBOA-treated pairs than in controls. By using implants, it was possible to treat one or both sexes in a mated pair. The positive effects of this compound on litter size and number of litters occurred when the females received implants, which indicates that the male has no influence on these parameters. The most unusual result of these experiments was that 6-MBOA has a significant effect on the sex ratio of the litters. Animals receiving 6-MBOA produced significantly more females than did control pairs. This result occurred regardless of the method of administration, and in the case of the implant studies, regardless of which sex received the active implants. These findings are discussed in relation to the ecology and life history strategy of Microtus montanus.     

Reproductive success of solitarily and communally nesting white-footed mice and deer mice

To determine the fitness consequences of communal nesting inwhite-footed mice, Peromyscus leucopus, and deer mice, P. maniculalus,I compared the reproductive success of field populations offemales nesting solitarily, in communal groups of more thanone female, in extended families of successive litters, andin communal groups with extended families. Mean first littersize of weanlings and juveniles 6 weeks old did not differ significantlyfor pups raised under the four nesting situations. Similarly,for pups born into extended families, litter sizes of pups fromsecond litters did not differ significantly from those of firstlitters or from pups born to solitarily nesting females. Delayeddispersal of juvenile females did not result in resource competitionor inhibition of reproduction. Thus, reproductive success offemales was not significantly affected by additional membersin the nest. At least 26 of 28 communally nesting females wereclose relatives. Solitary nesting is the common breeding patternin Peromyscus, and extended families and communal nesting arealternative reproductive tactics in response to limited space,delayed dispersal, and local grouping among related females.     

Numbers of litters, litter size and survival in two species of microtines at two elevations

The number of litters, litter size and survival of Clethrionomys gapperi and Microtus pennsylvanicus were documented at two elevations (1450 m; 2240 m) in the Canadian Rocky Mountains to determine if those life history traits changed with the length of the breeding season. The high elevation was colder, had fewer frost-free days, and more precipitation than the low elevation. The length of the breeding season of C. gapperi and M. pennsylvanicus averaged 8 d and 34 d longer, respectively, at the low elevation than at the high elevation. In both species and at both elevations, most females survived only long enough to produce one or two litters. Litter size was significantly greater at the high elevation than at the low elevation in M. pennsylvanicus , but not different in C. gapperi. Overall, nestling survival was greater at the low elevation than at the high elevation in M. pennsylvanicus , but not different in C. gapperi. Summer and winter survival rates were not different between elevations in either species. Differences in litter size and nestling survival between elevations in M. pennsylvanicus may be related to differences in habitat quality.     

Effects of dietary dieldrin on reproduction in the Swiss-Vancouver (SWV) mouse.

Reproduction was studied in SWV female mice that had raised one litter and then received either 0.2.5,5,10,15,20 or 25 parts/10(6) dietary dieldrin (n = 18 or 19 per group). Exposure began 4 weeks prior to the second mating and continued until day 28 postpartum (weaning). Fertile males were caged with the females during weeks 5-6 of exposure. Significantly mortality of the females occurred only at 20 and 25 parts/10(6) (89% and 56%, respectively) and all deaths occurred before parturition. Dieldrin did not affect behavioural oestrus. At 10 and 15 parts/10(6), 18% of the bred females did not become pregnant; all animals at lower doses and all survivors at higher doses were fertile. The gestation period was not affected. At 25 parts/10(6), the litter size was decreased by 17% over the control size (13.2 pups). The infertility and reduced fecundity resulted from a lesion(s) preceding implantation. Thus, in a separate experiment, 15 parts/10(6) increased the number of bred females that had no implantation sites 5 days post coitum while 25 parts/10(6) decreased the number of sites per pregnant female. As expected from the original dose-responses for infertility and decreased litter size, the converse effects did not occur. Pre-weaning mortality of all the pups occurred in 31%, 47%, 80% and 100% of the litters at 0,2.5,5, and larger than or equal to parts/10(6), respectively. Within the litters raised at 2.5 and 5 parts/10(6), the pup survival was not different from the controls (75%). Thus, in this strain, litter -oss is dieldrin's most important reproductive effect and it correlated with a dieldrin-induced maternal hepatomegaly. The birth weight of pups in litters that were lost was reduced by 3-13% and pre-death growth was reduced or absent. Pup-killing and pup-neglect were important proximate causes of mortality, but only at doses larger than or equal to 15 parts/10(6).     

Benefits of extra food to reproduction depend on maternal condition

The amount of food resources available to upper‐level consumers can show marked variations in time and space, potentially resulting in food limitation. The availability of food resources during reproduction is a key factor modulating variation in reproductive success and life‐history tradeoffs, including patterns of resource allocation to reproduction versus self‐maintenance, ultimately impacting on population dynamics. Food provisioning experiments constitute a popular approach to assess the importance of food limitation for vertebrate reproduction. In this study of a mesopredatory avian species, the lesser kestrel Falco naumanni, we provided extra food to breeding individuals from egg laying to early nestling rearing. Extra food did not significantly affect adult body condition or oxidative status. However, it increased the allocation of resources to flight feathers moult and induced females to lay heavier eggs. Concomitantly, it alleviated the costs of laying heavier eggs for females in poor body condition, and reduced their chances of nest desertion (implying complete reproductive failure). Extra food provisioning improved early nestling growth (body mass and feather development). Moreover, extra food significantly reduced the negative effects of ectoparasites on nestling body mass, while fostering forearm (a flight apparatus trait) growth among highly parasitized nestlings. Our results indicate that lesser kestrels invested the extra food mainly to improve current reproduction, suggesting that population growth in this species can be limited by food availability during the breeding season. In addition, extra food provisioning reduced the costs of the moult–breeding overlap and affected early growth tradeoffs by mitigating detrimental ectoparasite effects on growth and enhancing development of the flight apparatus with high levels of parasitism. Importantly, our findings suggest that maternal condition is a major trait modulating the benefits of extra food to reproduction, whereby such benefits mostly accrue to low‐quality females with poor body condition.     

The population ecology of Rattus exulans on Tiritiri Matangi Island,and a model of comparative population dynamics in New Zealand

Abstract

On Tiritiri, a small predator-free island in northern New Zealand, kiore (Rattus exulans) were live and snap trapped in grassland and forest. In both habitats, kiore abundance peaked in late summer/autumn. The increase followed a 3 month breeding season during which females produced two to three litters, each averaging 7 young. During the population decline in autumn and winter, animals lost weight. Few bred in the breeding season of their birth and none lived to breed in a second breeding season, so the population consisted of distinct age cohorts. These patterns may relate to a highly seasonal food supply.

Kiore elsewhere in New Zealand show seasonal breeding, but the length of breeding, sexual maturation, and litter size vary. Other studies of kiore in the Pacific show less marked seasonal fluctuations, longer breeding seasons, and smaller litters. We propose a model to explain the variation in rodent demography in New Zealand. The model is based on the seasonal availability of food, along with the modifying influences of predation and dispersal.     

Reproductive responses to variation in temperature and food supply by house mice. I. Mating and pregnancy

The effects of food restriction upon mating and pregnancy of female house mice were studied at a warm (21 degrees C) and a cold (5 degrees C) temperature to examine the hypothesis that the effects of temperature and food availability are not independent. Analyses of the data showed significant interaction between temperature and food availability for virtually all variables measured, supporting the initial hypothesis. Contingency analysis of mating, fertility, and litter survivorship showed that the interaction influenced not only the percentage of females successfully producing litters, but also the timing of abandonment of reproductive effort by those females that did not have surviving litters. The percentage of females who mated was reduced only in food-restricted females under cold conditions. Both low temperature and food restriction reduced the percentage of mated females that became pregnant. Food-restricted females under cold conditions who did become pregnant tended to kill their litters at birth. The net effect was an interaction between temperature and food restriction that resulted in females reducing or abandoning reproductive effort at progressively earlier stages of the reproductive cycle. The interaction between temperature and food restriction was further displayed in the growth curves of pregnant females, with the reduction of growth by food restriction being greater in the cold.     

Limitation of reproductive success by food availability and litter size in the bank vole,Clethrionomys glareolus

Food limitation has been suggested as one of the most important factors affecting life history evolution in terrestrial vertebrates. However, this inference is based mainly on evidence from birds, and reproductive trade-offs may differ among groups with different forms of parental care. To study whether the costs of enlarged litters (decreased mass of offspring) would appear when food is not limiting, we performed outdoor enclosure experiments in which we manipulated simultaneously the litter size (control versus litter plus two pups) and food availability (control versus food-supplemented) of female bank voles, Clethrionomys glareolus. The weaning success of females increased significantly in response to supplementary food. When females were provided with extra food, no differences were observed in body masses of weanlings of control and enlarged litters. Further, food-supplemented females grew to larger sizes during nursing than unsupplemented females. Our experiment suggests that energetic requirements during nursing constrain the number of offspring that can be raised successfully in a particular breeding attempt. The results also indicate that unlimited food resources may increase future reproductive potential of females because they can use more energy for somatic growth.     

Age at puberty and first litter size in early and late paired rats

The rate of sexual maturation among female Sprague-Dawley rats was measured in a variety of intraspecific social environments. It was found that females of this strain differ from at least one other strain of laboratory rat in that neither age at vaginal perforation nor age at first estrus was affected in Sprague-Dawley females by the presence or absence of male, regardless of his age or breeding history. Sizes of first litter among females who mated at their first estrus were compared with those among females who were first inseminated at older ages. On average, females bred at first estrus produced litters that contained more than 3 fewer pups than females mated at older ages. This observation suggests that female Sprague-Dawley rats do not attain full reproductive competence until sometime after the onset of puberty.     

The use of nest boxes by the Red Squirrel Sciurus vulgaris in a coniferous habitat

This paper describes the results of a 4-year study examining the use of nest boxes by Red Squirrels. Two different designs of nest box were erected in a conifer woodland which supported a high density of animals; spring population estimates were between 3.5 and 4.0 individuals per hectare. Boxes were used by adults, subadults, lactating females and their young. Breeding females more frequently used the larger of the two box designs, although this preference was not statistically significant. There was no difference in the mean sizes of litters found within the two box types. The mean size of litters present in boxes was 2.7 (n = 31, SE = 0.23) which was similar to the mean litter size of 2.5 (n = 8, SE = 0.50) found in dreys. Thus, nest boxes proved to be a valuable research tool for monitoring parturition dates, litter sizes, nestling sex ratios, and preweaning mortality.     

Variation in reproductive investment among and within populations of the scorpion Centruroides vittatus

Although information concerning variation among and within populations is essential to understanding an organism's life history, little is known of such variation in any species of scorpion. We show that reproductive investment by the scorpion Centruroides vittatus varied among three Texas populations during one reproductive season. Females from the Kickapoo population produced smaller offspring and larger litters than females from the Independence Creek or Decatur populations; this pattern remained when adjusting for among population variation in either female mass or total litter mass. Relative clutch mass (RCM) and within-litter variability in offspring mass (V*) did not differ among populations. Among-population variation may result from genetic differences or from phenotypically plastic responses to differing environments. Within populations, the interrelationships among reproductive variables were similar for Decatur and Independence Creek: females investing more in reproduction (measured by total litter mass, TLM) produced larger litters and larger offspring, and V* decreased with increased mean offspring mass (and with decreased litter size at Decatur). At Kickapoo, larger females produced larger litters and had larger TLM; females investing more in reproduction produced larger litters but not larger offspring. Within litter variability in offspring mass was not correlated with any reproductive variables in this latter population. These patterns may be explained by the fractional clutch hypothesis, the inability of females precisely to control investment among offspring or morphological constraints on reproduction.     

Genealogical concordance and the specific status of Peromyscus sejugis

Peromyscus sejugis, a peripheral isolate of Peromyscus maniculatus, is a threatened taxon endemic to 2 small islands in the Sea of Cortés. Although its insularity makes the specific recognition of P. sejugis inherently problematic, resolution of this problem has important conservation implications. To evaluate the specific validity and evolutionary history of P. sejugis, we compared sequence variation (ND3/ND4L/ND4) in mtDNA for both island populations of P. sejugis with that for 8 populations of P. maniculatus from mainland Baja California. Each island population of P. sejugis had a single haplotype (0.7% sequence divergence), whereas 11 different haplotypes (mean sequence divergence = 0.68%) were obtained for the populations of P. maniculatus. The mean sequence divergence between the populations of the 2 species was 2.0%. Nested clade analysis supports the conclusion that P. sejugis is an insular isolate of P. maniculatus from mainland Baja California. Although our analysis confirms a low level of mtDNA divergence between P. sejugis and P. maniculatus from Baja California, the genealogical concordance of morphological, chromosomal, microsatellite, and mtDNA haplotype distinctiveness supports the conclusion that the 2 island populations of P. sejugis constitute independent evolutionarily significant units and together represent a phylogenetic species distinct from the P. maniculatus from Baja California.     

Urinary proteins in four rodent species.

1. Mean urinary protein concentration levels are significantly higher in male Peromyscus leucopus than females (98.4 and 72.4 mg/dl). 2. Only females showed a significant correlation between weight and urinary protein concentration (r = 0.75 vs r = 0.03). 3. In intraspecific sexual electrophoretic comparisons of P. leucopus and P. maniculatus non-denatured urinary protein, four and two common bands were identified, respectively. Males of both species showed an extra protein band. 4. Four common electrophoretically separable denatured urinary protein bands were observed between 14,200 and 116,000 mol. wt in male and female P. leucopus and female P. gossypinus. Three of the four major protein bands were also found in P. maniculatus. Male Reithrodontomys megalotis pattern showed none of the major bands.     

布氏田鼠标志种群的繁殖参数

采用标志重捕和染色观测法跟踪了内蒙古典型草原区布氏田鼠野外种群,按绝对时间年龄研究其种群繁殖参数。结果3表明：4－5月份出生的雄鼠能在当年达到性成熟,性成熟发育历期约为1．5个月,6月后出生的雄鼠当年达不到性成熟。在达到性成熟的当年雄鼠中,多数个体再度转入性休止期,其平均繁殖结束时间要林越冬雄鼠早1个月,而越冬雄鼠则在整个繁殖期保持性活动状态。雌鼠性成熟发育历期约为1个月,首次产仔时间约为2月龄。雌鼠在一年中的产仔窝数与其年龄有关;越冬鼠能产3－4窝,4月份出生的雌鼠能产2－3窝,5月份出生的雌鼠当年能产1－2窝,6月份出生的雌鼠能产0－1窝,7月份之后出生雌鼠当年不参加繁殖,在自然条件下,布氏田鼠一年中最多能产4窝。