Postweaning mass gain in juvenile alpine marmots Marmota marmota

The effects of several environmental factors on the postweaning growth of wild Alpine marmots were investigated. Factors considered were year of birth, sun exposure in the home range, litter size, and sex of young. Components of growth were juvenile mass at emergence from the natal burrow (as a result of preweaning growth) and postweaning growth rate. We also considered the length of the active season during which growth occurs. Mass at emergence and postweaning growth rate varied according to year of birth, were higher in south-facing than in north-facing home ranges, and were higher in small litters. Mass at emergence was higher for males than for females. We suggest that environmental factors affected the juvenile growth pattern through influences on maternal body condition. Our results support Trombulak's hypothesis that mothers maintain as many young as physiologically possible. We suggest that mothers in poor condition sacrificed the mass of their offspring rather than their number. A body mass sexual dimorphism of juveniles occurred at emergence, suggesting that mothers may provide more care for their male than their female offspring. Received: 9 June 1997 / Accepted: 22 September 1997     

Explaining the seasonal decline in litter size in European ground squirrels

In European ground squirrels Spermophilus citellus as in many ground squirrel species. late born litters are composed of fewer young than early born litters. Two alternative though not mutually exclusive hypotheses may explain this seasonal pattern of change in litter size. On the one hand. the production of few large young late in the season may be an adaptation to time limitations on the offspring. that have to complete growth and fattening prior to hibernation. Then one would expect a trade-off between offspring number and size as the breeding season progresses. At its extreme. this hypothesis would predict that total maternal effort should be equal independent of litter size. Alternatively. litter size may be determined by physiological limitations on the mother. in that highly constrained mothers breed later and produce smaller litters. Then one would expect reduced overall maternal effort in highly constrained mothers of smaller litters. In this case. a trade-off between litter size and offspring size would not be expected. We found that total maternal effort in terms of gestation length and the duration of lactation increased with increasing litter size. thus supporting the second hypothesis. Lactation was not terminated at natal emergence. It extended a relatively long period of time beyond the time of first litter emergence depending on litter size. During prolonged lactation. individual young of large litters made up body mass to young of small litters. As a consequence. juvenile weaning body mass was unaffected by litter size although offspring body mass at natal emergence was inversely related to litter size. This additional weight gain in young of large litters compensated for initial survival disadvantages and presumably affected fecundity at yearling age.     

The shape of things eaten: the functional response of herbivores foraging adaptively

We studied reproductive and somatic investments in >700 female Richardson's ground squirrels ( Spermophilus richardsonii ) of known age over a 14-year period to evaluate three hypotheses, restraint, senescence, and residual reproductive value, proposed to explain age-specific life history patterns in iteroparous vertebrates. We found that reproductive investment, measured as litter mass at first emergence from the natal nest, did not differ among age classes. Although yearling female Richardson's ground squirrels made a greater somatic investment during reproduction than older females, they produced similar numbers and mass of offspring as older females. Reproductive investment did not decline with age, though active season somatic investment was lowest in the oldest females. Somatic investment during reproduction was highest in yearlings. This combination of age-related changes in somatic investment unaccompanied by changes in reproductive investment was not well explained by any of the hypotheses examined, though the senescence hypothesis best explained the combination of declining somatic investment and declining survival of the oldest females. Our results supported the ideas that reproductive and somatic senescence evolve independently, and that somatic senescence may be more common in relatively smaller species.     

Age-specific resource investment strategies: evidence from female Richardson's ground squirrels (Spermophilus richardsonii)

To avoid a possible cost to their future survival and/or reproduction, individuals must balance their somatic and reproductive investments. The van Noordwijk and De Jong model of resource investment predicts that investments into reproduction and soma can vary among individuals of a population based on the variation in the total amount of energy that individuals acquire. With principal components analysis (PCA), we created two axes of life history for female Richardson's ground squirrels Spermophilus richardsonii : an index of total energy investment (PC1) and an index of investment tactic (PC2). Using these indices, we examined patterns of resource allocation to reproductive and somatic investments. Because yearling female Richardson's ground squirrels complete growth to adult size during pregnancy and early lactation, their somatic needs exceed those of older, fully grown females. Therefore, we predicted that yearlings would show more evidence of a tradeoff between reproductive and somatic investments compared with older females. Both yearling and older females invested four to five times more mass into their litters than into their own body mass. With increasing total investment, yearling females increased investment in both reproduction and themselves, whereas older females invested relatively more in reproduction than themselves. Regardless of age, females that emerged heavier from hibernation invested fewer resources into themselves and more into their litters. Variation in total energy investment and investment tactic indices was similar for yearling and older females. Contrary to our prediction, however, yearling females showed positive associations between reproductive and somatic investments, whereas older females exhibited showed no significant association between reproductive and somatic investments.     

Determinants of reproductive success in female Columbian ground squirrels

Summary We studied the reproductive success of female Columbian ground squirrels (Spermophilus columbianus) in southwestern Alberta for nine years. We defined reproductive success as the number of offspring surviving their first hibernation, classified as yearlings. The number of weaned juveniles explained one third of the variance in number of yearlings at emergence from their first hibernation the following spring, and much of the variance in individual reproductive success originated after weaning. Weight of adult females at emergence from hibernation was correlated with annual reproductive success. The mother's survival beyond weaning and the subsequent winter's snow accumulation had positive effects on annual reproductive success, whereas population density and summer temperature had negative effects. We found no effects on annual reproductive success of date of litter emergence, weight at emergence as a yearling, presence or absence of adult kin, distance from the natal site, location within the study area, winter temperature or summer precipitation. Age of first breeding did not affect lifetime reproductive success, which ranged from 0 to 19 yearlings produced over a lifetime. The greatest source of variation in lifetime reproductive success for females surviving to breeding age was offspring survival, followed by reproductive lifespan.     

Reproductive Decisions in Female European Ground Squirrels: Factors Affecting Reproductive Output and Maternal Investment

Physiological and behavioural parameters associated with reproductive effort and success were investigated in female European ground squirrels Spermophilus citellus . The proportion of reproductive (lactating) females in the study population was over 90% and was not related to age. Timing of oestrus and ovulation was found to be affected by the female's emergence date and condition. Females with low emergence mass showed delayed oestrus. Differences in ovulation dates were shown to affect reproductive output in terms of litter size and sex ratio. Early litters were larger and male biased. X-ray techniques were used to determine intrauterine litter size in individual females. The results indicated that litter size and sex ratio were fixed prenatally. Lactation costs were reflected in the intensity of mass loss and duration of lactation. Mass loss varied with litter size, in that females with large litters showed a more rapid loss than others. The second parental investment parameter, lactation duration, varied among individual females and was dependent on the timing of reproduction and litter size (except yearlings). Early born litters, which were, in most cases, larger than later ones, were nursed longer. Prolonged lactation periods affected female condition in that they started prehibernation fattening later and entered hibernation with a lower mass than individuals that had shorter lactation periods. Yearling females probably could not afford the energetic costs of long lactation, independent of their offspring number. These results indicated that females with higher reproductive output and higher investment were unable to compensate these costs before hibernation. Consequences for these individuals could therefore be lower over-winter survival or a delayed oestrus in the following season.     

Birth Timing for Mountain Lions (Puma concolor); Testing the Prey Availability Hypothesis

We investigated potential advantages in birth timing for mountain lion (Puma concolor) cubs. We examined cub body mass, survival, and age of natal dispersal in relation to specific timing of birth. We also investigated the role of maternal age relative to timing of births. We captured mountain lion cubs while in the natal den to determine birth date, which allowed for precise estimates of the population birth pulse and age of natal dispersal. A birth pulse occurred during June–August. Body mass of cubs was related to litter size and timing of birth; heaviest cubs occurred in litters of 2, and those born after 1 July. Cubs born within pulse months exhibited similar survival to those born out of the pulse. We found that cubs born April–June dispersed at younger ages than those born after 1 July. There was less variation in birth timing for 1^st litters of females than older females. We hypothesize that cubs born after the peak in births of neonate prey are advantaged by the abundance of vulnerable prey and those cubs and mothers realize an evolutionary advantage.     

Reproductive effort and costs of reproduction in female European ground squirrels

Reproductive effort, factors affecting reproductive output and costs of reproduction were studied in primiparous yearling compared to multiparous older female European ground squirrels (Spermophilus citellus). Yearling females weaned smaller litters than older ones. Litter size increased with posthibernation body mass at the expense of slightly lighter young for yearling but not for older mothers. In older females, on the other hand, emergence body mass influenced offspring mass, whereas litter size was affected by oestrus date. High reproductive effort entailed reproductive costs in terms of reduced subsequent fecundity but not subsequent survival for both yearling and older females. The production of large litters and long duration of lactation delayed subsequent oestrus, which, in turn, correlated negatively with litter size. During the second half of lactation, oestradiol levels were significantly elevated, indicating the initiation of follicular maturation processes. Oestradiol levels during that time correlated negatively with current, but positively with subsequent litter size. We therefore assume that inhibitory effects of lactation on gonadal development may mediate the negative relationship between reproductive effort and subsequent reproductive timing in adults. This effect is absent in yearlings because they are reproducing for the first time. Reproductive output in yearlings was influenced by interactions between structural growth and puberty. Received: 22 March 1999 / Accepted: 7 June 1999     

Factor affecting body mass of young fat dormics (Glis glis) at weaning and by hibernation

This paper describes the factors affecting body mass at weaning and by hibernation of young fat dormice (Glis glis) We studied a free-living population in 1991 and 1992 in a mixed forest where 100 nest-boxes were placed. In the two years, 128 females used the nest-boxes to give birth and rear their litters and more than 900 dormice have been individually marked. Body mass of young at weaning was positively correlated to body mass of the mother in both years and to birth date in 1991, while litter size was negatively correlated to body mass of the young in 1992 only, when birth took place about two weeks later than in 1991. On the contrary, by the time of hibernation, young born later weighed significantly less than those born earlier in the season, and heavier young at weaning remained heavier by hibernation, while maternal body mass and litter size did not significantly affect body mass of young at this stage. Body mass of the mother was negatively related to the date of parturition and early breeding females lost weight during lactation, while females that bred later did not. It seems, therefore, that early breeding carries a cost that only females in good condition can afford. It appears that females may follow two strategies: those in poor body condition will delay pregnancy so that lactation occurs when environmental feeding conditions are at their best; those in good condition will breed earlier, since they can afford to lose weight during lactation. Their young will have more time to grow before hibernation and the mothers themselves will have a longer period of time for restoring their fat reserves.     

Endocrine profiles and reproductive output in European ground squirrels after unilateral ovariectomy

In the course of a long-term study on reproductive physiology and behaviour in female European ground squirrels (Spermophilus citellus) we investigated interactions among reproductive timing, maternal effort and hibernation. To gain information on the state of follicular development shortly before hibernation, histological investigations of ovaries were carried out. To collect the ovaries, unilateral ovariectomy was chosen as the least invasive method. The aim of this study was to monitor patterns of hormone secretion, behavioural parameters and reproductive output in females after unilateral ovariectomy, and compare them with intact individuals to assess potential effects of the treatment. Reproductive performance including duration of gestation and lactation, litter size, and offspring mass at natal emergence were similar in treated and untreated individuals. Before weaning, plasma estradiol levels were higher in the hemicastrated group than in intact females. Progesterone concentrations tended to be lower in unilaterally ovariectomized females during gestation and postlactation. In both groups we measured cortisol metabolites in fecal samples to estimate basal adrenal activity. Untreated individuals had significantly higher cortisol levels than hemicastrated females during lactation. In all other phases no significant differences in patterns of cortisol excretion were found between the two groups. Prehibernatory body-mass did not differ between treated and untreated females. These results indicate that unilateral ovariectomy had no significant negative effects on reproductive timing and output in female European ground squirrels. Compensatory activity of the remaining ovary does occur, concerning estradiol secretion even a kind of overcompensation was reported. These elevated estradiol levels in unilaterally ovariectomized females could play a role in limiting the degree of follicular compensation.     

Litter sex ratios in Richardson’s ground squirrels: long-term data support random sex allocation and homeostasis

When costs of producing male versus female offspring differ, parents may vary allocation of resources between sons and daughters. We tested leading sex-allocation theories using an information-theoretic approach and Bayesian hierarchical models to analyse litter sex ratios (proportion males) at weaning for 1,049 litters over 24 years from a population of Richardson’s ground squirrels (Urocitellus richardsonii), a polygynandrous, annually reproducing mammal in which litter size averages from six to seven offspring and sons are significantly heavier than daughters at birth and weaning. The model representing random Mendelian sex-chromosome assortment fit the data best; a homeostatic model received similar support but other models performed poorly. Embryo resorption was rare, and 5 years of litter data in a second population revealed no differences in litter size or litter sex ratio between birth and weaning, suggesting that litter size and sex ratio are determined in early pregnancy. Sex ratio did not vary with litter size at weaning in any of 29 years, and the observed distribution of sex ratios did not differ significantly from the binomial distribution for any litter size. For 1,580 weaned litters in the two populations, average sex ratio deviated from parity in only 3 of 29 years. Heavier females made a greater reproductive investment than lighter females, weaning larger and heavier litters composed of smaller sons and daughters, but litter sex ratio was positively related to maternal mass in only 2 of 29 years. Such occasional significant patterns emphasize the importance of multi-season studies in distinguishing infrequent events from normal patterns.     

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.     

Intraspecific differences in the pattern of hibernation in the ground squirrelSpermophilus beldingi

Summary Intraspecific differences in the patterns of heterothermy were found in captive Belding's ground squirrels that hibernated undisturbed at ambient temperatures of 5°C, 10°C, and 15°C. The timing of all entrances into and arousals from hibernation was determined from records of copper-constantan thermocouples that were mounted on the floor of each animal's nest box and connected to continuously recording potentiometers. In the absence of food, large adult males terminated hibernation spontaneously in the spring. In contrast, females and small non-breeding males (yearlings) did not stop hibernating but instead they shortened their bouts of torpor in the spring so that they aroused every three or four days. This interval of frequent arousals, termed the emergence period, lastel until the squirrels became emaciated, and it was only in the 2 or 3 weeks preceeding death (starvation period) that arousal frequency once again decreased towards midwinter values (Fig. 3). These animals terminated hibernation when fed during the emergence or starvation periods, but they were able to resume torpor if that feeding lasted less than a week.Termination of hibernation and reproductive development in males were related to the size of the animals, not their age. Males two years and older which did not deposit normal quantities of fat were like yearlings in that they had open-ended hibernation seasons and showed little testicular enlargement. Likewise, juvenile males that grew to near adult size in their first summer spontaneously terminated hibernation and had well developed testes like most older individuals. In addition, both the frequency and duration of arousals in the emergence period were related, in part, to the extent of the animals' fat reserves, such that large squirrels spent more time at high body temperatures than small individuals (Figs. 5, 6). This trend was most pronounced at high ambient temperatures.These intraspecific differences in hibernation physiology are consistent with the sex and agerelated differences in the timing of emergence above ground in nature. The increase in the time spent euthermic in the spring isinterpreted as an adaptation for increasing the opportunities for environmental assessment. The intraspecific differences in the extent of this euthermia appear to be associated with differences in the balance between the energy supplies available to an animal and its need to accelerate the use of that energy (i.e., arouse) in order to achieve an early and accurately-time emergence from the hibernaculum.     

Body size,litter size,timing of reproduction,and juvenile survival in the Unita ground squirrel,Spermophilus armatus

The timing of reproduction affected litter size, offspring mass, and offspring survival in the Uinta ground squirrel, Spermophilus armatus, in Grand Teton National Park, Wyoming. Survival of juvenile females to yearling age varied negatively with date of weaning and positively with individual offspring mass. At the same time, juveniles weaned early in the season were lighter, and juveniles weaned later in the season were heavier. The coefficient of variation for juvenile body mass, originally measured at weaning, significantly decreased by the time juveniles entered hibernation, indicating that individuals weaned early and light caught up in body mass to individuals weaned later and heavier. From the perspective of the mother's investment in the litter, litter size (corrected for mother's mass) decreased with later wcaning dates, while the relationship of weaning date to litter mass (corrected for mother's mass) was significant in only one year. Maternal allocation of resources in litters changed over the season so that mothers produced many, small offspring early in the season, and fewer, large offspring late in the season.     

Annual cycle of energy and time expenditure in a golden-mantled ground squirrel population

Summary We have analyzed seasonal shifts of energy and time allocation in a population of golden-mantled ground squirrels (Spermophilus saturatus) by directly measuring total daily energy expenditure (DEE) with an isotopic technique (doubly labeled water=dlw), and by estimating components of total DEE through an integration of field behavioral observations with laboratory-measured rates of energy expenditure (oxygen consumption) associated with major behavioral and physiological states. Hibernation laster about 7 1/2 months, and the 4 1/2-month activity season consisted of mating, a 28-d gestation of 3–5 young, 5 1/2 weeks of postnatal growth building to a peak in lactation just before the young emerged above ground, an additional 2–3-week period of maternal care before dispersal, and finally restoration of body mass preceding hibernation. Although the hibernation season comprised nearly two-thirds of the year, it involved only 13–17% of annual energy expenditure, leaving about 85% of energy expenditure for the active season. Ground squirrels were actually present on the surface for only about 11% of the year's time, and the foraging time required to obtain the total annual energy supply amounted to only about 2% of the year's time. The squirrels fed mainly on herbs in the early season and hypogeous fungi later; both were used extensively during peak lactation when female energy expenditure and demand were maximal. Average daily foraging time increased steadily throughout the season to a maximum of 28% of aboveground time as availability of greens diminished and fungus predominated in the diet; time availability did not limit foraging since the animals sat on average for 65% of the daily surface time of about 7 h. Timing of reproduction is apparently optimized such that peak reproductive energy demands are matched with maximal food availability and moderate thermal conditions that minimize energy demand. Despite the greater body mass of males, the greatest total DEE (measured by dlw) of any squirrels at any time of year was that of females during peak lactation. For production of young and lactation through above-ground emergence of an average litter of 2.7, females required a total energy increase of 24% above annual nonreproductive metabolism. Yearling females all bred and performed similarly to older females, yet some costs were greater because the yearlings began and ended hibernation at smaller mass, compensated by giving birth later, and finally showed a greater absolute increase in body mass over the active season than older females. Annual metabolic energy expenditure of breeding males was about 18% greater than that of females, due to greater male body mass. Yet the annual energy intake requirement for both sexes was essentially identical (about 42MJ) due to the greater reproductive export by females in the form of newborn and milk. During the mating season males showed wide-ranging exploratory behavior and social interactions, including aggression, that involved considerable locomotory energy expenditures. Although we did not directly account for the energetics of these specific reproductive behaviors, they are critical to male reproductive success and on a daily basis they probably involved much greater energy expenditure than sperm production. Some yearling males avoided these costs by foregoing testicular development, yet they allocated four times as much energy to growth as older males, thereby increasing somatic condition for the future.     

Sexual differences in over-winter torpor patterns of Richardson's ground squirrels in natural hibernacula

Summary Over-winter torpor patterns of Richardson's ground squirrels hibernating in southern Alberta were monitored with temperature-sensitive radiocollars to determine if these patterns differed between males and females in a manner related to the greater costs of mating effort by males than females. The hibernation season (from immergence to emergence) was composed of three periods: post-immergence euthermy, heterothermy, and pre-emergence euthermy. The hibernation season was shorter for juveniles than adults both among males (< 150 versus 234 days) and females (185 versus 231 days), a reflection of the later immergence into hibernation by juveniles. However, regardless of the absolute duration of hibernation, heterothermy accounted for a smaller proportion of the hibernation season of males (93±5%) than females (98±1%) and, within the heterothermal period, males had shorter torpor bouts and longer inter-torpor arousals. Overall, males spent a smaller proportion of the hibernation season in torpor (85±6%) than females (92±1%). This sexual difference was largely attributable to the longer duration of preemergence euthermy for males than females. Males terminated torpor in January and February, when hibernacula were at their coldest, then remained euthermic for 8.8 days (range 0.5–25.0 days) before emergence. In contrast, females terminated torpor in March, when hibernaculum temperatures were increasing, then remained euthermic for only 1.1 days (range 0.5–2.0 days) before emergence. Males lost less mass per euthermic day during hibernation than females (7.0 versus 9.3 g/day). Males and females hibernated at similar depths (56 cm), but males had larger chambers than females (18 versus 16 cm³/g). Many males, but no females, cached seeds in the hibernaculum. Males met the costs of thermogenesis and euthermy from a combination of fat reserves and food caches, whereas females relied solely on fat. Access to food caches permitted males to terminate torpor several weeks in advance of emergence, during which time they recouped mass and developed sperm in preparation for the forthcoming mating season.     

蝘蜓头、体大小的两性异形和雌体繁殖

报道了蜓 (Sphenomorphusindicus)头、体大小的两性异形和雌性繁殖。性成熟雌体大于雄体。雄性成体头长大于雌性成体 ,但头宽与雌性成体无显著差异。初生幼仔的头长和头宽无两性差异。雄性幼体头长和头宽大于雌性幼体。设置SVL恒定时 ,雄性幼体和雄性成体的头长和头宽无显著差异 ,雌性幼体的头长和头宽大于雌性成体。初生幼仔具有相对较大的头部。产仔雌体的最小SVL为 6 7 7mm ,大于此SVL的雌体均年产单窝仔。平均窝仔数、窝仔重和幼仔重分别为 7 2 ( 3～ 11)、 3 34( 1 30～ 5 19)和 0 48( 0 36～ 0 5 8)g。用卵黄沉积卵巢卵和输卵管计数的窝仔数比用幼仔计数的窝仔数多约 1 0个后代。幼仔体重与雌体SVL无关。相对窝仔重与雌体SVL边缘性地呈正相关。窝仔数、窝仔重与雌体SVL呈正相关 ,幼仔体重与窝仔数呈负相关。窝仔数与雌体状态无关。     

Mortality and reproductive patterns of wild European polecats<Emphasis Type="Italic">Mustela putorius</Emphasis> in Denmark

Despite its relative abundance and wide geographical range, the population dynamics and reproductive biology of the European polecatMustela putorius Linnaeus, 1758 are largely unknown as to the wild living. We therefore investigated age and reproductive status of 239 Danish polecats primarily killed in traffic or trapped during 1998–2004. Males comprised two third of all individuals in all age groups. Based on a static life table, apparent annual mortality was 68% during the first year of life, 33% during the second year and 65% from the third year in both sexes. The mean (± SE) litter size of 5.95 ± 0.62 (n = 18), estimated from placental scar counts, was significantly lower than litter sizes at birth reported for captive individuals but consistent with litter sizes reported for wild polecats in Russia. Female yearlings conceived at the same rate and produced litters of the same size as older individuals.Males had spermatozoa in their testes from February through August. Testes mass peaked in April and May, ie the same period when most females conceive. A lower prevalence of individuals with spermatozoa in yearlings suggests that most males postponed sexual maturity to two years of age.     

The "hibernation induction trigger": specificity and validity of bioassay using the 13-lined ground squirrel

Even though the existence of the blood-borne "hibernation induction trigger" has been reported in the 13-lined ground squirrel, transfusion of plasma from hibernating rodents with other hibernating species as the recipients failed to induce the occurrence of summer hibernation. In order to verify whether the response to the "trigger" substance is species specific, the present study was carried out to compare the effect of plasma from hibernating Richardson's ground squirrels on the incidence of summer hibernation in both juvenile Richardson's and adult 13-lined ground squirrels. In two series of experiments, 13-lined ground squirrels entered hibernation quite readily independent of the treatment. The rate of occurrence of hibernation ranged from 78% after sham injection to 86% after warm saline, fresh summer active plasma, and fresh hibernating plasma, respectively. There were no differences in the number of hibernation bouts and the number of days in hibernation after each treatment. In contrast, none of the juvenile Richardson's ground squirrels entered hibernation after any of the treatments up to the end of the 8-week observation period. These results not only argue against the existence of blood-borne "trigger" substance, at least in the Richardson's ground squirrel, but also caution against the use of the 13-lined ground squirrel as a standard test animal for the bioassay of the "trigger" substance.     

Optimal offspring size in a small mammal: an exception to the tradeoff invariant life-history rule

Offspring size and number were examined in a captive population of wild guinea pigs ( Cavia aperea ), and findings were compared with models of optimal offspring size for small litters. Median and modal litter size was two, regardless of maternal size or parity. Females producing their second litter tended to have litters that were larger than average. In contrast, young females that were still growing never had litters that were larger than average. Mean offspring size decreased and variation in offspring size tended to decrease with increasing litter size. Optimal offspring size models, in which offspring survival depended on the amount of resources invested, as well as litter size, predict such a trend. Little support was found for Charnov and Downhower's (1995) tradeoff invariant life-history rule that the range in offspring sizes between litters is inversely proportional to the size of the litter. Cavia aperea may be an exception to this rule because pup mass at birth did not reflect total reproductive investment, because conversion of resources into litter mass may not be linearly related to litter size and because resources were not equally partitioned among offspring within large litters. Experimental data are needed to determine the relevance of these results among mammals in general.