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.     

Reproductive effort during gestation and lactation by Richardson's ground squirrels

Summary Reproductive effort by yearling and older female Richardson's ground squirrels was studied over a 4-year period in southern Alberta by obtaining serial weight records from marked individuals to compare the mother's mass at critical points in the annual cycle (emergence from hibernation, estrus, parturition, and litter emergence) with her litter's mass at birth and weaning. Yearlings weighed only 80% of older adults at emergence from hibernation, but they mated at the same time as older females, attained adult mass coincident with pregnancy, and weaned litters that were not significantly smaller in size or mass than those of older females. Age and maternal mass were weak predictors of litter size and litter mass. Of the net increase in mass of the combined mother-litter unit during gestation, over half (60% of 139 g for yearlings; 52% of 127 g for older females) was attributable to an increase in the mother's own mass, whereas during lactation almost all of the net increase (93% of 545 g for yearlings; 96% of 567 g for older females) was attributable to an increase in the litter's mass. On a daily basis, deposition of mass in the litter was 6 times greater during lactation than gestation. On average, neonates weighed 2.3% (6.5 g) of maternal mass at birth and 23.1% (81 g) at emergence from the natal burrow; offspring masses at birth and at emergence were significantly negatively correlated with litter size. On average, litters weighed 16.3% (48 g) of maternal mass at birth and 157.5% (578 g) at emergence from the natal burrow. Compared with other hibernating sciurids, Richardson's ground squirrels have a similar offspring mass relative to maternal mass both at birth and at emergence from the natal burrow. However, because of the large litter size (typically 6–8), absolute reproductive effort, measured either as litter mass at birth or at natal emergence, is large for the body size of the species.     

Senescence and costs of reproduction in the life history of a small precocial species

Species following a fast life history are expected to express fitness costs mainly as increased mortality, while slow‐lived species should suffer fertility costs. Because observational studies have limited power to disentangle intrinsic and extrinsic factors influencing senescence, we manipulated reproductive effort experimentally in the cavy (Cavia aperea) which produces extremely precocial young. We created two experimental groups: One was allowed continuous reproduction (CR) and the other intermittent reproduction (IR) by removing males at regular intervals. We predicted that the CR females should senesce (and die) earlier and produce either fewer and/or smaller, slower growing offspring per litter than those of the IR group. CR females had 16% more litters during three years than IR females. CR females increased mass and body condition more steeply and both remained higher until the experiment ended. Female survival showed no group difference. Reproductive senescence in litter size, litter mass, and reproductive effort (litter mass/maternal mass) began after about 600 days and was slightly stronger in CR than IR females. Litter size, litter mass, and offspring survival declined with maternal age and were influenced by seasonality. IR females decreased reproductive effort less during cold seasons and only at higher age than CR females. Nevertheless, offspring winter mortality was higher in IR females. Our results show small costs of reproduction despite high reproductive effort, suggesting that under ad libitum food conditions costs depend largely on internal regulation of allocation decisions.     

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.     

Hormonal manipulation of offspring number: maternal effort and reproductive costs

We used exogenous gonadotropin hormones to physiologically enlarge litter size in the bank vole (Clethrionomys glareolus). This method allowed the study design to include possible production costs of reproduction and a trade-off between offspring number and body size at birth. Furthermore, progeny rearing and survival and postpartum survival of the females took place in outdoor enclosures to capture salient naturalistic effects that might be present during the fall and early winter. The aim of the study was to assess the effects of the manipulation on the growth and survival of the offspring and on the reproductive effort, survival, and future fecundity of the mothers. Mean offspring body size was smaller in enlarged litters compared to control litters at weaning, but the differences disappeared by the winter. Differences in litter sizes disappeared before weaning age due to higher mortality in enlarged litters. In addition to the effects of the litter size, offspring performance was probably also influenced by the ability of the mother to support the litter. Experimental females had higher reproductive effort at birth, and they also tended to have higher mortality during nursing. Combined effects of high reproductive effort at birth and high investment in nursing the litter entailed costs for the experimental females in terms of decreased probability of producing a second litter and a decreased body mass gain. Thus, enlarged litter size had both survival and fecundity costs for the mothers. Our results suggest that the evolution of litter size and reproductive effort is determined by reproductive costs for the mothers as well as by a trade-off between offspring number and quality.     

Short- and long-term effects of litter size manipulation in a small wild-derived rodent

Iteroparous organisms maximize their overall fitness by optimizing their reproductive effort over multiple reproductive events. Hence, changes in reproductive effort are expected to have both short- and long-term consequences on parents and their offspring. In laboratory rodents, manipulation of reproductive efforts during lactation has however revealed few short-term reproductive adjustments, suggesting that female laboratory rodents express maximal rather than optimal levels of reproductive investment as observed in semelparous organisms. Using a litter size manipulation (LSM) experiment in a small wild-derived rodent (the common vole; Microtus arvalis), we show that females altered their reproductive efforts in response to LSM, with females having higher metabolic rates and showing alternative body mass dynamics when rearing an enlarged rather than reduced litter. Those differences in female reproductive effort were nonetheless insufficient to fully match their pups’ energy demand, pups being lighter at weaning in enlarged litters. Interestingly, female reproductive effort changes had long-term consequences, with females that had previously reared an enlarged litter being lighter at the birth of their subsequent litter and producing lower quality pups. We discuss the significance of using wild-derived animals in studies of reproductive effort optimization.     

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.     

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.     

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.     

Age‐related reproductive success in dichromatic male Black Redstarts Phoenicurus ochruros: Why are yearlings handicapped?

Reproductive constraints on sexually mature, young birds may be the underlying cause for the evolution and maintenance of delayed plumage maturation in passerine birds. I examined whether yearling male Black Redstarts in non-definitive plumage are reproductively handicapped and explored possible reasons for it. Yearlings raised only half as many young per season as adults. Differential reproductive effort was excluded as a possible source for age-related reproductive success because yearlings and adults did not differ in the frequency of non-breeding or in paternal effort. The lower annual reproductive success of yearlings was attributed to their lower probability of obtaining (multiple) females and adult females. Males mated to adult females raised almost twice as many offspring as males mated to yearling females. I conclude that the proposed preconditions for the evolution and maintenance of delayed plumage maturation do still prevail in the Black Redstart.     

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.     

Determinants of reproductive success in female adders,Vipera berus

Summary Female lifetime reproductive success in a small population of individually-marked adders in southern Sweden was studied over a period of seven years. Reproductive characteristics varied little from year to year and were consistent through time in individual females. Most females mature at four years of age and reproduce every two years. The total number of offspring produced by a female depends on her adult body size (and thus, litter size) and longevity (and thus, number of litters per lifetime). Adult body size in females is influenced mainly by subadult growth rates. Offspring size depends on maternal body size and a tradeoff between offspring size and offspring number. Maternal age does not affect litter sizes and offspring sizes except through ontogenetic changes in maternal body size.Survival of females after parturition is low because of the high energy costs of reproduction, compounded by low feeding rates of gravid females because of their sedentary behaviour at this time. About one-half of females produce only a single litter during their lifetimes, although some females live to produce four or five litters. On a proximate basis, rates of energy accumulation for growth (in subadults) and reproduction (in adults) may be the most important determinants of fitness in female adders.     

黑线仓鼠繁殖输出与基础代谢率的关系

为了解黑线仓鼠繁殖输出与基础代谢率(BMR)的关系,阐明最大持续能量收支(SusMR)的限制水平, 揭示哺乳期能量收支对策,本文测定了哺乳期黑线仓鼠的体重、摄食量、BMR 和身体组成,以及哺乳期的胎仔数、胎仔重和泌乳能量支出(MEO)。结果显示,黑线仓鼠哺乳期体重降低了15.0 ± 0.8% , 摄食量显著增加, 哺乳高峰期平均摄食量为13.9 ± 0.3 g /d, 摄入能为222.1 ± 5.3 kJ/ d, 比哺乳初期增加121% , 比对照组高288% ;哺乳高峰期MEO 为62.4 ± 2.3 kJ/ d, 哺乳末期BMR 为49.7 ± 1.1 kJ/ d; 断乳时平均胎仔数4.7 ± 0.2、窝胎仔重50.5 ±1.6 g; 哺乳末期BMR 比对照组增加48% ,BMR 与消化系统各器官的相关性高于对照组; BMR 与胎仔数、胎仔重、乳腺重量和MEO 显著正相关。结果表明:初次繁殖的黑线仓鼠哺乳期SusMR 限制为4.47 ×BMR, 在自身维持和繁殖输出之间采取了“权衡分配”的原则,通过体重降低以减少BMR 的增加幅度, 从而有利于繁殖输出。     

Reproductive resilience to food shortage in a small heterothermic primate

The massive energetic costs entailed by reproduction in most mammalian females may increase the vulnerability of reproductive success to food shortage. Unexpected events of unfavorable climatic conditions are expected to rise in frequency and intensity as climate changes. The extent to which physiological flexibility allows organisms to maintain reproductive output constant despite energetic bottlenecks has been poorly investigated. In mammals, reproductive resilience is predicted to be maximal during early stages of reproduction, due to the moderate energetic costs of ovulation and gestation relative to lactation. We experimentally tested the consequences of chronic-moderate and short-acute food shortages on the reproductive output of a small seasonally breeding primate, the grey mouse lemur (Microcebus murinus) under thermo-neutral conditions. These two food treatments were respectively designed to simulate the energetic constraints imposed by a lean year (40% caloric restriction over eight months) or by a sudden, severe climatic event occurring shortly before reproduction (80% caloric restriction over a month). Grey mouse lemurs evolved under the harsh, unpredictable climate of the dry forest of Madagascar and should thus display great potential for physiological adjustments to energetic bottlenecks. We assessed the resilience of the early stages of reproduction (mating success, fertility, and gestation) to these contrasted food treatments, and on the later stages (lactation and offspring growth) in response to the chronic food shortage only. Food deprived mouse lemurs managed to maintain constant most reproductive parameters, including oestrus timing, estrogenization level at oestrus, mating success, litter size, and litter mass as well as their overall number of surviving offspring at weaning. However, offspring growth was delayed in food restricted mothers. These results suggest that heterothermic, fattening-prone mammals display important reproductive resilience to energetic bottlenecks. More generally, species living in variable and unpredictable habitats may have evolved a flexible reproductive physiology that helps buffer environmental fluctuations.     

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.     

Age-specific reproductive performance of Black Grouse Tetrao tetrix females

As part of a demographic study of Black Grouse in the southern French Alps, we estimated variations in reproductive parameters in relation to age of females. Both yearlings and adults initiated a clutch, but adults raised about eight times more young (yearlings 0.25 chicks/females, adults 1.85 chicks/female). Yearlings laid significantly fewer eggs and initiated incubation later than older females. Success of nests did not differ between the two age classes, but yearlings tended to re-nest less frequently and exhibited higher chick loss than older females. The lower reproductive success of yearlings was primarily due to their lower ability to raise young. At nest sites, nest concealment, as well as vertical and horizontal structure and density of the vegetation, did not differ between yearling and older females, which is consistent with the similarity in nesting success in the two age classes. Results support the hypothesis that the extent of the difference in number of young raised by yearling and older females increases with predation pressure.     

Maternal Effort is State Dependent: Energetic Limitation or Regulation?

Many small altricial rodents have a postpartum oestrus and are often simultaneously pregnant and lactating. Negative influences of concurrent pregnancy and lactation on both lactational performance and the litter in utero are commonly observed and have been interpreted as resulting from high simultaneous energetic demands of gestation and lactation. We studied these effects in the precocial guinea‐pig (Cavia aperea f. porcellus) that, like many altricial rodents, has a postpartum oestrus, but in which the peaks of energy expenditure on lactation and gestation are widely separated. This life history allowed to investigate whether physiological regulation other than by energetic limitations may be responsible for allocation conflicts arising when lactation and gestation overlap. By comparing simultaneously pregnant and lactating females with lactating non‐pregnant females, we show that females in the former group nurse less and wean earlier than females of the latter group. In a comparison of litter size, litter mass, and pup mortality of females that had not been lactating during pregnancy with females that had been simultaneously pregnant and lactating, we show that the latter do not reduce investment in the following litter. In our study, energetic constraints on ad libitum fed females are unlikely and we therefore suggest that the results must be explained by regulatory constraints on lactational effort. We point out that this explanation has not been excluded for the effects observed in altricial small mammals.     

PROXIMATE AND ULTIMATE CONTROLS ON LIFE-HISTORY VARIATION: THE EVOLUTION OF LITTER SIZE IN WHITE-FOOTED MICE (PEROMYSCUS LEUCOPUS)

Recruitment of litter-mates of nest-box-inhabiting white-footed mice was monitored to study the evolution of litter size. The frequency distribution of litter sizes was nonsymmetrical, and the most frequent litter size was less than the optimum. This was not the result of differential parental survival, which was independent of litter size produced. Recruitment remained constant or increased slightly to a peak in litters of five young, and then dropped precipitously for larger litters. The single optimum litter size of five did not appear to have any physiological correlates. Instead, the equally low probability of successful recruitment of any young from any given litter may have given rise to a bet-hedging strategy of frequent iterated reproductions. A theoretical analysis of optimal parental investment in offspring was initiated under the assumption that optimal brood size represents a maximization of differences between age-specific costs and benefits of reproduction, both of which should be measured in constant currency of inclusive fitness. In the past, benefit has been measured by current fecundity, and cost by residual reproductive value. However, reproductive value is an appropriate estimate of inclusive fitness only for organisms in which parental investment has little effect on the subsequent survival of offspring to reproductive age. Reproductive value weighted by offspring survival and devalued by the degree of genetic relatedness defines a new currency, replacement value, which is more appropriate for evaluating the costs and benefits of parent-offspring conflict over parental investment in current as opposed to future young. Total parent-offspring conflict intensifies with increases in current brood size. For species with severe reproductive constraints, such as post-partum estrus in white-footed mice, such conflict may force parents to curtail investment in current offspring at or near parturition of subsequent litters, even if that means reducing the survival of current young.     

川西平原大足鼠的种群生态学 III.繁殖

在川西平原利用标志重捕、夹捕解剖和半自然条件下笼养的方法,提供了1989～1995年大足鼠种群雄性和雌性个体处于繁殖状态的比例、胎次数、每胎幼仔数、幼年个体的补充和发育等有关繁殖的基本资料。结果显示,大足鼠极灵活的繁殖策略 提高了这个物种幼年个体的存活,使该物种在可预见而频繁的周期性变化的农田环 境中能够保持相对稳定的种群。大足鼠的繁殖特征主要有：处于繁殖状态的雄性比例较大（平均0.491,95%置信区间0.451和0.534）,怀孕雌性的比例较小（平均0.227,95％置信区间0.213和0.301）,较高和相对稳定的繁殖能力（每年4胎,每胎8.245±0.452只）,雌性幼仔较慢的生长和性成熟（4月）,以及幼年个体加入种群与春季的小麦和秋季的水稻成熟相同步。这些繁殖特征使得大足鼠的繁殖活动集中在有利的时间。结果,虽然伴随繁殖活动雌雄性的个体质量下降,但仍能在短暂的时间里恢复到原来的水平。将大足鼠的胎仔数与一些关于中国啮齿动物繁殖参数地理变异 的研究进行了分析比较,发现后者提供的资料尚不足以支持中国所有啮齿动物都有 "胎仔数随纬度增加"的过分简单的一般结论。每一个物种种群都有自己的最优胎 仔数,但最优胎仔数不一定随纬度而增加。我们认为在大尺度上研究如胎仔数这样的生活史性状时,生活史多样性的提法比单一性状的过分简单的一般化更好。     

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.