Life history of the moose Alces alces: relationship between growth and reproduction

We studied the relationship between increase in body weight and reproductive performance in different populations of Norwegian moose to evaluate costs associated with early onset of reproduction, viz. whether early onset of reproduction was correlated with low adult body weight or reduced adult fecundity. The mean carcass weight of non-ovulating yearlings was significantly lower than for ovulating yearlings. However, those 2.5 yr old females that conceived as yearlings were lighter than non-reproducing females of the same age. Thus, to begin to reproduce as a yearling was assumed to be expensive because it reduces the possibilities for further growth. The cost associated with reproduction was further illustrated by the fact that the difference in mean carcass weight from age 2.5 to 3.5 yr of females that produced calves in both years, was less for the females from regions with lowest mean yearling weights, i.e. regions with probably the lowest resource availability. In populations with high mean yearling carcass weights, the proportion of cows with calf and the number of calves per pregnant female in the early reproductive phase (2.5 or 3.5 yr old) were higher than in populations in which the mean yearling weights were low. There was a negative correlation between growth rate in the population after 1.5 yr of age and the mean yearling carcass weight. Thus, low yearling weight was associated with a prolonged period of growth and thereby a reduced reproductive output during the first year of the female's life. For old females (≥ 9.5 yr old) the number of calves produced per pregnant female was highest in populations where yearling carcass weights were highest. Furthermore, mean yearling weight and the mean adult female weight were positively correlated in those regions. This relationship suggests that within this species early onset of reproduction is not related to retarded reproduction or lower weight later in life. We suggest that the moose has been selected for an early onset of reproduction.     

Density-dependence vs. density-independence - linking reproductive allocation to population abundance and vegetation greenness

1. Recent studies have shown that optimal reproductive allocation depends on both climatic conditions and population density. We tested this hypothesis using six years of demographic data from eight reindeer (Rangifer tarandus) populations coupled with data on population abundance and vegetation greenness [measured using the Enhanced Vegetation Index (EVI)]. 2. Female spring body mass positively affected summer body mass gain, and lactating females were unable to compensate for harsh winters as efficiently as barren ones. Female spring body mass was highly sensitive to changes in population abundance and vegetation greenness and less dependent on previous autumn body mass and reproductive status. Lactating females were larger than barren females in the spring. Moreover, female autumn body mass was positively related to female autumn body mass and reproductive success and was not very sensitive to changes in vegetation greenness and population abundance. 3. Offspring autumn body mass was positively related to both maternal spring and autumn body mass, and as predicted from theory, offspring were more sensitive to changes in vegetation greenness and population abundance than adult females. A lagged cost of reproduction was present as larger females who were barren, the previous year produced larger offspring than equally sized females that successfully reproduced the previous year. 4. Reproductive success was negatively related to female autumn body mass and positively related to female spring body mass. Moreover, females who successfully reproduced the previous year experienced the highest reproductive success. The fact that negative density-dependence was only present for females that had successfully reproduced the previous year further support the hypothesis that reproduction is costly. 5. This study shows that female reindeer buffer their reproductive allocation according to expected winter conditions and that their buffering abilities were limited by population abundance and a lagged cost of reproduction and enhanced by vegetation greenness.     

Interactions Among Female Guppies (Poecilia reticulata) Affect Growth and Reproduction

Competition among females over resources may have consequences for their resource budgets and thereby the resource allocation between growth and reproduction. In addition, the consequences of female–female interactions may differ for dominant and subordinate individuals, with the dominant ones being at an advantage. In this study, we investigated the consequences of female–female competition in guppies by manipulating the competitive environment of females. We found that large guppy females dominated smaller females and that interactions between females likely are costly because females exposed to competition grew less. These females compensated by growing at a higher rate when no longer subjected to competition. The higher growth rate might in turn be the cause of the reduced reproductive effort in the more competitive treatments. Furthermore, interactions were more costly for females when they were in the subordinate role than in the dominant role, because the reduction in growth and reproductive effort was highest in females exposed to larger competitors. Whether there was a differential allocation of resources into growth and reproduction depending on dominance status needs further investigation. However, in general, smaller females had a higher growth rate than larger females, independent of competitive level. We also found a negative relationship between reproduction and growth in all treatments, indicating a cost of reproduction.     

Geographic variation as a result of evolution of the traits with broad and narrow norms of reaction in the moor frog (Rana arvalis)

Females reproductive, size, and age characteristics were studied in isolated local populations of Rana arvalis in the southern and northern parts of its range. The yearlings of the southern populations used to get larger by their first overwintering due to earlier beginning of the breeding season, as compared with the yearlings of the northern population. As a result, "southern" females become sexually mature at the age of two years while the "northern" ones become mature at the age of three years. This causes geographic differences in age composition among two populations, the "southern" reproductive females being younger on average than the "northern" ones. The earlier female maturation in the first case is not compensated by respective rise of the growth rate; to the contrary, the "southern" females grow more slowly during the first two years of their life and appear to be smaller than the "norhern" ones. These reproduction and growth patterns arise supposedly due to paedomorphosis, which causes specific reproductive characteristics, namely decrease in the egg size, increase in the reproductive effort and more strong correlation between female fertility and body size. Local and geographic differences are expressed not in the extent but in the structure of reproductive pattern, as no negative correlation was revealed between female reproductive age and body size in the southern populations. Southern habitats cannot be considered as "unfavourable with respect to body size", so the geographic differences under consideration cannot be explained by optimization of the reproductive strategies at population level. Paedomorphosis appears as a result of the female maturation rate possessing a wider norm of reaction than the growth rate. At the same time, fixation of the specific growth rate narrows norm of reaction of some other characters important for the phenotype reproductive fitness thus predetermining their subsequent evolution.     

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.     

Sexual dimorphism explains residual variance around the survival‐reproduction tradeoff in lizards: Implications for sexual conflict over life‐history evolution*

The tradeoff between survival and reproduction is a central feature of life‐history variation, but few studies have sought to explain why females of some species exhibit relatively lower survival than expected for a given level of reproductive effort (RE). Intralocus sexual conflict theory proposes that sex differences in selection on survival and RE may, by virtue of shared genes underlying these components of fitness, prevent females from optimizing this life‐history tradeoff. To test this hypothesis, we used a phylogenetically based comparative analysis of published estimates for mean annual survival and RE from females of 82 lizard species to (1) characterize the tradeoff between survival and reproduction and (2) test whether variation around this tradeoff is explained by sexual size dimorphism (SSD), a potential proxy for sexual conflict over life‐history traits. Across species, we found a strong negative correlation between mean annual survival and RE, confirming this classic life‐history tradeoff. Although residual variance around this tradeoff is unrelated to the absolute magnitude of SSD, it is strongly related to the direction of SSD. Specifically, we found that females have lower survival than expected for a given level of RE in female‐larger species, whereas they have higher survival than expected in male‐larger species. Given that female‐larger SSD is thought to reflect selection for increased fecundity, our results suggest that intralocus sexual conflict may be particularly likely to constrain female life‐history evolution in situations where increased RE is favored, but the phenotypes that facilitate this increase (e.g., body size) are constrained by antagonistic selection on males.     

Different paths to sexual size dimorphism in two praying mantids,Pseudomantis albofimbriata and Hierodula majuscula

Sexual size dimorphism （SSD） is widespread among diverse animal taxa and has attracted the attention of evolutionary biologists for over a century. SSD is likely to be adaptive and the result of divergent selection on different size optima for males and females, given their different roles in reproduction. The developmental trajectory leading to SSD may help us to understand how selection acts on male and female size. Here, we describe the growth and development of two Australian praying mantids, Pseudomantis albofimbriata and Hierodula majuscula including the number of moults, time to adulthood, size at each moult, and the degree of SSD. While both species exhibit the common pattern of female-biased SSD, the number of moults required for individuals to reach adulthood differed between males and females and between species. Despite their larger adult size, P. albofimbriata females require fewer moults and less time than males to reach adulthood, but are significantly larger than males from the second instar onwards. In contrast, H. majuscula males reached adulthood in fewer moults, and less time than females, however males and females did not differ in size until females went through their final moult into adulthood. H. majuscula also required more time and more moults to reach adulthood than 17. albofimbriata. We discuss these different developmental pathways in light of the existing knowledge of reproductive biology for each species. We also suggest that these differences may relate to the different phenologies that occur in strongly seasonal temperate environments compared with those in the tropics. This study provides evidence that SSD can result from two different patterns of growth and development in closely related species.     

Roots, shoots and reproduction: sexual dimorphism in size and costs of reproductive allocation in an annual herb

Females tend to be smaller than males in woody dioecious plant species, but they tend to be larger in herbs. The smaller size of females in woody species has been attributed to higher reproductive costs, yet no satisfactory explanation has been provided for their larger size in herbs. Because herbs have higher nitrogen concentrations in their tissues than woody plants, and because pollen is particularly rich in nitrogen, we predicted that male growth would be more compromised by reproduction than female growth. To test this hypothesis, we conducted three experiments on the annual dioecious herb Mercurialis annua. First, we compared the timing of reproduction between males and females and found that males started flowering earlier than females; early flowering is expected to compromise growth more than later flowering. Second, we compared plants allowed to flower with those prevented from flowering by experimental debudding and found that males incurred a higher reproductive cost than females in terms of both biomass and, particularly, nitrogen. Third, we grew plants under varying levels of nitrogen availability and found that although sexual size dimorphism was unaffected by nitrogen, females, but not males, decreased their relative allocation to both roots and reproduction under high nitrogen availability. We propose that males deal with the high cost of pollen production in terms of nitrogen by allocating biomass to nitrogen-harvesting roots, whereas females pay for carbon-rich seeds and fruits by investing in photosynthetic organs. Sexual dimorphism would thus seem to be the outcome of allocation to above- versus below-ground sinks that supply resources (carbon versus nitrogen) limiting the female and male reproduction differentially.     

Growth and survivorship as proximate causes of sexual size dimorphism in peninsula dragon lizards Ctenophorus fionni

Males and females differ in body size in many animals, but the direction and extent of this sexual size dimorphism (SSD) varies widely. Males are larger than females in most lizards of the iguanian clade, which includes dragon lizards (Agamidae). I tested whether the male larger pattern of SSD in the peninsula dragon lizard, Ctenophorus fionni, is a result of sexual selection for large male size or relatively higher mortality among females. Data on growth and survivorship were collected from wild lizards during 1991–1994. The likelihood of differential predation between males and females was assessed by exposing pairs of male and female lizards to a predator in captivity, and by comparing the frequency of tail damage in wild‐caught males and females. Male and female C. fionni grew at the same rate, but males grew for longer than females and reached a larger asymptotic size (87 mm vs. 78 mm). Large males were under‐represented in the population because they suffered higher mortality than females. Predation may account for some of this male‐biased mortality. The male‐biased SSD in C. fionni resulted from differences in growth pattern between the sexes. The male‐biased SSD was not the result of proximate factors reducing female body size. Indeed SSD in this species remained male‐biased despite high mortality among large males. SSD in C. fionni is consistent with the ultimate explanation of sexual selection for large body size in males.     

Is bigger better? The relationship between size and reproduction in female Asian elephants

The limited availability of resources is predicted to impose trade‐offs between growth, reproduction and self‐maintenance in animals. However, although some studies have shown that early reproduction suppresses growth, reproduction positively correlates with size in others. We use detailed records from a large population of semi‐captive elephants in Myanmar to assess the relationships between size (height and weight), reproduction and survival in female Asian elephants, a species characterized by slow, costly life history. Although female height gain during the growth period overlapped little with reproductive onset in the population, there was large variation in age at first reproduction and only 81% of final weight had been reached by peak age of reproduction at the population level (19 years). Those females beginning reproduction early tended to be taller and lighter later in life, although these trends were not significant. We found that taller females were more likely to have reproduced by a given age, but such effects diminished with age, suggesting there may be a size threshold to reproduction which is especially important in young females. Because size was not linked with female survival during reproductive ages, the diminishing effect of height on reproduction with age is unlikely to be due to biased survival of larger females. We conclude that although reproduction may not always impose significant costs on growth, height may be a limiting factor to reproduction in young female Asian elephants, which could have important implications considering their birth rates are low and peak reproduction is young – 19 years in this population.     

Similar gender dimorphism in the costs of reproduction across the geographic range of Fraxinus ornus

BACKGROUND AND AIMS: The reproductive costs for individuals with the female function have been hypothesized to be greater than for those with the male function because the allocation unit per female flower is very high due to the necessity to nurture the embryos until seed dispersal occurs, while the male reproductive allocation per flower is lower because it finishes once pollen is shed. Consequently, males may invest more resources in growth than females. This prediction was tested across a wide geographical range in a tree with a dimorphic breeding system (Fraxinus ornus) consisting of males and hermaphrodites functioning as females. The contrasting ecological conditions found across the geographical range allowed the evaluation of the hypothesis that the reproductive costs of sexual dimorphism varies with environmental stressors. METHODS: By using random-effects meta-analysis, the differences in the reproductive and vegetative investment of male and hermaphrodite trees of F. ornus were analysed in 10 populations from the northern (Slovakia), south-eastern (Greece) and south-western (Spain) limits of its European distribution. The variation in gender-dimorphism with environmental stress was analysed by running a meta-regression between these effect sizes and the two environmental stress indicators: one related to temperature (the frost-free period) and another related to water availability (moisture deficit). KEY RESULTS: Most of the effect sizes showed that males produced more flowers and grew more quickly than hermaphrodites. Gender differences in reproduction and growth were not minimized or maximized under adverse climatic conditions such as short frost-free periods or severe aridity. CONCLUSIONS: The lower costs of reproduction for F. ornus males allow them to grow more quickly than hermaphrodites, although such differences in sex-specific reproductive costs are not magnified under stressful conditions.     

Observational evidence of risk-sensitive reproductive allocation in a long-lived mammal

Organisms should adopt a risk-sensitive reproductive allocation when summer reproductive allocation competes with survival in the coming winter. This trade off is shown through autumn female body mass, which acts as an insurance against unpredictable winter environmental conditions. We tested this hypothesis on female reindeer in a population that has experienced a time period of dramatic increase in abundance. Environmental conditions during winter were fairly stable (with the exception of 1 year). We conclude that increased population abundance (perhaps in interaction with winter environmental conditions) could have represented a worsening of winter environmental conditions as both autumn offspring and spring female body mass decreased during the course of the study. Moreover, we found that the cost of reproduction was related to environmental conditions as: (1) autumn body mass was larger for barren than for lactating females, and this difference was temporally highly variable; (2) lactating females produced smaller offspring than barren ones in the following year; and (3) reproductive output (offspring size) decreased over time. We also found evidence of quality effects as lactating females had a higher reproductive success in the following year. In sum, a worsening of winter conditions lead to: (1) decreased reproductive output; (2) lowered autumn body mass for lactating females; and (3) increased body mass for barren females. Since females reduce their reproductive allocation as winter conditions becomes more severe, we conclude that reindeer have adopted a risk-sensitive reproductive allocation.     

Time Allocation during the Reproductive Season in Vicuñas

Vicuñas (Vicugna vicugna) are wild South American camelids that live in high-altitude grasslands (Puna). Their social organization is based on family and bachelor groups. The amount of time they allocated to walking, running, lying, grazing and alert behaviour was studied in Abrapampa, northwestern Argentina, during the 1988–1990 reproductive seasons. Females and yearlings spent significantly more time grazing, and less time walking, running and alert than territorial males and bachelors. This result suggests that females maximized the time spent foraging to fulfil the energetic demands of reproduction. The alertness of the territorial males increased and the grazing decreased with the number of females in the families, suggesting that males that incurred a higher cost of lost feeding time through vigilance activities increased their mating success.     

Proximate developmental mediators of sexual dimorphism in size: case studies from squamate reptiles

Sexual dimorphism in size (sexual size dimorphism; SSD) is nearlyubiquitous, but the relative importance of genetic versus environmentalcontrol of SSD is not known for most species. We investigatedproximate determinants of SSD in several species of squamatereptiles, including three species of Sceloporus lizards andthe diamond-backed rattlesnake (Crotalus atrox). In naturalpopulations of these species, SSD is caused by sexual differencesin age-specific growth. Males and females, however, may oftenshare similar potentials for growth: growth is strongly responsiveto the availability of food, and sexual differences in growthcan be greatly suppressed or completely absent under commonenvironmental conditions in the laboratory. Sexually divergentgrowth is expressed in natural environments because of inherentecological differences between males and females and becauseof potential epigenetic effects of sex-specific growth regulators.In field-active Sceloporus, sexual differences in growth rateare associated with sexual divergence in plasma testosterone.Experiments confirm that testosterone inhibits growth in speciesin which females are larger (for example, S. undulatus and S.virgatus) and stimulates growth in those in which males arelarger (for example, S. jarrovii). Interestingly, however, sexualdivergence in plasma testosterone is not accompanied by divergencein growth in S. jarrovii or in male-larger C. atrox in the laboratory.Furthermore, experimental effects of castration and testosteronereplacement on growth are not evident in captive S. jarrovii,possibly because growth effects of testosterone are supersededby an abundant, high-quality diet. In female-larger S. undulatus,growth may be traded-off against testosterone-induced reproductivecosts of activity. In male-larger species, costs of reproductionin terms of growth are suggested by supplemental feeding ofreproductive female C. atrox in their natural environment andby experimental manipulation of reproductive cost in femaleS. jarrovii. Growth costs of reproduction, however, do not contributesubstantially to the development of SSD in male-larger S. jarrovii.We conclude that the energetic costs of testosterone-induced,male reproductive behavior may contribute substantially to thedevelopment of SSD in some female-larger species. However, despitestrong evidence that reproductive investment exacts a substantialcost in growth, we do not support the reproductive cost hypothesisas a general explanation of SSD in male-larger species.     

Sociality and individual fitness in yellow-bellied marmots: insights from a long-term study (1962–2001)

Theoretical and empirical studies suggest that the age of first reproduction (the age at which reproduction begins) can have a substantial influence on population dynamics and individual fitness. Using complete survival and reproductive histories of 428 female yellow-bellied marmots (Marmota flaviventris) from a 40-year study (1962-2001), we investigated causes and fitness consequences of delayed maturity. Most females (86%) died without reproducing. The age of first reproduction of females that survived to reproduce at least once (n=60) ranged from 2 to 6 years. Females maturing later did not have a larger lifetime number of successful reproductive events or offspring production, nor did they experience improved survival. Females reproducing earlier had a higher fitness than those that delayed maturity. These results suggest that the net cost of early maturity was less than fitness benefits associated with early onset of reproduction, and that age of first reproduction in our study population is under substantial directional selection favoring early maturity. We conclude that female yellow-bellied marmots delay onset of reproduction not because of fitness benefits of foregoing reproduction at an earlier age, but due to the social suppression of reproduction by older, reproductive females, which enhances their own fitness to the detriment of the fitness of young females. Our results indicate that female yellow-bellied marmots that survive to reproduce may act to increase their own direct fitness, and that social suppression of reproduction of young females is a part of that strategy.     

宁波滑蜥两性异形和雌性繁殖

蜥蜴的雌性繁殖特征对理解两性异形的进化原因起着重要作用。于2011年4月在安徽滁州采集宁波滑蜥(Scincella modesta),定量研究该种形态特征的两性异形和雌性繁殖特征,检验成体形态特征两性异形与雌性繁殖的相关性。研究共采集43条(17♀♀,26♂♂)宁波滑蜥,雄性和雌性个体的最大体长分别为47.4 mm和46.6 mm。雌雄两性在体长和头宽上没有差异,而在腹长和头长上差异显著,雄性有较大的头长,雌性有较大的腹长。宁波滑蜥年产单窝卵。窝卵数和窝卵重与雌体体长及腹长呈正相关,卵重与雌体体长无相关性。窝卵数及卵重的变异系数分别为0.20和0.12。卵长径与窝卵数呈负相关,而卵短径与窝卵数无关。雌体主要通过增加窝卵数来增加繁殖输出。这些结果表明,宁波滑蜥是雌雄个体大小同形的两性异形模式,性选择使得雄性有着较大的头长,以具有较高的交配成功率,生育力选择使得雌性有着较大的腹长,以具有较大的生育力和繁殖输出。     

Absence of Mate Choice and Postcopulatory Benefits in a Species with Extreme Sexual Size Dimorphism

Most hypotheses related to the evolution of female‐biased extreme sexual size dimorphism (SSD) attribute the differences in the size of each sex to selection for reproduction, either through selection for increased female fecundity or selection for male increased mobility and faster development. Very few studies, however, have tested for direct fitness benefits associated with the latter – small male size. Mecaphesa celer is a crab spider with extreme SSD, whose males are less than half the size of females and often weigh 10 times less. Here, we test the hypotheses that larger size in females and smaller size in males are sexually selected through differential pre‐ and postcopulatory reproductive benefits. To do so, we tested the following predictions: matings between small males and large females are more likely to occur due to mate choice; females mated to small males are less likely to accept second copulation attempts; and matings between small males and large females will result in larger clutches of longer‐lived offspring. Following staged mating trials in the laboratory, we found no support for any of our predictions, suggesting that SSD in M. celer may not be driven by pre‐ or post‐reproductive fitness benefits to small males.     

Sexual abstinence and the cost of reproduction in adult male water snakes, Nerodia sipedon

Low frequency of reproduction among iteroparous organisms is most often observed among female ectothermic vertebrates and is thought to be a strategy used to defer reproductive costs. We assessed reproductive costs of male water snakes ( Nerodia sipedon ) to determine why half of adult males abstain from reproduction each year. There was no evidence of a short-term energetic cost of reproduction. Change in mass did not differ between reproductive and non-reproductive males during the one-month mating season or during the entire four-month activity season. Changes in mass of reproductive males were similar at two sites in which the spatial distribution of females differed. However, there were size-specific differences in growth and survival between reproductive and non-reproductive males. Among reproductive males growth rate decreased with body size at a lower rate than among non-reproductive males. Survival increased with body size for reproductive males, but decreased with body size among non-reproductive males. Most of the differential survival between reproductive and non-reproductive males did not occur during the mating season but rather during hibernation. Size-related differences between reproductive and non-reproductive males may reflect selection having eliminated low quality males from the larger size classes. Overall our results appear most consistent with there being high variance in male quality, such that the best males can bear the cost of reproducing and still grow and survive as well or better than low quality males that abstain from reproduction.     

Metabolic cost of first reproduction in young female European roe deerCapreolus capreolus

In female roe deerCapreolus capreolus (Linnaeus, 1758), like in several deer species, first reproduction occurs before they have reached their full body size. This study quantifies, in young females, the energetic costs induced by the contemporaneously occurring events: growth and first reproduction. Resting metabolic rate and body mass were measured in young primiparous females from first mating to 4 months after parturition, and compared to values measured in fully-grown adult multiparous females. Throughout the 10-month period from mating to fawning, body mass increased in yearlings. Prior to the blastocyst implantation (month −5 before parturition) young females were lighter than adult females whereas after parturition they had the same body mass. Our results suggest that body growth was reinitiated during pregnancy in primiparous females. From mating to fawning, except in the first part of pregnancy, mass-specific metabolism was higher in primiparous females than in multiparous individuals indicating the occurrence of an additional cost due to growth in young females. The depressed level observed at the beginning of gestation could allow the resumption of growth at lower cost. Thus, the allocation of resources to both reproduction and growth was not detrimental to first reproduction in young female roe deer under experimental conditions withad lib feeding.     

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.