Maternal effects on post-weaning physical and social development in juvenile mountain goats (Oreamnos americanus)

Little is known about maternal effects on post-weaning development, yet they may be important because maternal care could have long-term consequences only evident when offspring approach adulthood. We have assessed the effects of maternal age, current reproduction (presence of a kid of the year) and social rank on the body mass, horn length and social rank of 1- and 2-year-old mountain goats (Oreamnos americanus). Maternal reproductive status and social rank did not affect the mass or horn length of either yearlings or 2-year-olds. Maternal age was positively correlated with yearling body mass for males but not females. We could not detect any maternal age effects on body mass of 2-year-olds. Maternal age and spring forage quality were positively correlated with horn length of yearlings of both sexes, but not of 2-year-olds. Juvenile females showed compensatory growth in mass between 1 and 2 years of age, but males did not. Neither sex showed compensatory growth in horn length. None of the maternal characteristics we examined directly affected the social rank of juveniles, which increased with body mass. Social rank in female mountain goats seems to be established early in life and maintained to adulthood. By affecting yearling development, maternal age could affect the reproductive success of males.     

Why chimpanzee (Pan troglodytes schweinfurthii) mothers are less gregarious than nonmothers and males: the infant safety hypothesis

Socialization of young is an important component of maternal care in social mammals. It is therefore perplexing that female chimpanzees with dependent offspring spend more time alone than females without dependent offspring, and than males. We propose that chimpanzee mothers are less gregarious than nonmothers and males to reduce the risk of injury that aggressive males pose to their offspring. We predict that mothers will associate less with males, associate with fewer males, and reduce mother−infant proximity in the presence of males, and that these effects will reduce with increasing offspring age. We test whether the pattern of gregariousness and mother−offspring proximity support these predictions in the Kanyawara chimpanzee community in Uganda. The probability that a female was found in the presence of males was lower for mothers than nonmothers and increased with offspring age. The probability that a female was found with other females did not differ between mothers and nonmothers. Mother-to-offspring distance was higher when a mother was in an all-female group than in a mixed-sex group and increased with her offspring's age. Mother-to-offspring distance was greatest when there were relatively low numbers of males and relatively high numbers of females in a subgroup. We propose that mothers avoid grouping with males because of the vulnerability of their young, and that the presence of males in a subgroup increases a female's protectiveness of her young. We discuss the implications of our findings and the relevance of fission−fusion group formation to chimpanzee mothers.     

Sex bias or equal opportunity? Patterns of maternal investment in bison

Summary In polygynous mammals, it may be adaptive for mothers to invest more in sons and/or to adjust the sex ratio of offspring in relation to body condition. Calving patterns were examined over an 8-year period (1982–1989) for a population of Bison bison in which barren females are not selectively culled. From these data, we tested predictions of the sex ratio adjustment hypothesis as well as two assumptions: (1) that offspring weight at the end of the period of parental investment (PI) is correlated with later condition, and (2) that maternal and offspring condition during the period of PI are correlated. In contrast to predictions, there was little evidence that mothers in better condition bear more sons. Short- and long-term measures of maternal condition (previous reproductive status, age, dominance status, pre-pubertal body weight, age at first reproduction, birth date, and the duration of the mother's own suckling period) were little related to offspring sex ratio, although the last calves of old females were nearly always female. Similarly, there was little evidence for sex-biased PI. Weights at about 7 months of age were greater for males than females; males also had somewhat later birth dates, suggesting either longer gestation or later conception. However, maternal reproductive costs, as measured by subsequent fecundity, weight loss, and interbirth intervals, did not vary with calf sex. Both assumptions of the model received some support. However, while maternal condition was correlated with offspring condition, there may be sex differences in investment patterns. Mothers appear better able to influence the condition of daughters than of sons. This sex difference may negate any benefit from male-biased investment.     

Do female roe deer in good condition produce more sons than daughters

In polygynous roe deer Capreolus capreolus, males are only slightly heavier than females and the overall sex ratio at birth is close to unity. We studied offspring sex ratio and litter size (range 1–4, n = 74) of culled females, in utero, which provided an opportunity to examine responses of sex ratio to maternal condition. Male embryos were heavier than their sisters, and male fawns (9 months old) heavier than female fawns, suggesting a higher growth rate in males. There was no evidence for differential mortality between the sexes from birth to 9 months old. Heavier adult females produced larger embryos than lighter, or primiparous females. The overall sex ratio of embryos did not differ from unity, but adult does had more male embryos (55%) than primiparous does (32%), and the proportion of male embryos in a litter increased with the mother's body mass. Litter size also tended to increase with maternal age and body mass. We argue that this pattern reflects adaptive variation in offspring sex ratio.     

Factors influencing sexual cannibalism and its benefit to fecundity and offspring survival in the wolf spider Pardosa pseudoannulata (Araneae: Lycosidae)

Sexual cannibalism is hypothesized to have evolved as a way to obtain a high-quality meal, as an extreme mate choice or as a consequence of female aggressive spillover. Here, we examined underlying factors likely to influence sexual cannibalism in the wolf spider Pardosa pseudoannulata (Bösenberg & Strand, 1906) from China, including mating status, female egg-laid status, female hunger level, female adult age and mate size dimorphism. The results showed that about 10 % of P. pseudoannulata virgin females cannibalized the approaching males before mating and that 28 % of P. pseudoannulata virgin females immediately cannibalized the males after mating. No incidents of sexual cannibalism during copulation were observed. Before mating, previously mated females and starved females tended to engage in significantly higher rates of attacks compared to virgin and well-fed females. Females that had laid egg sacs tended to engage in a significantly higher rate of attacks and sexual cannibalism than virgin females before mating. Regardless of pre- or post-mating, there was a strong positive relationship between mate size dimorphism and the occurrence of sexual cannibalism. We also tested the effects of sexual cannibalism on the fecundity of cannibalistic females and the survival of their offspring. Our results indicated that sexual cannibalism affected positively the offspring survival of cannibalistic females, but not fecundity. Our findings support the hypothesis that sexual cannibalism has evolved as an adaptive component of female foraging strategy and that it benefits offspring survival as a result of paternal investment.     

Fallow deer polyandry is related to fertilization insurance

Polyandry is widespread, but its adaptive significance is not fully understood. The hypotheses used to explain its persistence have rarely been tested in the wild and particularly for large, long-lived mammals. We investigated polyandry in fallow deer, using female mating and reproduction data gathered over 10 years. Females of this species produce a single offspring (monotocous) and can live to 23 years old. Overall, polyandry was evident in 12 % of females and the long-term, consistent proportion of polyandrous females observed, suggests that monandry and polyandry represent alternative mating strategies. Females were more likely to be polyandrous when their first mate had previously achieved high numbers of matings during the rut or was relatively old. However, polyandry was not related to the following factors: female age, the stage of the rut, the dominance ranks of mates, or the number of daily matings achieved by males. Polyandrous and monandrous multiple-mating females were not more likely than single-mating females to be observed with an offspring during the following year, and there were no significant differences in offspring size between these females. These results provide support for a fertility insurance hypothesis, with females remating if fertilization from the first mating was uncertain due to possible sperm depletion. The potential for different female mating strategies among large, polygynous mammals has generally been overlooked. Our findings highlight the complexity of female reproductive strategies and the possible trade-offs between fertilization success, preferences for high-quality males, and potential costs of polyandry, particularly for monotocous species.     

Multi-year lactation and its consequences in Bornean orangutans (Pongo pygmaeus wurmbii)

In most mammals, females pay for reproduction by dramatically increasing net energy intake from conception to mid- or late lactation. To do this, they time their reproductive events in relation to environmental cycles so that periods of peak food availability coincide with peak demand or are used to build energy stores. This timing is not possible in species with slow development in which lactation is prolonged over a multi-year period with fluctuating food availability. Here, mothers are expected to sustain a stable but generally lower level of nutrient transfer. In a sample of over 1,050 complete follow days of eight mother–infant pairs collected over 7 years, we document maternal effort for wild Bornean orangutans (Pongo pygmaeus wurmbii) over their average 6.5-year lactation period. As predicted, maternal feeding time was independent of the age of her growing offspring, indicating a stable sustained “plateau” effort of ≤ 25 % above baseline level, instead of a short peak lactation as seen in seasonal breeders. Infant orangutans started to regularly supplement milk with self-harvested food when they were 1–1.5 years old, indicating milk intake was insufficient from this age onwards, even though maternal effort did not decrease. We expect the same regulation of sustained maternal effort in other large and large-brained mammals with slow infant development. We also predict that mother–infant conflict over suckling may show another peak at the onset of the milk?+?solid food phase, in addition to the well-known conflict around the endpoint of lactation (weaning), which is reached after a long and gradual increase in solid food intake by the infant.     

Interactive effects of male and female age on extra-pair paternity in a socially monogamous seabird

Females sometimes obtain older sires for their offspring through extra-pair interactions, but how female age influences paternity is largely unexplored and interactive effects across the age span of both sexes have not been analyzed. To test whether female choice of sire age varies with female age in the blue-footed booby (Sula nebouxii), we examined associations between ages of both partners and the probability of extra-pair paternity (EPP) in 350 broods of parents up to 22 years old in a single breeding season. Extra-pair paternity enables a female to select an alternative sire for her offspring and could function to avoid or achieve particular combinations of parental ages. A male age?×?female age interaction revealed that in young females (≤4 years), EPP decreased with increasing age of the social partner, whereas in old females (≥8 years), it increased. Moreover, sires of extra-pair (EP) chicks of young females paired to young males were on average 6.33 years older than the females’ social partners. Since female boobies control copulatory access, this pattern could imply that young females choose old sires for their proven genetic quality and that old females avoid very old males because matings with them may risk infertility or genetic defects in offspring. Taking female age into account and observing across the whole age span may be necessary for understanding female age-based mate choice.     

Communal nesting, kinship, and maternal success in a social primate

Communal nesting, where several mothers regularly pool and cooperatively rear offspring, is unusual in mammals. This type of crèching behavior is especially rare among primates, with the notable exceptions of humans, some nocturnal strepsirrhines, and—as we show in this study—black-and-white ruffed lemurs (Varecia variegata). Here, we combine data on nesting behavior, genetic relatedness, and infant survivorship to describe variation in ruffed lemur infant care and to examine the potential benefits of ruffed lemur communal breeding. Reproductive events were rare, and females produced litters (synchronously) only once in 6 years of observation. We show that not all mothers participate in communal crèches, but those that did had greater maternal success; communal breeders spent more time feeding and their offspring were more likely to survive. Although cooperating mothers were often related, females also cooperated with non-kin, and those who shared infant care responsibilities had greater maternal success than mothers who did not participate. If there is indeed a causal link between maternal cooperation and reproductive success, this unusual behavior, like that of human communal rearing, may have evolved via some combination of kin selection and mutualism.     

Influence of alternate reproductive tactics and pre- and postcopulatory sexual selection on paternity and offspring performance in a lizard

Sexual selection theory predicts different optima for multiple mating in males and females. We used mating experiments and genetic paternity testing to disentangle pre- and postcopulatory mechanisms of sexual selection and alternate reproductive tactics in the highly promiscuous lizard Eulamprus heatwolei. Both sexes mated multiply: 30–60 % of clutches were sired by two to four fathers, depending on the experiment. Larger males sired more offspring when we allowed male contest competition: 52 % of large males but only 14 % of small males sired at least one offspring. In the absence of male contest competition, females mated promiscuously and there was no large male advantage: 80 % of large males and 90 % of small males sired at least one offspring, and there was no evidence for last-male precedence. Multiple mating did not yield obvious direct or indirect benefits to females. E. heatwolei represents a complex system in which males attempt to improve their fertility success by limiting rivals from access to females and through adopting alternate reproductive tactics. Conversely, females exhibit no obvious precopulatory mate choice but may influence fitness through postcopulatory means by either promoting sperm competition or through cryptic female choice. Our results support the hypothesis that female multiple mating in nonavian reptiles is best explained by the combined effect of mate encounter frequency and high benefits to males but low costs to females.     

Parental investment and the secondary sex ratio in northern elephant seals

Summary Data on northern elephant seals, Mirounga angustirostris, bearing on sex ratio theory were collected at Año Nuevo, California, and other Californian and Mexican Islands, during the period 1967 to 1988. The mass of males exceeded that of females by 7–8% at birth and at weaning. The sex ratio was biased to males at birth (51.2%) and was near unity at weaning (49.6% males). The sex ratio did not vary as a function of maternal age or maternal mass except in 6-year-old females, who produced significantly more males. Although sons cost more to rear in energetic terms than daughters, and mothers were more successful weaning the latter, the sex of the pup reared exerted no significant effect on the mother's reproductive performance the following year or on her subsequent survival. These data suggest that parents invest equally in sons and daughters when investment is measured in terms of future reproduction (Fisher 1930) and provide no support for the theory of adaptive shifts in sex ratio (Trivers and Willard 1973). The small sex difference in mass due to maternal effort reflects the fact that females fast during lactation and all energy transferred is from limited body stores. Because of these circumstances, selection for superior condition at the end of the period of parental investment may act more strongly on pups, who have the opportunity to steal milk, than on their mothers.     

Egg size and laying order in relation to offspring sex in the extreme sexually size dimorphic brown songlark,<Emphasis Type="Italic"> Cinclorhamphus cruralis</Emphasis>

In some bird species, mothers can advantage the offspring of one sex either by elevating them in the laying order to promote earlier hatching or by allocating greater resources to eggs of the preferred sex. In size dimorphic species, the predictions as to which sex should benefit most from such pre-laying adjustments are ambiguous. The smaller sex would benefit from an initial size advantage to help compensate for the faster growth rate of the larger sex. However, an early advantage to offspring of the larger sex might have a greater effect on their lifetime reproductive success than an equivalent advantage to offspring of the smaller sex. We investigated these hypotheses in the polygynous brown songlark, Cinclorhamphus cruralis, which is one of the most sexually size dimorphic birds known. We conducted within-clutch comparisons and found that females hatched from larger eggs and were initially heavier (but not structurally larger) than their brothers. This may afford females an early competitive advantage, as egg volume remained correlated with chick mass until at least 5 days of age. Similarly, we found that hatch order was still positively associated with nestling mass and size when the brood was 10 days of age, but there was no clear relationship between offspring sex and hatching order. During this study, food was plentiful and there were few obvious cases of nestling starvation. When food is limited, we suggest that the greater nutrient reserves of female hatchlings could not only help compensate for their slower growth, but could also give them a survival advantage over their brothers early in the nestling period. Consequently, egg size dimorphism may be an adaptation that facilitates an early shift in brood sex-ratio towards cheaper daughters in conditions of low food availability.     

Fertile diploid males in the ant Cataglyphis cursor: a potential cost of thelytoky?

Under the hymenopteran single-locus complementary sex-determination system, production of diploid males results from homozygosity at the sex-determiner locus. This arises when both parents transmit identical alleles at the locus to the offspring. In species reproducing asexually through thelytokous parthenogenesis, production of diploid males may also occur when the sex locus undergoes recombination and becomes homozygous in the offspring. Diploid males represent a substantial genetic load in hymenopteran populations because they often produce unviable sperm or sire sterile triploid female offspring. In the Mediterranean ant Cataglyphis cursor, the queen and workers can produce female offspring through automictic thelytokous parthenogenesis with central fusion, a mode of parthenogenesis that increases homozygosity. We report, for the first time, the presence of about 39 % of colonies producing adult diploid males (seven colonies out of 18). Overall, 8 % of adult males were diploid (12 diploid males out of the 146 males genotyped). Genotyping workers from the seven colonies producing diploid males showed that three diploid males were sons of queens and produced by thelytoky, six were probably sons of workers also produced by thelytoky and three were non-natal. Furthermore, the mating of a diploid male with two virgin queens in the laboratory led to the production of sterile triploid workers, which shows that diploid males in C. cursor are fertile, mate successfully and produce viable and functional but probably sterile female offspring. Because diploid males originate from thelytokous reproduction, they are only produced during sexual production and hence do not impair colony growth, which could explain why they are not removed at early brood stages.     

Male sexual attractiveness affects the investment of maternal resources into the eggs in peafowl (<Emphasis Type="Italic">Pavo cristatus</Emphasis>)

According to the differential investment hypothesis, females paired with attractive mates are expected to invest more in the current reproduction relative to females paired with unattractive males. We experimentally tested this hypothesis in the peafowl (Pavo cristatus) by providing females with males that differed in sexual attractiveness. In agreement with the differential allocation hypothesis, females paired with more ornamented males laid larger eggs, and deposited higher amounts of testosterone into the egg yolk, independently of the sex of the embryo. These results show that the association between paternal phenotype and offspring quality could arise via a differential maternal investment. They also suggest that, if ornamented males do transmit good genes to the progeny, the maternal differential investment can amplify the effect of such good genes on the offspring fitness.     

Preparation and activation: determinants of age at reproductive maturity in male baboons

Age at maturity is a particularly important life history trait, but maturational data are rare for males in natural populations of mammals. Here we provide information on three maturational milestones and their social and demographic correlates among 43 wild male baboons, Papio cynocephalus, in a natural population in Amboseli National Park, Kenya. We examined (1) age at testicular enlargement, which signals puberty and the onset of subadulthood, (2) age at attainment of adult dominance rank, which we consider to be the beginning of adulthood, and (3) age at first sexual consortship, which is the best measure available for age at first reproduction in male baboons. Testicular enlargement (median age = 5.69 years) occurred earlier among sons of high ranking mothers, and was not influenced by rainfall or seasonality. Attainment of adult dominance rank (median age = 7.41 years) was also accelerated among sons of high-ranking mothers, and among males whose mothers had died while the males were juveniles. First sexual consortship (median age = 7.92 years) was not influenced directly by maternal characteristics, but attainment of adult dominance rank always preceded first consortship. The lag time between attainment of adult rank and first consortship (median = 2.5 months; range = 5–526 days), was predicted by the number of sexually cycling females in the group when the male attained rank, and by how high ranking the male became in his first months as an adult. We suggest that the age at which a male baboon is ready to begin reproducing is influenced by a relatively stable maternal characteristic that exerts its influence early in development, but the timing with which this potential is realized depends on activation by more proximate, often stochastic triggers such as female availability. This two-level organization of influences is likely to contribute to the variance both in age at first reproduction and in lifetime fitness. Differences in the relative magnitude of the two levels will lead to both intra- and interspecific variability in the opportunity for maternal selection and sexual selection.     

Long-term and trans-life-cycle effects of exposure to ocean acidification in the green sea urchin Strongylocentrotus droebachiensis

Anthropogenic CO₂ emissions are acidifying the world’s oceans. A growing body of evidence demonstrates that ocean acidification can impact survival, growth, development and physiology of marine invertebrates. Here, we tested the impact of long-term (up to 16 months) and trans-life-cycle (adult, embryo/larvae and juvenile) exposure to elevated pCO₂ (1,200 μatm, compared to control 400 μatm) on the green sea urchin Strongylocentrotus droebachiensis. Female fecundity was decreased 4.5-fold when acclimated to elevated pCO₂ for 4 months during reproductive conditioning, while no difference was observed in females acclimated for 16 months. Moreover, adult pre-exposure for 4 months to elevated pCO₂ had a direct negative impact on subsequent larval settlement success. Five to nine times fewer offspring reached the juvenile stage in cultures using gametes collected from adults previously acclimated to high pCO₂ for 4 months. However, no difference in larval survival was observed when adults were pre-exposed for 16 months to elevated pCO₂. pCO₂ had no direct negative impact on juvenile survival except when both larvae and juveniles were raised in elevated pCO₂. These negative effects on settlement success and juvenile survival can be attributed to carry-over effects from adults to larvae and from larvae to juveniles. Our results support the contention that adult sea urchins can acclimate to moderately elevated pCO₂ in a matter of a few months and that carry-over effects can exacerbate the negative impact of ocean acidification on larvae and juveniles.     

The relationship between maternal phenotype and offspring quality: do older mothers really produce the best offspring?

Maternal effects are increasingly recognized as important drivers of population dynamics and determinants of evolutionary trajectories. Recently, there has been a proliferation of studies finding or citing a positive relationship between maternal size/age and offspring size or offspring quality. The relationship between maternal phenotype and offspring size is intriguing in that it is unclear why young mothers should produce offspring of inferior quality or fitness. Here we evaluate the underlying evolutionary pressures that may lead to a maternal size/age-offspring size correlation and consider the likelihood that such a correlation results in a positive relationship between the age or size of mothers and the fitness of their offspring. We find that, while there are a number of reasons why selection may favor the production of larger offspring by larger mothers, this change in size is more likely due to associated changes in the maternal phenotype that affect the offspring size-performance relationship. We did not find evidence that the offspring of older females should have intrinsically higher fitness. When we explored this issue theoretically, the only instance in which smaller mothers produce suboptimal offspring sizes is when a (largely unsupported) constraint on maximum offspring size is introduced into the model. It is clear that larger offspring fare better than smaller offspring when reared in the same environment, but this misses a critical point: different environments elicit selection for different optimal sizes of young. We suggest that caution should be exercised when interpreting the outcome of offspring-size experiments when offspring from different mothers are reared in a common environment, because this approach may remove the source of selection (e.g., reproducing in different context) that induced a shift in offspring size in the first place. It has been suggested that fish stocks should be managed to preserve these older age classes because larger mothers produce offspring with a greater chance of survival and subsequent recruitment. Overall, we suggest that, while there are clear and compelling reasons for preserving older females in exploited populations, there is little theoretical justification or evidence that older mothers produce offspring with higher per capita fitness than do younger mothers.     

Complete reproductive skew within white-browed sparrow weaver groups despite outbreeding opportunities for subordinates of both sexes

The distribution of reproductive success within societies is a key determinant of the outcomes of social evolution. Attempts to explain social diversity, therefore, require that we quantify reproductive skews and identify the mechanisms that generate them. Here, we address this priority using life history and genotypic data from >600 individuals in 40 wild groups of the cooperatively breeding white-browed sparrow weaver, Plocepasser mahali. We show that groups comprise up to six males and seven females, but within-group reproduction is completely monopolised by a single dominant male and female, while extra-group males sire 12–18 % of offspring. Strong within-group kin structure could frequently explain these monopolies, as subordinates had typically delayed dispersal from their natal groups and so frequently (1) lacked within-group outbreeding partners, and/or (2) stood to gain little from contesting dominant reproduction, being almost as related to the dominant’s young as they would have been to their own. Kin structure alone cannot account entirely for these monopolies, however, as they remained complete following the immigration of unrelated males and females. That subordinate females remain reproductively quiescent despite also showing comparable body condition to dominants, overlapping them substantially in age, and showing no evidence of elevated stress hormone levels raises the possibility that they exercise reproductive restraint due instead to a threat of action by dominants and/or deficits in offspring fitness that might arise if subordinates bred. Our findings highlight the complexity of the mechanisms that generate reproductive disparities in animal societies and the challenge of identifying them when skews are complete.     

Androgen-dependent maternal effects on offspring fitness in zebra finches

There is accumulating evidence that maternal hormones may play a role in offspring sex adjustment, but little is known about the costs of such hormone-mediated mechanisms. Recent studies have reported sex-specific effects of hormones on offspring viability. Specifically, we previously found that elevating the plasma androgen level in mothers results in a male-biased offspring primary sex ratio, but it affects the viability of sons negatively and daughters positively in zebra finches (Taeniopygia guttata; Rutkowska and Cichoń, Anim Behav, 71:1283–1288, 2006). In this study, we studied further fitness consequences of exposure to elevated yolk androgen levels in zebra finches. We measured growth rate and cellular immune response of nestlings that hatched from eggs laid by females injected with testosterone during egg laying and nestlings of unaffected control females. We found that sons of testosterone-treated females grew slower in comparison to sons of control females. The significant interaction between experimental group and offspring sex indicates that sons of testosterone-treated mothers suffered impaired immune responsiveness while daughters seemed to benefit from elevated androgen level in terms of enhanced immune responsiveness. We found no effects of androgens on offspring performance at adulthood—neither fecundity of females nor attractiveness of males was affected. We conclude that the benefits of biasing sex ratio towards males by increasing androgen level in the yolk may be limited due to negative effects on male offspring performance early in life.     

Factors influencing length of maternal care in brown bears (<Emphasis Type="Italic">Ursus arctos</Emphasis>) and its effect on offspring

Length of maternal care, i.e. the interval between successfully raised litters, is the most important factor explaining the variation in reproductive rate among brown-bear (Ursus arctos) populations. In this paper, we examine the variation in length of maternal care in radio-marked brown bears and its effect on their offspring in northern Sweden. Young stayed with their mothers for 1.4–1.5 or 2.4–2.5 (in one case 3.5) years and were weaned with body masses varying from 17 to 69 kg. The probability of yearling litters staying with their mother for a 2nd year increased with decreasing yearling body mass, and was higher for litters with two offspring than for litters with one or three to four offspring. Staying with their mothers for a 2nd year had a positive effect on mass gain in yearlings and this effect was more pronounced in litters with two than three to four offspring. Body mass of 2-year-olds was not related to age of weaning, suggesting that keeping offspring for an additional year mainly compensated for low yearling body mass. If large offspring body mass positively affects later offspring survival and reproduction, mothers may be able to optimize the length of maternal care according to the litter size and the size of their yearlings.Communicated by F. Trillmich