The Weaker Sex? The Propensity for Male-Biased Piglet Mortality

For the most part solutions to farm animal welfare issues, such as piglet mortality, are likely to lie within the scientific disciplines of environmental design and genetic selection, however understanding the ecological basis of some of the complex dynamics observed between parent and offspring could make a valuable contribution. One interesting, and often discussed, aspect of mortality is the propensity for it to be sex-biased. This study investigated whether known physiological and behavioural indicators of piglet survival differed between the sexes and whether life history strategies (often reported in wild or feral populations) relating to parental investment were being displayed in a domestic population of pigs. Sex ratio (proportion of males (males/males+females)) at birth was 0.54 and sex allocation (maternal investment measured as piglet birth weight/litter weight) was statistically significantly male-biased at 0.55 (t₃₅ = 2.51 P = 0.017), suggesting that sows invested more in sons than daughters during gestation. Despite this investment in birth weight, a known survival indicator, total pre-weaning male mortality was statistically significantly higher than female mortality (12% vs. 7% respectively z = 2.06 P = 0.040). Males tended to suffer from crushing by the sow more than females and statistically significantly more males died from disease-related causes. Although males were born on average heavier, with higher body mass index and ponderal index, these differences were not sustained. In addition male piglets showed impaired thermoregulation compared to females. These results suggest male-biased mortality exists despite greater initial maternal investment, and therefore reflects the greater susceptibility of this sex to causal mortality factors. Life history strategies are being displayed by a domestic population of pigs with sows in this study displaying a form of parental optimism by allocating greater resources at birth to males and providing an over-supply of this more vulnerable sex in expectation of sex-biased mortality.     

The origin of parental care in relation to male and female life history

The evolution of maternal, paternal, and bi‐parental care has been the focus of a great deal of research. Males and females vary in basic life‐history characteristics (e.g., stage‐specific mortality, maturation) in ways that are unrelated to parental investment. Surprisingly, few studies have examined the effect of this variation in male and female life history on the evolution of care. Here, we use a theoretical approach to determine the sex‐specific life‐history characteristics that give rise to the origin of paternal, maternal, or bi‐parental care from an ancestral state of no care. Females initially invest more into each egg than males. Despite this inherent difference between the sexes, paternal, maternal, and bi‐parental care are equally likely when males and females are otherwise similar. Thus, sex differences in initial zygotic investment do not explain the origin of one pattern of care over another. However, sex differences in adult mortality, egg maturation rate, and juvenile survival affect the pattern of care that will be most likely to evolve. Maternal care is more likely if female adult mortality is high, whereas paternal care is more likely if male adult mortality is high. These findings suggest that basic life‐history differences between the sexes can alone explain the origin of maternal, paternal, and bi‐parental care. As a result, the influence of life‐history characteristics should be considered as a baseline scenario in studies examining the origin of care.     

Sex allocation and sex‐specific parental investment in an endangered seabird

Biased offspring sex ratio is relatively rare in birds and sex allocation can vary with environmental conditions, with the larger and more costly sex, which can be either the male or female depending on species, favoured during high food availability. Sex‐specific parental investment may lead to biased mortality and, coupled with unequal production of one sex, may result in biased adult sex ratio, with potential grave consequences on population stability. The African Penguin Spheniscus demersus, endemic to southern Africa, is an endangered monogamous seabird with bi‐parental care. Female adult African Penguins are smaller, have a higher foraging effort when breeding and higher mortality compared with adult males. In 2015, a year in which environmental conditions were favourable for breeding, African Penguin chick production on Bird Island, Algoa Bay, South Africa, was skewed towards males (1.5 males to 1 female). Males also had higher growth rates and fledging mass than females, with potentially higher post‐fledging survival. Female, but not male, parents had higher foraging effort and lower body condition with increasing number of male chicks in their brood, thereby revealing flexibility in their parental strategy, but also the costs of their investment in their current brood. The combination of male‐biased chick production and higher female mortality, possibly at the juvenile stage as a result of lower parental investment in female chicks, and/or at the adult stage as a result of higher parental investment, may contribute to a biased adult sex ratio (ASR) in this species. While further research during years of contrasting food availability is needed to confirm this trend, populations with male‐skewed ASRs have higher extinction risks and conservation strategies aiming to benefit female African Penguin might need to be developed.     

Sex ratio at birth and mortality rates are negatively related in humans

Evolutionary theory posits that resource availability and parental investment ability could signal offspring sex selection, in order to maximize reproductive returns. Non-human studies have provided evidence for this phenomenon, and maternal condition around the time of conception has been identified as most important factor that influence offspring sex selection. However, studies on humans have reported inconsistent results, mostly due to use of disparate measures as indicators of maternal condition. In the present study, the cross-cultural differences in human natal sex ratio were analyzed with respect to indirect measures of condition namely, life expectancy and mortality rate. Multiple regression modeling suggested that mortality rates have distinct predictive power independent of cross-cultural differences in fertility, wealth and latitude that were earlier shown to predict sex ratio at birth. These findings suggest that sex ratio variation in humans may relate to differences in parental and environmental conditions.     

Sex differences in life history drive evolutionary transitions among maternal,paternal, and bi‐parental care

Evolutionary transitions among maternal, paternal, and bi‐parental care have been common in many animal groups. We use a mathematical model to examine the effect of male and female life‐history characteristics (stage‐specific maturation and mortality) on evolutionary transitions among maternal, paternal, and bi‐parental care. When males and females are relatively similar – that is, when females initially invest relatively little into eggs and both sexes have similar mortality and maturation – transitions among different patterns of care are unlikely to be strongly favored. As males and females become more different, transitions are more likely. If females initially invest heavily into eggs and this reduces their expected future reproductive success, transitions to increased maternal care (paternal → maternal, paternal → bi‐parental, bi‐parental → maternal) are favored. This effect of anisogamy (i.e., the fact that females initially invest more into each individual zygote than males) might help explain the predominance of maternal care in nature and differs from previous work that found no effect of anisogamy on the origin of different sex‐specific patterns of care from an ancestral state of no care. When male mortality is high or male egg maturation rate is low, males have reduced future reproductive potential and transitions to increased paternal care (maternal → paternal, bi‐parental → paternal, maternal → bi‐parental) are favored. Offspring need (i.e., low offspring survival in the absence of care) also plays a role in transitions to paternal care. In general, basic life‐history differences between the sexes can drive evolutionary transitions among different sex‐specific patterns of care. The finding that simple life‐history differences can alone lead to transitions among maternal and paternal care suggests that the effect of inter‐sexual life‐history differences should be considered as a baseline scenario when attempting to understand how other factors (mate availability, sex differences in the costs of competing for mates) influence the evolution of parental care.     

Sexual dimorphism,survival, and parental investment in relation to offspring sex in a precocial bird

Differential growth rate between males and females, owing to a sexual size dimorphism, has been proposed as a mechanism driving sex‐biased survival. How parents respond to this selection pressure through sex ratio manipulation and sex‐biased parental investment can have a dramatic influence on fitness. We determined how differential growth rates during early life resulting from sexual size dimorphism affected survival of young and how parents may respond in a precocial bird, the black brant Branta bernicla nigricans. We hypothesized that more rapidly growing male goslings would suffer greater mortality than females during brood rearing and that parents would respond to this by manipulating their primary sex ratio and parental investment. Male brant goslings suffered a 19.5% reduction in survival relative to female goslings and, based on simulation, we determined that a female biased population sex ratio at fledging was never overcome even though previous work demonstrated a slight male‐biased post‐fledging survival rate. Contrary to the Fisherian sex ratio adjustment hypothesis we found that individual adult female brant did not manipulate their primary sex ratio (50.39% male, n = 645), in response to the sex‐biased population level sex ratio. However, female condition at the start of the parental care period was a good predictor of their primary sex ratio. Finally, we examined how females changed their behavior in response to primary sex ratio of their broods. We hypothesized that parents would take male biased broods to areas with increased growth rates. Parents with male biased primary sex ratios took broods to areas with higher growth rates. These factors together suggest that sex‐biased growth rates during early life can dramatically affect population dynamics through sex‐biased survival and recruitment which in turn affects decisions parents make about sex allocation and sex‐biased parental investment in offspring to maximize fitness.     

Sexual Selection and Breeding Patterns: Insights from Salmonids (Salmonidae)

Although "intrasexual selection" has been accepted as the mechanism by which males evolve elaborate secondary sexual traits which are used in aggressive contests, the importance of "intersexual selection" as a mechanism by which males have acquired exaggerated traits to display to females during courtship was less readily accepted. In spite of this scepticism, several genetic models have supported the latter idea, and many empirical studies showed that females were generally more discriminating in mate choice than males, because of differences in relative investment between sexes. Nowadays, this idea is reinforced by various concepts (parental investment, potential reproductive rate, environmental potential for polygamy...) which stress that the strength of sexual selection is related to many interdependent factors, such as mating systems, resource distribution (food, habitat, mate), life history and other ecological characteristics. The case of Salmonids is presented here to show how novel information on sexual selection has contributed to the understanding of the plasticity of breeding patterns in the context of evolutionary biology.     

Red howler monkey birth data II: Interannual,habitat, and sex comparisons

Data presented in this paper are derived from the births and subsequent histories of red howler infants born in two habitats. Overall the sex ratio of infants at birth was about 1:1. Infant survivorship (at 1 yr) was about 80%, and 44% of infant mortality was attributed to infanticide by males. Survivorship curves indicated a dramatic sex difference, with far fewer females than males known to be alive at age 7 yr. However, this sex difference may be inflated because emigrant males are more easily identified than emigrant females, and females may be dispersing beyond the boundaries of the study area at a higher rate. Annual birthrate varied somewhat from year to year and was positively related to rainfall. Annual birthrate tended to be higher in the habitat with lower density and higher growth rate. Consistent with the trends, in annual birthrate, variation in interbirth interval length (TBR after births of surviving infants was related primarily to habitat differences and annual variation in rainfall. Season of birth and maternal age class had no effect on IBI. Infant sex had mostly nonsignificant effects on IBI. A small sample indicated that IBI's were significantly longer after the births of females who eventually became natal breeders than after the births of females who eventually emigrated. This difference might reflect differential parental (maternal) investment of some sort.     

Maternal Condition Does Not Influence Birth Sex Ratios in Anubis Baboons (Papio anubis)

Trivers and Willard predicted that when parental condition has differential effects on the fitness of male and female offspring, parents who are in good condition will bias investment toward the sex that benefits most from additional investment. Efforts to test predictions derived from Trivers and Willard''s model have had mixed results, perhaps because most studies have relied on proxy measures of parental condition, such as dominance rank. Here, we examine the effects of female baboons condition on birth sex ratios and post-natal investment, based on visual assessments of maternal body condition. We find that local environmental conditions have significant effects on female condition, but maternal condition at conception has no consistent relationship with birth sex ratios. Mothers who are in poorer condition at the time of conception resume cycling significantly later than females who are in better condition, but the sex of their infants has no effect on the time to resumption of cycling. Thus, our findings provide strong evidence that maternal condition influences females'' ability to reproduce, but females do not facultatively adjust the sex ratio of their offspring in relation to their dominance rank or current condition.     

Sex-biased environmental sensitivity: natural and experimental evidence from a bird species with larger females

The larger sex is often more vulnerable, in terms of developmentand survival, to poor conditions during early life. Differentialvulnerability has implications for parental investment strategiessuch as sex ratio theory. When males are larger, it is not possibleto separate the effects of larger size per se and other aspectsof the male phenotype on vulnerability. Furthermore, offspringcompetition might favor the larger sex and thereby mask intrinsic,size-related effects. We studied sex-specific mortality in abird species with reversed size dimorphism, the great skua Stercorariusskua, under natural and experimentally created poor conditions.Small eggs from extended laying sequences were used to createpoor early conditions for the offspring, which were raised assingletons. Daughters had a lower survival in all treatmentgroups. Survival in natural broods was additionally affectedby hatch date and position. Hatch weight was not different forsons and daughters but was lower in experimental than in naturalnests. In natural nests, daughters fledged 10% heavier thansons, but in experimental nests, they did not reach a highermass. The average survival difference between sons and daughterswas not increased in experimental broods. However, hatch weighthad a strong sex-specific effect. Very light females never survived,and survival probability of daughters increased with increasinghatch weight. By contrast, survival of sons over the same rangeof hatch weights was not related to weight. These findings supportthe hypothesis that larger (final) size per se is related tosex-specific offspring vulnerability during early life.     

Parental investment, sexual selection and sex ratios

Conventional sex roles imply caring females and competitive males. The evolution of sex role divergence is widely attributed to anisogamy initiating a self‐reinforcing process. The initial asymmetry in pre‐mating parental investment (eggs vs. sperm) is assumed to promote even greater divergence in post‐mating parental investment (parental care). But do we really understand the process? Trivers [Sexual Selection and the Descent of Man 1871–1971 (1972), Aldine Press, Chicago] introduced two arguments with a female and male perspective on whether to care for offspring that try to link pre‐mating and post‐mating investment. Here we review their merits and subsequent theoretical developments. The first argument is that females are more committed than males to providing care because they stand to lose a greater initial investment. This, however, commits the ‘Concorde Fallacy’ as optimal decisions should depend on future pay‐offs not past costs. Although the argument can be rephrased in terms of residual reproductive value when past investment affects future pay‐offs, it remains weak. The factors likely to change future pay‐offs seem to work against females providing more care than males. The second argument takes the reasonable premise that anisogamy produces a male‐biased operational sex ratio (OSR) leading to males competing for mates. Male care is then predicted to be less likely to evolve as it consumes resources that could otherwise be used to increase competitiveness. However, given each offspring has precisely two genetic parents (the Fisher condition), a biased OSR generates frequency‐dependent selection, analogous to Fisherian sex ratio selection, that favours increased parental investment by whichever sex faces more intense competition. Sex role divergence is therefore still an evolutionary conundrum. Here we review some possible solutions. Factors that promote conventional sex roles are sexual selection on males (but non‐random variance in male mating success must be high to override the Fisher condition), loss of paternity because of female multiple mating or group spawning and patterns of mortality that generate female‐biased adult sex ratios (ASR). We present an integrative model that shows how these factors interact to generate sex roles. We emphasize the need to distinguish between the ASR and the operational sex ratio (OSR). If mortality is higher when caring than competing this diminishes the likelihood of sex role divergence because this strongly limits the mating success of the earlier deserting sex. We illustrate this in a model where a change in relative mortality rates while caring and competing generates a shift from a mammalian type breeding system (female‐only care, male‐biased OSR and female‐biased ASR) to an avian type system (biparental care and a male‐biased OSR and ASR).     

Juvenile sex ratio in relation to breeding success in Capercaillie Tetrao urogallus and Black Grouse T. tetrix

We examined the relationship between breeding success and juvenile sex ratio in two gamebirds with differing degrees of sexual size dimorphism. We used breeding success as an indicator of the level of environmental stress the birds had experienced. Capercaillie Tetrao urogallus and Black Grouse T. tetrix wing samples were collected from hunters in northern Sweden during 1977-86. In both species, annual juvenile sex ratio (percentage of males) increased with annual breeding success, i.e. with reduced environmental stress. We attribute this to the higher energy requirements during growth, vulnerability to environmental stress, and mortality of juvenile males compared with females, especially under adverse conditions. Moreover, the relationship between breeding success and juvenile sex ratio was stronger in Capercaillie than in Black Grouse. This reflects the higher degree of sexual size dimorphism in Capercaillie, and consequent larger mortality difference between juvenile males and females.     

Sex allocation,juvenile mortality and the costs imposed by offspring on parents and siblings

Generally, sex‐specific mortality is not expected to affect optimal patterns of sex allocation. Several authors have, however, made verbal arguments that this is not true if juvenile mortality is sex specific during the period of parental care. Here, we provide formal mathematical models exploring the effect of such mortality on optimal sex allocation. We confirm the prediction that biased production of the sex with higher mortality during care is favoured. Crucially, however, this is only true when juvenile mortality in the period of parental care frees up resources for their current/future siblings (i.e. the saved investment is transferable). Furthermore, we show that although optimal sex allocation is consistent with the theory of equal investment (as asserted by previous authors), thinking in terms of equal investment is not readily feasible in some scenarios. We also show that differences in early mortality overcome biased sex allocation such that the sex ratio at independence is generally, but not always, biased in the opposite direction from that at birth. Our models should prove useful to empiricists investigating the effect of sex‐specific juvenile mortality and antagonistic sibling interactions on sex allocation.     

Monogynous Mating Behaviour and its Ecological Basis in the Golden Orb Spider Nephila fenestrata

Males, especially in species where they provide little or no parental investment, usually have high potential reproductive rates and are expected to maximize their fitness by mating with several females. This view is challenged, however, by species in which males provide no parental investment, but nevertheless mate with one female only. Male monogamy (monogyny), associated with an extreme investment in paternity protection, appears to be comparatively common in web‐building spiders, and has recently been subject to experimental and theoretical studies. To date, however, studies approaching this issue from an ecological perspective are rare. Theory predicts that the evolution of a monogynous mating strategy is favoured by a male‐biased sex ratio, but not necessarily by a high mortality risk for mate‐searching males. To test these predictions, we conducted a field study on the golden orb spider Nephila fenestrata, which has a mating system with potentially cannibalistic, polyandrous females, and males that are often functionally sterile after mating with one female only. Based on daily observations of marked individuals, we confirm that, consistent with laboratory findings, monogyny is common in N. fenestrata. Nevertheless, observations of male movements between females raise the possibility that a proportion of males may mate with two females. We show that the sex ratio in our study population is male‐biased, and that males incur only a relatively moderate mortality risk during mate‐search. These findings provide insights into the ecological basis for the evolutionary maintenance of monogyny.     

Sex‐specific selection under environmental stress in seed beetles

Sexual selection can increase rates of adaptation by imposing strong selection in males, thereby allowing efficient purging of the mutation load on population fitness at a low demographic cost. Indeed, sexual selection tends to be male‐biased throughout the animal kingdom, but little empirical work has explored the ecological sensitivity of this sex difference. In this study, we generated theoretical predictions of sex‐specific strengths of selection, environmental sensitivities and genotype‐by‐environment interactions and tested them in seed beetles by manipulating either larval host plant or rearing temperature. Using fourteen isofemale lines, we measured sex‐specific reductions in fitness components, genotype‐by‐environment interactions and the strength of selection (variance in fitness) in the juvenile and adult stage. As predicted, variance in fitness increased with stress, was consistently greater in males than females for adult reproductive success (implying strong sexual selection), but was similar in the sexes in terms of juvenile survival across all levels of stress. Although genetic variance in fitness increased in magnitude under severe stress, heritability decreased and particularly so in males. Moreover, genotype‐by‐environment interactions for fitness were common but specific to the type of stress, sex and life stage, suggesting that new environments may change the relative alignment and strength of selection in males and females. Our study thus exemplifies how environmental stress can influence the relative forces of natural and sexual selection, as well as concomitant changes in genetic variance in fitness, which are predicted to have consequences for rates of adaptation in sexual populations.     

Sex differences in parental care: Gametic investment,sexual selection,and social environment

Male and female parents often provide different type and amount of care to their offspring. Three major drivers have been proposed to explain parental sex roles: (1) differential gametic investment by males and females that precipitates into sex difference in care, (2) different intensity of sexual selection acting on males and females, and (3) biased social environment that facilitates the more common sex to provide more care. Here, we provide the most comprehensive assessment of these hypotheses using detailed parental care data from 792 bird species covering 126 families. We found no evidence for the gametic investment hypothesis: neither gamete sizes nor gamete production by males relative to females was related to sex difference in parental care. However, sexual selection correlated with parental sex roles, because the male share in care relative to female decreased with both extra‐pair paternity and frequency of male polygamy. Parental sex roles were also related to social environment, because male parental care increased with male‐biased adult sex ratios (ASRs). Taken together, our results are consistent with recent theories suggesting that gametic investment is not tied to parental sex roles, and highlight the importance of both sexual selection and ASR in influencing parental sex roles.     

Wear Fast,Die Young: More Worn Teeth and Shorter Lives in Iberian Compared to Scottish Red Deer

Teeth in Cervidae are permanent structures that are not replaceable or repairable; consequently their rate of wear, due to the grinding effect of food and dental attrition, affects their duration and can determine an animal''s lifespan. Tooth wear is also a useful indicator of accumulative life energy investment in intake and mastication and their interactions with diet. Little is known regarding how natural and sexual selection operate on dental structures within a species in contrasting environments and how these relate to life history traits to explain differences in population rates of tooth wear and longevity. We hypothesised that populations under harsh environmental conditions should be selected for more hypsodont teeth while sexual selection may maintain similar sex differences within different populations. We investigated the patterns of tooth wear in males and females of Iberian red deer (Cervus elaphus hispanicus) in Southern Spain and Scottish red deer (C. e. scoticus) across Scotland, that occur in very different environments, using 10343 samples from legal hunting activities. We found higher rates of both incisor and molar wear in the Spanish compared to Scottish populations. However, Scottish red deer had larger incisors at emergence than Iberian red deer, whilst molars emerged at a similar size in both populations and sexes. Iberian and Scottish males had earlier tooth depletion than females, in support of a similar sexual selection process in both populations. However, whilst average lifespan for Iberian males was 4 years shorter than that for Iberian females and Scottish males, Scottish males only showed a reduction of 1 year in average lifespan with respect to Scottish females. More worn molars were associated with larger mandibles in both populations, suggesting that higher intake and/or greater investment in food comminution may have favoured increased body growth, before later loss of tooth efficiency due to severe wear. These results illustrate how independent selection in both subspecies, that diverged 11,700 years BP, has resulted in the evolution of different longevity, although sexual selection has maintained a similar pattern of relative sex differences in tooth depletion. This study opens interesting questions on optimal allocation in life history trade-offs and the independent evolution of allopatric populations.     

Disentangling Natural From Hunting Mortality in an Intensively Hunted Wild Boar Population

Abstract We assessed age-specific natural mortality (i.e., excluding hunting mortality) and hunting mortality of 1,175 male and 1,076 female wild boar (Sus scrofa) from Chǎteauvillain-Arc en Barrois (eastern France), using a 22-year dataset (1982–2004) and mark-recapture-recovery methods. Overall yearly mortality was >50% for all sex and age-classes. Low survival was mostly due to high hunting mortality; a wild boar had a >40% of chance of being harvested annually, and this risk was as high as 70% for adult males. Natural mortality rates of wild boar were similar for males and females (approx. 0.15). These rates were comparable to rates typical of male ungulates but high for female ungulates. Wild boar survival did not vary across sex and age-classes. Despite high hunting mortality, we did not detect evidence of compensatory mortality. Whereas natural mortality for males was constant over time, female mortality varied annually, independent of fluctuations in mast availability. Female wild boar survival patterns differed from those reported in other ungulates, with high and variable natural mortality. In other ungulates, natural mortality is typically low and stable across a wide range of environmental conditions. These differences may partly reflect high litter sizes for wild boar, which carries high energetic costs. High hunting mortality may induce a high investment of females in reproduction early in life, at the detriment to survival. Despite high hunting mortality, the study population increased. Effective population control of wild boar should target a high harvest rate of piglets and reproductive females.     

Predicting the direction of sexual selection

Our current understanding of the operation of sexual selection is predicated on a sex difference in parental investment, which favours one sex becoming limiting and choosy over mates, the other competitive and nonchoosy. This difference is reflected in the operational sex ratio (OSR), the ratio of sexually receptive males to females, considered to be of fundamental importance in predicting the direction of sexual selection. Difficulties in measuring OSR directly have led to the use of the potential reproductive rates (PRR) as a measure of the level of investment in offspring of males and females. Several recent studies have emphasized that other factors, such as variation in mate quality and sex differences in mortality patterns, also influence the direction of sexual selection. However, as yet there has been no attempt to form a comprehensive theory of sex roles. Here we show that neither OSR nor PRR is the most fundamentally important determinant of sex roles, and that they are not interchangeable. Instead, the cost of a single breeding attempt has a strong direct effect on competition and choosiness as well as consistent relationships to both OSR and PRR. Our life history based approach to mate choice also yields simple, testable predictions on lack of choice in either sex and on mutual mate choice.     

根田鼠母体捕食应激效应与Trivers-Willard 模型的关系

本研究了根田鼠母体捕食应激对其子代出生、断乳和成体体重、窝性比及死亡率的作用,检验Trivers—Willard模型的2个前提条件、母体应激激素在母体投资中的作用,以及母体捕食应激效应与该模型的关系。将妊娠根田鼠母体暴露于其捕食艾鼬,母体应激子代的出生和断乳体重均显降低;到成体,雄性体重有此效应,而雌性体重则接近对照。出生窝性比无变化,但成体窝性比向雌性偏斜。不同年龄阶段的死亡率无显变化,但累计死亡率明显增大。处理雄性子代在断乳和成体时的皮质酮含量显增高,而雌性子代则无显变化,从而验证了Trivers-Willard模型的2个前提条件,提出应激母体激素参与母体对子代的投资观点,并认为,母体捕食应激使根田鼠子代成体窝性比向雌性偏斜的生理投资符合进化稳定对策。