Prenatal Investment Research Articles

Mothers in a cooperatively breeding bird increase investment per offspring at the pre-natal stage when they will have more help with post-natal care.

In many cooperative societies, including our own, helpers assist with the post-natal care of breeders' young and may thereby benefit the post-natal development of offspring. Here, we present evidence of a novel mechanism by which such post-natal helping could also have beneficial effects on pre-natal development: By lightening post-natal maternal workloads, helpers may allow mothers to increase their pre-natal investment per offspring. We present the findings of a decade-long study of cooperatively breeding white-browed sparrow-weaver, Plocepasser mahali, societies. Within each social group, reproduction is monopolized by a dominant breeding pair, and non-breeding helpers assist with nestling feeding. Using a within-mother reaction norm approach to formally identify maternal plasticity, we demonstrate that when mothers have more female helpers, they decrease their own post-natal investment per offspring (feed their nestlings at lower rates) but increase their pre-natal investment per offspring (lay larger eggs, which yield heavier hatchlings). That these plastic maternal responses are predicted by female helper number, and not male helper number, implicates the availability of post-natal helping per se as the likely driver (rather than correlated effects of group size), because female helpers feed nestlings at substantially higher rates than males. We term this novel maternal strategy "maternal front-loading" and hypothesize that the expected availability of post-natal help either allows or incentivizes helped mothers to focus maternal investment on the pre-natal phase, to which helpers cannot contribute directly. The potential for post-natal helping to promote pre-natal development further complicates attempts to identify and quantify the fitness consequences of helping.

Life History Parameters to Inform Pattern of Prenatal Investment in Marine Mammals

Marine mammals are a diverse group of aquatic animals that exhibit wide variation in body size, living conditions, breeding habitat, social behaviour and phylogeny. Although case studies about prenatal investment in cetaceans and pinnipeds have been investigated, comparative studies across different marine mammal taxonomic groups have not yet been conducted systematically. Here, six life history parameters from 75 marine mammal species were collected based on a meta-analysis of the existing literature, and prenatal investment patterns for different taxonomic groups were explored using an unsupervised artificial neural network of a self-organizing map (SOM). Most marine mammal species can be clearly divided into two clusters of small-bodied taxa (small-bodied toothed whales, pinnipeds) and large-bodied taxa (baleen whales, sperm whales and beaked whales, large-bodied toothed whales) based on their distribution within SOM feature maps. Gestation periods and breeding intervals are significantly shorter in pinnipeds than in small-bodied toothed dolphins despite being similar in body size, indicating their adaption to birthing and nursing on land or ice floes. Specific deep-dive feeding behaviour seems to have no impact on the prenatal investment of beaked whales and sperm whales, as these species exhibit a similar capital breeding strategy to baleen whales. Medium-bodied sirenians adopt an intermediate strategy between small-bodied and large-bodied toothed whales, suggesting their prenatal investment strategy is not affected by herbivorous habits. Overall, our results support the body-size hypothesis and breeding-substrate hypothesis and indicate that prenatal investment strategies of marine mammals are possibly not influenced by feeding habits or social behaviour. We suggest that effective conservation measures for small-bodied toothed whales and pinnipeds should prioritize the protection of habitats and minimize human disturbance, whereas conservation measures for large-bodied whales and beaked whales should focus on strategies to prevent substantial declines in population size.

Baby's best Foe-riend: Endogenous retroviruses and the evolution of eutherian reproduction

High maternal investment in pregnancy and the perinatal period are prominent features of eutherian reproduction. Viviparity increases offspring survival, favoring high maternal prenatal investment. Matrotrophy through the placenta reduces maternal investment at early pregnancy, allowing the mother to abort embryos of subpar quality, therefore reducing resources wastage. On the other hand, intimate maternal-fetal interplay enables the fetus to manipulate maternal physiology to acquire more resources. This parent-offspring conflict likely drives the evolution of eutherian placentation, which is facilitated by the endogenous retroviruses (ERVs), ancient retroviruses that invaded host genome millions of years ago. ERVs bring new genes and novel regulatory elements into host genome, contribute to maternal-fetal tolerance, placenta-specific cell type formation, trophoblast gene expression network rewiring, and the establishment of imprinting. However, retroviruses/ERVs can function as infectious pathogens that interfere with host immune and inflammation pathways and cause genomic instability. In addition, ERVs coopted for host function may contribute to pathogenesis during infections due to their susceptibility to mechanisms activated by the invading pathogens. ERVs have been implicated in multiple perinatal adverse outcomes, therefore, eutherians must have evolved control mechanisms to regulate their function. Here we propose the TRIM family as an important participant of host antiviral defense and a likely candidate that mediates the coevolution of ERVs and their eutherian host. TRIMs have been shown to interact with retroviruses during each step of the infectious cycle. Understanding TRIMs’ role in ERV regulation in the placenta may provide insight to both the physiology and pathology of eutherian reproduction.

Adverse childhood experiences and reproductive strategies in a contemporary U.S. population.

Stressful experiences may initiate developmentally plastic responses toward a faster reproductive strategy. This study tests whether adverse childhood experiences (ACEs) are associated with characteristics of faster reproductive strategies: earlier menarche and reduced prenatal investment in offspring in a well-nourished, low-immune system burden population. We analyzed the first 214 enrollees of the prenatal-birth Albany Infant and Mother Study. Mother's menarcheal age, offspring gestational age at birth, and cephalization index (head circumference/weight, cm/g) were derived from medical records. Linear regression models tested the contribution of self-reported ACEs from 0 to 18 years of age to menarcheal age and the contribution of menarcheal age to offspring gestational age and cephalization index. Birth outcome models included covariates self-reported maternal race, education, prenatal smoking, prenatal diet, newborn sex, parity, delivery method, and labor induction derived from medical records. More ACEs were associated with earlier age at menarche, controlling for covariates (β = -.18, SE = 0.048, p < .001), though timing of ACEs relative to menarche is unknown. Earlier menarcheal age was associated with offspring higher cephalization index (β = -.01, SE = 0.006, p < .05). Stratified models showed a significant relationship in high (≥2) ACEs group (β = -.02, SE = 0.009, p < .05), not present among low ACEs. Menarcheal age predicted gestational age only among those with high ACEs (β = .22, SE = 0.091, p < .05). In a well-nourished population, early life stress can result in faster reproductive strategies, initiating sexual maturation earlier, and reducing prenatal investment in individual offspring. Early age at menarche following childhood stress has a stronger relationship with adverse birth outcomes than early menarche without exposure to adverse childhood stress.

Whoever Has Will Be Given More: Child Endowment and Human Capital Investment

Using a unique longitudinal survey from Ethiopia, we investigate whether resource constrained parents reinforce or attenuate differences in early abilities between their children. We propose a simple model that allows for sibling interactions. To overcome the endogeneity associated with measures of the endowment, we construct a measure of human capital at birth that is plausibly net of prenatal investment. We estimate a sibling fixed-effect model to account for bias due to unobserved family-specific heterogeneity. We find that parents reinforce educational inequality: inherently healthy children are more likely to attend preschool, be enrolled in elementary school, and have more expenses incurred towards their education. Health inputs are allocated in a compensatory manner.

The relationship between egg size and helper number in cooperative breeders: a meta-analysis across species.

BackgroundLife history theory predicts that mothers should adjust reproductive investment depending on benefits of current reproduction and costs of reduced future reproductive success. These costs and benefits may in turn depend on the breeding female’s social environment. Cooperative breeders provide an ideal system to test whether changes in maternal investment are associated with the social conditions mothers experience. As alloparental helpers assist in offspring care, larger groups might reduce reproductive costs for mothers or alternatively indicate attractive conditions for reproduction. Thus, mothers may show reduced (load-lightening) or increased (differential allocation) reproductive investment in relation to group size. A growing number of studies have investigated how cooperatively breeding mothers adjust pre-natal investment depending on group size. Our aim was to survey these studies to assess, first, whether mothers consistently reduce or increase pre-natal investment when in larger groups and, second, whether these changes relate to variation in post-natal investment.MethodsWe extracted data on the relationship between helper number and maternal pre-natal investment (egg size) from 12 studies on 10 species of cooperatively breeding vertebrates. We performed meta-analyses to calculate the overall estimated relationship between egg size and helper number, and to quantify variation among species. We also tested whether these relationships are stronger in species in which the addition of helpers is associated with significant changes in maternal and helper post-natal investment.ResultsAcross studies, there is a significant negative relationship between helper number and egg size, suggesting that in most instances mothers show reduced reproductive investment in larger groups, in particular in species in which mothers also show a significant reduction in post-natal investment. However, even in this limited sample, substantial variation exists in the relationship between helper number and egg size, and the overall effect appears to be driven by a few well-studied species.DiscussionOur results, albeit based on a small sample of studies and species, indicate that cooperatively breeding females tend to produce smaller eggs in larger groups. These findings on prenatal investment accord with previous studies showing similar load-lightening reductions in postnatal parental effort (leading to concealed helper effects), but do not provide empirical support for differential allocation. However, the considerable variation in effect size across studies suggests that maternal investment is mitigated by additional factors. Our findings indicate that variation in the social environment may influence life-history strategies and suggest that future studies investigating within-individual changes in maternal investment in cooperative breeders offer a fruitful avenue to study the role of adaptive plasticity.

Age‐related prenatal maternal effects and postnatal breeding experience have different influences on nestling development in an altricial passerine

Reproductive success and nestling performance are related to the age of parents across several vertebrate taxa. However, because breeding experience and prenatal maternal investment in reproduction often covary, the source of these age‐related differences can be difficult to determine. In this study, we evaluated the influence of prenatal maternal effects and postnatal breeding experience on the performance of nestling tree swallows Tachycineta bicolor by conducting a carefully controlled partial cross‐fostering experiment. We swapped half‐broods of nestlings between the nest of a young first‐time breeding female and the nest of a female known to have previously raised and fledged young. Our manipulation did not influence the within‐brood nestling hierarchies, and controlled for the effects of egg laying order. We found that nestlings of older females were heavier just prior to fledging regardless of the breeding experience of the attending female. In addition, fledglings raised by experienced females grew their flight feathers faster, and had greater probability of fledging. Our study demonstrates that prenatal investment in reproduction by older females can have long‐term consequences on nestling mass, and suggests limited potential for compensatory mass gains prior to fledging. Because our analyses controlled for feeding rates, our results also suggest that foraging quantity and quality are not the only benefits nestlings gain by being raised by an experienced female.

Female reproductive competition explains variation in prenatal investment in wild banded mongooses.

Female intrasexual competition is intense in cooperatively breeding species where offspring compete locally for resources and helpers. In mammals, females have been proposed to adjust prenatal investment according to the intensity of competition in the postnatal environment (a form of ‘predictive adaptive response’; PAR). We carried out a test of this hypothesis using ultrasound scanning of wild female banded mongooses in Uganda. In this species multiple females give birth together to a communal litter, and all females breed regularly from one year old. Total prenatal investment (size times the number of fetuses) increased with the number of potential female breeders in the group. This relationship was driven by fetus size rather than number. The response to competition was particularly strong in low weight females and when ecological conditions were poor. Increased prenatal investment did not trade off against maternal survival. In fact we found the opposite relationship: females with greater levels of prenatal investment had elevated postnatal maternal survival. Our results support the hypothesis that mammalian prenatal development is responsive to the intensity of postnatal competition. Understanding whether these responses are adaptive requires information on the long-term consequences of prenatal investment for offspring fitness.

DISENTANGLING GENETIC AND PRENATAL SOURCES OF FAMILIAL RESEMBLANCE ACROSS ONTOGENY IN A WILD PASSERINE

Cross-fostering experiments are widely used by quantitative geneticists to study genetics and by behavioral ecologists to study the effects of prenatal investment. Generally, the effects of genes and prenatal investment are confounded and the interpretation given to such experiments is largely dependent on the interests of the researcher. Using a large-scale well-controlled experiment on a wild population of blue tits (Cyanistes caeruleus), we are able to partition variation in body mass across ontogeny into the effects of genes and the effects of between-clutch variation in egg characteristics. We show that although egg effects are important early in ontogeny they quickly dissipate, suggesting that the genetic interpretation of cross-fostering experiments may be preferable for many types of trait. However, the heritability of body mass is smaller than has previously been reported. Our results suggest that this is due to a combination of controlling postnatal environmental effects more carefully and accounting for viability selection operating early in ontogeny.

Variability of Prenatal Maternal Investment in the Honey Bee (Apis mellifera)

Variability of Prenatal Maternal Investment in the Honey Bee (Apis mellifera)

Maternal investment during pregnancy in wild meerkats

Maternal investment in offspring development is a major determinant of the survival and future reproductive success of both the mother and her young. Mothers might therefore be expected to adjust their investment according to ecological conditions in order to maximise their lifetime fitness. In cooperatively breeding species, where helpers assist breeders with offspring care, the size of the group may also influence maternal investment strategies because the costs of reproduction are shared between breeders and helpers. Here, we use longitudinal records of body mass and life history traits from a wild population of meerkats (Suricata suricatta) to explore the pattern of growth in pregnant females and investigate how the rate of growth varies with characteristics of the litter, environmental conditions, maternal traits and group size. Gestational growth was slight during the first half of pregnancy but was marked and linear from the midpoint of gestation until birth. The rate of gestational growth in the second half of pregnancy increased with litter size, maternal age and body mass, and was higher for litters conceived during the peak of the breeding season when it is hot and wet. Gestational growth rate was lower in larger groups, especially when litter size was small. These results suggest that there are ecological and physiological constraints on gestational growth in meerkats, and that females may also be able to strategically adjust their prenatal investment in offspring according to the likely fitness costs and benefits of a particular breeding attempt. Mothers in larger groups may benefit from reducing their investment because having more helpers might allow them to lower reproductive costs without decreasing breeding success.

Patterns of variation in reproductive parameters in Eurasian lynx (Lynx lynx)

Detailed knowledge of the variation in demographic rates is central for our ability to understand the evolution of life history strategies and population dynamics, and to plan for the conservation of endangered species. We studied variation in reproductive output of 61 radio-collared Eurasian lynx females in four Scandinavian study sites spanning a total of 223 lynx-years. Specifically, we examined how the breeding proportion and litter size varied among study areas and age classes (2-year-old vs. >2-year-old females). In general, the breeding proportion varied between age classes and study sites, whereas we did not detect such variation in litter size. The lack of differences in litter sizes among age classes is at odds with most findings in large mammals, and we argue that this is because the level of prenatal investment is relatively low in felids compared to their substantial levels of postnatal care.

Implication of life history strategies for prenatal investment in cetaceans

Prenatal investment directly determines the size at birth and fetus growth rate, which affects neonatal survival and growth and potentially affects maternal fitness. This study explored the associated prenatal life history traits of cetaceans. Using multivariate analysis and ANCOVA, baleen whales and toothed cetaceans had distinct energy patterns, with two exceptions including beaked whales and eusocial cetaceans. Baleen whales are characterized by fast prenatal growth, which suggests high prenatal energetics, and utilize the capital breeder tactic. Toothed cetaceans, except for beaked whales, utilize income breeder energetics, which yields relatively slow prenatal growth. However, eusocial cetaceans have especially slow prenatal growth, suggesting very low prenatal energetic effort with social compensation. Although beaked whales are behaviorally income breeders, both discriminant analysis and ANCOVA showed that they are energetically similar to baleen whales, utilizing capital energetics. ANCOVA further revealed that beaked whales have comparatively large calf size, suggesting high prenatal investment. Because all cetaceans wean their calves at comparable size, high prenatal investment may further suggest reduced cost of lactation, which may be behaviorally and energetically adaptive to their specific deep-dive-feeding niche.

Maternal investment and reproductive success in Chinese water deer

Abstract Among cervids, maternal investment, estimated as the amount of resources and care allocated to the offspring, was expected to be related to species body size. Therefore, maternal investment in a herd of captive Chinese water deer Hydropotes inermis, a relatively small species of cervid, was investigated over 3 years. Except during the lactation period, reproductive females spent about 2-fold more time resting than feeding. During lactation, the amount of time spent feeding increased highly (25.3 min/h during lactation vs 17.3 min/h during the gestation period). Females spent less than 30% of time in communal behaviours with offspring. They did not reject alien fawns during this care period. Frequency and duration of suckling events decreased exponentially from the second week onwards. More than 10% of suckling bouts were non-filial. Prenatal investment leads to a mean litter mass (about 12% of maternal mass) higher than in most cervid species. Postnatal investment in fawns represents a daily mass gain of ca. 85 g/d during the first 2 weeks, without any sexual difference. Female production, timing and synchrony of births and survival of fawns characterized reproductive success. Seventy percent of mature females gave birth, with a mean of 1.9 offspring per female. The sex ratio was even. Births were synchronous, 80% of births occurring in 25 days. In this herd, 0.74 fawn per female was successfully weaned and 0.56 fawn per female survived through their first year. Based on these results we conclude that reproductive strategy of Chinese water deer was efficient and characterized by mother-offspring relationships typical of hiders and high levels of pre- and postnatal investments. This strategy seems typical of small species of cervids without marked sexual dimorphism.

Reproductive ecology and the endometrium: Physiology, variation, and new directions

Endometrial function is often overlooked in the study of fertility in reproductive ecology, but it is crucial to implantation and the support of a successful pregnancy. Human female reproductive physiology can handle substantial energy demands that include the production of fecund cycles, ovulation, fertilization, placentation, a 9-month gestation, and often several years of lactation. The particular morphology of the human endometrium as well as our relative copiousness of menstruation and large neonatal size suggests that endometrial function has more resources allocated to it than many other primates. The human endometrium has a particularly invasive kind of hemochorial placentation and trophoblast that maximizes surface area and maternal-fetal contact, yet these processes are actually less efficient than the placentation of some of our primate relatives. The human endometrium and its associated processes appear to prioritize maximizing the transmission of oxygen and glucose to the fetus over efficiency and protection of maternal resources. Ovarian function controls many aspects of endometrial function and thus variation in the endometrium is often a reflection of ecological factors that impact the ovaries. However, preliminary evidence and literature from populations of different reproductive states, ages and pathologies also suggests that ecological stress plays a role in endometrial variation, different from or even independent of ovarian function. Immune stress and psychosocial stress appear to play some role in the endometrium's ability to carry a fetus through the mechanism of inflammation. Thus, within reproductive ecology we should move towards a model of women's fecundity and fertility that includes many components of ecological stress and their effects not only on the ovaries, but on processes related to endometrial function. Greater attention on the endometrium may aid in unraveling several issues in hominoid and specifically human evolutionary biology: a low implantation rate, high rates of early pregnancy loss, prenatal investment in singletons but postnatal support of several dependent offspring at once, and higher rate of reproductive and pregnancy-related pathology compared to other primates, ranging from endometriosis to preeclampsia. The study of the endometrium may also complicate some of these issues, as it raises the question of why humans have a maximally invasive placentation method and yet slow fetal growth rates. In this review, I will describe endometrial physiology, methods of measurement, variation, and some of the ecological variables that likely produce variation and pregnancy losses to demonstrate the necessity of further study. I propose several basic avenues of study that leave room for testable hypotheses in the field of reproductive ecology. And finally, I describe the potential of this work not just in reproductive ecology, but in the resolution of broader women's health issues.

Human Prenatal Investment Affected by Maternal Age and Parity

This paper investigates the effects of mother's age and parity on the prenatal allocation of resources in sons and daughters using a sample of 6994 in 20- to 34-year-old mothers from Granada (Spain). This study tests the Trivers–Willard allocation model, which predicts that, depending on the mother's condition, there can be a bias from 50/50 expected investment in the sexes. The results show that mothers of 25–29 age class produce relatively heavier sons compared to younger mothers, suggesting a bias in the allocation of resources before birth favoring neonate boys. This extra investment in boys appears to be related more to the mother's age than to her parity. Offspring of multiparous mothers had shorter gestation length, but multiparous mothers of the 25–29 age class had boys with a longer gestation than the younger ones. The 25–29 age class mothers are in the peak of fertility and they are presumably in better condition than the other mothers in the study sample. The results suggest that Granada women performed a sex-biased prenatal investment favoring sons by prolonging the gestation period. An index of boys' extra birth weight is proposed as a suitable variable to measure the maternal prenatal investment.

Prenatal investment in the subantarctic fur seal, Arctocephalus tropicalis

We investigated prenatal investment in a large sexually dimorphic mammal, the subantarctic fur seal, Arctocephalus tropicalis, on Amsterdam Island in the Indian Ocean. Pups' sex ratio and body mass, body length, and body condition at birth were studied in relation to timing of birth and maternal characteristics (body length and body condition) during three consecutive breeding seasons. Pups' sex ratio did not differ from unity throughout the pupping period. The sex of the pup was related to neither maternal body length (i.e., maternal age) nor maternal body condition when mating occurred or at parturition (1 year later), which suggests that the sex ratio was not biased toward one sex during gestation. Newborn male pups were heavier and longer than female pups in all years. Longer mothers tended to arrive later in the season regardless of their body condition, and gave birth to heavier pups whatever the sex of the pup. Mothers in good condition gave birth to heavier male pups than mothers in poor condition, but no significant differences were found for female pups, suggesting that the costs of carrying male foetuses is higher than that of carrying female foetuses. Differences in allocation of maternal resources between male and female pups may be due to sex-related differences in body composition, since male pups were heavier than female pups for a given body length at birth. Thus, male and female foetuses may use maternal resources differently, with males growing in length whereas females appear to grow in body mass. The mothers we monitored over 2 consecutive years gave birth to pups that were similar in quality (in terms of birth mass) over years regardless of the sex of the previous pup and the mother's body length, suggesting that individual reproductive value is independent of maternal age. Furthermore, maternal body condition was not affected by the sex of the foetus, suggesting that there is no differential reproductive cost in carrying a male or a female foetus. Interannual differences in pup body size at birth suggest that environmental conditions such as prey availability during the last stages of gestation, and consequent maternal body condition, are important components of maternal investment in fur seals.

Prenatal investment in the subantarctic fur seal, &lt;i&gt;Arctocephalus tropicalis&lt;/i&gt;

We investigated prenatal investment in a large sexually dimorphic mammal, the subantarctic fur seal, Arctocephalus tropicalis, on Amsterdam Island in the Indian Ocean. Pups' sex ratio and body mass, body length, and body condition at birth were studied in relation to timing of birth and maternal characteristics (body length and body condition) during three consecutive breeding seasons. Pups' sex ratio did not differ from unity throughout the pupping period. The sex of the pup was related to neither maternal body length (i.e., maternal age) nor maternal body condition when mating occurred or at parturition (1 year later), which suggests that the sex ratio was not biased toward one sex during gestation. Newborn male pups were heavier and longer than female pups in all years. Longer mothers tended to arrive later in the season regardless of their body condition, and gave birth to heavier pups whatever the sex of the pup. Mothers in good condition gave birth to heavier male pups than mothers in poor condition, but no significant dif- ferences were found for female pups, suggesting that the costs of carrying male foetuses is higher than that of carrying female foetuses. Differences in allocation of maternal resources between male and female pups may be due to sex- related differences in body composition, since male pups were heavier than female pups for a given body length at birth. Thus, male and female foetuses may use maternal resources differently, with males growing in length whereas females appear to grow in body mass. The mothers we monitored over 2 consecutive years gave birth to pups that were similar in quality (in terms of birth mass) over years regardless of the sex of the previous pup and the mother's body length, suggesting that individual reproductive value is independent of maternal age. Furthermore, maternal body condition was not affected by the sex of the foetus, suggesting that there is no differential reproductive cost in carrying a male or a female foetus. Interannual differences in pup body size at birth suggest that environmental conditions such as prey availability during the last stages of gestation, and consequent maternal body condition, are important compo- nents of maternal investment in fur seals. 609

Low birth weight, maternal birth-spacing decisions, and future reproduction : A cost-benefit analysis.

The aim of this study is an analysis of the possible adaptive consequences of delivery of low birth weight infants. We attempt to reveal the cost and benefit components of bearing small children, estimate the chance of the infants' survival, and calculate the mothers' reproductive success. According to life-history theory, under certain circumstances mothers can enhance their lifetime fitness by lowering the rate of investment in an infant and/or enhancing the rate of subsequent births. We assume that living in a risky environment and giving birth to a small infant may involve a shift from qualitative to quantitative production of offspring. Given high infant mortality rates, parents will have a reproductive interest in producing a relatively large number of children with a smaller amount of prenatal investment. This hypothesis was tested among 650 Gypsy and 717 non-Gypsy Hungarian mothers. Our study has revealed that 23.8% of the Gypsy mothers had low birth weight (<2,500 g) children, whose mortality rate is very high. These mothers also had more spontaneous abortions and stillbirths than those with normal weight children. As a possible response to these reproductive failures, they shortened birth spacing, gaining 2-4 years across their reproductive lifespan for having additional children. Because of the relatively short interbirth intervals, by the end of their fertility period, Gypsy mothers with one or two low birth weight infants have significantly more children than their ethnic Hungarian counterparts. They appear to compensate for handicaps associated with low birth weights by having a larger number of closely spaced children following the birth of one or more infants with a reduced probability of survival. The possible alternative explanations are discussed, and the long-term reproductive benefits are estimated for both ethnic groups.

Sex-Bias in Prenatal Parental Investment in the Eastern Cottontail (Sylvilagus floridanus)

We based our research on the premise that if variation in reproductive success for one sex is greater than that of the other sex, and if differential parental investment can enhance the probability of that sex reproducing at a higher rate, selection should favor investment in that sex. We tested for sex-bias in prenatal investment by weighing 75 fetuses of eastern cottontails (Sylvilagus floridanus) and their placentae in 17 gravida gestated ≥18 days. Significantly more male than female fetuses were heavier than the mean mass of fetuses in their respective gravida; placentae of these fetuses did not exhibit the same relationship between sexes. However, total resources devoted to males (fetal and placental masses combined) by maternal females was significantly greater than to females. Also, the slope of the regression line of log10 of placental mass on log10 of fetal mass for male fetuses was significantly greater than for female fetuses, suggesting a direct means by which resources are allocated differentially. Fetal mass was not affected significantly by intrauterine position in relation to fetuses of the same or opposite sex. The greatest opportunity for the maternal female to impart a potential advantage to male offspring is during the prenatal period and any advantage obtained by male neonates during postnatal development probably is a direct result of additional resources obtained before birth.