Allometry of human fertility and energy use

The flux of energy and materials constrains all organisms, and allometric relationships between rates of energy consumption and other biological rates are manifest at many levels of biological organization. Although human ecology is unusual in many respects, human populations also face energetic constraints. Here we present a model relating fertility rates to per capita energy consumption rates in contemporary human nations. Fertility declines as energy consumption increases with a scaling exponent of ?1/3 as predicted by allometric theory. The decline may be explained by parental trade‐offs between the number of children and the energetic investment in each child. We hypothesize that the ?1/3 exponent results from the scaling properties of the networked infrastructure that delivers energy to consumers. This allometric analysis of human fertility offers a framework for understanding the demographic transition to smaller family sizes, with implications for human population growth, resource use and sustainability.     

A synthetic biosocial model of fertility transition: Testing the relative contribution of embodied capital theory,changing cultural norms,and women's labor force participation

This article presents a biosocial model of fertility decline, which integrates ecological‐economic and informational‐cultural hypotheses of fertility transition in a unified theoretical framework. The model is then applied to empirical data collected among 500 women from San Borja, Bolivia, a population undergoing fertility transition. Using a combination of event history analysis, multiple regression, and structural equation modeling, we examine the pathways by which education responds to birth cohort, parental education and network ties, and how age at first birth and total fertility, in turn, respond to birth cohort, social network ties, education, expectations about parental investment, work, and contraceptive use. We find that in addition to secular trends in education, respondent's education is associated with the education of parents, the investment she received from them, and the education of older siblings. Total fertility has dropped over time, partly in response to increased education; moreover, the behavior of other women in a woman's social network predicts both initiation of reproduction and total fertility, while expected parental investment in offspring negatively predicts total fertility. Involvement in paid work that is incompatible with childcare is associated with a later age of first reproduction, but not subsequent fertility. Contraceptive use partially mediates the effect of education and birth cohort on total fertility, but is not a mediator of the effect of social network or expected parental investment on total fertility. Overall, the empirical results provide support for a biosocial model of fertility decline, particularly the embodied capital and cultural pathways. Am J Phys Anthropol 154:322–333, 2014. © 2014 Wiley Periodicals, Inc.     

The life-history trade-off between fertility and child survival

Evolutionary models of human reproduction argue that variation in fertility can be understood as the local optimization of a life-history trade-off between offspring quantity and ‘quality’. Child survival is a fundamental dimension of quality in these models as early-life mortality represents a crucial selective bottleneck in human evolution. This perspective is well-rehearsed, but current literature presents mixed evidence for a trade-off between fertility and child survival, and little empirical ground to evaluate how socioecological and individual characteristics influence the benefits of fertility limitation. By compiling demographic survey data, we demonstrate robust negative relationships between fertility and child survival across 27 sub-Saharan African countries. Our analyses suggest this relationship is primarily accounted for by offspring competition for parental investment, rather than by reverse causal mechanisms. We also find that the trade-off increases in relative magnitude as national mortality declines and maternal somatic (height) and extrasomatic (education) capital increase. This supports the idea that socioeconomic development, and associated reductions in extrinsic child mortality, favour reduced fertility by increasing the relative returns to parental investment. Observed fertility, however, falls considerably short of predicted optima for maximizing total offspring survivorship, strongly suggesting that additional unmeasured costs of reproduction ultimately constrain the evolution of human family size.     

Parental investment in temporally varying environments

Summary Using a model that allows the mean and variance of investment by parents in offspring to evolve in response to change in degree of temporal environmental variation, this paper shows that both parental investment parameters should increase with increases in temporal variation. If offspring receiving greater parental investment are viable over a broader range of environmental conditions, then increased temporal environmental variation can select for increases in parental investment. The variance in parental investment also may increase with increases in temporal variation, but there is a threshold level of temporal variation that must be exceeded before variance in parental investment is adaptive. Thus phenotypic variance in parental investment is not adaptive in all temporally varying environments. Further, increased overlap among generations reduces the expected effects of temporal variation on the mean and variance in parental investment. Thus a negative correlation between length of reproductive life and both measures of investment is expected. There is support for the predictions of this model in some animal groups, but not among plants. Possible reasons for the lack of support among plants are discussed and directions for future research aimed at distinguishing adaptive and maladaptive phenotypic variance in parental investment are suggested.     

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).     

Ecological demography: A synthetic focus in evolutionary anthropology

The interests of evolutionary anthropologists, behavioral ecologists, and demographers converge on the ecology of human fertility. Ecological conditions influence the optimum pattern of maternal effort. Patterns of abortion, neglect, and infanticide vary with mothers' ability to invest in their children and children's ability to use that investment. As in most other mammals, the ecology of human fertility varies between the sexes: status and resource control are important for males, whereas reproductive value is crucial for females. In pre-industrial societies, and even in monogamous societies in demographic transition, wealthy men had more children than did poorer men. This correlation, often assumed to have disappeared, persists today, with richer men still having more sexual access than others. Sex differences in the ecology of fertility mean that sex of the offspring, as well as birth order, influences parental investment. Because individual fertility varies with environment, it is not surprising that “natural” (uncontrolled) fertility varies across societies or that demographic transitions proceed locally, with occasional reverses, as individuals strive to maximize their lifetime reproductive success in changing, competitive, conditions.     

Parental investment in an anadromous population of threespine sticklebacks: An experimental study

Summary Two previously published field studies of threespine sticklebacks conducted to test predictions of parental investment (PI) theory yielded different conclusions about how males invest in their young. Males of a freshwater population invested more in older/larger broods whereas environmental factors played no role in PI decisions. In contrast, males of an anadromous population adjusted levels of PI in response to environmental factors rather than brood characteristics. The current laboratory study attempts to determine possible reasons for the discrepancy between these two studies. We addressed the following questions. Do males invest more in larger/older broods? Do environmental factors affect levels of parental investment? Males were more aggressive (a measure of PI) in defending eggs than empty nests and free-swimming fry. However, aggression was similar for different-aged eggs and different-aged fry. Moreover, when we manipulated brood sizes (fry), males did not change their level of investment. Males increased their level of aggressive defence in response to a rise in water level, indicating that they can adjust their level of investment in response to changes in environmental conditions that affect brood survival.     

Flexibility in reproductive timing in human females: integrating ultimate and proximate explanations

From an ultimate perspective, the age of onset of female reproduction should be sensitive to variation in mortality rates, and variation in the productivity of non-reproductive activities. In accordance with this prediction, most of the cross-national variation in women's age at first birth can be explained by differences in female life expectancies and incomes. The within-country variation in England shows a similar pattern: women have children younger in neighbourhoods where the expectation of healthy life is shorter and incomes are lower. I consider the proximate mechanisms likely to be involved in producing locally appropriate reproductive decisions. There is evidence suggesting that developmental induction, social learning and contextual evocation may all play a role.     

Inclusive fitness and differential productivity across the life course determine intergenerational transfers in a small-scale human society

Transfers of resources between generations are an essential element in current models of human life-history evolution accounting for prolonged development, extended lifespan and menopause. Integrating these models with Hamilton''s theory of inclusive fitness, we predict that the interaction of biological kinship with the age-schedule of resource production should be a key driver of intergenerational transfers. In the empirical case of Tsimane’ forager–horticulturalists in Bolivian Amazonia, we provide a detailed characterization of net transfers of food according to age, sex, kinship and the net need of donors and recipients. We show that parents, grandparents and siblings provide significant net downward transfers of food across generations. We demonstrate that the extent of provisioning responds facultatively to variation in the productivity and demographic composition of families, as predicted by the theory. We hypothesize that the motivation to provide these critical transfers is a fundamental force that binds together human nuclear and extended families. The ubiquity of three-generational families in human societies may thus be a direct reflection of fundamental evolutionary constraints on an organism''s life-history and social organization.     

The evolution of parental investment in caecilian amphibians: a comparative approach

Parental care is widespread among vertebrates and the observed patterns of parental care and investment are extremely diverse. Among amphibians, caecilians (Gymnophiona) exhibit considerable variation in reproductive modes, including both oviparity and viviparity, combined with highly unusual investment strategies (e.g. skin‐feeding and intrauterine feeding). In the present study, current knowledge on the reproductive modes is integrated into an analysis of the evolutionary scenario of parental investment of caecilians. Phylogenetically basal caecilians possessing a biphasic life cycle that includes an aquatic larval stage invest in macrolecithal eggs directly corresponding to size at hatching. Some phylogenetically derived caecilians (i.e. the Teresomata) have a smaller clutch size and show a reduction to either medium‐yolked (mesolecithal) or small‐yolked (microlecithal) eggs. Via alternative pathways of parental investment, such as intrauterine feeding in viviparous taxa and maternal dermatotrophy in oviparous taxa, teresomatan caecilians increase both offspring size and quality. However, more data regarding reproductive biology are needed to obtain a fully resolved understanding of the evolution of reproduction in caecilian amphibians.     

Parental investment with a superior alien in the brood

When a parent's parentage differs across breeding attempts, established theory predicts that the parent should invest more in a brood when perceived parentage is high. We present a model of parental investment in which offspring unrelated to the parent have a competitive advantage over the parent's own offspring and take a larger share of investment. We show that this can weaken or, if the competitive advantage is great, reverse the predicted relationship between perceived parentage and parental investment. A moderate competitive advantage of extra-pair young over within-pair young could partly explain the lack of any clear relationship between paternal care and paternity in many studies, and could easily arise if females choose extra-pair partners for good genes. Our results are also relevant to interspecific avian brood parasitism. As parasites reared together with host offspring are often superior competitors, their hosts could benefit from increasing investment in response to suspected parasitism.     

Parental antagonism and parent-offspring co-adaptation interact to shape family life

The family is an arena for conflicts between offspring, mothers and fathers that need resolving to promote the evolution of parental care and the maintenance of family life. Co-adaptation is known to contribute to the resolution of parent-offspring conflict over parental care by selecting for combinations of offspring demand and parental supply that match to maximize the fitness of family members. However, multiple paternity and differences in the level of care provided by mothers and fathers can generate antagonistic selection on offspring demand (mediated, for example, by genomic imprinting) and possibly hamper co-adaptation. While parent-offspring co-adaptation and parental antagonism are commonly considered two major processes in the evolution of family life, their co-occurrence and the evolutionary consequences of their joint action are poorly understood. Here, we demonstrate the simultaneous and entangled effects of these two processes on outcomes of family interactions, using a series of breeding experiments in the European earwig, Forficula auricularia, an insect species with uniparental female care. As predicted from parental antagonism, we show that paternally inherited effects expressed in offspring influence both maternal care and maternal investment in future reproduction. However, and as expected from the entangled effects of parental antagonism and co-adaptation, these effects critically depended on postnatal interactions with caring females and maternally inherited effects expressed in offspring. Our results demonstrate that parent-offspring co-adaptation and parental antagonism are entangled key drivers in the evolution of family life that cannot be fully understood in isolation.     

Plasticity,political economy,and physical growth status of Guatemala Maya children living in the United States

Migration of Maya refugees to the United States since the late 1970s affords the opportunity to study the consequences of life in a new environment on the growth of Maya children. The children of this study live in Indiantown, Florida, and Los Angeles, California. Maya children between 4 and 14 years old (n = 240) were measured for height, weight, fatness, and muscularity. Overall, compared with reference data for the United States, the Maya children are, on average, healthy and well nourished. They are taller and heavier and carry more fat and muscle mass than Maya children living in a village in Guatemala. However, they are shorter, on average, than children of black, Mexican-American, and white ethnicity living in Indiantown. Children of Maya immigrants born in the United States tend to be taller than immigrant children born in Guatemala or Mexico. Families that invest economic and social resources in their children tend to have taller children. More economically successful families have taller children. Migration theory and political economy theory from the social sciences are combined with plasticity theory and life history theory (parental investment) from biology to interpret these data. Am J Phys Anthropol 102:17–32, 1997. © 1997 Wiley-Liss, Inc.     

Parental investment and quality of insurance offspring in an obligate brood-reducing species, the American white pelican

The last-hatched chick, or B-offspring, of American white pelicanstypically survives only as "insurance" when its elder siblingfails. Life-history theory suggests that parents should investrelatively less in these disadvantaged insurance offspring.For an insurance strategy to be effective, however, reducedinvestment may be constrained by the need to maintain potentialinsurance offspring in a viable condition until at least 3–6days of age, after which they are rarely needed. In agreementwith the life-history prediction, egg size, resultant hatchingmass, and growth rates at two-chick nests were significantlylower for B-offspring. When hatched in the laboratory, B-eggswere also slightly but significantly less efficient at convertingegg size into hatching mass. Despite these differences, B-chicksthat were reared as singles, free from sibling competition fromhatching onward, showed no decrement in survival or growth rate.When A-chicks were removed from nests with underweight 3- or6-day-old B-chicks, a minority (21%) of B-chicks failed to recover,but mean growth rates of survivors increased rapidly to controllevels. Results suggest that although parental investment inB-offspring is reduced, it is usually adequate to produce andmaintain potential insurance offspring in viable condition duringthe time that they are most likely to be needed as replacementsfor failed elder siblings.     

Mate feeding, offspring investment, and sexual differences in katydids (Orthoptera: Tettigoniidae)

Food stress in the katydid Requena veriicalis (Orthoptera: Tettigoniidae)decreases the relative availability of males able to supplynutritious spermatophores to females and increases the valueof the male courtship meal (i.e., relative male parental investment).These changes cause female sexual competition in katydid populations.Here we examine the effect of food stress on male and femaleinvestment in single offspring and test the prediction thatmale-derived nutrients in eggs increase relative to nutrientsfrom the female's reserves. We varied the diet of female R.veriicalis and determined the fate of nutrients from male andfemale sources using I4C and 3H radiolabeled amino acids. Low-dietfemales retained more nutrients from male and female sourcesin somatic tissues and invested less in reproduction both becausethey produced fewer eggs and because they invested less peroffspring (egg) than females maintained on a high-quality diet.Moreover, opposite to our prediction, relative male investmentin individual eggs decreased in foodstressed females; femalesretained more nutrients in somatic tissues from the male sourcethan the female source. Food-stressed females may retain nutrientreserves, particularly those from the male, as an adaptive trategyfor immediate survival needs and future reproduction. Such afemale strategy is unlikely to compromise male reproductivesuccess; first-male sperm precedence means that males matingwith virgin females are likely to father most eggs laid, evenin future reproductive bouts. The decrease in male investmentin eggs of low-diet females does not conflict with the contentionthat relative parental investment controls male intrasexualcompetition because, in mate-feeding species, male investmentinfluencing this competition includes more than investment incurrent offspring; females should compete for males if courtshipgifts aid survival and later reproduction.     

Cooperation and conflict between women in the family

Here I review recent research on reproductive conflict between females in families and how it influences their reproductive behaviour. Kin selection can favor cooperation between parent and offspring, siblings, or unrelated co‐residents who share interests in other family members such as grand‐offspring. However, these are also the individuals most likely to be sharing resources, and so conflict can also emerge. While substantial interest has arisen in evolutionary anthropology, especially over the last two decades, in the possibility of cooperative breeding in humans, less attention has been paid to reproductive conflict among female kin. Communal breeding in animals is generally understood as emerging from competition over the resources needed to breed. Competition for household resources is a problem that also faces human families. Models suggest that in some circumstances, inclusive fitness can be maximized by sharing reproduction rather than harming relatives by fighting with them, even if the shares that emerge are not equal. Thus, competition and cooperation turn out to be strongly related to each other. Reproductive competition within and between families may have underpinned the biological evolution of fertility patterns (such as menopause) and the cultural evolution of marriage, residence, and inheritance norms (such as late male marriage or primogeniture), which can enhance cooperation and minimize the observed incidence of such conflicts.     

Litter size and latitude in a large mammal: the wild boar Sus scrofa

• 1 A positive relationship between clutch size or litter size and latitude exists in birds and many species of small mammal. Hitherto, however, analyses for large mammals have failed to provide evidence that litter sizes increase with latitude.
• 2 We collated data from published studies of wild boar in Europe, to analyse the relationship between litter size and latitude in this widely distributed terrestrial mammal.
• 3 Depending on the specific data set (whether only the most reliable data or all available data were included), latitude explained 58% to 72% of the variation in mean litter sizes across studies. On average, litter size increases by approximately 0.15 piglets per degree of latitude.
• 4 A strong correlation between litter size and latitude for wild boar in Europe provides a starting point for demographic modelling of this species of both ecological and economic importance.
• 5 The pattern for wild boar is consistent with Ashmole's explanation for the effects of latitude on reproduction. The contrast between our results and those generated for other large mammals may result from our focus on an herbivore in contrast to previous work which was focused on carnivores. Further work could usefully examine the extent of seasonality in the availability of resources for species of different dietary types.

     

Status competition,inequality, and fertility: implications for the demographic transition

The role that social status plays in small-scale societies suggests that status may be important for understanding the evolution of human fertility decisions, and for understanding how such decisions play out in modern contexts. This paper explores whether modelling competition for status—in the sense of relative rank within a society—can help shed light on fertility decline and the demographic transition. We develop a model of how levels of inequality and status competition affect optimal investment by parents in the embodied capital (health, strength, and skills) and social status of offspring, focusing on feedbacks between individual decisions and socio-ecological conditions. We find that conditions similar to those in demographic transition societies yield increased investment in both embodied capital and social status, generating substantial decreases in fertility, particularly under conditions of high inequality and intense status competition. We suggest that a complete explanation for both fertility variation in small-scale societies and modern fertility decline will take into account the effects of status competition and inequality.     

Effects of among-offspring relatedness on the origins and evolution of parental care and filial cannibalism

Parental care is expected to increase the likelihood of offspring survival at the cost of investment in future reproductive success. However, alternative parental behaviours, such as filial cannibalism, can decrease current reproductive success and consequently individual fitness. We evaluate the role of among-offspring relatedness on the evolution of parental care and filial cannibalism. Building on our previous work, we show how the evolution of care is influenced by the effect of among-offspring relatedness on both the strength of competition and filial cannibalism. When there is a positive relationship between among-offspring competition and relatedness, parental care will be favoured when among-offspring relatedness is relatively low, and the maintenance of both care and no-care strategies is expected. If the relationship between among-offspring competition and relatedness is negative, parental care is most strongly favoured when broods contain highly related offspring. Further, we highlight the range of conditions over which the level of this among-offspring relatedness can affect the co-occurrence of different care/no care and cannibalism/no cannibalism strategies. Coexistence of multiple strategies is independent of the effects of among-offspring relatedness on cannibalism but more likely when among-offspring relatedness and competition are positively associated.