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Directional selection on size is common but often fails to result in microevolution in the wild. Similarly, macroevolutionary rates in size are low relative to the observed strength of selection in nature. We show that many estimates of selection on size have been measured on juveniles, not adults. Further, parents influence juvenile size by adjusting investment per offspring. In light of these observations, we help resolve this paradox by suggesting that the observed upward selection on size is balanced by selection against investment per offspring, resulting in little or no net selection gradient on size. We find that trade-offs between fecundity and juvenile size are common, consistent with the notion of selection against investment per offspring. We also find that median directional selection on size is positive for juveniles but no net directional selection exists for adult size. This is expected because parent-offspring conflict exists over size, and juvenile size is more strongly affected by investment per offspring than adult size. These findings provide qualitative support for the hypothesis that upward selection on size is balanced by selection against investment per offspring, where parent–offspring conflict over size is embodied in the opposing signs of the two selection gradients.

1. Parent–offspring conflicts are likely to occur when resources are limiting either at pre- orpost-natal stages due to intergenerational trade-offs over resources. Current theory posits thatsuch conflicts may influence the evolution of parental allocation as well as reproductive modes.While energy allocation to the offspring has received considerable attention, the distribution ofwater – another potentially limited vital resource to both the mother and offspring – and theresulting outcomes remain grossly understudied.2. Here, we explored the intergenerational trade-off related to water resources in the viviparousaspic viper (Vipera aspis) by examining the effects of water deprivation on female physiology(body mass, haematocrit and osmolality), water transfer to developing embryos andreproductive performance.3. As a result of water deprivation, females became dehydrated, with the effects more pronouncedin pregnant compared to nonreproductive females. Among pregnant females,the impacts of water deprivation on water balance were correlated with fecundity. Incontrast, water deprivation had no effect on water transfer to the offspring or on reproductiveperformance.4. Our results demonstrate that, under water-constraining conditions, female water balance iscompromised in favour of the developing embryos, highlighting a significant intergenerationaltrade-off for water. Although ectothermic reptiles are particularly tolerant in water balanceperturbations, our results suggest that, like energy, water can be a conflicting resourcebetween mother and offspring. Parent–offspring conflict over water should therefore befurther investigated to better understand reproductive modes and reproductive trade-offs interrestrial organisms.

Recent years have seen an explosion of multidisciplinary interest in ancient human warfare. Theory has emphasized a key role for kin-selected cooperation, modulated by sex-specific demography, in explaining intergroup violence. However, conflicts of interest remain a relatively underexplored factor in the evolutionary-ecological study of warfare, with little consideration given to which parties influence the decision to go to war and how their motivations may differ. We develop a mathematical model to investigate the interplay between sex-specific demography and human warfare, showing that: the ecology of warfare drives the evolution of sex-biased dispersal; sex-biased dispersal modulates intrafamily and intragenomic conflicts in relation to warfare; intragenomic conflict drives parent-of-origin-specific patterns of gene expression—i.e. ‘genomic imprinting’—in relation to warfare phenotypes; and an ecological perspective of conflicts at the levels of the gene, individual, and social group yields novel predictions as to pathologies associated with mutations and epimutations at loci underpinning human violence.

Parents want daughters- and sons-in-law who are similar to their children, and children want spouses who are similar to themselves. In turn, the question arises: Do parents agree among themselves on how much similarity they desire in their prospective in-laws concerning their children? Moreover, do parents and children agree on the level of similarity they desire in an in-law and a spouse, respectively? Furthermore, to exercise an in-law preference for similarity, parents need to assess how their children score in traits deemed desirable in a spouse. This raises the question of whether mothers and fathers perceive their children similarly, and whether they perceive their children in the same way their children perceive themselves in these traits. The current study aimed to address these questions using a sample of 356 families from China, focusing on four desirable traits: good looks, good providers, good economic prospects, and good family background. Our results indicated that parents preferred sons- and daughters-in-law who were similar to their children, and mothers and fathers concurred on the level of similarity they desired between their children and their in-laws. Additionally, parents desired as much similarity between their children and their in-laws as their children desired between themselves and their spouses. Furthermore, we found that mothers and fathers concurred on how they perceive their children across the four desirable traits. Finally, both parents scored their children higher in these traits than their children scored themselves.

Offspring are often produced in excess as insurance against stochastic events or unpredictable resources. This strategy may result in high early-life mortality, yet age-specific mortality before offspring independence and its associated costs have rarely been quantified. In this study, we modelled age-specific survival from hatching to fledging using 24 years of data on hatching order (HO), growth and age of mortality of more than 15 000 common tern (Sterna hirundo) chicks. We found that mortality peaked directly after hatching, after which it declined rapidly. Mortality hazard was best described with the Gompertz function, and was higher with later HO, mainly due to differences in baseline mortality hazard, rather than age-dependent mortality. Based on allometric mass–metabolism relationships and detailed growth curves of starving chicks, we estimated that the average metabolizable energy intake of non-fledged chicks was only 8.7% of the metabolizable energy intake of successful chicks during the nestling phase. Although 54% of hatchlings did not fledge, our estimates suggest them to have consumed only 9.3% of the total energy consumption of all hatched chicks in the population before fledging. We suggest that rapid mortality of excess offspring is part of an adaptive brood reduction strategy to the benefit of the parents.

Survey results have shown that the traits women seek in a partner are different from the traits parents seek in a son-in-law. These differences have been attributed to parent–offspring conflict, where parents prefer mates for their offspring who provide benefits to the entire family group, but adult women prefer traits in a potential partner that indicate heritable fitness (e.g. creativity, exciting personality). We compare the characteristics of husbands of women in self-choice and arranged marriages using data from the longitudinal Indonesia Family Life Survey (IFLS) which surveyed families between 1993 and 2015. Results show that the husbands of women in arranged marriages had lower levels of completed education than those from self-choice marriages, counter to predictions. There were no significant differences in the husband’s wealth prior to marriage or the proportion of couples who were of the same religion. An examination of personality traits showed little difference based on arranged marriage status. The only prediction that had significant support was that couples in arranged marriages were more likely to share an ethnic background than couples in self-choice marriages. These results suggest that the characteristics of husbands vary little by arranged versus self-choice marriage status, contrary to previous survey findings.

Family members show behavioural strategies predicted to maximize individual fitness. These behaviours depend directly on genes expressed in focal individuals but also indirectly on genes expressed in other family members. However, how sibling and parental behavioural strategies are modified by genes expressed in family members, and to what degree, remains unclear. To answer this question, we have used a split litter design in an experimental population of genetically variable mouse families, and identified loci that indirectly affected sibling and maternal behaviour simultaneously. These loci map to genomic regions that also show a direct effect on offspring behaviour. Directly and indirectly affected traits were significantly correlated at the phenotypic level, illustrating how indirect effects are caused. Genetic variants in offspring that influence solicitation also impacted their siblings' and maternal behaviour. However, in contrast to predictions from sibling competition, unrelated litter mates benefited from increased solicitation. Overall, such indirect genetic effects explained a large proportion of variation seen in behaviours, with candidate genes involved in metabolism to neuronal development. These results reveal that we need to view behavioural strategies as the result of conjoint selection on genetic variation in all interacting family members.

Social structures such as families emerge as outcomes of behavioural interactions among individuals, and can evolve over time if families with particular types of social structures tend to leave more individuals in subsequent generations. The social behaviour of interacting individuals is typically analysed as a series of multiple dyadic (pair-wise) interactions, rather than a network of interactions among multiple individuals. However, in species where parents feed dependant young, interactions within families nearly always involve more than two individuals simultaneously. Such social networks of interactions at least partly reflect conflicts of interest over the provision of costly parental investment. Consequently, variation in family network structure reflects variation in how conflicts of interest are resolved among family members. Despite its importance in understanding the evolution of emergent properties of social organization such as family life and cooperation, nothing is currently known about how selection acts on the structure of social networks. Here, we show that the social network structure of broods of begging nestling great tits Parus major predicts fitness in families. Although selection at the level of the individual favours large nestlings, selection at the level of the kin-group primarily favours families that resolve conflicts most effectively.

Mate preferences probably evolved to increase fitness; however, studies using arranged and non-arranged marriage as proxies for limited and free mate choice (respectively) do not find any reproductive differences. We explore why arranged and non-arranged marriages are an imperfect proxy for limited and free-choice matings and what fitness effects different marriage types could produce. Data from focus group discussions with men and women in Nepal show that there are three spouse choice categories with differing levels of parental influence over mate choice, reinforcing that arranged and non-arranged are not dichotomous. Discussions also show that parents and offspring, especially sons, may be more aligned in in-law/mate preferences than expected, demonstrating the need to establish clear domains of parent–offspring disagreement over spouse choice in the community before investigating fitness. Several social and financial benefits that are only available to arranged couples in this community were detected, and these benefits could compensate for any costs of not choosing a spouse independently. These benefits of arranged marriage are more salient for women than for men. These discussions indicate that predictions about the effects of spouse choice on fitness outcomes may differ for men and women and depend on community-specific socioeconomic benefits.

In spite of strong selection by time-dependent mortality on the length of the embryo development (incubation) period, time to hatching varies substantially among species, independently of body size. One view, strongly supported by the work of Thomas Martin and his colleagues, maintains that this variation reflects parental strategies to minimize their own mortality risk at the nest—strategies that influence egg temperature and embryo growth rate. A second, not incompatible, view maintains that variation in the incubation period reflects a trade-off between the growth rate of the embryo and its subsequent quality as a free-living individual. We evaluate several lines of evidence relating nest attendance by adults and the quality of the immune system to the length of the incubation period. Particularly important is the role of sibling competition in selecting for rapid embryo growth and early hatching, and the fact that many species with prolonged incubation periods are raised either as single chicks or in broods with staggered hatching, which predetermines the outcome of sibling competition.