The other facets of family life and their role in the evolution of animal sociality

Family life forms an integral part of the life history of species across the animal kingdom and plays a crucial role in the evolution of animal sociality. Our current understanding of family life, however, is almost exclusively based on studies that (i) focus on parental care and associated family interactions (such as those arising from sibling rivalry and parent‐offspring conflict), and (ii) investigate these phenomena in the advanced family systems of mammals, birds, and eusocial insects. Here, we argue that these historical biases have fostered the neglect of key processes shaping social life in ancestral family systems, and thus profoundly hamper our understanding of the (early) evolution of family life. Based on a comprehensive survey of the literature, we first illustrate that the strong focus on parental care in advanced social systems has deflected scrutiny of other important social processes such as sibling cooperation, parent–offspring competition and offspring assistance. We then show that accounting for these neglected processes – and their changing role over time – could profoundly alter our understanding of the origin and subsequent evolution of family life. Finally, we outline how this ‘diachronic’ perspective on the evolution of family living provides novel insights into general processes driving the evolution of animal sociality. Overall, we infer that the explicit consideration of thus‐far neglected facets of family life, together with their study across the whole diversity of family systems, are crucial to advance our understanding of the processes that shape the evolution of social life.     

The pay‐offs of maternal care increase as offspring develop,favouring extended provisioning in an egg‐feeding frog

Offspring quantity and quality are components of parental fitness that cannot be maximized simultaneously. When the benefits of investing in offspring quality decline, parents are expected to shift investment towards offspring quantity (other reproductive opportunities). Even when mothers retain complete control of resource allocation, offspring control whether to allocate investment to growth or development towards independence, and this shared control may generate parent–offspring conflict over the duration of care. We examined these predictions by, in a captive colony, experimentally removing tadpoles of the strawberry poison frog (Oophaga pumilio) from the mothers that provision them with trophic eggs throughout development. Tadpoles removed from their mothers were no less likely to survive to nutritional independence (i.e. through metamorphosis) than were those that remained with their mothers, but these offspring were smaller at metamorphosis and were less likely to survive to reach adult size, even though they were fed ad libitum. Tadpoles that remained with their mothers developed more slowly than those not receiving care, a pattern that might suggest that offspring extracted more care than was in mothers’ best interests. However, the fitness returns of providing care increased with offspring development, suggesting that mothers would be best off continuing care until tadpoles initiated metamorphosis. Although the benefits of parental investment in offspring quality are often thought to asymptote at high levels, driving parent–offspring conflict over weaning, this assumption may not hold over natural ranges of investment, with selection on both parents and offspring favouring extended durations of parental care.     

Antipredator defense as a limited resource: unequal predation risk in broods of an insect with maternal care

The allocation of parental investment is a potential sourceof conflict within broods whenever offspring are able obtaindifferential access to the parental resource. Unlike the provisioningof food, parental antipredator behavior is usually considereda resource that benefits all offspring simultaneously. In thethornbug treehopper (Umbonia crassicornis), offspring formaggregations in exposed positions on host-plant stems. Theyare subject to intense predation, and maternal defense is theirprimary means of protection. I examined the distribution ofrisk within these offspring groups, using natural variationin the outcome of more than 500 predation attempts (324 recordedon videotape) by vespid wasps (Pseudopolybia compressa) on18 U. crassicornis aggregations. I found three influences onan individual offspring's risk of predation. The first wasthe presence of a defending female: as expected, offspringwere much more likely to survive contact with a wasp if thefemale was present than if the female had disappeared. Thesecond influence was position relative to other offspring: when wasps were successful in removing an individual, they almostalways removed it from the edge of the group. The third influencewas distance from the female: the closer an offspring was tothe female at the time it was contacted by a wasp, the higherits likelihood of survival. The distribution of risk is determinedlargely by the behavior of defending females and the prey-searchingbehavior of wasps. The nature of risk within these aggregations sets the stage for two forms of sibling rivalry: selfish herdbehavior and competition for access to maternal defense. Italso raises the question of how a parent should allocate defenseamong offspring when it is unable to defend them all simultaneously.     

Sex-specific sibling interactions and offspring fitness in vertebrates: patterns and implications for maternal sex ratios

Vertebrate sex ratios are notorious for their lack of fit to theoretical models, both with respect to the direction and the magnitude of the sex ratio adjustment. The reasons for this are likely to be linked to simplifying assumptions regarding vertebrate life histories. More specifically, if the sex ratio adjustment itself influences offspring fitness, due to sex-specific interactions among offspring, this could affect optimal sex ratios. A review of the literature suggests that sex-specific sibling interactions in vertebrates result from three major causes: (i) sex asymmetries in competitive ability, for example due to sexual dimorphism, (ii) sex-specific cooperation or helping, and (iii) sex asymmetries in non-competitive interactions, for example steroid leakage between fetuses. Incorporating sex-specific sibling interactions into a sex ratio model shows that they will affect maternal sex ratio strategies and, under some conditions, can repress other selection pressures for sex ratio adjustment. Furthermore, sex-specific interactions could also explain patterns of within-brood sex ratio (e.g. in relation to laying order). Failure to take sex-specific sibling interactions into account could partly explain the lack of sex ratio adjustment in accordance with theoretical expectations in vertebrates, and differences among taxa in sex-specific sibling interactions generate predictions for comparative and experimental studies.     

Giving offspring a head start in life: field and experimental evidence for selection on maternal basking behaviour in lizards

The timing of birth is often correlated with offspring fitness in animals, but experimental studies that disentangle direct effects of parturition date and indirect effects mediated via variation in female traits are rare. In viviparous ectotherms, parturition date is largely driven by female thermal conditions, particularly maternal basking strategies. Our field and laboratory studies of a viviparous lizard (Niveoscincus ocellatus) show that earlier‐born offspring are more likely to survive through their first winter and are larger following that winter, than are later‐born conspecifics. Thus, the association between parturition date and offspring fitness is causal, rather than reflecting an underlying correlation between parturition date and maternal attributes. Survival selection on offspring confers a significant advantage for increased maternal basking in this species, mediated through fitness advantages of earlier parturition. We discuss the roles of environmentally imposed constraints and parent–offspring conflict in the evolution of maternal effects on parturition date.     

A chemical signal of offspring quality affects maternal care in a social insect

Begging signals of offspring are condition-dependent cues that are usually predicted to display information about the short-term need (i.e. hunger) to which parents respond by allocating more food. However, recent models and experiments have revealed that parents, depending on the species and context, may respond to signals of quality (i.e. offspring reproductive value) rather than need. Despite the critical importance of this distinction for life history and conflict resolution theory, there is still limited knowledge of alternative functions of offspring signals. In this study, we investigated the communication between offspring and caring females of the common earwig, Forficula auricularia, hypothesizing that offspring chemical cues display information about nutritional condition to which females respond in terms of maternal food provisioning. Consistent with the prediction for a signal of quality we found that mothers exposed to chemical cues from well-fed nymphs foraged significantly more and allocated food to more nymphs compared with females exposed to solvent (control) or chemical cues from poorly fed nymphs. Chemical analysis revealed significant differences in the relative quantities of specific cuticular hydrocarbon compounds between treatments. To our knowledge, this study demonstrates for the first time that an offspring chemical signal reflects nutritional quality and influences maternal care.     

Density‐dependent offspring interactions do not explain macroevolutionary scaling of adult size and offspring size

Most life forms exhibit a correlated evolution of adult size (AS) and size at independence (SI), giving rise to AS–SI scaling relationships. Theory suggests that scaling arises because relatively large adults have relatively high reproductive output, resulting in strong density‐dependent competition in early life, where large size at independence provides a competitive advantage to juveniles. The primary goal of our study is to test this density hypothesis, using large datasets that span the vertebrate tree of life (fishes, amphibians, reptiles, birds, and mammals). Our secondary goal is to motivate new hypotheses for AS–SI scaling by exploring how subtle variation in life‐histories among closely related species is associated with variation in scaling. Our phylogenetically informed comparisons do not support the density hypothesis. Instead, exploration of AS–SI scaling among life‐history variants suggests that steeper AS–SI scaling slopes are associated with evolutionary increases in size at independence. We suggest that a positive association between size at independence and juvenile growth rate may represent an important mechanism underlying AS–SI scaling, a mechanism that has been underappreciated by theorists. If faster juvenile growth is a consequence of evolutionary increases in size at independence, this may help offset the cost of delayed maturation, leading to steeper AS–SI scaling slopes.     

Maternal care,mother–offspring aggregation and age‐dependent coadaptation in the European earwig

Benefits and costs of parental care are expected to change with offspring development and lead to age‐dependent coadaptation expressed as phenotypic (behavioural) matches between offspring age and parental reproductive stage. Parents and offspring interact repeatedly over time for the provision of parental care. Their behaviours should be accordingly adjusted to each other dynamically and adaptively, and the phenotypic match between offspring age and parental stage should stabilize the repeated behavioural interactions. In the European earwig (Forficula auricularia), maternal care is beneficial for offspring survival, but not vital, allowing us to investigate the extent to which the stability of mother–offspring aggregation is shaped by age‐dependent coadaptation. In this study, we experimentally cross‐fostered nymphs of different age classes (younger or older) between females in early or late reproductive stage to disrupt age‐dependent coadaptation, thereby generating female–nymph dyads that were phenotypically matched or mismatched. The results revealed a higher stability in aggregation during the first larval instar when care is most intense, a steeper decline in aggregation tendency over developmental time and a reduced developmental rate in matched compared with mismatched families. Furthermore, nymph survival was positively correlated with female–nymph aggregation stability during the early stages when maternal care is most prevalent. These results support the hypothesis that age‐related phenotypically plastic coadaptation affects family dynamics and offspring developmental rate.     

Complex offspring size effects: variations across life stages and between species

Classical optimality models of offspring size and number assume a monotonically increasing relationship between offspring size and performance. In aquatic organisms with complex life cycles, the size–performance function is particularly hard to grasp because measures of performance are varied and their relationships with size may not be consistent throughout early ontogeny. Here, we examine size effects in premetamorphic (larval) and postmetamorphic (juvenile) stages of brooding marine animals and show that they vary contextually in strength and direction during ontogeny and among species. Larger offspring of the sea anemone Urticina felina generally outperformed small siblings at the larval stage (i.e., greater settlement and survival rates under suboptimal conditions). However, results differed when analyses were conducted at the intrabrood versus across‐brood levels, suggesting that the relationship between larval size and performance is mediated by parentage. At the juvenile stage (15 months), small offspring were less susceptible than large ones to predation by subadult nudibranchs and both sizes performed similarly when facing adult nudibranchs. In a sympatric species with a different life history (Aulactinia stella), all juveniles suffered similar predation rates by subadult nudibranchs, but smaller juveniles performed better (lower mortalities) when facing adult nudibranchs. Size differences in premetamorphic performance of U. felina were linked to total lipid contents of larvae, whereas size‐specific predation of juvenile stages followed the general predictions of the optimal foraging strategy. These findings emphasize the challenge in gathering empirical support for a positive monotonic size–performance function in taxa that exhibit complex life cycles, which are dominant in the sea.     

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.     

Parents and offspring in an evolutionary game: the effect of supply on demand when costs of care vary

Current models of parent-offspring communication do not explicitly predict the effect of parental food supply on offspring demand (ESD). However, existing theory is frequently interpreted as predicting a negative ESD, such that offspring beg less when parental supply is high. While empirical evidence largely supports this interpretation, several studies have identified the opposite case, with well-fed offspring begging more than those in poorer condition. Here, we show that signalling theory can give rise to either a negative or a positive ESD depending on the precise form of costs and benefits. Introducing variation among parents in the cost of care, we show that the ESD may change sign depending upon the quantitative relation between two effects: (i) decreased supply leads to increased begging because of an increase in marginal fitness benefit of additional resources to offspring, (ii) decreased supply leads to reduced begging because it is associated with a decrease in parental responsiveness, rendering begging less effective. To illustrate the interplay between these two effects, we show that Godfray's seminal model of begging yields a negative ESD when care is generally cheap, because the impact of supply on the marginal benefits of additional resources then outweighs the associated changes in parental responsiveness to begging. By contrast, the same model predicts a positive ESD when care is generally costly, because the impact of care costs on parental responsiveness then outweighs the change in marginal benefits.     

The positive correlation between maternal size and offspring size: fitting pieces of a life‐history puzzle

The evolution of investment per offspring (I) is often viewed through the lens of the classic theory, in which variation among individuals in a population is not expected. A substantial departure from this prediction arises in the form of correlations between maternal body size and I, which are observed within populations in virtually all taxonomic groups. Based on the generality of this observation, we suggest it is caused by a common underlying mechanism. We pursue a unifying explanation for this pattern by reviewing all theoretical models that attempt to explain it. We assess the generality of the mechanism upon which each model is based, and the extent to which data support its predictions. Two classes of adaptive models are identified: models that assume that the correlation arises from maternal influences on the relationship between I and offspring fitness [w(I)], and those that assume that maternal size influences the relationship between I and maternal fitness [W(I)]. The weight of evidence suggests that maternal influences on w(I) are probably not very general, and even for taxa where maternal influences on w(I) are likely, experiments fail to support model predictions. Models that assume that W(I) varies with maternal size appear to offer more generality, but the current challenge is to identify a specific and general mechanism upon which W(I) varies predictably with maternal size. Recent theory suggests the exciting possibility that a yet unknown mechanism modifies the offspring size–number trade‐off function in a manner that is predictable with respect to maternal size, such that W(I) varies with size. We identify two promising avenues of inquiry. First, the trade‐off might be modified by energetic costs that are associated with the initiation of reproduction (‘overhead costs’) and that scale with I, and future work could investigate what specific overhead costs are generally associated with reproduction and whether these costs scale with I. Second, the trade‐off might be modified by virtue of condition‐dependent offspring provisioning coupled with metabolic factors, and future work could investigate the proximate cause of, and generality of, condition‐dependent offspring provisioning. Finally, drawing on the existing literature, we suggest that maternal size per se is not causatively related to variation in I, and the mechanism involved in the correlation is instead linked to maternal nutritional status or maternal condition, which is usually correlated with maternal size. Using manipulative experiments to elucidate why females with high nutritional status typically produce large offspring might help explain what specific mechanism underlies the maternal‐size correlation.     

Family dynamics through time: brood reduction followed by parental compensation with aggression and favouritism

Parental food allocation in birds has long been a focal point for life history and parent–offspring conflict theories. In asynchronously hatching species, parents are thought to either adjust brood size through death of marginal offspring (brood reduction), or feed the disadvantaged chicks to reduce the competitive hierarchy (parental compensation). Here, we show that parent American coots (Fulica americana) practice both strategies by switching from brood reduction to compensation across time. Late‐hatching chicks suffer higher mortality only for the first few days after hatching. Later, parents begin to exhibit parental aggression towards older chicks and each parent favours a single chick, both of which are typically the youngest of the surviving offspring. The late‐hatched survivors can equal or exceed their older siblings in size prior to independence. A mixed allocation strategy allows parents to compensate for the costs of competitive hierarchies while gaining the benefits of hatching asynchrony.     

Bigger is better: changes in body size explain a maternal effect of food on offspring disease resistance

Maternal effects triggered by changes in the environment (e.g., nutrition or crowding) can influence the outcome of offspring–parasite interactions, with fitness consequences for the host and parasite. Outside of the classic example of antibody transfer in vertebrates, proximate mechanisms have been little studied, and thus, the adaptive significance of maternal effects on infection is not well resolved. We sought to determine why food‐stressed mothers give birth to offspring that show a low rate of infection when the crustacean Daphnia magna is exposed to an orally infective bacterial pathogen. These more‐resistant offspring are also larger at birth and feed at a lower rate. Thus, reduced disease resistance could result from slow‐feeding offspring ingesting fewer bacterial spores or because their larger size allows for greater immune investment. To distinguish between these theories, we performed an experiment in which we measured body size, feeding rate, and susceptibility, and were able to show that body size is the primary mechanism causing altered susceptibility: Larger Daphnia were less likely to become infected. Contrary to our predictions, there was also a trend that fast‐feeding Daphnia were less likely to become infected. Thus, our results explain how a maternal environmental effect can alter offspring disease resistance (though body size), and highlight the potential complexity of relationship between feeding rate and susceptibility in a host that encounters a parasite whilst feeding.     

Short-term benefits,but transgenerational costs of maternal loss in an insect with facultative maternal care

A lack of parental care is generally assumed to entail substantial fitness costs for offspring that ultimately select for the maintenance of family life across generations. However, it is unknown whether these costs arise when parental care is facultative, thus questioning their fundamental importance in the early evolution of family life. Here, we investigated the short-term, long-term and transgenerational effects of maternal loss in the European earwig Forficula auricularia, an insect with facultative post-hatching maternal care. We showed that maternal loss did not influence the developmental time and survival rate of juveniles, but surprisingly yielded adults of larger body and forceps size, two traits associated with fitness benefits. In a cross-breeding/cross-fostering experiment, we then demonstrated that maternal loss impaired the expression of maternal care in adult offspring. Interestingly, the resulting transgenerational costs were not only mediated by the early-life experience of tending mothers, but also by inherited, parent-of-origin-specific effects expressed in juveniles. Orphaned females abandoned their juveniles for longer and fed them less than maternally-tended females, while foster mothers defended juveniles of orphaned females less well than juveniles of maternally-tended females. Overall, these findings reveal the key importance of transgenerational effects in the early evolution of family life.     

Influences of Parental Care and Food Deprivation on Regulation of Body Mass in a Burying Beetle

Changes in adult body mass during breeding can reveal how parents prepare energetically for care, the stress of care, and the need to terminate care in a state conducive for future reproduction. Interpreting changes in parent mass can be difficult, however, because temporal variation in body mass may reflect a constraint imposed by the stress of care, revealing conflict within the family, or a shift to a new body mass optimum adaptive for a different stage of the breeding cycle. Here, we examined the effect of food deprivation and parenting on variation in female body mass of Nicrophorus orbicollis, an insect in which parents and offspring share a common food resource (a prepared carrion ball). Female parents demonstrated a remarkable degree of regulation of body mass: Despite varied periods of food deprivation (0–8 d) prior to discovery of a carcass, females attained a similar body mass (108.3–109.2% of pre‐deprivation mass) at the time of larval hatching. Females attained a greater body mass in anticipation of rearing a greater number of young. Mothers lost mass during active parental care, and mass at the end of caregiving was less in mothers that reared more and heavier young. Body mass at the end of care was less than the preferred mass for females searching for a carcass, indicating that the mothers sacrificed self‐maintenance and future reproductive potential for their current brood. Contrary to prediction, pre‐breeding food deprivation had no effect on offspring size or on female condition at the end of care. We conclude that there is a limited degree of conflict over the sharing of food among N. orbicollis parents and offspring, but that this conflict is not exacerbated by food deprivation prior to breeding.