When to bee social: interactions among environmental constraints, incentives, guarding, and relatedness in a facultatively social carpenter bee

In the facultatively social carpenter bee, Xylocopa pubescens,foundresses usually establish nests solitarily. However, nestsmay become social if a second foundress (referred to as alpha)successfully usurps the nest, with the original foundress (referredto as beta) remaining as a guard. Reproductive skew theory predictsthat beta foundresses should remain as helpers only if alphausurpers allow them a share of reproduction. Because alpha femalesdestroy much of beta's brood and beta females do not lay eggsafter takeovers, studies have concluded that usurpers offerno staying incentives or concessions in return for helping behavior.This conclusion is paradoxical, and we suggest that by refrainingfrom destroying all of beta's brood, alpha females do indeedoffer concessions to beta females. We constructed a model toexamine the conditions under which social nesting is favoredby both alpha and beta females. Female preference for socialversus solitary nesting is proportional to expected fitnessin either setting and is affected by current environmental conditions,the value of guarding behavior in protecting brood from pollenrobbery, the size of the concession offered by alpha, and thedegree of genetic relatedness between the foundresses. Our modelshows that at a minimum, establishing sociality after unrelatedusurpations always requires a concession, whereas in relatedusurpations, a concession is not always required. Generally,agreement between alpha and beta is difficult because alpharequires a much higher premium in pollen robbery protectionthan beta in order for sociality to be advantageous. Alpha femalesprefer social nesting only under the most severe environmentalconditions because usually they gain less by the presence ofa guard than by replacing beta brood with their own. In contrast,beta females always strongly prefer social nesting because thechances of successful renesting elsewhere are low and rarelyoutweigh the advantages of guarding their own brood that surviveusurpation. Effects of relatedness between foundresses on preferencefor social nesting are nonintuitive: first, alpha's preferenceincreases with relatedness, whereas beta's preference declines,and second, unrelated beta females prefer sociality more stronglythan related ones. This is because replacement of beta's offspringwith related alpha offspring partially compensates her for theloss of her own brood, even should she leave the nest.     

Life history and the evolution of family living in birds

The reason why some bird species live in family groups is an important question of evolutionary biology that remains unanswered. Families arise when young delay the onset of independent reproduction and remain with their parents beyond independence. Explanations for why individuals forgo independent reproduction have hitherto focused on dispersal constraints, such as the absence of high-quality breeding openings. However, while constraints successfully explain within-population dispersal decisions, they fail as an ultimate explanation for variation in family formation across species. Most family-living species are long-lived and recent life-history studies demonstrated that a delayed onset of reproduction can be adaptive in long-lived species. Hence, delayed dispersal and reproduction might be an adaptive life-history decision rather than 'the best of a bad job'. Here, we attempt to provide a predictive framework for the evolution of families by integrating life-history theory into family formation theory. We suggest that longevity favours a delayed onset of reproduction and gives parents the opportunity of a prolonged investment in offspring, an option which is not available for short-lived species. Yet, parents should only prolong their investment in offspring if this increases offspring survival and outweighs the fitness cost that parents incur, which is only possible under ecological conditions, such as a predictable access to resources. We therefore propose that both life-history and ecological factors play a role in determining the evolution of family living across species, yet we suggest different mechanisms than those proposed by previous models.     

The behavioural immune system and the psychology of human sociality

Because immunological defence against pathogens is costly and merely reactive, human anti-pathogen defence is also characterized by proactive behavioural mechanisms that inhibit contact with pathogens in the first place. This behavioural immune system comprises psychological processes that infer infection risk from perceptual cues, and that respond to these perceptual cues through the activation of aversive emotions, cognitions and behavioural impulses. These processes are engaged flexibly, producing context-contingent variation in the nature and magnitude of aversive responses. These processes have important implications for human social cognition and social behaviour-including implications for social gregariousness, person perception, intergroup prejudice, mate preferences, sexual behaviour and conformity. Empirical evidence bearing on these many implications is reviewed and discussed. This review also identifies important directions for future research on the human behavioural immune system--including the need for enquiry into underlying mechanisms, additional behavioural consequences and implications for human health and well-being.     

Does it Pay to Care?: Exploring the Costs and Benefits of Parental Care in the Hibiscus Harlequin Bug Tectocoris diophthalmus (Heteroptera: Scutelleridae)

Knowledge of the selective pressures favouring parental care can inform our understanding of the evolutionary origins and transitions of sociality in insects. Here, we report upon investigations designed to estimate the costs and benefits of parental care in the egg‐guarding hibiscus harlequin bug Tectocoris diophthalmus (Heteroptera: Scutelleridae). We found that the presence of a guarding female had no effect on hatching success under benign laboratory conditions. In the wild, however, guarding decreased the likelihood of total clutch failure, and produced a fourfold greater egg‐hatching rate relative to unguarded clutches. Females guarded against generalist invertebrate egg predators, including conspecific nymphs, but were ineffective against hymenopteran egg parasitoids. Females continued to feed during the guarding period and exhibited no signs of weight loss or increased mortality due to this behaviour. We did not observe the production of subsequent clutches in any experimental females; therefore, the lifetime fecundity costs of providing parental care in T. diophthalmus remain indeterminate. Overall, maternal egg guarding appears to function as a relatively low cost–low benefit strategy that increases hatching success by protecting against predation – but not parasitism.     

Implications of the behavioural immune system for social behaviour and human health in the modern world

The ‘behavioural immune system’ is composed of mechanisms that evolved as a means of facilitating behaviours that minimized infection risk and enhanced fitness. Recent empirical research on human populations suggests that these mechanisms have unique consequences for many aspects of human sociality—including sexual attitudes, gregariousness, xenophobia, conformity to majority opinion and conservative sociopolitical attitudes. Throughout much of human evolutionary history, these consequences may have had beneficial health implications; but health implications in modern human societies remain unclear. This article summarizes pertinent ways in which modern human societies are similar to and different from the ecologies within which the behavioural immune system evolved. By attending to these similarities and differences, we identify a set of plausible implications—both positive and negative—that the behavioural immune system may have on health outcomes in contemporary human contexts. We discuss both individual-level infection risk and population-level epidemiological outcomes. We also discuss a variety of additional implications, including compliance with public health policies, the adoption of novel therapeutic interventions and actual immunological functioning. Research on the behavioural immune system, and its implications in contemporary human societies, can provide unique insights into relationships between fitness, sociality and health.