Social immunity of the family: parental contributions to a public good modulated by brood size

Evolutionary change in the construction of the nursery environment when parents are prevented from caring for their young directly

Parental care can be partitioned into traits that involve direct engagement with offspring and traits that are expressed as an extended phenotype and influence the developmental environment, such as constructing a nursery. Here, we use experimental evolution to test whether parents can evolve modifications in nursery construction when they are experimentally prevented from supplying care directly to offspring. We exposed replicate experimental populations of burying beetles (Nicrophorus vespilloides) to different regimes of posthatching care by allowing larvae to develop in the presence (Full Care) or absence of parents (No Care). After only 13 generations of experimental evolution, we found an adaptive evolutionary increase in the pace at which parents in the No Care populations converted a dead body into a carrion nest for larvae. Cross-fostering experiments further revealed that No Care larvae performed better on a carrion nest prepared by No Care parents than did Full Care larvae. We conclude that parents construct the nursery environment in relation to their effectiveness at supplying care directly, after offspring are born. When direct care is prevented entirely, they evolve to make compensatory adjustments to the nursery in which their young will develop. The rapid evolutionary change observed in our experiments suggests there is considerable standing genetic variation for parental care traits in natural burying beetle populations-for reasons that remain unclear.

Convergence of Social Strategies in Carrion Breeding Insects

Carrion is a highly ephemeral and nutrient rich resource, characterized by extreme biotic and abiotic stressors. We hypothesized that specific constraints of the carrion ecosystem, and especially its nutrient richness, ephemerality, and competition with microbes, have promoted the evolution of social behaviors in necrophagous insects. We show that group living is prevalent among early succession carrion breeding insects, suggesting that this trait has emerged as an adaptation to facilitate survival in the highly competitive environment of fresh carrion. We then highlight how developmental niche construction allows larvae to compete with microbes, efficiently feed on fresh cadavers, and rapidly reach maturity. We observed that larval societies and parental care are two different strategies responding to similar competitive and environmental constraints. We conclude that intra and interspecific competition on carrion are mitigated by social behavior.

Offspring reverse transcriptome responses to maternal deprivation when reared with pathogens in an insect with facultative family life

Offspring of species with facultative family life are able to live with and without parents (i.e. to adjust to extreme changes in their social environment). While these adjustments are well understood on a phenotypic level, their genetic underpinnings remain surprisingly understudied. Investigating gene expression changes in response to parental absence may elucidate the genetic constraints driving evolutionary transitions between solitary and family life. Here, we manipulated maternal presence to observe gene expression changes in the fat body of juvenile European earwigs, an insect with facultative family life. Because parents typically protect offspring against pathogens, expression changes were recorded in pathogen-free and pathogen-exposed environments. We found that manipulating maternal presence changed the expression of 154 genes, including several metabolism and growth-related genes, and that this change depended on pathogen presence. Specifically, localization and cell transporter genes were downregulated in maternal absence without pathogens but upregulated with pathogens. At least one immunity gene (pathogenesis-related protein 5) was affected by pathogen exposure regardless of maternal presence. Overall, our findings explicate how offspring adjust to parental deprivation on a molecular level and reveal that such adjustments heavily depend on pathogens in the environment. This emphasizes the central role of pathogens in family life evolution.

An evolutionary switch from sibling rivalry to sibling cooperation, caused by a sustained loss of parental care

Sibling rivalry is commonplace within animal families, yet offspring can also work together to promote each other's fitness. Here we show that the extent of parental care can determine whether siblings evolve to compete or to cooperate. Our experiments focus on the burying beetle Nicrophorus vespilloides, which naturally provides variable levels of care to its larvae. We evolved replicate populations of burying beetles under two different regimes of parental care: Some populations were allowed to supply posthatching care to their young (Full Care), while others were not (No Care). After 22 generations of experimental evolution, we found that No Care larvae had evolved to be more cooperative, whereas Full Care larvae were more competitive. Greater levels of cooperation among larvae compensated for the fitness costs caused by parental absence, whereas parental care fully compensated for the fitness costs of sibling rivalry. We dissected the evolutionary mechanisms underlying these responses by measuring indirect genetic effects (IGEs) that occur when different sibling social environments induce the expression of more cooperative (or more competitive) behavior in focal larvae. We found that indirect genetic effects create a tipping point in the evolution of larval social behavior. Once the majority of offspring in a brood start to express cooperative (or competitive) behavior, they induce greater levels of cooperation (or competition) in their siblings. The resulting positive feedback loops rapidly lock larvae into evolving greater levels of cooperation in the absence of parental care and greater levels of rivalry when parents provide care.

Defences against brood parasites from a social immunity perspective

Parasitic interactions are so ubiquitous that all multicellular organisms have evolved a system of defences to reduce their costs, whether the parasites they encounter are the classic parasites which feed on the individual, or brood parasites which usurp parental care. Many parallels have been drawn between defences deployed against both types of parasite, but typically, while defences against classic parasites have been selected to protect survival, those against brood parasites have been selected to protect the parent's inclusive fitness, suggesting that the selection pressures they impose are fundamentally different. However, there is another class of defences against classic parasites that have specifically been selected to protect an individual's inclusive fitness, known as social immunity. Social immune responses include the anti-parasite defences typically provided for others in kin-structured groups, such as the antifungal secretions produced by termite workers to protect the brood. Defences against brood parasites, therefore, are more closely aligned with social immune responses. Much like social immunity, host defences against brood parasitism are employed by a donor (a parent) for the benefit of one or more recipients (typically kin), and as with social defences against classic parasites, defences have therefore evolved to protect the donor's inclusive fitness, not the survival or ultimately the fitness of individual recipients This can lead to severe conflicts between the different parties, whose interests are not always aligned. Here, we consider defences against brood parasitism in the light of social immunity, at different stages of parasite encounter, addressing where conflicts occur and how they might be resolved. We finish with considering how this approach could help us to address longstanding questions in our understanding of brood parasitism. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

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.

Fitness costs of phoretic nematodes in the burying beetle, Nicrophorus vespilloides

Nicrophorus vespilloides is a social beetle that rears its offspring on decomposing carrion. Wild beetles are frequently associated with two types of macrobial symbionts, mites, and nematodes. Although these organisms are believed to be phoretic commensals that harmlessly use beetles as a means of transfer between carcasses, the role of these symbionts on N. vespilloides fitness is poorly understood. Here, we show that nematodes have significant negative effects on beetle fitness across a range of worm densities and also quantify the density-dependent transmission of worms between mating individuals and from parents to offspring. Using field-caught beetles, we provide the first report of a new nematode symbiont in N. vespilloides, most closely related to Rhabditoides regina, and show that worm densities are highly variable across individuals isolated from nature but do not differ between males and females. Next, by inoculating mating females with increasing densities of nematodes, we show that worm infections significantly reduce brood size, larval survival, and larval mass, and also eliminate the trade-off between brood size and larval mass. Finally, we show that nematodes are efficiently transmitted between mating individuals and from mothers to larvae, directly and indirectly via the carcass, and that worms persist through pupation. These results show that the phoretic nematode R. regina can be highly parasitic to burying beetles but can nevertheless persist because of efficient mechanisms of intersexual and intergenerational transmission. Phoretic species are exceptionally common and may cause significant harm to their hosts, even though they rely on these larger species for transmission to new resources. However, this harm may be inevitable and unavoidable if transmission of phoretic symbionts requires nematode proliferation. It will be important to determine the generality of our results for other phoretic associates of animals. It will equally be important to assess the fitness effects of phoretic species under changing resource conditions and in the field where diverse interspecific interactions may exacerbate or reduce the negative effects of phoresy.

Social immunity: why we should study its nature, evolution and functions across all social systems

Mounting defences against pathogens is a necessity for all animals. Although these defences have long been known to rely on individual processes such as the immune system, recent studies have emphasized the importance of social defences for group-living hosts. These defences, called social immunity, have been mostly studied in eusocial insects such as bees, termites and ants, and include, for instance, mutual cleaning and waste management. Over the last few years, however, a growing number of works called for a broader exploration of social immunity in non-eusocial species. In this review, we summarize the rationales of this call and examine why it may provide major insights into our current understanding of the role of pathogens in social evolution. We start by presenting the original conceptual framework of social immunity developed in eusocial insects and shed light on its importance in highly derived social systems. We then clarify three major misconceptions possibly fostered by this original framework and demonstrate why they made necessary the shift towards a broader definition of social immunity. Because a broader definition still needs boundaries, we finally present three criteria to discriminate what is a form of social immunity, from what is not. Overall, we argue that studying social immunity across social systems does not only provide novel insights into how pathogens affect the evolution of eusociality, but also of the emergence and maintenance of social life from a solitary state. Moreover, this broader approach offers new scopes to disentangle the common and specific anti-pathogen defences developed by eusocial and non-eusocial hosts, and to better understand the dependent and independent evolutionary drivers of social and individual immunity.

The other insect societies: overview and new directions

The diversity of societies and forms of social interaction across the Arthropoda is commensurate with the great taxonomic diversity within this pylum. Social evolution research has, however, largely focused on a small subset of social forms; namely, those deemed to be 'eusocial'-groups exhibiting overlapping generations, cooperative brood care, and reproductive division of labor. Here I provide a brief overview of the 'other', non-eusocial, societies of insects and allies, defining the main social traits of interest and summarizing recent work. Four active and emerging fields of inquiry in the other insect societies are discussed.

Social immunity in honeybees-Density dependence, diet, and body mass trade-offs

Group living is favorable to pathogen spread due to the increased risk of disease transmission among individuals. Similar to individual immune defenses, social immunity, that is antiparasite defenses mounted for the benefit of individuals other than the actor, is predicted to be altered in social groups. The eusocial honey bee (Apis mellifera) secretes glucose oxidase (GOX), an antiseptic enzyme, throughout its colony, thereby providing immune protection to other individuals in the hive. We conducted a laboratory experiment to investigate the effects of group density on social immunity, specifically GOX activity, body mass and feeding behavior in caged honey bees. Individual honeybees caged in a low group density displayed increased GOX activity relative to those kept at a high group density. In addition, we provided evidence for a trade-off between GOX activity and body mass: Individuals caged in the low group density had a lower body mass, despite consuming more food overall. Our results provide the first experimental evidence that group density affects a social immune response in a eusocial insect. Moreover, we showed that the previously reported trade-off between immunity and body mass extends to social immunity. GOX production appears to be costly for individuals, and potentially the colony, given that low body mass is correlated with small foraging ranges in bees. At high group densities, individuals can invest less in social immunity than at low densities, while presumably gaining shared protection from infection. Thus, there is evidence that trade-offs at the individual level (GOX vs. body mass) can affect colony-level fitness.