The evolution of social inbreeding mating systems in spiders: limited male mating dispersal and lack of pre‐copulatory inbreeding avoidance in a subsocial predecessor

Cooperation and group living are extremely rare in spiders and only few species are known to be permanently social. Inbreeding is a key characteristic of social spiders, resulting in high degrees of within‐colony relatedness that may foster kin‐selected benefits of cooperation. Accordingly, philopatry and regular inbreeding are suggested to play a major role in the repeated independent origins of sociality in spiders. We conducted field observations and laboratory experiments to investigate the mating system of the subsocial spider Stegodyphus tentoriicola. The species is suggested to resemble the ‘missing link’ in the transition from subsociality to permanent sociality in Stegodyphus spiders because its social period is prolonged in comparison to other subsocial species. Individuals in our two study populations were spatially clustered around maternal nests, indicating that clusters consist of family groups as found in the subsocial congener Stegodyphus lineatus. Male mating dispersal was limited and we found no obvious pre‐copulatory inbreeding avoidance, suggesting a high likelihood of mating with close kin. Rates of polygamy were low, a pattern ensuring high relatedness within broods. In combination with ecological constraints, such as high costs of dispersal, our findings are consistent with the hypothesis that the extended social period in S. tentoriicola is accompanied with adaptations that facilitate the transition towards permanent sociality. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 851–859.     

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.     

The real females of human evolution

When “woman‐the‐gatherer” was first proposed as a counter to “man‐the‐hunter,” we were only beginning to understand the many faces of primate females. In ensuing decades we have learned about the skills and talents of female chimpanzees in tool using, hunting, and transmitting behaviors from one generation to the next as teachers, and across space as newcomers to neighboring communities. The perspective of evolutionary time highlights the continuity of female lives from the origin of mammals to the origin of Homo sapiens. The combination of behavioral, fossil, and archeological information aid in reconstructing key ingredients acquired by females along the way that contributed to the success of our species. © 2012 Wiley Periodicals, Inc.     

Convergent evolution in social swallows (Aves: Hirundinidae)

Behavioral shifts can initiate morphological evolution by pushing lineages into new adaptive zones. This has primarily been examined in ecological behaviors, such as foraging, but social behaviors may also alter morphology. Swallows and martins (Hirundinidae) are aerial insectivores that exhibit a range of social behaviors, from solitary to colonial breeding and foraging. Using a well‐resolved phylogenetic tree, a database of social behaviors, and morphological measurements, we ask how shifts from solitary to social breeding and foraging have affected morphological evolution in the Hirundinidae. Using a threshold model of discrete state evolution, we find that shifts in both breeding and foraging social behavior are common across the phylogeny of swallows. Solitary swallows have highly variable morphology, while social swallows show much less absolute variance in all morphological traits. Metrics of convergence based on both the trajectory of social lineages through morphospace and the overall morphological distance between social species scaled by their phylogenetic distance indicate strong convergence in social swallows, especially socially foraging swallows. Smaller physical traits generally observed in social species suggest that social species benefit from a distinctive flight style, likely increasing maneuverability and foraging success and reducing in‐flight collisions within large flocks. These results highlight the importance of sociality in species evolution, a link that had previously been examined only in eusocial insects and primates.     

The adaptive value of sociality in mammalian groups

According to behavioural ecology theory, sociality evolves when the net benefits of close association with conspecifics exceed the costs. The nature and relative magnitude of the benefits and costs of sociality are expected to vary across species and habitats. When sociality is favoured, animals may form groups that range from small pair-bonded units to huge aggregations. The size and composition of social groups have diverse effects on morphology and behaviour, ranging from the extent of sexual dimorphism to brain size, and the structure of social relationships. This general argument implies that sociality has fitness consequences for individuals. However, for most mammalian species, especially long-lived animals like primates, there are sizable gaps in the chain of evidence that links sociality and social bonds to fitness outcomes. These gaps reflect the difficulty of quantifying the cumulative effects of behavioural interactions on fitness and the lack of information about the nature of social relationships among individuals in most taxa. Here, I review what is known about the reproductive consequences of sociality for mammals.     

Exploiting new terrain: an advantage to sociality in the slime mold Dictyostelium discoideum

Understanding the ecological benefits of social actions is centralto explaining the evolution of social behavior. The social amoebaDictyostelium discoideum has been well studied and is a modelfor social evolution and development, but surprisingly littleis known about its ecology. When starving, thousands of thenormally solitary amoebae aggregate to form a differentiatedmulticellular organism known as a slug. The slug migrates towardthe soil surface where it metamorphoses into a fruiting bodyof hardy spores held up by a dead stalk comprising about one-fifthof the cells. Multicellularity in D. discoideum is thought tohave evolved to lift the spores above the hazards of the soilwhere spores can be picked up for long-distance dispersal. Here,we show that multicellularity has another advantage: local dispersalto new food sources. We find that cells shed by D. discoideumslugs during migration consume and remove bacteria in the pathof the slug, although slugs themselves do not breakup. We alsoshow that slugs are adept at local dispersal by comparing migrationof slugs with migration of individual cells of the mutant, CAP2,which cannot aggregate and so rely only on cellular movement.In particular, the solitary cells of the aggregation mutantare unable to cross a soil barrier, easily crossed by slugs.We propose that the exploitation of local food patches is animportant selective benefit favoring multicellular cooperationin D. discoideum.     

Effect of soil hardness on aggression in the solitary wasp Mellinus arvensis

1. Two alternative nesting strategies are exhibited by soil‐nesting Mellinus arvensis females, digging a new nest (diggers) and searching for an old unoccupied burrow (searchers). Wasps appear unable to distinguish between occupied and unoccupied nests, and aggressive interactions between searchers and nest owners at nest entrances are frequent. 2. In aggressive encounters, there is an advantage in size and residency status. 3. The costs associated with the two nesting strategies vary across geographically separated populations: nest digging incurs costs in terms of time, and these vary according to the hardness of the soil substrate; nest searching is variably costly in terms of risk of injury in aggressive encounters with nest‐owning females. 4. Individual female wasps can switch between nesting strategies, and thus soil hardness, by affecting the cost of nest construction, affects the relative frequencies of the two nesting strategies within a population, favouring an increase in the searching strategy. This, in turn, affects the frequency and intensity of aggression between females at a nesting aggregation. 5. Female body size is correlated with soil hardness. As aggressive encounters are more frequent in sites with hard soil substrates, there is increased selective advantage in having large body size at these sites. 6. Body size is determined primarily by the availability of food resources during larval development, which is, to a degree, a function of the size of the adult female. There is a trade‐off between provisioning a few cells with many provisions in each, leading to the development of few but large adults, as opposed to many cells with few provisions, leading to many small offspring. The relative advantage of these two provisioning strategies is, at least in part, a function of the hardness of the soil substrate.