Understanding the Preliminary Stages of Social Evolution in Western Black Widow Spiders
Abstract
The transition from solitary to social living is considered one of the major evolutionary transitions. While much social evolutionary research considers costs and benefits associated with sociality, these studies are dominated by taxa with complex cooperative social behavior. Investigating species in the early stages of social evolution provides insight into evolutionary forces that underlie the initiation of sociality. I test key predictions of three hypotheses of social evolution in the western black widow spider, Latrodectus hesperus, which displays variation in sociality across its range, from solitary to facultative group living. I studied three focal populations, one facultatively group living population in British Columbia (BC), and two solitary populations, one from California (CA), and one from Texas (TX). I used an experimental approach to test patterns of kin discrimination, inbreeding depression, and aggression in two contexts across the solitary and social populations. I found that the social BC population had higher mating success with kin compared to nonkin, and did not suffer from inbreeding depression. The CA population showed the opposite pattern of higher mating success with nonkin and both solitary CA and TX populations suffered significant inbreeding depression. However, the BC population had the lowest fecundity overall, suggesting a history of inbreeding in this population. The social BC population also displayed the lowest levels of foraging aggression. However, group living individuals within BC were more aggressive than solitary living individuals. The solitary CA population was the only population that displayed ritualized escalation of agonistic behavior, suggesting an evolutionary history of high interindividual conflict. Finally, I used reduced representation sequencing to directly assess levels of inbreeding, genetic diversity, and gene flow across the three focal populations. I found that the social BC population had the highest levels of inbreeding, lowest genetic diversity, and that gene flow was restricted between all sampled populations. Together, these results suggest that an increase in relatedness and higher aggressiveness are factors that may be contributing to social group formation and continued social evolution in L. hesperus.
