Tale of Two Cities: The relationship of density and morphology varies among populations of the maritime earwig Anisolabis maritima

Abstract

For organisms that live in groups, the spatial distribution of individuals according to size and sex can provide insight into social interactions, including aggression (intrasexual selection for armaments) and mating preferences (intersexual selection for ornaments). To gain insight into how group dynamics may influence the mating system, we investigated the relationship of density and morphology in the maritime earwig, Anisolabis maritima, an insect found in high densities beneath pieces of driftwood above the high-tide line in coastal ecosystems throughout the world. There are fundamental differences in the behavior and morphology between the sexes. Males and females differ fundamentally in their aggression during agonistic encounters with conspecifics; males more readily cohabitate with conspecifics and resolve their disputes non-lethally whereas females often kill conspecifics in close proximity as they vigorously guard their offspring. Males also differ markedly from females in both body size (males are more variable in size, and sometimes substantially larger, than females) and weaponry (males possess asymmetrical, curved forceps whereas females have straight forceps). Given previously observed variation in both body size and forceps asymmetry, we investigated the possible correlation between population density and morphology of individuals in two populations on San Juan Island, WA to determine whether these parameters affect group dynamics and social interactions. Specifically, we lifted 10-15 logs at two sites (False Bay and Cattle Point) at two different times during the breeding season (June and July) and determined the overall density, average body size and average forceps asymmetry for individuals under each log. Comparing the area occupied to the body size and forceps asymmetry of individuals, we found that the relationship between body size and population density varied both by site and by the period in the breeding cycle. Additionally, we found a greater, more morphologically diverse population of males at False Bay, a site with lower tidal action and more predictable habitat availability. We posit, therefore, that the breeding cycle varies from site to site based on the stability of the environment, and that a more turbulent environment can disrupt the population and delay the breeding cycle, leading to a less diverse, more unstable population. Our results also suggest that larger individuals are more likely to live in higher densities before the breeding cycle, possibly due to their increased fighting ability and willingness to compete with others for mates, and at lower densities early in the breeding cycle due to their increased conspecific aggression during courtship and nesting. This research lays the foundation for future studies regarding the social dynamics of this species where we can monitor individual interactions and group distributions in a more controlled laboratory setting.