Anisolabis maritima (Dermaptera: Anisolabididae): The Maritime Earwig
Exaggerated male traits are seen throughout the animal kingdom, and have been the subject of an increasing number of studies in sexual selection. In polygynous mating systems where males mate multiply and females do not, this type of sexual dimorphism can be the result of female choice, where females mate preferentially based on the male’s expression of these, male competition, where males use armaments in intrasexual battles for access to females, or a combination of both. In such mating systems, males usually compete with each other for females in contests usually determined by differences in body size or weaponry which dictate fighting ability. Males may compete directly for access to females or may instead control resources essential for female survival and reproduction.
Earwigs are a very interesting group in which to study reproductive behavior because they are sexually dimorphic (with males have curved forceps while females have straight forceps), which is a strong indicator that there are different selection pressures on males and females linked to the mating system. In general, female earwigs are larger than males, presumably due to a fecundity advantage seen in many arthropods and the fact that females exhibit strong maternal care, often aggressively guarding eggs and juveniles for up to 28 days. In fact, among the nearly two thousand species of earwigs, all described species have extended parental care in the form of egg and nymph guarding. Anisolabis maritima, also known as the maritime or seaside earwig, is an insect found under driftwood and beach wrack (washed up seaweed) just above the high tide line. Although their likely origin is the Mediterranean region, the maritime earwig is now widely distributed throughout the world in temperate to tropical coastal habitats due to transportation via shipping. Like other members of the order Dermaptera, these flightless insects have cerci that have been modified and hardened into forceps that are used in capturing prey primarily consisting of small arthropods. Unlike other earwigs, however, A. maritima do not have any wing covers, which enables them to burrow very quickly as they seek to avoid desiccation and escape predators. Despite their large size (15 to 32 mm), shiny black color and important role as an intertidal predator, maritime earwigs are relatively inconspicuous on most beaches because they only emerge from their shelters to forage at night. The maritime earwig is particularly well-suited for studies of sexual selection because males differ markedly from females in body size (males are more variable in size, and often substantially larger, than females) and weaponry (males possess asymmetrical, curved forceps whereas females have straight forceps). These sexually dimorphic traits have likely played a significant role in the evolution of this insect’s mating system, where these forceps are used in competition for mates, prey capture, and maternal nest defense.
We are currently conducting lab and field experiments in which we examine the mating system of A. maritima, focusing primarily on the roles of sex, size and weaponry on intrasexual competition and intersexual mating preferences. We presented these results at the last four meetings for the Society of Integrative & Comparative Biology, and we have two publications (with others in prep) . By examining spatial distribution among trios of earwigs where movement was restricted to promote either intersexual choice or intrasexual competition, we found strong sexual selection for larger sizes through both competition and choice in both sexes (PDF). Currently we are examining continuous videotaping of pairwise interactions to determine how such interactions affect group dynamics. By monitoring the distribution of two individuals in larger arenas with either two shelters (no habitat limitation) or one shelter (habitat limitation), we found that females, whose high levels of aggression are often associated with maternal care, were particularly averse to cohabitation, whereas males were generally more tolerant of other earwigs (PDF). Currently we are also examining spatial distribution of maritime earwigs in large groups both in the lab and in the field to gain a better understanding of their social network.