Megalorchestia californiana (Amphipoda: Talitridae): The California Beach Flea
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.
Even in a harem polygyny mating system, both intra- and intersexual selection may occur because both sexes can benefit from this type of mating system.Males benefit by having access to multiple females because male reproductive success is limited by the number of mates. Females, by allowing males to compete, may also benefit by joining the harem of (and mating with) a male of high genetic quality. Females are by no means passive in a harem polygyny mating system, however, because exaggerated male characters can be used by both sexes to assess male quality: males that use such characters to accurately assess a rival’s quality before engaging in a potentially costly fight are favored by selection, and females often choose which harem to join based on assessing both phenotypic and genetic benefits.
The amphipod Megalorchestia californiana is found on sandy beaches from California to Washington. Adults, which reach over 1″ in size, are often found roaming the beaches in the evening scavenging washed up animals or eating seaweed. Females spend most of the daytime in burrows that are guarded by males. Competition for burrows can be fierce, and burrows appear to be a valuable resource by offering protection from predators, prevention of desiccation and a safe place to copulate and breed. Victorious males get the reward of mating with the females, who brood their eggs in an internal pouch and release fully active juveniles. This amphipod is particularly well-suited for studies of sexual selection because males differ markedly from females in body size (males are 15% larger), color (males have redder antennae), and weaponry (males possess enlarged appendages with sharp hooks). These traits have likely played a significant role in the evolution of this amphipod’s harem polygyny mating system, where males, who compete for access to females, often have several females in their burrows.
We found that both sexes preferred to be in separate burrows rather than together in one burrow, regardless of the relative sizes of the individuals. We also observed that males were more aggressive towards one another, whereas females demonstrated very little intrasexual aggression. The remainder of our experiments investigated the interactions between the sexes when 2 males and 1 female were placed in an arena together. Overall, the female ended up in a burrow with the larger male in the majority of the trials. Although it is difficult at this time to disentangle the mechanism by which this occurs, it appears that male competition may play a larger role than female choice. We found that, when given a choice between empty burrow and one occupied by a member of the opposite sex, males will seek the burrow with the female whereas the female chooses her burrowing site randomly. Regardless of whether male competition and/or female choice is occurring, larger males do appear able to secure more matings with females.We presented these results at the meeting for the International Society for Behavioral Ecology, and our results were published in Behavioral Ecology in 2008 (PDF). We are currently interested in conducting field studies to further understand the dynamics of social interactions in M. californiana, and there are many exciting avenues of investigation to explore.