Sexual selection in harems: male competition plays a larger role than female choice in an amphipod

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Spatial distribution can be affected by sexual selection, particularly in mating systems where males have exaggerated traits used to compete for access to females. Although it is rare to be able to disentangle selection arising from male competition versus female choice in such mating systems, we here report experiments in which we manipulated group size and sex ratio to determine the relative roles of males and females in a harem polygyny. Megalorchestia californiana is an amphipod (family Talitridae) that is sexually dimorphic—males are not only larger than females but also possess enlarged red antennae and gnathopods that are used to hold and guard females prior to mating inside their burrows. We conducted 2 main types of experiments: 1) we varied the mobility of 3 amphipods (2M/1F) to determine the relative roles of male competition and female choice and 2) we monitored groups of 9 or more individuals to determine how intersexual and intrasexual interactions affect overall spatial distribution. In our 3-amphipod tests, we found that male competition was more important than female choice in explaining burrow preferences. Our large group experiments, however, indicated that females also play an active role in mate selection, as females settled in random locations in single-sex groups, whereas females clustered around larger males in mixed-sex groups. Finally, we found that male body mass was correlated with the length and redness of the antennae, which suggests that males may signal their quality through visual communication. Key words: honest signaling, intrasexual competition, mate choice, Megalorchestia californiana, polygyny, sexual dimorphism. [Behav Ecol]