Sexual differences in seasonal changes in major cheliped size and use pattern during dyadic interactions in the hermit crab <i>Pagurus filholi</i>

Sexual differences in morphology and behavior are common. Although males typically have larger weapons and are more aggressive because of sexual selection, such aggressive traits can benefit both sexes in year-round non-sexual resource competition. These benefits might affect female trait expression and cause sexual differences in seasonal trait changes, with male traits peaking in the reproductive seasons and female traits remaining stable or even diminishing. Here, sexual differences in major cheliped size and use in the hermit crab Pagurus filholi were examined across reproductive and non-reproductive seasons. Individuals were collected from April to July 2018 (reproductive season), and from October 2018 to January 2019 (non-reproductive) on an intertidal rocky shore in southern Hokkaido, Japan (41°44′N, 140°36′E). The female major cheliped was smaller in the reproductive season than in the non-reproductive season, with the smallest in ovigerous females; their cheliped use did not show seasonal change. In contrast, males possessed a larger major cheliped, and used it more, in the reproductive season than in the non-reproductive season. Whereas females mainly used their cheliped defensively and more often against female opponents, males used it in both defensive and offensive behavior regardless of the opponent’s sex. These results suggest that the function of aggressive traits in each sex can be shaped by different selective pressures, such as reproductive investment in females versus mating success in males.