To examine whether solitary males that had been single in the field are less active in male–male contests than those had been paired in the field in the hermit crab P. filholi, we conducted a laboratory experiment of male–male contests to compare the behaviors of challenger males that had been found not paired in the field (N-male), or had guarded other females in the field (G-male). We introduced a challenger male and a guarding pair into an experimental container and observed the behaviors of challenger males. N-males showed a significantly lower frequency of initiating contests than G-males. Time to initiate contests in N-males was longer than that in G-male. Difference in body size between male contestants significantly affected the time to initiate contests, contest duration and contest outcome, but female quality did not affect these variables. We suggest that solitary males of P. filholi in the field would not be always active for competition for mates, which will decrease intensity of sexual selection in this species.

In species with both male–male contests and male mate choice, dominance hierarchy in males may affect the male mate choice. This study described patterns of precopulatory guarding pairs of the hermit crab Pagurus nigrofascia in the field to infer variation in male mate choice based on male body size. We found size assortative pairing in precopulatory guarding pairs collected in the field, and that larger males tended to guard females with a shorter time until molting in the field. To examine whether prior outcomes of male–male contests affect male mate choice in P. nigrofascia, we conducted an experiment where a male with a winning or losing experience in male–male contests was placed in a container with two receptive females, and recorded the outcomes of male mate choice. An interaction between two variables of male experiences and time until female molting was statistically significant, indicating that males with a winning experience were likely to choose females with a shorter time until molting while males with losing experience were likely to choose females with longer time until molting. Small inferior males might guard females with low quality as a prudent mate choice in the field.