Male mate choice in hermit crabs: prudence by inferior males and simple preference by superior males

In species with both male--male competition and male mate choice, inferior males may make different mate choice decisions from superior males. Males of the intertidal hermit crab, Pagurus middendorffii, are known to conduct precopulatory guarding and to adjust the threshold for guarding according to social parameters, such as encounter rate with females, competitor size, and sex ratio. Larger males are stronger in male--male competition during guarding in this species. We here tested whether male of P. middendorffii initially guarding a smaller female exchanged partners when the male encountered a larger receptive female and whether large and small males chose potential mates on the basis of body size and/or time needed for guarding when a male simultaneously encounters 2 females. When a male guarding a smaller receptive female encountered a larger receptive female, the male assessed the larger female and exchanged his partner only in cases of a large difference in body size between the 2 females, suggesting that males of this species could choose their mates based on female quality even during guarding. When a male simultaneously encountered 2 receptive females, small males showed the prudent mate choice by balancing female traits between larger body size and shorter time until breeding, whereas large males showed preference for larger females. The distinct preference exhibited by males of different size classes is concluded to be an adaptive response to the size-dependent risk of losing the female during guarding. Copyright 2011, Oxford University Press.