Aggression, but Not Familiar Recognition, Occurs During Cohabitation in Females of a Hermit Crab

Aggressive interactions are ubiquitous in the animal kingdom and are a key context favoring the evolution of familiar recognition. This cognitive ability has been widely reported in male crustaceans, including Pagurus hermit crabs. However, our understanding of female–female aggression and female cognitive ability remains relatively limited. Here, we describe the pattern of intrasexual aggression and assess the presence of familiar recognition in females of the hermit crab Pagurus middendorffii through two consecutive trials with 1-day's cohabitation. In the first trial, pairs of similar-sized females typically engaged in aggression using their appendages, though shell fights were rare. Following a 1-day's cohabitation, in the second trial, female behaviors did not differ between familiar (previously paired) females and unfamiliar (newly paired) females. As familiar recognition is beneficial in avoiding the associated costs of aggression, the lower frequency of energetically costly shell fights might explain the lack of evidence for familiar recognition in female P. middendorffii, as the benefits of recognizing opponents in such less intense interactions may be limited.