Continuously choosy males and seasonally faithful females: sex and season differences underlie size-assortative pairing

In monogamous mating systems, pair mates frequently share similar characteristics, such as size or ornamentation, a pattern suggesting mutual mate choice. When pairs persist across multiple seasons, preferences for familiar pair mates can either reinforce or disrupt patterns of assortative pairing. Snapping shrimp (Alpheus angulosus) are socially monogamous shrimp that can be found in size-assortative pairs year-round; both sexes could benefit from pairing with larger individuals. To determine the role of pair mate preferences for either size or established pair mates in size-assortative pairing, we performed simultaneous choice trials to test whether (1) either or both sexes prefer larger pair mates, (2) females prefer males with larger claws, or (3) either or both sexes prefer current pair mates over size-matched unfamiliar potential mates. Males, but not females, preferred larger potential pair mates in both the reproductive and nonreproductive seasons; males also showed more mate-searching behaviour, switching more frequently between choice options than females. Females, but not males, preferred their current pair mate over a novel potential pair mate; females also engaged in more frequent switching between pair mates and novel males, but only during the nonreproductive season. Thus, while both males and females exhibit preferences in pairing, size-assortative pairing in snapping shrimp does not result from mutual mate choice for size, but rather preferences for size in one sex, with size-assortative pairing reinforced by preferences for established pair mates in the other. Furthermore, pairing behaviour may be dynamic, shifting across seasons even in a year-round socially monogamous species. (C) 2019 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.