Sex on the beach: a cocktail of behavioural repertoires which drive size-assortative mating in rocky shore littorinids?

Abstract

Size-assortative mating occurs in a wide range of animal taxa and sexual selection is one of the major hypotheses proposed to explain the formation of this distinctive mating pattern. The rationale is that male mate preference for large, more fecund females, along with malemale competition (i.e., a physical advantage of large over small males in competing for large females), will result in a positive correlation between the sizes of mating males and females. This paradigm has been assumed to hold true for species of rocky shore littorinids, many of which exhibit size-assortative mating, despite any rigorous test of the model. This hypothesis was, therefore, tested in three littorinids: Echinolittorina malaccana, E. radiata and E. vidua on rocky shores at Cape d’ Aguilar, Hong Kong. Males of the three species generally trail followed, mounted and copulated with females similar or larger than their own body sizes (a form of size-dependent male mate preference); and copulated with larger females for longer durations. The males mating preference seems to be specific to their own body size (i.e., they selected females which were larger than their own size), which is a more dynamic approach than the traditional belief that all males simply prefer 'large' over small females. All three species also demonstrated male-male competition in the form of 'pushing' behaviour, where large males usually had a size advantage over small males in obtaining females to mate with. These findings support recent studies on mangrove littorinids and suggest that size-dependent male mate preference, along with male-male competition, may play an important role in driving size-assortative mating in littorinids and other animals that exhibit male mate choice.