Female size and pre-copulatory sexual selection by males of intertidal snail (Littorina saxatilis)

Littorina saxatilis, the rough periwinkle, is highly promiscuous and polyandrous; female L. saxatilis snails rarely reject male advances. Sexual selection by males has been observed in this species. Sexual selection is one of the driving forces of evolution and may be a significant contributor to partial reproductive isolation between two ecotypes of L. saxatilis. Male sexual selection based on female size was investigated for wave-exposed crab ecotype snails in this study. Males and females were measured and placed in mating arenas. Males were presented with a choice of a smaller or larger female relative to themselves. Mating frequency and duration was recorded. A total of 19 successful trials were performed. A significantly higher mating frequency (p<0.005) was associated with larger females, indicating that males exhibit strong pre-copulatory sexual selection. Mating duration was significantly longer for larger female mating events (p<0.05) but was also equally determined by male size (p<0.05) as larger males tended to mate for longer. These results indicate that a strong sexual selection by males exists for L. saxatilis, in which larger females are preferred. This behavior may be a response to perceived fecundity of females, and/or may be a product of selection against hybrids.