Adult size and mate choice in seaweed flies (Coelopa frigida)

Abstract

The willingness to mate of pairs of seaweed flies (Coelopa frigida) was measured using a stock known not to exhibit female mate choice with respect to a chromosomal inversion. Mating success was strongly correlated with the size of the female, but not at all with male size. Mating success was not associated with the size difference between the male and female in a pair, except when the male was considerably larger than the female, when mating success was reduced. The components of mating behaviour were quantified during slow-playback of video recordings. Both the mount rate and the proportion of mounts ending in female rejection were positively correlated with female size, and a strong negative correlation was found between female size and the rate of male dismounting. This lower rate of dismounting from large females may be the consequence of a male preference for large females and could account for their greater mating success. The higher rejection rate exhibited by large females, particularly of males much smaller than themselves, may be the manifestation of female choice for large males. Both sexes appear to prefer large mates. In trials involving many animals male mate choice is either weaker or is masked by other effects. The evolutionary significance of male mate choice and its interactions with other components of mating behaviour are discussed.