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.

Female mating preference for large size in Coelopa frigida (seaweed fly)

“…excluding sexual selection) is that males of intermediate size are the fittest (see for example Day et al, 1983). In contrast, sexual selection in the form of both female mate choice and male mate choice (Pitafi, 1991; unpublished observations) must operate directionally favouring large size. We therefore have, in seaweed flies, the situation predicted by Fisher (1930) in which natural and sexual selection act in opposition.…”

The evolution of female choice when the preference and the preferred trait are linked to the same inversion system

“…No evidence exists of any direct advantage to mating with large males, but this possibility certainly cannot be excluded. There is, however, evidence of a cost to mating in that mating appears to reduce longevity (Pitafi, 1991;Gilburn & Day, in preparation). A similar relationship has been found in Drosophila melanogaster (Partridge et at., 1987;Fowler & Partridge, 1989).…”

“…It is therefore paid only by RR females and results in a proportion, m, of females failing to produce progeny. As in other insect species (Partridge et al, 1987;Fowler & Partridge, 1989;Rowe et a!., 1994), mating in seaweed flies carries a cost; Pitafi (1991) found that females allowed unlimited matings died younger than did virgin females. Furthermore, a positive correlation exists between the frequency of rejection and female longevity (Gilburn & Day, in preparation).…”

The effect of an inversion system and the time interval between matings on postcopulatory sexual selection in the seaweed fly, Coelopa frigida

“…Variation in the ability of females to reject males of different sizes results in sexual selection for large male size (Crean and Gilburn, 1998;Crean et al, 2000). Male mate choice also operates on the basis of female fecundity (Pitafi et al, 1990) and longevity (Dunn et al, 2001). LMSP has been previously reported in Coelopa frigida with P 2 values increasing with the time interval between matings (Butlin, 1983).…”

“…Copulations typically last between 1 and 2 min but occasionally continue for longer. Males were stopped from mating for longer than 5 min to limit any possibility that variation in fertilisation success might be associated with copulation duration, although previous studies have revealed no such correlation (Pitafi, 1991). The matings were carried out in 1.5 ml microcentrifuge tubes to reduce the ability of females to successfully reject males.…”