The function of female behaviours adopted during premating struggles in the seaweed fly, Coelopa frigida

Abstract

The mating system of sciomyzoid flies is typified by vigorous premating struggles and a large male mating advantage. Females commonly exhibit three behaviours (shaking, kicking and curling) during struggles. We tested the predictions of three hypotheses proposed to explain the evolution of female resistance in Coelopa frigida. Both shaking and curling were associated with reduced female mating rate and thus appear to be resistance traits. Mounts that ended while the female was curling appeared to be terminated by the male dismounting from the female rather than as a result of resistance. The communication hypothesis proposes that female resistance functions by signalling to the male that the female is unwilling to mate to encourage males to dismount them. Curling therefore seems to fit the predictions of the communication hypothesis. Shaking was associated with sexual selection for large male size and also males that could withstand resistance the longest. These observations fit with the predictions of both the female reluctance and screening hypotheses. Furthermore, shaking was associated with a pattern of mating that increased offspring fitness. This did not occur in females that did not shake. This corresponds with the predictions of the screening hypothesis that resistance generates indirect sexual selection. However, our results are also largely consistent with the female reluctance hypothesis. It appears that different resistance behaviours may have different functions, and furthermore that individual resistance behaviours might generate different mating rates and patterns of sexual selection between female karyotypes.