Size-dependent mate preference in the stalk-eyed fly Cyrtodiopsis dalmanni

We investigated size-dependent mate choice in the stalk-eyed fly Cyrtoniopsis dalmanni. In this species, females prefer to mate with males with larger eyespans. We experimentally manipulated female eyespan by rearing larvae at different densities. The mating preferences of females with small and large eyespans were compared in choice chambers containing two males that differed in eyespan. Neither type of female distinguished between males with small differences in eyespan ((X) over bar =1.45 mm), but both preferred the male with the wider eyespan when the difference between males was large ((X) over bar =3.17 mm). When presented with an intermediate difference in male eyespan ((X) over bar =2.40 mm), large-eyespan females showed a significantly stronger preference than small-eyespan females for the male with the wider eyespan. Size-dependent mate choice in stalk-eyed flies is probably caused by small-eyespan females being less able to discriminate differences between males. It may also reflect female condition, as eyespan is highly correlated with body size, which is a strong predictor of developmental conditions. (C) 2001 The Association for the Study of Animal Behaviour.