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We investigated size-dependent mate choice in the stalk-eyed fly Cyrtodiopsis 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=1.45 mm), but both preferred the male with the wider eyespan when the difference between males was large (X=3.17 mm). When presented with an intermediate difference in male eyespan (X=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.

Many forms of interaction within and between species appear to be based on ‘scrounger’ individuals or species exploiting a limited resource provided ‘producers’. A mathematical model is presented which shows whether or not scroungers are maintained in a group, depending on their frequency and the group size. Some of the predictions of the model were tested in captive flocks of house sparrows Passer domesticus L. Here the scroungers obtained most of their food (mealworms) by interaction and the producers found most of their food by actively foraging: the pay-off to each type was measured as mealworm capture rate. Neither type changed strategy opportunistically in response to instantaneous flock composition but, not surprisingly, scroungers fared better when one of more producers were present. However, scrougers did much worse than expected when greatly outnumbered by producers, perhaps because producers then found the available food very quickly.