EFFECTS OF SEASONAL-VARIATION IN OPERATIONAL SEX-RATIO AND POPULATION-DENSITY ON THE MATING SUCCESS OF DIFFERENT-SIZED AND AGED MALES IN THE YELLOW DUNG FLY, SCATHOPHAGA-STERCORARIA

In the yellow dung fly, Scathophaga stercoraria, large males generally enjoy higher short-term mating success than small males. This paper e xamines how the large male mating advantage is affected by the natural Variation in operational sex ratio (OSR) and population density or by the relationship between male size and age. There were seasonal chang es both in the number of males per pat (range 1-93 males) and in OSR ( 0.24-1.34, females/males). The mean size of copulating males and males guarding ovipositing females was larger at pats with a large number o f rival males. Variance in male size within pats increased with OSR bu t was unaffected by the number of males. The number of males per pat d ecreased and OSR became more female-biased towards the end of the flig ht season. Both the intensity of selection on male size and the opport unity for selection decreased with season. The proportion of young mal es (no wing wear) did not differ between single, copulating or female guarding males. Except at the beginning of the flight season, large ma ting males were younger than small ones, which may have resulted from either different patterns of adult mortality or dispersal. While these results confirm that large males in general enjoy higher short-term m ating success than small ones, they also show that small males are suc cessful at pats with a low number of rival males and a female-biased O SR. Age differences between mating males of different size suggest tha t short-term mating success does not correspond to lifetime mating suc cess. Variation in sexual selection on male size and the possible diff erences in lifespan of different sized males could be among the factor s maintaining genetic size Variation in this species.