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

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Abstract
In the yellow dung fly, Scathophaga stercoraria, large males generally enjoy higher short–term mating success than small males. This paper examines 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 changes 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 of rival males. Variance in male size within pats increased with OSR but was unaffected by the number of males. The number of males per pat decreased and OSR became more female–biased towards the end of the flight season. Both the intensity of selection on male size and the opportunity for selection decreased with season. The proportion of young males (no wing wear) did not differ between single, copulating or female guarding males. Except at the beginning of the flight season, large mating 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 mating success than small ones, they also show that small males are successful at pats with a low number of rival males and a female–biased OSR. Age differences between mating males of different size suggest that short–term mating success does not correspond to lifetime mating success. Variation in sexual selection on male size and the possible differences in lifespan of different sized males could be among the factors maintaining genetic size variation in this species.