The effects of size on the mating behaviour of the dung fly Sepsis cynipsea

Summary

The breeding system of the dung fly Sepsis cynipsea has a precopulatory guarding phase during which a male prevents other males from gaining access to a female while she lays her eggs in the dung. After oviposition, the pair leaves the dropping and may copulate. Large males have three advantages over smaller males in mating success; they are more likely to become paired (Table 1), they copulate with larger females (Fig. 4) and they are more successful in fights between males for the possession of females.

Pairs are formed at random with respect to the sizes of the male and female in the pair but at copulation there is positive assortative mating (Fig. 4). The mechanism for this change is unknown.

Small males modified their mate searching behaviour to attempt to improve their chances of finding a female. They searched more on the dung surface than in the grass and for longer round any one dropping (Fig. 5) than did larger males.

The mean adult size in both sexes declined over the season (Table 1, Fig. 3). It is suggested that most of the size variation present is due to environmental causes. This has led to phenotype limited mate searching; a male's searching behaviour is closely related to his size.

Guarding is probably precopulatory rather than postcopulatory as in the dung fly Scatophaga stercoraria, which breeds in the same droppings, because males are much smaller than females and so cannot force copulation on unwilling females.