SIZE ASSORTATIVE MATING IN THE YELLOW DUNG FLY SCATOPHAGA-STERCORARIA

In the yellow dung fly, Scatophaga stercoraria, pairs copulating in the grass or ovipositing on a dropping were size assortatively mated. However, pairs initiating copulations showed no assortative pattern: 1) Pairs formed at different times from the deposition of droppings or in different parts of the mating area showed no consistent relationship in size. 2) Successful males in copulation attempts involving several males were larger than in single male attempts but these type of copulation attempts were not more frequent with large females. 3) Female display did not result in assortative mating although displaying females copulated with a relatively larger male than other females. During copulation and oviposition, single males attempted to take-over paired females which resulted in the separation of many initial pairs. Pairs in unsuccessful take-over attempts were size assortatively mated but not in succesful take-overs. Pairs flying between droppings were also size assortatively mated suggesting that assortatively mated pairs are more likely to move long distances than other pairs. In addition, density affected pair formation. The difference from the expected female-male size ratio decreased with increasing density among copulating pairs suggesting that assortative mating is more common at high density droppings. Assortative mating in the yellow dung fly therefore results from male aggression against copulating and ovipositing pairs. During take-over attempts non-assortatively mated pairs are likely to be separated. High male density with increased level of male encounter rate is likely to contribute to this process.