SIZE ASSORTATIVE MATING IN THE YELLOW DUNG FLY SCATOPHAGA-STERCORARIA

In the yellow dung fly, Scatophaga stercoraria, pairs copulating in th e grass or ovipositing on a dropping were size assortatively mated. Ho wever, 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 copul ation attempts were not more frequent with large females. 3) Female di splay did not result in assortative mating although displaying females copulated with a relatively larger male than other females. During co pulation and oviposition, single males attempted to take-over paired f emales which resulted in the separation of many initial pairs. Pairs i n unsuccessful take-over attempts were size assortatively mated but no t in succesful take-overs. Pairs flying between droppings were also si ze assortatively mated suggesting that assortatively mated pairs are m ore likely to move long distances than other pairs. In addition, densi ty affected pair formation. The difference from the expected female-ma le size ratio decreased with increasing density among copulating pairs suggesting that assortative mating is more common at high density dro ppings. Assortative mating in the yellow dung fly therefore results fr om male aggression against copulating and ovipositing pairs. During ta ke-over attempts non-assortatively mated pairs are likely to be separa ted. High male density with increased level of male encounter rate is likely to contribute to this process.