Female-mediated differential sperm storage in a fly with complex spermathecae, Scatophaga stercoraria

Multiple spermathecae potentially allow selective sperm use, provided that sperm from rival males are stored differentially, that is, in different pro portions across storage compartments. In the yellow dung fly, Scatophaga st ercoraria, females have three spermathecae arranged as a doublet and single t. To test whether females store the sperm of rival males actively and diff erentially, we mated fixed male pairs to three females. After copulation, f emales were (1) dissected immediately before they could start laying a clut ch of eggs, (2) left awake for 30 min but prevented from oviposition, or (3 ) anaesthetized with carbon dioxide for 30 min to interfere with the muscul ar control presumably required for sperm transport from the site of insemin ation to the spermathecae. For each female, we estimated the proportion of the second male's sperm stored in her spermathecae (S-2 value), using sperm length as a male marker. After copulation, the S-2 values in the singlet a nd doublet spermathecae differed significantly, indicating differential spe rm storage during copulation. Postcopulatory treatment affected differentia l sperm storage significantly. Females dissected immediately had lower S-2 values in the doublet than in the singlet spermatheca, while females left a wake showed the reverse pattern for the same two males. This reversal did n ot occur when females were treated with carbon dioxide. The results indicat e differential storage of sperm from different males during copulation and that female muscular activity can affect storage and separation of competin g ejaculates beyond copulation. (C) 2000 The Association for the Study of A nimal Behaviour.