Why it is difficult to model sperm displacement in Drosophila melanogaster: The relation between sperm transfer and copulation duration

We have investigated the effects of experimental manipulation of copulation duration on sperm displacement in Drosophila melanogaster. Both spermless and normal males were used as second (displacing) males in the experiments. Displacement induced in the absence of sperm, that is, by males that pass accessory gland fluid alone, was a relatively inefficient process and produ ced much lower levels of displacement than normal males. Therefore, the pre sence of second-male sperm is necessary (but unlikely sufficient) for the h igh levels of displacement commonly observed in D. melanogaster. Furthermor e, when second matings were interrupted at various times after the initiati on of copulation, the distribution of displacement was strongly bimodal. We conclude that sperm transfer is relatively rapid. beginning shortly after the initiation of copulation, and is essentially complete before the midpoi nt of copulation. Therefore, sperm transfer bears no simple relation to cop ulation duration. Because it would be difficult to manipulate the numbers o f sperm transferred by manipulating copulation duration, methods used to st udy sperm displacement in other insect species are unlikely to be appropria te for D. melanogaster. We also investigated why males mate for more than t wice the duration that appears to be necessary to complete sperm transfer. Experimental interruption of first matings indicated that the extra copulat ion time serves to delay female remating, rather than to increase that rate at which of offspring are sired before remating.