Resource presence and operational sex ratio as determinants of copulation duration in the fly Rhagoletis juglandis

The effects of the interaction between the operational sex ratio (OSR) and a resource (i.e. oviposition site) on mating dynamics have rarely been cons idered. We examined the effect of the resource presence and its interaction , with the effect of OSR on copulation duration in Rhagoletis juglandis, a tephritid fly species characterized by a resource-defence mating system in which males defend territories on walnut fruit. In this species, copulation duration varies from 30 s to over Ih and was shown previously to respond s trongly to changes in OSR. In the field, short copulations tended to begin and end on fruit, whereas most long copulations generally began on fruit bu t ended in the foliage, suggesting a possible effect of resource presence o n the copulation duration. In laboratory assays of isolated pairs, copulati ons were significantly shorter in the presence of a surrogate fruit, confir ming the effect of resource presence. In another laboratory assay, in which we manipulated OSR independently of resource presence, resource presence a nd OSR were additive in their effects. Results are discussed in the context of sperm competition theory. (C) 1999 The Association for the Study of Ani mal Behaviour.