Male control of mating duration following exposure to rivals in fruitflies

Males of many species assess the likely level of sperm competition and respond adaptively, for example by increasing the level of courtship they deliver, by transferring more sperm or seminal fluids or by extending matings. In mechanistic terms, it may be easier for males to adjust the level of their investment to the likely level of sperm competition for male-limited traits such as sperm and seminal fluid production over which they have control. However, for shared traits, such as mating duration, that are expressed at a level determined by direct interactions between males and females, adaptive responses by males to competition could be constrained. This need not be the case, however, if males have significant influence over the expression of such traits. Understanding which sex can most influence the expression of shared traits in response to sexual competition is important in order to document the range of strategic, plastic responses that are available to each sex. However, direct tests of these ideas require, as in this study, measurements of the effect on a shared trait of manipulating the ability of one, but not the other, sex to influence it. We studied the responses of male Drosophila melanogaster to sexual competition, in which mating duration is increased following exposure to rivals, resulting in significantly increased paternity share. Males were allowed to respond normally to the presence of rivals prior to mating, but female responses to males were reduced via decapitation and immobilisation. We found that matings with both intact and decapitated, immobilised females were significantly longer with males that had been exposed to rivals prior to mating. Hence males could maintain their responses to rivals with intact and decapitated females, suggesting significant male influence over the ability to extend mating duration in this context. However, overall, mating duration was significantly longer with intact in comparison to decapitated females. Whether this is due to a female influence over mating duration in general, or whether males respond differently to immobilised females, is not yet known. Gaining a fuller understanding of sex-specific control of plastic traits will be important in the future for understanding how reproductive traits evolve and function.