Spatially clustered resources increase male aggregation and mating duration in Drosophila melanogaster

Abstract

In environments where females mate multiply, males should adjust their behaviour and physiology in response to the perceived level of sperm competition to maximize their fitness. Evidence of such plasticity has been found in several laboratory and field studies, but little is known about the cues stimulating these responses in natural populations. One way in which males appear to assess sperm competition risk is through encounter rates with conspecific males. Such encounter rates may be driven by the spatial distribution of resources required by males (i.e. food patches or potential mates), which in turn affects local density. However, explicit links between resource distribution, male encounter rates and shifts in behaviour related to sperm competition have not been demonstrated. We found that when group size of Drosophila melanogaster males was held constant, a small decrease in the distance between patches of food resources had striking effects on male behaviour. Compared to those from dispersed resources, males on clustered resources had a significantly reduced intermale distance (and hence encounter rate) and subsequently a longer noncompetitive copulation duration, previously shown to be a reliable indicator of male perception of sperm competition risk. The aggregation of resources, operating via increased encounter rate, can stimulate shifts in behaviour affecting male sperm competition performance. Given that the spatial distribution of resources is typically variable in natural populations (and often unpredictable), selection is likely to favour the evolution of plasticity in sexual behaviour where resource aggregation increases the probability of sperm competition.