Social and physical environment independently affect oviposition decisions in Drosophila melanogaster

Abstract

In response to environmental stimuli, including variation in the presence of conspecifics, animals show highly plastic responses in behavioural and physiological traits influencing reproduction. These responses have been extensively documented in males, but equivalent study of females is so far lacking. We expect females to be highly responsive to environmental variation, with significant impacts on fitness given females direct impact on offspring number, size, and developmental conditions. Using Drosophila melanogaster as a model, we manipulate (a) exposure to conspecific females, expected to influence their expectation of number of potential mates and larval density for their own offspring, and (b) test how prior consexual population density interacts with the spatial distribution of potential oviposition sites, with females expected to prefer clustered food resources that can support a larger number of eggs and larvae. After exposure to competition, females were slower to start copulating and reduced their copulation duration - the opposite effect to that observed in males previously exposed to rivals. There was a parallel and perhaps related effect on egg production, with females previously housed in groups laying fewer eggs than those that were housed in solitude. The spatial distribution of resources also influenced oviposition behaviour: females clearly preferred aggregated patches of substrate, being more likely to lay, and laying on more of the available patches, in the clustered environment. However, we found no significant interaction between prior housing conditions and resource patchiness, indicating that females did not perceive the value of different resource distributions differently when they were expecting either high or low levels of larval competition. While exposure to consexual competition influences copulatory behaviours, it is the distribution of oviposition resources that has a greater impact on oviposition decisions.