Female fruit flies use social cues to make egg clustering decisions

Abstract

The ability to respond plastically to environmental cues is a key determinant of fitness. In changing environments, females plastically adjust the number and placement of eggs they lay, allowing them to optimise the level of resources available for offspring by minimising over-exploitation whilst also accruing potential cooperative benefits such as through communal larval feeding. This suggests that the pattern of female egg laying should vary across differing social environments, to balance potential for cooperative benefits versus competitive costs of egg clustering. Here we test this prediction in the fruit flyDrosophila melanogaster. We developed a mathematical null model, which verified that females lay eggs on substrates in significantly non-random patterns. We then tested the effects of manipulating social density and egg density on the egg laying patterns of mated females. As the density of adult females was increased, females laid more eggs, laid them faster, and laid more eggs in clusters. Females also preferred to place eggs within existing egg clusters -and most egg clusters were of mixed maternity. These results show that females in groups, or females that encounter already laid eggs, adjust their laying to place more eggs in mixed maternity clusters. Overall, the results are consistent with our predictions and reveal that females exhibit active choice in egg investment and egg deposition patterns in response to variation in social environments.