Female fruit flies use social cues to make egg-clustering decisions

The ability to respond plastically to environmental variation is a key determinant of fitness. Females may use cues to strategically place their eggs, for example adjusting the number or location of eggs according to whether other females are present and driving the dynamics of local competition or cooperation. The expression of plasticity in egg-laying patterns within individual patches (i.e. in contact clusters or not) represents an additional, under-researched, and potentially important opportunity for fitness gains. Clustered eggs might benefit from increased protection or defence, and clustering could facilitate cooperative feeding. However, increased clustering is also expected to increase the risk of overexploitation through direct competition. These potential benefits and costs likely covary with the number of individuals present; hence, egg-clustering behaviour within resource patches should be socially responsive. We investigate this new topic using the fruit fly Drosophila melanogaster.

Our mathematical model, parameterised by data, verified that females cluster their eggs non-randomly and increase clustering as group size increases. We also showed that as the density of adult females increased, females laid more eggs, laid them faster, and laid more eggs in clusters. Females also preferred to place eggs within existing clusters. Most egg clusters were of mixed maternity.

Collectively, the results reveal that females express plasticity in egg clustering according to social environment cues and prefer to lay in clusters of mixed maternity, despite the potential for increased competition. These findings are consistent with egg-clustering plasticity being selected due to cooperative benefits.