Plasticity and subfertility at the evolution of eusociality: an empirical test of the evolution of role specialisation

Lucy Winder, Jeremy Field

At the origin of eusociality, some individuals (workers) gave up reproducing to help another individual (queen) raise their offspring. In Hymenoptera, females are just as related to their siblings as they are to their own offspring meaning only a small benefit of role specialisation is required for selection to favour this strategy. However, it is yet to be established what conditions promoted the initial emergence of role specialisation. One likely driver is a trade-off between reproduction and provisioning, where efficiency could be optimised through specialisation in either task. To test whether role specialisation increases efficiency, we experimentally manipulated provisioning behaviour in the pre-social digger wasp Ammophila pubescens. This created a gradient of specialisation across individuals, from egg-laying specialists to provisioning specialists. We then determine if egg-laying specialists can produce enough eggs to support both her own and her helper’s reproductive output, and whether provisioning specialists can provision at a rate to rear these extra offspring. We also utilise natural variation in body sizes within the population to determine if smaller, less fecund individuals gain fitness benefits by helping larger, more fecund individuals. Our findings provide new insight into how underlying trade-offs may have favoured the earliest forms of role specialisation at the origin of eusociality.