Biomimetic robots reveal flexible adjustment of sexual signalling in a wild invertebrate.

Studies of communication in primates, birds and fish show that signalling behaviour is often flexibly adjusted depending on the presence, characteristics and behavioural states of others. Such flexibility is likely important in competitive social contexts such as sexual signalling, where attractiveness is directly affected by rival behaviour. Although sexual displays are often sensitive to the presence and proximity of rivals, less is known about the effect of changes in rival signalling. In this study, we used a biomimetic robot to manipulate rival morphology and waving behaviour in a wild population of fiddler crabs () and investigated whether males responded flexibly by adjusting their own activity and waving. Males were less likely to enter their burrow when the robotic rival was waving, particularly if that rival had a small claw, and spent less time in their burrow if they did enter. While outside, males produced more waves when the robotic rival was waving fast, not by changing their own rate but by prolonging their bouts. These results reveal the subtle ways in which an invertebrate flexibly adjusts behaviour to remain competitive in a dynamic environment, investing more in signalling when it is likely most profitable.