Sex-specific roles for yawning in the Emperor cichlid Boulengerochromis microlepis

Yawning is a poorly understood, yet spontaneous and ubiquitous behaviour. Three main hypotheses explain the function of yawns, most notably stimulating arousal. Yawns can also play a role in social cohesion through their "contagious" nature, eliciting an unconscious yawn in an observer. Yawn contagion has been observed in mammals and birds, and has been suggested in fish. Yawning also contributes to sexual dimorphisms, whereby males and females display differences in yawn frequency or function. Here, we examine these hypotheses in juveniles of the African cichlid, Boulengerochromis microlepis. By re-analyzing hundreds of hours of video recordings and nearly 3000 observed yawns we find support for all three yawn hypotheses in fish and two temporally and sexually distinct effects for yawns. First, our results suggest yawns at night and dawn induce arousal by delaying transitions into sleep. In contrast, daytime yawns induced rapid yawn contagion in neighbouring fish. Our analyses also demonstrate clear differences between sexes, with increased yawning in males at night and dawn associated with less sleep, whereas females displayed more robust social yawning. Together, our results expand the evolutionary context for yawning, and raise further questions about its functions in sleep and social cohesion across animals, and across sexes.