Judith Burkart: What’s in a gaze? Cooperative breeding and the evolution of cooperative signaling

Having a bright white sclera and elongated eyes can facilitate cooperative signaling, a key precondition for the evolution of language. Cooperative signaling is particularly important for cooperatively breeding species, such as humans and callitrichid monkeys. Some evidence indeed suggests a correlation between allomaternal care and scleral brightness, even though methodological issues and ecological factors still require further work. However, we suggest that a comprehensive approach to understanding the role of eye morphology for language evolution benefits from including not only investigations on the evolutionary origin of human-like eye morphology. To complement such insights, we should also ask (i) to what extent conspicuous eye morphology coevolved with other means of cooperative signaling, and (ii) how receivers perceive and use such cooperative signals. We exemplify this approach by reviewing recent comparative studies and intraspecific results from callitrichid marmoset monkeys. Comparative studies suggest a link between allomaternal care and white scleral coloration, but also between allomaternal care and neural control of facial muscles, and thus facial expressiveness. Moreover, an inclination toward cooperative signaling can also be found in callitrichid vocalisations. From the side of the perceivers, callitrichids show very low levels of gaze aversion, which for instance allows them to use human given eye-gaze direction alone as communicative cue with high angular precision, to distinguish where food is hidden in an object choice task. Finally, in contrast to capuchin monkeys and Tonkean macaques, marmosets use mutual gaze as a coordination smoother in ambiguous cooperation trials in a joint Simon task. Overall, this evidence suggests an increasingly coherent picture of a facilitating role of cooperative breeding for the evolution of cooperative signaling.