Commentary: Yawning, acute stressors, and arousal reduction in Nazca booby adults and nestlings

Citation: Gallup AC and Clark AB (2015) Commentary: Yawning, acute stressors, and arousal reduction in Nazca booby adults and nestlings. Liang et al. (2015) recently reported on the relationship between yawning and stress responses in a wild population of Nazca boobies (Sula granti) in the Galapagos. Their analysis covered two separate investigations: a human capture-restraint stressor applied to adult boobies, and observations of nestlings maltreated by Non-parental Adult Visitors (NAVs). The authors conclude that the temporal sequence of yawning following these stressors provides support for the newly termed arousal reduction hypothesis, an idea initially proposed by Dourish and Cooper (1990), and demonstrates a communicative function to yawning. This study adds much-needed field data on patterns of yawning, stress and corticosterone in a natural context. Our commentary addresses the inference of a communicative function of yawning, discusses the match between the reported patterns and a thermoregulatory function, and draws attention to important shortcomings of the arousal reduction hypothesis as an explanation of yawning. The authors state in the abstract that they " tested the hypothesis that yawning communicates to others a transition from a state of physiological and/or psychological arousal (for example, due to action of a stressor) to a more relaxed state " (p. 38), i.e., the arousal reduction hypothesis. This is quite misleading as the experiments reported were not designed as a direct test of this hypothesis, which is proposed later in the discussion. Useful development of the hypothesis would include a sketch of how it would work in the context of the results. In fact, the reported patterns—nocturnal yawning by adult boobies and particularly yawning by nestlings in the absence of parents or responses by nearby nestlings—raise doubts about evolved signal function, which generally requires selection in terms of receivers and senders (e.g., Bradbury and Vehrencamp, 2011). Liang et al. (2015) state that their results on nestlings support the arousal reduction hypothesis and challenge the thermoregulatory hypothesis that yawning is a cooling mechanism for the brain (Gallup and Gallup, 2007). Despite critiques (e.g., Guggisberg et al., 2010), the primary predictions of the thermoregulatory hypothesis have been supported in different model systems (see recent review and discussion of critiques: Gallup and Eldakar, 2013) and we think it remains an important possibility here. While admitting that their data on adults cannot speak to the role of thermoregulation, the authors reject it based on the positive correlation between the nestlings' latency …