Ambient temperature modulates yawning

In humans yawning can be triggered both physiologically as well as psychologically (i.e., spontaneous and contagious yawns), and it is the distinct manifestation of these 2 yawn types that has led researchers to traditionally study them independently. However, evolution produces adaptations that are constrained by and build upon existing architecture, and since contagious yawning is a derived feature of the more primitive spontaneous form, it stands to reason that at least some fundamental mechanistic pathways should be shared between them. Recent research by Eldakar et al. has taken this perspective by assessing how non-social variables, i.e., those related to thermoregulation, alter the expression of yawn contagion. The thermoregulatory theory of yawning posits that the motor action pattern of this behavior functions as a brain cooling mechanism, serving to counteract rising brain temperature by altering the rate and temperature of arterial blood flow to the skull. Accordingly, the powerful stretching of the jaw and deep inhalation of air forces hyperthermic blood away from the skull while introducing cooler blood from the lungs and extremities. The deep inspiration also allows for countercurrent heat exchange with the ambient air and subsequent cooling of venous return surrounding arterial blood supply. Since the inception of this theory in 2007, the primary brain/ skull cooling predictions derived from this theory have been confirmed and replicated in different species (including humans). The thermal window hypothesis is a derivative of the thermoregulatory theory, which makes additional predictions regarding the relationship between yawning and ambient temperature (Fig. 1). First, yawns should increase in frequency with initial rises in ambient temperature, as this would trigger cooling mechanisms that function to maintain thermal homeostasis. Second, as air temperature continues to rise toward body temperature, yawning should then decrease since deep inhalations of warm air would heat rather than cool internal tissues. Third, yawning frequency would be expected to diminish as ambient temperature falls below a thermal neutral zone, as cooling mechanisms would no longer be needed. In sum, yawning is predicted to occur within a relatively narrow range of ambient temperatures (i.e., a thermal window). Initial support for this hypothesis has come from studies of birds and rodents, which demonstrated that