Over the past 5 years numerous reports have confirmed and replicated the specific brain cooling and thermal window predictions derived from the thermoregulatory theory of yawning, and no study has found evidence contrary to these findings. Here we review the comparative research supporting this model of yawning among homeotherms, while highlighting a recent report showing how the expression of contagious yawning in humans is altered by seasonal climate variation. The fact that yawning is constrained to a thermal window of ambient temperature provides unique and compelling support in favor of this theory. Heretofore, no existing alternative hypothesis of yawning can explain these results, which have important implications for understanding the potential functional role of this behavior, both physiologically and socially, in humans and other animals. In discussion we stress the broader applications of this work in clinical settings, and counter the various criticisms of this theory.

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

Yawning is a ubiquitous physiological phenomenon observed across various species, including humans. Traditionally associated with boredom or drowsiness, yawning encompasses a complex interplay of neurological, physiological, and social factors. This article delves into the multifaceted nature of yawning, examining its evolutionary significance, neurological underpinnings, and physiological functions. The initiation of yawning is primarily regulated by the hypothalamus and brainstem, with neurotransmitters such as dopamine, serotonin, and oxytocin playing critical roles. Anatomically, yawning involves coordinated actions of cranial nerves and muscle groups. The physiological functions of yawning extend beyond mere ventilation, with hypotheses suggesting roles in brain cooling, arousal, and alertness. Additionally, yawning has significant social and communicative implications, particularly through the phenomenon of contagious yawning, which is believed to foster empathy and social bonding. Developmentally, yawning emerges early in fetal life and persists throughout the lifespan, with its frequency and triggers evolving with age. Comparative studies in animals reveal yawning's deep evolutionary roots, linked to thermoregulation and social communication. Clinically, excessive yawning may indicate underlying neurological or systemic conditions, necessitating further investigation. This comprehensive exploration underscores yawning as a complex behavior with significant physiological, neurological, and social dimensions, offering insights into human and animal physiology and social interactions.

Contagious yawning (i.e. yawning triggered by perceiving others' yawning) is a well-documented phenomenon, but the mechanism underlying it is still unclear. In this chapter, I review the current evidence about: (1) developmental studies with typically and atypically developing populations, and (2) comparative studies in non-human animals. Developmental studies have revealed that contagious yawning is disturbed in individuals with autism spectrum disorders, suggesting that contagious yawning may share a developmental basis with the capacity for theory of mind. Comparative studies have suggested that contagious yawning can be observed in non-primate species, such as domestic dogs. As dogs are known to have exceptional skills in communicating with humans, it has also been suggested that contagious yawning may be related to the capacity for social communication. These results from developmental and comparative studies are consistent with the claim that the mechanism underlying contagious yawning relates to the capacity for empathy.

The ubiquitousness of normal yawning and the existence of abnormal yawning warrant an understanding of this reflex. Its mechanisms and functional role are not entirely known. A review of the literature reveals that yawning is a brain stem arousal reflex with both peripheral and central loops subserving reversal of brain hypoxia or hypoxemia. Behaviorally, yawning is a semi-involuntary act that occurs also because of loss of interest in the surroundings and it is not necessarily associated with fatigue. Socio-environmental factors can influence the emergence of yawning. Dopaminergic, acetylcholinergic, ACTHergic and oxytocinergic systems are involved in the generation and modulation of yawning in animal experimentation.

Yawning is a normal reflex triggered by arousal, drowsiness, boredom, hunger and emotional conditions and it is associated to several neurological diseases and drug abuse. Its wide presence in the phylogenetic vertebrate scale and even in human fetuses as young as 12 weeks directed the search for the common anatomic and biochemical mechanisms involved. The demonstration that yawning is not connected with high CO2 or low O2 blood levels left aside a prevalent metabolic hypothesis. Its close relationship with the sleep-wake cycle, specially in moments previous to falling asleep and after awaking has been related to changes in personal situation and activity. A single component of this reflex which is to be found exclusively in humans, is the fact that yawning is contagious. Thus, it is considered a component of the adaptative mechanism that is part of the surveillance reflex, becoming a significant paralinguistic evolutive expression aimed at protection and social cohesion. The common anatomical structures and neurochemical systems taking part in yawning, the sleep-wake cycle and the temporal lobe epilepsy may imply that yawning results from a set of protective systems induced by endogenous opiods which intervene in the inhibition and prevention of the temporal lobe epileptic seizures.