Brain size and neuron numbers drive differences in yawn duration across mammals and birds

Abstract

Recent studies indicate that yawning evolved as a brain cooling mechanism. Given that larger brains have greater thermolytic needs and brain temperature is determined in part by heat production from neuronal activity, it was hypothesized that animals with larger brains and more neurons would yawn longer to produce comparable cooling effects. To test this, we performed the largest study on yawning ever conducted, analyzing 1291 yawns from 101 species (55 mammals; 46 birds). Phylogenetically controlled analyses revealed robust positive correlations between yawn duration and (1) brain mass, (2) total neuron number, and (3) cortical/pallial neuron number in both mammals and birds, which cannot be attributed solely to allometric scaling rules. These relationships were similar across clades, though mammals exhibited considerably longer yawns than birds of comparable brain and body mass. These findings provide further evidence suggesting that yawning is a thermoregulatory adaptation that has been conserved across amniote evolution.

Functions of Spontaneous Yawning: A Review

“…Yawning is a neurophysiologically related behavior that is widely observed in vertebrates [4]. The experiments in other vertebrates provided indirect evidence for the functions of yawning.…”

“…Scientists already found evolutionary evidence of yawning in old black apes as a facial communication behavior to reveal anger, anxiety, or leadership [3], indicating its association with social situations. Moreover, some recent analyses of brain sizes among different species clearly show a relationship between brain mass and the duration of yawning, implying the neurophysiological function of yawning [4]. Yet, the specific functionality of yawning is still elusive.…”

Yawning and airway physiology: a scoping review and novel hypothesis

“…Yawning, gaping, or oscitation is a common and stereotyped physical behavior performed by most vertebrates, including mammals, birds, reptiles, amphibians, and even fish [ 1 , 2 ]. In most animals, a regular yawn lasts 4 to 7 s, and is characterized by (1) a long inspiratory phase with gradual mouth gaping, followed by (2) a brief climax (or acme) with powerful muscle stretching, and (3) a rapid expiratory phase with muscle tension release [ 1 , 3 , 4 ].…”

Interspecific Contagious Yawning in Humans

“…Yawning is a complex reflex that has been documented across all classes of vertebrates [ 1 , 2 , 3 , 4 ]. From an evolutionary perspective, this stereotyped motor action patten appears to be a neurological adaptation that stimulates changes in state [ 5 ] and arousal [ 6 ] through intracranial circulation and brain cooling [ 7 , 8 , 9 , 10 ].…”