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mystery of yawning 

mise à jour du
10 avril 2025
Zoolog Sci.
2025;42(1)
Yawning and Its Temperature-Dependent Modulation in Leopard Geckos
Kotake KT, Yamaguchi ST, Mukai Y, Zhou Z, Norimoto H.

Chat-logomini
 
Yawning is a stereotyped behavior characterized by the involuntary and forceful opening of the mouth during inhala- tion, a peak in temporary muscle contraction, and the pas- sive closure of the jaw during exhalation (Barbizet, 1958; Provine, 1986). Previous studies have indicated a circadian rhythm in yawning for humans and primates (Deputte, 1994; Baenninger et al., 1996; Zilli et al., 2008), and that yawning stimulates behavioral arousal in primates (Vick and Paukner, 2010).
The frequency of yawning is influenced by ambient temperature in endothermic animals. In birds, yawning fre- quency peaked around 30°C during the increasing tempera- ture condition and began to decrease as the temperature rose further (Gallup et al., 2009). Similarly, in humans, yawn- ing frequency reached its highest around 20°C and decreased as temperatures rose beyond or fell below this point (Massen et al., 2014). These findings suggest that yawning may play a role in regulating body and brain tem- perature (Gallup et al., 2016; Massen et al., 2021).
 
Yawning is also observed in ectothermic animals (Bakkegard, 2017; Yamada and Wada, 2023). Reports of increased locomotor activity following spontaneous yawning in amphibians and fish suggest that some functions related to the behavior are conserved across vertebrates, such as induction of behavioral arousal (Bakkegard, 2017; Yamada and Wada, 2023). However, the relationship between tem- perature and yawning in ectothermic animals remains unclear. Specifically, it is unclear whether ambient tempera- ture influences yawning frequency and yawning-induced changes of activity levels in ectothermic animals.
Here, we observed yawning behavior and associated changes in activity levels in the leopard gecko, Eublepharis macularius, an ectothermic reptile, in which the detection of yawning behavior is highly straightforward, under various temperature conditions.
 
Discussion
We have demonstrated here that ambient temperature affects yawning in leopard geckos. A higher frequency of yawning was observed at 30°C than at 25°C (Fig. 2A). This result is consistent with previous findings that yawning fre- quency depends on ambient temperature in humans and birds (Gallup et al., 2009; Massen et al., 2014). It has been shown in humans that yawning occurrence is temperature- dependent, and yawning occurs most frequently within an optimal temperature range (Massen et al., 2014). Since our experiments were conducted within the temperature range that geckos could experience in their natural environment (22&endash;24°C in March to 40&endash;45°C in June&endash;July) to minimize stress, yawning might decrease at temperatures below 20°C and above 40°C, which were not investigated in this study.
 
Additionally, with the increase in ambient temperature, a significant decrease in yawning duration was observed (Fig. 2B). While interspecies comparisons of yawning duration were conducted among some mammals and birds in a previ- ous report (Massen et al., 2021), our experiment is the first to examine the length of yawning in the same individual under different temperature conditions. Furthermore, we observed a concurrent increase in movement speed as ambient temperature rose (Fig. 2D). This feature is consistent with findings from previous research on rattlesnakes, which dem- onstrated a positive correlation between temperature and both the attacking speed and the gaping velocity during strikes (Whitford et al., 2020). Considering these findings, it is plausible that the increase in ambient temperature not only boosts overall activity speed but also accelerates the movement of muscles around the mouth. This could poten- tially contribute to both the shorter durations of yawnings and quicker mouth openings during attacks.
 
Increase in locomotor activity after yawning was observed at 30°C and 35°C (Fig. 3). This trend is similar to observations in primates and consistent with the arousal- state change hypothesis (Vick and Paukner, 2010). Further- more, distinct periods conducive to yawning were identified in geckos (Fig. 4A, B). Interestingly, yawning exhibited a uni- modal peak every 24 hour, while activity levels showed bimodal peaks per day (Fig. 4C, D). This suggests that yawning is not simply correlated with changes in activity lev- els but also may be influenced by other factors, similar to the findings in humans and primates (Baenninger et al., 1996). Further experiments under constant conditions are needed to investigate whether these patterns reflect the internal cir- cadian clock or sleepiness, or are influenced by external cues.
 
In summary, we have demonstrated temperature- dependent modulation of yawning in leopard geckos. In par- ticular, we found that yawning at 30°C and 35°C resulted in an increase in post-yawning locomotor activity. These results suggest that yawning may promote behavioral arousal at temperatures of 30°C and above. Moreover, we observed specific daily periods during which yawning frequently occurred. These characteristics align with findings from previous research on endothermic animals.
 
However, it is possible that yawning in geckos may exhibit features and functions not previously reported in mammals. For example, 47.7% of the yawns observed in this study were accompanied by tongue-flicking behavior (see Supplementary Movie S2) within 30 seconds before or after. Given that some of this behavior is known to be a means of reptilian chemical cognition (Graves and Duvall, 1983; Barbour and Clark, 2012), it raises the possibility that this type of yawning in geckos may also serve a function in chemical cognition. This insight suggests that the yawning behavior observed may not merely be an arousal mecha- nism but could also play a role in the animal's sensory inter- action with its environment. Further studies are now required to unveil conserved and species-specific functions of yawn- ing, contributing to their adaptation to varying environments.