Yawning in lungfishes: occurrence and potential functional differences between respiratory modes

Yawning is a stereotyped behavioural pattern observed across vertebrates and is considered multifunctional, but how its function changed during major physiological transitions remains poorly understood. Lungfishes provide an ideal model to address this question because species vary in their relative reliance on branchial and pulmonary respiration. We investigated whether yawning occurs in one individual each of the branchial-dominant Australian lungfish and three pulmonary-dominant species (marbled, spotted, and West African lungfishes) and, when present, characterized yawning frequency, yawning investment (frequency × duration), and activity levels before and after yawning. Yawning was observed exclusively underwater in all species. The branchial-dominant Australian lungfish showed a significantly higher yawning frequency than the three individual pulmonary-dominant lungfishes (negative binomial model: p = 0.007), whereas yawning investment was broadly comparable among the four individuals. In the branchial-dominant Australian lungfish, activity levels significantly increased following yawning (Wilcoxon signed-rank test: p = 0.041), whereas no significant post-yawning increase was observed in the two pulmonary-dominant individuals. The magnitude of activity-level change associated with yawning differed significantly from that observed during control periods in the branchial-dominant Australian lungfish (Wilcoxon rank-sum test: p = 0.006), but not when the pulmonary-dominant individuals were pooled. These observations indicate that lungfish yawning occurs underwater regardless of respiratory mode and is associated with differences in yawning characteristics and post-yawning activity among the individuals. Although these findings cannot establish species-level differences, they suggest that yawning function may vary with respiratory mode and warrant further comparative study.