Yawning in the forest: a comparative study on wild Propithecus verreauxi and Lemur catta

Yawning is an involuntary behaviour common to all vertebrates. Although easily recognizable, its adaptive significance is not yet fully understood. In the past, several physiological hypotheses have been tested to explain yawning including oxygenation, circulation and thermoregulation. More recently, researchers have begun to focus on the behavioural meaning of yawning, especially in primates. This is the first study to test hypotheses on yawning in natural populations of primates. We selected two sympatric species, Lemur catta and Propithecus verreauxi, of the Ankoba forest (24.99°S, 46.29°E, Berenty Reserve) in southern Madagascar that differ in ecological and behavioural characteristics. Their low sexual dimorphism and high sexual competition may have reduced the differences between sexes in both species. As predicted from the Dimorphism Hypothesis , males and females yawned with similar frequencies. As in other primate species, yawning was linked to the sleep-wake cycle and punctuated the transitions from one behavioural pattern to another ( State Changing Hypothesis supported). Finally, we tested the Anxiety Hypothesis, which predicts that yawning should increase after stressful, anxiogenic events. We found a significant increase in yawning after both predatory attacks and aggression. Clearly, primate yawning is associated with behavioural state changes within different contexts (e.g. threat, anxious or rest). Yet it is not known if yawning is a single, unitary behaviour that varies only with respect to the context in which it is performed. We cannot exclude that yawning in lemurs can be elicited by certain type of stressors; further data in clearly defined contexts will be necessary to demonstrate this hypothesis