Bipedal locomotion in birds: the importance of functional parameters in terrestrial adaptation in Anatidae

The kinematic characteristics of a bird's walk vary according to whether the species is cursorial or not. To increase their speed, running birds increase the frequency of their movements, whereas non-running birds preferentially increase the amplitude. Previous studies have shown that these differences are accompanied by differences in posture; however, the observations were carried out on different species. Do these differences correspond to morphological differences linked to the history of the particular species, or do they reflect more effective solutions from a mechanical point of view? Two breeds of the duck Anas platyrhynchos platyrhynchos, the mallard and the Indian runner, which have different locomotor behaviours, are compared. The mallard is a dabbling duck with the typical horizontal duck posture, while the Indian runner is a terrestrial duck that carries its trunk very erect. The kinematic characteristics of the walks of both breeds were studied. The observed differences in posture between the mallard and the Indian runner have repercussions in the kinematic features of locomotion. The strategies used to increase speed differ in the two breeds: the mallard increases the amplitude of its movements, like other non-running birds, while the Indian runner increases the frequency of its movements, as cursorial birds do. Thus, behavioural adaptation to terrestrial locomotion is associated with functional adaptation (upright posture) that permits a more effective mechanical solution without requiring obvious morphological modifications.