International Journal of Comparative Psychology

Complex movements, such as placing food into the mouth, involve coordinating multiple limb segments. Given the degrees of freedom for one limb segment, the computational costs of such complex movements can be high. One way to reduce such costs is to limit the adjusting movements needed to achieve coordination of distal body parts to only one part of the body. For example, for scratching the head, the hand or foot needs to make contact with the head and this involves movements of the head, neck and torso, as well as those of the foot and leg, or hand and arm. In this situation, the foot or hand is raised to a specific location in space and then makes oscillatory movements, but it is movements by the head and neck that ensure appropriate contact is made with the head (Pellis, 2010). In this paper, whether such cost-saving rules apply across functional contexts is tested in the purple swamp hen by comparing head and foot coordination during head scratching and during food reaching and handling. This species uses its foot to grasp and hold a wide range of food items that are picked up in its bill. Comparison of hundreds of videotaped sequences revealed that, in both cases, the bird uses the same rule: that of making the accommodating movements with only one of those body parts, even when coordination requires movements of disparate parts of the body. These data show that there are likely common computational cost-saving rules that widely apply to movements occurring in many different functional contexts.