Food Acquisition and the Evolution of Positional Behaviour: The Case of Bipedalism

Links between positional behaviour and food acquisition are undeniably important. As Napier (1970) puts it, an animal must move to feed and feed to move. With some interesting exceptions, however, positional behaviour is determined by the distribution rather than by the nature of food items. As it relates to food acquisition, an arboreal species’ locomotor behaviour is determined by the large scale distribution of food items within trees, and postural behaviour is determined by the local distribution of food, usually within the peripheral parts of a tree crown. While the terrestrial environment is in many ways less complex, the clumping of food items on or close to the ground similarly influences positional behaviour. It is thus the structures that intervene between an animal and its food that are important for positional behaviour. An animal of a given size and morphology will use a particular positional repertoire and style of performance of particular activities to select a set of these intervening structures as a substrate or superstrate during foraging and feeding. Changes in diet and in ways of acquiring food are therefore likely to have been important in the evolution of different types of positional morphology and behaviour to the extent that different sets of intervening structures were encountered. One cannot therefore expect positional morphology to track changing dietary patterns as closely as dental morphology may. As Ripley (1979) has shown, however, it is possible to explore the evolutionary links between positional behaviour and food acquisition. This paper explores some of these points with respect to the evolution of bipedalism.