Body composition in wild animals is regulated within a band that encompasses variations due to seasonal food supply and physiological state (e.g. that of the reproducing, lactating female and the growing juvenile). This body composition is characteristic of the genotype and is notable for the avoidance of obesity (Pitts & Bullard 1968). Urgent behavioural responses are also invoked to minimize weight loss. Figure 1 illustrates this homeostasis diagrammatically for the non-lactating, mature animal.In this integrated complex, both food intake (quantity) and diet choice (quality) play a major role in achieving the required balance between food-energy intake and ‘net output’ of energy in its various forms such as activity, growth, lactation and change in body reserves. The development of such a system, through evolution, to achieve optimum reproductive fitness in a given environment, is consistent with the theory of optimal foraging which is characteristic of many animals in the wild. Newman (1991) has used such an approach in modelling diet selection by sheep.Domesticated animals have inherited the complex genome of their progenitors, albeit modified in certain respects through natural and artificial selection, in their new circumstances. Many aspects of the physiology and behaviour of the domesticated animal still relate to the environment in which they evolved. For example Jensen & Redbo (1987) have demonstrated the complex nesting behaviour retained by the modern domestic pig, under free ranging conditions similar to those of its wild progenitor.Two major non-genetic factors have emerged as major influences on the appetite of domesticated animals and man in modern times.
