Introduction

The feeding habits of the largest land mammals have naturally attracted a lot of attention, if only for the sheer quantity and variety of plants they consume and the impact they make on their habitats. Both in Africa and in Asia, there have been numerous studies of the foraging ecology of the elephant. Given the diversity of habitats in which elephants are found across the two continents, we can naturally expect considerable variation in the types and parts of plants they consume. Yet, we must search for commonality amid the myriad patterns of foraging behavior recorded. Can the elephant's feeding strategies throughout its range be explained by a simple set of rules?

An animal's feeding behavior is fundamentally explained by its anatomical and physiological adaptations, products of its evolutionary history. Among mammalian herbivores, structures of the teeth and of the gastrointestinal tract are obviously important determinants of plant types eaten. Their ability to degrade cellulose in the plant cell wall through microbial symbionts for obtaining energy is closely related to the structure of the gastrointestinal tract. Body size exerts a great influence through its physiological consequences, such as metabolic rate. The sexes may have differing nutritional requirements or may use habitats in a different manner; females and young may have greater need to avoid predators, for example. In the case of the elephant, the very large body size imposes special constraints on its feeding ecology. At a more proximal level, the food choices of a herbivore will depend on what is available. The theoretically "ideal" diet for a species may never be achieved, except in very localized regions, by most populations because such an ideal variety and quantity of plants may just not be available.

Allometry has been used as a predictive tool in the comparative ecology of species within functional groups. Using data on body size, for instance, the quantity of forage or even a particular nutrient such as protein required by a herbivorous mammal could be derived from a standard relationship. Allomet-ric functions could also fail completely with certain ecological characteristics. With the elephant, it fails to predict home range (which varies over two orders of magnitude) or, say, the proportion of the diet constituted by grasses. Predictive relationships, of course, could be explored with a species that spans a diverse range of habitats, from rain forest, through deciduous forest and savanna-woodland, to near desert.

Optimal foraging theory, which has been successfully applied to animals such as insectivorous birds or predatory mammals, has only had limited application to large mammalian herbivores. Apart from qualitative accounts, the foraging behavior of elephants has not been examined within such a theoretical framework in spite of numerous studies on their feeding. One reason could be the relative paucity of specific physiological data for elephants. A more important reason could be the sheer logistical problems involved in studying the food choices year-round of a long-ranging species, quantifying the distribution and availability of a variety of herbaceous and browse plants, and simultaneously keeping track of changes in plant phenology and nutritional qualities.

I begin with a description of the plants actually consumed by elephants across their distributional range, the quantities ingested, and the variations across seasons and habitats. The issue of whether the elephant is a browser or a grazer is then tackled through the newer evidence of carbon isotopes. The ecological determinants of diet in elephants in relation to body size, gut anatomy and physiology, and nutritive requirements are finally explored.

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