Body size is a very obvious determinant of foraging strategy and diet in a herbivorous mammal. Norman Owen-Smith argues persuasively that animals weighing over 1,000 kg—the megaherbivores—are distinct in almost all aspects of ecology compared to smaller-size species. In other words, the mega-herbivores do not necessarily represent a continuum from trends seen in other large herbivores in several ecological features. Individual species may deviate markedly from the general allometric relationship, suggesting adaptations that free the species from the constraints imposed by body size.
While body size is one important influence, the diet of a herbivorous mammal has coevolved with various other anatomical and physiological features; habitat and vegetation characteristics; competition with other species, especially closely related ones; and even its social organization. Among the two extant rhinos of the genus Rhinoceros in Asia, the larger R. unicornis (greater one-horned rhino), with its moderately high-crowned molars, is adapted for grazing, while the smaller R. sondaicus (Javan rhino), with relatively low-crowned and high-cusped molars, is a browser. The former evolved in moist grasslands of river valleys of South Asia, while the latter was a creature of moist forests in Southeast Asia. Both species have very restricted distributions today, the latter reduced to just two small populations; a third species, Dicerorhinus sumatrensis (the Sumatran rhino), which inhabits more hilly forests, is also a browser. In Africa, the larger Ceratotherium simum (white rhino), with its broad lips and high-crowned teeth, is essentially a grazer, while in the smaller Diceros bicornis (black rhino), the fingerlike upper lip and low-crowned molars are adaptations for browsing. Interestingly, both these species inhabit similar habitat—dry savanna and arid bushland.
The elephants are possibly unique among the larger herbivores in that populations may be almost entirely browsers (as in rain forest) or predominantly grazers (as in savanna). Evolutionary changes in mandible and dental structure of Elephas and Loxodonta suggest that E. maximus is adapted for a grazing diet to a higher degree than is L. africana. This difference is not necessarily seen in the actual diets of the two species over their range of habitats, although the carbon isotopic data does hint at such a difference. Dietary compositions are virtually indistinguishable, with a possible exception of higher quantities of bark in the African elephant's diet in woodland habitats. The foraging strategy of a mixed feeder is much more difficult to dissect compared to that of a pure grazer or a pure browser.
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