Anatomical and physiological adaptations

In any discussion of the elephant's feeding strategy, the role of the trunk cannot be ignored. From selectively picking up tiny flowers or fallen fruits from the ground to reaching up 5 m for a stout branch, this unique organ enables the elephant to feed on a wide variety of plants and plant parts. Whatever other functions may be attributed to the trunk, its mere possession and versatility could have been a strong determinant of dietary diversity. In other words, if you have it, you might as well use it.

Over evolutionary time, the jaws and teeth of the proboscideans show adaptations toward an increasingly coarse and abrasive plant diet. The high-crowned molars, the complex pattern of folded enamel plates, the rasplike (occlusal) surface, sequential replacement of molars, and the mechanics of shearing plants between the jaws are all adaptations toward a diet of plants, such as coarse grasses, that cause greater wear on the teeth.

When the anatomy of the digestive system and the process of digestion and nutrient assimilation in herbivorous mammals adapted to a cellulosic plant diet are described, a comparison between ruminants and nonruminants becomes inevitable. Each has its own advantages and disadvantages; the evolutionary determinants of these contrasting strategies were traced by Christine Janis in a classic article published in 1976. I mention the salient differences between ruminant and nonruminant digestion before discussing the implications for elephant foraging.

In contrast to herbivores, which feed mainly on plant reproductive parts such as fruits or seeds, those that subsist on a diet of structural parts such as stems or leaves have to find a way to break down cellulose. The cellulose, a complex carbohydrate of the plant cell wall, has to be degraded not only to obtain energy from the products of its breakdown, but also to release nutrients from the cell. Only microbes such as bacteria produce the enzyme that can degrade cellulose. Among ruminants, the stomach, or rumen, is the site of microbial fermentation of the plant cell wall, while in the nonruminants, this is usually achieved in the cecum or colon.

Although there is little difference in the fermentation process itself, with volatile fatty acids (VFAs) being the products of polysaccharide breakdown, there are interesting consequences for protein and carbohydrate metabolism among the ruminants and nonruminants. In ruminants, not only cellulose, but also simple carbohydrates and proteins are fermented in the forestomach before the ingesta reach the small intestine. As simple sugars are fermented into VFAs, there are limitations to rapid mobilization of energy through absorption of glucose in the small intestine among many ruminants. The fermentation of protein into ammonia and subsequent recycling of nitrogen through urea and microbial protein synthesis, however, ensure that ruminants do not suffer from amino acid imbalances in the diet. Nonruminants, on the other hand, do not have the same advantage of digestion of microbial proteins. The proteins and simple carbohydrates are already absorbed in the small intestine before the ingesta reach the site of fermentation. The significance of hindgut absorption of amino acids from the digestion of microbial proteins is not clear. A nonru-minant, therefore, may have to sample a greater variety of plants to meet its protein (amino acid) requirements.

A nonruminant scores over a ruminant in its ability to achieve a higher throughput rate of forage. While a ruminant has physical restrictions, due to the complex structure of the stomach, in the rate of passage of food and, hence, in the quantity of food it can consume, a nonruminant has much less limitation. Thus, a nonruminant can tolerate a diet of lower quality, but has to increase its feeding rate or proportion of time spent in feeding. The above comparisons are for animals of similar body size.

The elephant is a nonruminant having a simple, balloon-shaped stomach, a small intestine, a relatively small cecum, and a noncompartmentalized colon, with most of the ingested food found within the proximal two-thirds of the large bowel (fig. 5.5). The most recent study of the digestive physiology of the

Figure 5.5

A diagram of a female African elephant showing the internal organs. (From Shoshani 1991. Reproduced with the permission of Jeheskel Shoshani.)

Figure 5.5

A diagram of a female African elephant showing the internal organs. (From Shoshani 1991. Reproduced with the permission of Jeheskel Shoshani.)

elephant (and two other large herbivores) by E. T. Clemens and G.M.O. Maloiy (1982) indicated that the concentrations of the VFAs, chiefly acetate, propionate, and butyrate, in the cecum and colon are comparable to those attained in the rumen of cattle. The efficiency of hindgut fermentation in the elephant is therefore high. The low pH (acidic) and low sodium ion concentration of the small intestine suggest decreased effectiveness of pancreatic and biliary secretions. How this influences intestinal digestion is not clear.

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