To fuel endothermy, mammals require more calories per ounce (or gram) of tissue than do ectothermic vertebrates such as reptiles. This is accomplished by more efficient digestion of food stuffs and more efficient absorption of nutrients. This efficiency begins with specialization of the teeth. Mammals have four different kinds of teeth (heterodonty) that are ideally shaped to cut, slice, grind, and crush food. An exception is the toothed whales in which all the teeth are similar (homodonty). The four types of teeth are incisors for slicing, canines for pierc ing, premolars for crushing or slicing, and molars for crushing. They are commonly represented by a notation called a dental formula, e.g., I2/2 C1/1 P3/3 M2/3, the dental formula for the Egyptian fruit bat (Rousettus aegyptiacus). The first in each group of two numbers represents the teeth in the upper jaw and the second is the number of teeth in the lower jaw. Multiplying the dental formula by 2 gives the total number of teeth, 34. The Egyptian fruit bat's dental formula indicates that the full set of teeth for the upper jaw is four upper incisors, two upper canines, six upper premolars, and four molars. There is a great deal of variation in the number and type of teeth present. For example, the prosimian primate, the aye-aye (Daubentonia madagascariensis), has a dental formula of I1/1 C0/0 P1/0 M3/3, which illustrates a reduction in number of some teeth and the complete loss of others. In the case of some herbivorous mammals the upper incisors are either reduced in number or completely replaced by a hard dental or gummy pad that functions as a cutting board for the lower incisors. In some gnawing mammals, such as rodents and rabbits, the upper and lower incisors grow throughout the entire life span and the canines have been lost. Modification of teeth may be extreme, such as complete loss in most anteaters, or the formation of large tusks, derived from the second upper incisors in elephants, or from the canines in walruses (Odobenus rosmarus).
The relationship between dental structure and function is so precise that the diets of long-extinct mammals can be de
duced from their teeth. The teeth are often the only fossil remains recovered from paleontological sites. Teeth perform mechanical (or physical) digestion by breaking down a food morsel into smaller pieces, providing additional surface area for action by digestive enzymes. Premolars in herbivorous mammals usually have ridges for grinding. In some carnivores such as wolves, the last upper premolar has a blade that shears against the first lower molar. Fruit-eating mammals such as flying foxes often have flattened premolars and molars.
Other modifications for efficient digestion occur in the stomach, a portion of the gastrointestinal tract. The stomach serves as a storage receptacle in most mammals and as a site of protein breakdown. A simple stomach is found in most mammal species, including some that consume fibrous plant material. In other mammals that consume a high fiber diet the stomach has become enlarged and modified to handle more difficult digestion. These modifications comprise a foregut digestive strategy, for which the stomach contains compartments where symbiotic microbes break down cellulose and produce volatile fatty acids (VFA) that can be utilized by the mammal. Foregut fermentation has been developed to the greatest degree among the mammal order Artiodactyla, which includes pigs, peccaries, camels, llamas, giraffes, deer, cattle, goats, and sheep. Rumination, reprocessing of partially digested food, is accomplished by the four-compartment stomachs of giraffes, deer, cattle, and sheep. Less complex tubular and sacculated stomachs are found in kangaroos, colobus monkeys, and sloths. Stomachs in foregut fermenting species are neutral or only slightly acidic, around pH 6.7, to provide a favorable environment for symbionts.
Food moves from the stomach to the intestines; which consist of the small intestine, where most digestion and absorption occurs, and the large intestine. The wall of the small intestine contains epithelial tissue with small finger-like projections called villi. In turn, each villus has smaller extensions called microvilli. The villi secrete enzymes for further carbohydrate and protein digestion. The microvilli absorb the digested nutrients. The presence of the villi and microvilli in the mammal small intestine increases the absorptive surface area by at least 600 times that of a straight smooth tube. The
villi of the human small intestine, for example, provide 3,230 ft2 (300 m2) of surface area whereas the surface area of a smooth tube of the same size as the small intestine is about 5.4 ft2 (0.5 m2). Nutrient absorption occurs through the membranes of the microvilli of each intestinal epithelial cell. Also distributed throughout the small intestine are glands that secrete special enzymes for further digestion of proteins, carbohydrates, and lipids.
For mammals, diet and the length of the small intestine are closely correlated. Mammals that consume a diet that is either digested in the stomach (such as animal protein consumed by faunivores) or easily absorbed (such as nectar consumed by nectarivores) have a shorter small intestine than
other mammals. Herbivores that eat very fibrous plant matter tend to have the longest small intestine. The small intestines of fruit-eaters tend to be intermediate in length.
The foregut fermentation strategy of herbivores requires a medium or large body size to accommodate the necessarily large stomach. A strategy generally used by smaller herbivores is hindgut fermentation (although there are large hindgut fermenters such as horses, elephants, and howler monkeys). The hindgut, also called the large intestine, consists of the cecum and the colon. The cecum is a blind pouch that serves as the principal fermentation chamber in the hindgut strategy. As in the stomach of foregut herbivores, colonies of symbionts in the cecum of hindgut fermenters break down cellulose and excrete products advantageous to the mammal. Nutrients appear to be absorbed through the wall of both the cecum and colon, especially in the larger mammals.
Small hindgut fermenters, such as many rodents and rabbits, have the problem that food can only be retained in the gut for a short time. As it leaves the hindgut, digestion is incomplete and many valuable nutrients may be left unabsorbed. This problem is solved by a behavioral adaptation: a soft pellet is produced in the cecum, defecated, and immediately
picked up by the animal and reingested. This reingestation of feces is called coprophagy. The soft pellet then goes through the digestive process a second time and the end product is a hard fecal pellet devoid of nutrients. Many owners of pet rabbits are familiar with the hard pellet, often called a "raisin." The softer pellet is usually consumed at night (when coprophagy goes unobserved by the pet owner) and is called the "midnight pellet." Coprophagy is efficient; voles are able to extract 67-75% of the energy contained in their food.
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