Eating
Most mammals are fairly catholic in their tastes — as long as the food falls into the right food group. While carnivores such as canids prefer to eat the flesh of other mammals, they also enjoy fish and insects and will feed on the gut of their prey even when it's full of plant matter. Even the zebra-loving lion will devour a distasteful human when it is too old to catch anything else. Omnivores such as bears are opportunistic and can thrive on a variable mix of meat and plants. Herbivores, however, usually restrict their diet to plants — except those female herbivores that eat their placenta after birth. A few herbivores are so strict that they'll turn up their nose at most other food, preferring one specific kind of plant. Giant pandas primarily eat bamboo, for instance, and koalas feed only on a few species of smooth-barked eucalyptus trees.
A mammal that has evolved to eat only insects dines frequently. Anteaters can digest as much as one pound (0.5 kg) of ants at a time. After an anteater rips open a nest, it plunges in its snout and uses its long sticky tongue to garner the meal. Most herbivores keep busy foraging because plants are relatively low-energy food compared to protein-rich flesh. The African elephant must snack nearly incessantly to get about 330 pounds (150 kg) of grass and shrubs a day, while the lion makes one big meal of zebra or other meat and then spends days lolling about without hunting. A catnap is time well spent, however, for catching meat requires effort and skill. Because lions often run several miles an hour slower than their prey, they resort to surprise, showing great patience as they slowly creep up on a herd.
Mammals need much of their dinner as fuel to maintain body temperature; they cannot afford to waste energy on digestion. Many herbivores let chewing take care of the problem, easing the burden on the stomach and intestines. In fact, all mammals grow at least two sets of teeth over a lifetime. In front are the incisors. Among some mammals, especially rodents, these sharp teeth are ever-growing. Beside and behind the incisors are canines, teeth well-suited for stabbing meat. Behind them are the cheek teeth: premolars, which tear, and molars, which grind. Herbivores such as horses often have so-called lophodont molars — molars with grinding cusps — to help break down the silica crystals in plants, while carnivores have specialized teeth called carnassials — consisting of an upper premolar and a lower molar, which enable them to slice meat efficiently.
Mammals also have an impressive variety of inner digestive systems. Herbivores do the hardest work — masticating raw plant fiber so their digestive system can extract nutrients. Lacking enzymes, they enlist the help of bacteria to break down the cellulose. In horses, this fermentation occurs in the large intestine, after the food has been chewed and passed through gastric juices in the stomach. The nutrients are absorbed into the bloodstream across the intestinal walls. Ruminants such as cows and giraffes chew only lightly before food goes to the stomach, which is usually four-chambered. It may take them up to a hundred hours to fully digest a meal, as opposed to between 30 and 45 hours for a horse.
