In general, animals eat plants, other animals, or the remains of living things. Plant-eaters, or herbivores, often do not have to search far to find things to eat, and in some cases—for example wood-boring insects—they are entirely surrounded by their food. The disadvantage of a plant-based diet is that it can be difficult to digest and is often low in nutrients.
In carnivores (right), the front of the skull has a pair of enlarged canine teeth and the lower jaw moves only in an up and down direction, which assists with the capture and holding of prey. In herbivores (left), the canine teeth are absent and the premolars and molars are well developed. The jaw construction also allows for the lateral movement of the lower jaw in relation to the upper jaw, which helps to provide a grinding motion necessary for rendering plant materials into a state suitable for swallowing and digestion.
To overcome the first of these problems, most herbivores have tough mouthparts for chewing and grinding their food. Many plant-eating animals, from termites to cattle, have complex digestive systems containing microorganisms that break down cellulose and other indigestible plant substances, turning them into nutrients that the animals can absorb. The second problem—lack of nutrients–is harder to sidestep, particularly in a diet made up largely of leaves. As a result, leaf-eaters often have to feed for many hours each day to obtain the nutrients that they need.
The alligator snapping turtle, the largest of the freshwater turtles, has a ridged, camouflaged shell and powerful jaws. When a fish, mistaking a small, wriggling projection on the turtle’s tongue for a worm, swims within reach, the turtle captures it by quickly snapping its jaws shut.
Carnivores live on flesh from other animals that is often nutrient-rich and easy to digest but difficult to obtain. Finding and capturing this kind of food calls for keen senses. But even though a hunter has acute vision or a highly developed sense of smell, a large proportion of a hunter's victims manage to escape. If this happens too often, a predator quickly starves.
Some mammalian predators, such as the lion and wolf, increase their chances of success by hunting in groups. While this strategy enables them to tackle larger prey, a successful kill has to be shared among members of the group. But in the animal world as a whole, many other predators adopt a less energy-intensive approach to catching their food. Instead of actively searching out their prey, they position themselves in a suitable location and wait for their prey to come within striking distance.
Anglerfish have appendages that serve as fishing rods or lures to attract prey, mainly other fish. They are found in oceans all over the world and generally inhabit deep waters. Certain species can grow to lengths of about 1.5 m (5 ft), and have huge mouths capable of swallowing fish of equal size.
In this method of hunting, camouflage and other forms of deception play a prominent role. Most animals that use a lie-and-wait strategy blend in with their surroundings, but a few use lures to entice their prey within range. A typical example is the alligator snapping turtle of North America, which waves a ribbon of pink flesh on its tongue that resembles a worm. Any fish venturing toward it is swallowed whole.
In predatory animals, teeth or other mouthparts often play a part in catching and subduing food as well as in preparing it for digestion. These mouthparts include canine teeth in carnivorous mammals, venomous fangs in snakes, and poisonous "harpoons" in some marine mollusks. These harpoons can impale and kill small fish. Each harpoon is used just once, and afterwards it is expelled and another is formed in its place.