The ability to select beneficial food. Many animals are able to select the right kind of food as a result of their innate preferences. Others are able to learn rapidly what kinds of food are good for them and what bad.
Feeding specialists consume only one or a few kinds of food. The monarch butterfly (Danaus plexippus) eats only milkweed. The koala bear (Phascolarctos cinereus) eats eucalyptus leaves. As their food is homogeneous, such animals require a single hunger detection system, and a simple food-recognition system. Feeding generalists, on the other hand, have a heterogeneous diet, and must distinguish among a variety of specific hungers or nutritional deficits, and require a complex food recognition system. Poison avoidance is particularly important for omnivores.
Omnivores can easily become deficient in certain nutrients, such as vitamins and minerals. They may consume too much of a particular food, so obtaining an imbalance of nutrients, and they are in danger of consuming poisonous substances. Omnivores usually rely on learning to select a balanced diet and avoid harmful substances. Rats (Rattus norvegicus), for example, sample novel food substances with caution, eating only a little. If the rat becomes ill after sampling a new food, then it quickly learns to avoid that food in future. This type of learning is an important feature of poison avoidance. Moreover, the rat can learn rapidly which foods are nutritionally beneficial. When a rat suffers a deficit of some essential vitamin or mineral, it takes more interest in novel food substances and samples them more than it otherwise would. If, as a result, it eats food containing vitamins or minerals capable of rectifying its deficiency, then it quickly learns to eat more of that food. Usually, the rat is unable to detect, by smell or taste, the presence of vitamins and minerals in the food. It learns to select the beneficial food on the basis of the post-ingestive consequences of eating it. In other words, the rat can learn to select those foods that make it feel better, and avoid those that make it feel ill.
Many species of omnivorous mammals and birds have been shown to have the ability to learn rapidly to avoid noxious foods and select beneficial foods. Amongst mammals, the ability to recognize different foods is based primarily on olfaction and taste, whereas in birds it is based primarily on vision. However, the principle of food selection based upon post-ingestive consequences remains the same.
Subjects: Zoology and Animal Sciences.