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An Omnivore, meaning 'all-eater' (Latin omni, vorare: "all, everything", "to devour"), is a polyphage ("many foods") species that is a consumer of a variety of material as significant food sources in their natural diet. These foods may include plants, animals, algae and fungi.
Omnivores often are opportunistic, general feeders with neither carnivore nor herbivore specializations for acquiring or processing food, and are capable of consuming and do consume both animal protein and vegetation. Many omnivores depend on a suitable mix of animal and plant food for long-term good health and reproduction.
Omnivore, omnivory and similar derivations are terms of convenience; their significance varies according to context nand to both kind and degree. Non-fuzzy definition therefore is neither possible nor necessary. Traditionally the definition for omnivory is some variation of the form: "including both animal and vegetable tissue in the diet", which seems clear enough for most purposes. However, it is neither absolute nor yet precise, either exclusively or inclusively. It is in fact meaningful only in limited senses, either taxonomically or ecologically. Because most herbivores and omnivores eat only a small range of types of plant food one seldom has reason to refer to an omnivorous pig scavenging for fruit and carrion, and digging for roots and small animals, as being in the same category as an omnivorous chameleon that eats leaves as well as insects; apart from their taxonomic differences the two have little ecological or dietary overlap.
The term "omnivory" also is not comprehensive because it does not deal with questions of mineral food such as salt licks, or the question of eating life forms that are not included in the kingdoms Animalia and Plantae. As for appeals to etymological points such as that "omnivore" means "eater of everything", no biologist or philologist would take them seriously.
One might be tempted to impose a taxonomic definition, irrespective of actual diet, appealing to the Carnivora as a taxon in which, in spite of their being Carnivora, most species in the order eat at least some vegetable matter. However, there are no corresponding taxa called "Omnivora" or "Herbivora", and even if there were, zoologists would not claim either that all Carnivora are carnivores, or that all carnivores are Carnivora. Taxonomically in fact, there probably are fewer than three hundred species of Carnivora..., whereas there are more than that number of species specialising in animal food among the Chiroptera alone.
Concerning the phenomena to which terms such as "omnivore" might apply, very few carnivores and herbivores in the normal senses are strictly limited to just one type of tissue in the diet. Even felids and mustelids, animals normally seen as specialist carnivores, often eat a little vegetable matter for various reasons, such as when they eat the guts of prey. Examples of animals that come closest to rigid specialisation in carnivory or herbivory respectively, are the likes of parasitoidal insects or insects that are specialist sap-suckers. Naturally biologists take no interest in quibbling about whether, or how strictly, to classify a ruminant as an omnivore on the grounds that a cow might swallow insects on the grass it eats, or even that it may eat old bones as mineral supplements. Nor is it rewarding to argue whether to call an animal an omnivore because it eats mainly animal food at one stage of its life, and plant matter at another, even though many diverse animals do so and in many different ways. Some species of grazing waterfowl, such as geese, are well-known examples. So are many insects such as beetles in the family Meloidae, that begin by eating animal food as larvae, but change to plant food when they mature. Many mosquitoes begin with plant food or assorted detritus, but when they are mature the two genders adopt different diets if they eat at all; the males mainly eat nectar and other plant juices, whereas many species of females in genera such as Anopheles, Aedes and Culex, though they similarly eat nectar, also must suck blood if they are to reproduce effectively. Other species, such as the genus Toxorhynchites on the contrary, are predatory carnivores when in the larval stage, but grow up into nectar-eating adults of both genders. Concerning omnivory, one terminology might validly be most convenient in some contexts, but not in others.
In summary "omnivory" is a general term of convenience in many contexts and takes many forms in biology, but as a general term it intrinsically is both non-specific and ambiguous; wherever it is necessary to refer to a particular type of omnivory, one must begin by specifying what version one has in mind and how it is defined. To some extent the same applies to logically related terms for dietary behaviour, such as herbivory and carnivory.
Although cases exist of carnivores eating plant matter, as well as of herbivores eating meat, the classification "omnivore" refers to the adaptations and main food source of the species in general, so these exceptions do not make either individual animals nor the species as a whole omnivorous. In order for the concept of "omnivore" to be regarded as a scientific classification, some clear set of measurable and relevant criteria would need to be considered to differentiate between an "omnivore" and the other vague but less ambiguous diet categories e.g., faunivore, folivore, scavenger, etc. Some researchers argue that evolution of any species from herbivory to carnivory or carnivory to herbivory would be rare except via an intermediate stage of omnivory.
Various mammals are omnivorous in the wild, such as the Hominidae, pigs, badgers, bears, coatis, hedgehogs, opossums, skunks, sloths, squirrels, raccoons, chipmunks, mice, and rats. Various birds are omnivorous, with diets varying from berries and nectar to insects, worms, fish, and small rodents. Examples include cassowarys, chickens, crows and related corvids, keas, rallidae, and rheas. In addition, some lizards, turtles, fish, such as piranhas and catfish, and invertebrates are also omnivorous.
Most bear species are considered[by whom?] omnivores, but individual diets can range from almost exclusively herbivorous to almost exclusively carnivorous, depending on what food sources are available locally and seasonally. Polar bears are classified as carnivores, both taxonomically (they are in the order Carnivora), and behaviorally (they subsist on a largely carnivorous diet). Wolf subspecies (including wolves, dogs, dingoes, and coyotes) can live on such vegetable material as grain and fruit products indefinitely but clearly prefer meat. Depending on the species of bear, there is generally a preference for one class of food, as plants and animals are digested differently.
While most mammals may display "omnivorous" behavior patterns depending on conditions of supply, culture, season and so on, they will generally prefer one class of food or another, and when their digestive processes are adapted to a particular class, their long-term preferences will reflect such adaptations. Like most arboreal species, most squirrels are primarily granivores, subsisting on nuts and seeds. But as with virtually all mammals, squirrels avidly consume some animal food when it becomes available. For example, the American Eastern gray squirrel has spread to parts of Europe, Britain and South Africa. Where it flourishes, its effect on populations of nesting birds is often serious, largely because of consumption of eggs and nestlings.
Quite commonly, predominantly herbivorous organisms will eagerly eat small quantities of animal food when it happens to become available. Although this is a trivial matter most of the time, omnivorous or herbivorous birds, such as sparrows, often will feed their chicks animal food (largely insects) as far as possible while the need for growth is most urgent. On close inspection it appears that nectar-feeding birds such as sunbirds rely on the ants and other insects that they find in flowers, not for a richer supply of protein, but for essential nutrients such as Cyanocobalamin that are essentially absent from nectar. Similarly monkeys of many species eat maggoty fruit, sometimes in clear preference to sound fruit. When to refer to such animals as omnivorous or otherwise becomes a question of context and emphasis rather than of definition.