Omnivore

Omnivore, omnivory and similar derivations are terms of convenience; their significance varies according to context and 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.