Why omnivore is a slippery label, from alligators eating fruit to seed-eating squirrels
Alligators have been caught eating fruit, perhaps on purpose. Squirrels, supposedly seed eaters, raid birds' nests for eggs and chicks. Geese eat animals young and plants later. Biologists have no single family tree for animals that eat everything, and pinning down what counts as an omnivore turns out to be surprisingly hard.
The word joins Latin omnis, all, with vorare, to devour, and was coined in French before English borrowed it in the 1800s. At its simplest it means an animal that eats both plants and animals, digesting their carbohydrates, protein, fat and fibre. Researchers now separate a behavioural sense, what an animal actually eats, from a physiological one, what it is able to get nutrition from. Humans count as omnivores in the second sense regardless of any individual's diet.
Unlike carnivores, which have an order of their own in Carnivora, omnivores are scattered across the tree of life and arrived at their habits from opposite directions. Dogs descend from mainly meat-eating ancestors and pigs from mainly plant-eating ones. A rooting pig and a chameleon munching leaves and insects share a label but little else. Tooth shape can still hint at diet, as studies of bears show.
Bears illustrate the spread well. Most species are omnivores, yet a particular bear may live almost entirely on plants or almost entirely on meat depending on the season and what grows nearby, while the polar bear is a true carnivore. Life-history omnivores switch as they grow: beetles of the family Meloidae eat animal tissue as larvae and plants as adults. And a meat-eater that nibbles grass, or a herbivore that eats an egg, does not suddenly become an omnivore, because the term describes a species' main adaptation.
Occasional animal food matters more than it looks. Sparrows feed their chicks insects during the hungriest growth phase, and nectar-feeding sunbirds appear to eat the ants in flowers for trace nutrients rather than extra protein. Squirrels introduced from North America have hit nesting birds hard in Britain and elsewhere. The payoff of a flexible diet is security, letting omnivores ride out lean times and settle in unpredictable places.
Source: Omnivore
