See R. F. Ewer, The Carnivores (1986); J. L. Gittleman, Carnivore Behavior, Ecology, and Evolution (1989).
A carnivore meaning 'meat eater' (Latin carne meaning 'flesh' and vorare meaning 'to devour'), is any animal with a diet consisting mainly of meat, whether it comes from animals living (predation) or dead (scavenging).
In a more general sense, animals are loosely considered carnivores if their feeding behaviour consists of preying on other animals rather than grazing on plants. Animals that must eat meat in order to thrive are referred to as obligate carnivores.
The designation "hypercarnivore" is used to describe animals that exclusively feed on animal tissue. Additionally, this term is also used in paleobiology to describe taxa of animals which have an increased slicing component of their dentition relative to the grinding component.
Carnivores are classified by their dietary behaviour. However, because the specific definition of what exactly meat is can be flexible, so too can the definition of carnivore. In the strictest sense, meat may refer to the flesh of mammalian species, but in a broader sense the term might be used to refer to any animal tissue considered food.
Carnivores that eat insects and similar invertebrates primarily or exclusively are called insectivores, while those that eat fish primarily or exclusively are called piscivores. Carnivory that entails the consumption of members of an organism's own species is referred to as cannibalism. This includes sexual cannibalism and cannibalistic infanticide.
The word "carnivore" sometimes refers to the mammalian Order Carnivora, but this is misleading. Although many Carnivora fit the first definition of being exclusively meat eaters, not all do. For example, bears are members of Carnivora, yet are not carnivores in the dietary sense, and pandas are almost exclusively herbivorous. In addition, some full-time (dolphins, shrews) and part-time (humans, pigs) meat-eating mammalian species, as well as all carnivorous non-mammals, are not members of Carnivora.
Outside of the animal kingdom, there are several genera containing carnivorous plants and several phyla containing carnivorous fungi. The former are predominantly insectivores, while the latter prey mostly on microscopic invertebrates such as nematodes, amoeba and springtails.
Prehistoric mammals of the crown-clade Carnivoramorpha (Carnivora and Miacoidea without Creodonta), along with the early Order Creodonta, and some mammals of the even early Order Cimolesta, were true carnivores. The earliest carnivorous mammal is considered to be the Cimolestes that existed during the Late Cretaceous and Tertiary Periods in North America about 65 million years ago. Most species of Cimolestes were mouse to rat-sized, but the Late Cretaceous Cimolestes magnus reached the size of a marmot, making it one of the largest Mesozoic mammals known (20-60g). The cheek teeth combined the functions of piercing, shearing and grinding, and the molars of Palaeoryctes had extremely high and acute cusps that had little function other than piercing. The dentition of Cimolestes foreshadows the same cutting structures seen in all later carnivores. While the earlier smaller species were insectivores, the later marmot-sized Cimolestes magnus probably took larger prey and were definitely a carnivore to some degree. The cheek teeth of Hyracolestes ermineus (an ermine-like shrew - 40g) and Sarcodon pygmaeus ("pygmy flesh tooth" - 75g), were common in the Latest Paleocene of Mongolia and China and occupied the small predator niche. The cheek teeth show the same characteristic notches that serve in today's carnivores to hold flesh in place to shear apart with cutting ridges. The theropod dinosaurs such as Tyrannosaurus rex that existed during the late Cretaceous, although not mammals, were "obligate carnivores".
An obligate or true carnivore is an animal that must eat meat in order to thrive. Hypercarnivores present specialized dentition for a meat-only diet. They may consume other products presented to them, especially animal products like eggs and bone marrow or sweet sugary substances like honey and syrup, but, as these items are not essential, they do not consume these on a regular basis. True carnivores lack the physiology required for the efficient digestion of vegetable matter, and, in fact, some carnivorous mammals eat vegetation specifically as an emetic. The domestic cat is a prime example of an obligate carnivore, as are all of the other felids.