A carnivore , meaning "meat eater" (Latin, carne meaning "meat" or "flesh" and vorare meaning "to devour"), is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging. Animals that depend solely on animal flesh for their nutrient requirements are called obligate carnivores while those that also consume non-animal food are called facultative carnivores.Omnivores also consume both animal and non-animal food, and, apart from the more general definition, there is no clearly defined ratio of plant to animal material that would distinguish a facultative carnivore from an omnivore. A carnivore that sits at the top of the food chain is termed an apex predator.
The word "carnivore" sometimes refers to the mammalian order Carnivora, but this is somewhat misleading. While many Carnivora meet the definition of being meat eaters, not all do, and even fewer are true obligate carnivores (see below). For example, most species of bears are actually omnivorous, except for the giant panda, which is almost exclusively herbivorous, and the exclusively meat-eating polar bear, which lives in the Arctic, where few plants grow. In addition, there are plenty of carnivorous species that are not members of Carnivora.
Outside 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, amoebae and springtails.
Carnivores are sometimes characterized by the type of prey that they consume. For example, animals that eat insects and similar invertebrates primarily or exclusively are called insectivores, while those that eat fish primarily or exclusively are called piscivores. The first tetrapods, or land-dwelling vertebrates, were piscivorous amphibians known as labyrinthodonts. They gave rise to insectivorous vertebrates and, later, to predators of other tetrapods.
Carnivores may alternatively be classified according to the percentage of meat in their diet. The diet of a hypercarnivore consists of more than 70% meat, that of a mesocarnivore 50-70%, and that of a hypocarnivore less than 30%, with the balance consisting of non-animal foods such as fruits, other plant material, or fungi.
Obligate carnivores, or "true" carnivores, are those carnivores whose survival depends on nutrients which are found only in animal flesh. While obligate carnivores might be able to ingest small amounts of plant material, because of their evolution they lack the necessary physiology required to digest that plant matter. In fact, some obligate carnivorous mammals will only ever ingest vegetation for its specific use as an emetic to self-induce vomiting to rid itself of food that has upset its stomach.
For instance, felids including the domestic cat are obligate carnivores requiring a diet of primarily animal flesh and organs. Specifically, cats have high protein requirements and their metabolisms appear unable to synthesize certain essential nutrients (including retinol, arginine, taurine, and arachidonic acid), and thus, in nature, they can rely only on animal flesh as their diet to supply these nutrients.
Characteristics commonly associated with carnivores include organs for capturing and disarticulating prey (teeth and claws serve these functions in many vertebrates) and status as a predator. In truth, these assumptions may be misleading, as some carnivores do not hunt and are scavengers (though most hunting carnivores will scavenge when the opportunity exists). Thus they do not have the characteristics associated with hunting carnivores. Carnivores have comparatively short digestive systems, as they are not required to break down tough cellulose found in plants. Many animals that hunt other animals have evolved eyes that face forward, thus making depth perception possible. This is almost universal among mammalian predators. Other predators, like crocodiles, as well as most reptiles and amphibians, have sideways facing eyes.
The first vertebrate carnivores were fish, and then amphibians that moved on to land. Early tetrapods were large amphibious piscivores. Some scientists assert that Dimetrodon "was the first terrestrial vertebrate to develop the curved, serrated teeth that enable a predator to eat prey much larger than itself." While amphibians continued to feed on fish and later insects, reptiles began exploring two new food types, tetrapods (carnivory), and later, plants (herbivory). Carnivory was a natural transition from insectivory for medium and large tetrapods, requiring minimal adaptation (in contrast, a complex set of adaptations was necessary for feeding on highly fibrous plant materials).
Carnivoramorphs are currently the dominant carnivorous mammals, and have been so since the Miocene. In the early to mid-Cenozoic, however, hyaenodonts, oxyaenid, entelodonts, ptolemaiidans, "arctocyonids" and "mesonychians" were dominant instead, representing a very high diversity of eutherian carnivores in the northern continents and Africa. In South America, sparassodonts were dominant instead, while Australia saw the presence of several marsupial predators, such as the dasyuromorphs and thylacoleonids.
In the Mesozoic, while theropod dinosaurs were the larger carnivores, several carnivorous mammal groups were already present. Most notable are the gobiconodontids, the triconodontid Jugulator, the deltatheroideans and Cimolestes. Many of these, such as Repenomamus, Jugulator and Cimolestes, were among the largest mammals in their faunal assemblages, capable of attacking dinosaurs.
Most carnivorous mammals, from dogs to Deltatheridium, share several adaptations in common, such as carnassialiforme teeth, long canines and even similar tooth replacement patterns. Most aberrant are thylacoleonids, which bear a diprodontan dentition completely unlike that of any mammal, and "eutriconodonts" like gobioconodontids and Jugulator, by virtue of their cusp anatomy, though they still worked in the same way as carnassials.