Relative to size, domestic cats are very effective predators. They ambush and dispatch vertebrate prey using tactics similar to those of leopards and tigers by pouncing; they then deliver a lethal neck bite with their long canine teeth that severs the victim's spinal cord, or asphyxiate it by crushing the windpipe.
The domestic cat can hunt and eat about one thousand species—many big cats will eat fewer than 100. Although, theoretically, big cats can kill most of these species as well, they often do not due to the relatively low nutritional content that smaller animals provide. An exception is the leopard, which commonly hunts rabbits and many other smaller animals.
Even well-fed domestic cats will hunt and kill birds, mice, rats and other small animals in the vicinity. They often present such trophies to their owner. The motivation is not entirely clear, but friendly bonding behaviors are often associated with such an action. It is probable that cats in this situation expect to be praised for their symbolic contribution to the group. Some theorize that cats see their owners gone for long times of the day and assume they are out hunting, as they always have plenty of food available. It is thought that a cat presenting its owner with a dead animal thinks it's 'helping out' by bringing home the kill.
Due to their hunting behaviour, in many countries feral cats are considered pests. Domestic cats are occasionally also required to have contained cat runs or to be kept inside entirely, as they can be hazardous to locally endangered bird species. For instance, various municipalities in Australia have enacted such legislation. In some localities, owners fit their cat with a bell in order to warn prey of its approach. Sometimes, the bell has the unwanted effect of "training" the cat to be an even stealthier killer.
Cats have highly specialized teeth and a digestive tract suitable to the digestion of meat. The premolar and first molar together compose the carnassial pair on each side of the mouth, which efficiently functions to shear meat like a pair of scissors. While this is present in canines, it is highly developed in felines. The cat's tongue has sharp spines, or papillae, useful for retaining and ripping flesh from a carcass. These papillae are small backward-facing hooks that contain keratin and assist in their grooming. Domesticated cats eat relatively little vegetable matter. It is quite common, however, for cats to occasionally supplement their carnivorous diets with small amounts of grass or other plant matter. Whereas bears and dogs commonly supplement their diet of meat with fruits, berries, roots, and honey when they can get them, cats prefer to feed mainly on meat. Cats, including the great cats, have a genetic anomaly that prevents them from tasting sweetness , which is probably related to their meat dominated eating habits, and almost certainly related to their aversion to fruits and berries.
In captivity, cats cannot be adapted to an unsupplemented vegetarian diet because they cannot synthesize several nutrients they need and that are absent or rare in plant food. This applies mainly to taurine, vitamin A (cats cannot convert the pro-vitamin A that is abundant in plants to vitamin A proper) and to certain fatty acids. The absence of taurine causes the cat's retina to slowly degenerate, causing eye problems and (eventually) irreversible blindness. This condition is called central retinal degeneration (CRD). Cow's milk is a poor source of taurine and adult cats are generally lactose intolerant.
Lactose-free milk is perfectly safe, but still not a substitute for meat. This contrasts with domesticated dogs, who commonly are fed a mixture of meat and vegetable products and can be adapted to non-supplemented vegetarian diets (though supplementation may be better for dogs too). However, the majority of brand-name cat foods are primarily grain based, often containing large amounts of corn or rice and supplemented with meat byproducts and minerals and vitamins.
Cats are also known to munch on grass, leaves, shrubs and houseplants to regurgitate whatever is upsetting their stomach.
Some houseplants are harmful to cats. The leaves of the Easter Lily can cause permanent and life-threatening kidney damage to cats. Philodendron are also poisonous to cats. Cat Fancy has a full list of plants harmful to cats.
Some cats have a fondness for catnip. While they generally don't consume it, they will often roll in it, paw at it, and occasionally chew on it. The effect is usually relatively short, lasting for only a few minutes. After two hours or less, susceptible cats gain interest again. Several other species of plants cause this effect, to a lesser degree.
Cats can be fussy eaters. This can happen when the vomeronasal, or Jacobson's, organ becomes sensitized to a specific food, or if the cats are spoiled by their owners, at which point the cat will reject any food that doesn't fit the pattern it is expecting. It is also known for cats to merely become bored with their given food and decide to stop eating until they are tempted into eating again. It is not unknown for them to starve themselves to death doing this; although it is extremely rare for a cat to deliberately starve itself to the point of injury, the sudden loss of weight can cause a fatal condition called hepatic lipidosis, a liver dysfunction which causes pathological loss of appetite. Additionally, cats have been known to develop a fondness for "people food" such as chicken, bread, French fries, pizza, ice cream, tomato soup, carrot juice, olives, mushrooms,and carnitas burritos, as well as cat diet exotica such as corn kernels and diced cantaloupe or cantaloupe skin. Many "people foods" are not good for cats; chocolate, for example, can be fatal due to the presence of theobromine (see theobromine poisoning), although few cats will eat chocolate. Paracetamol, found in many familiar over-the-counter pharmaceutical products such as Tylenol, is extremely toxic to cats. Because they naturally lack enzymes needed to digest it, even minute portions of doses safe for humans can be fatal. Any suspected ingestion warrants immediate veterinary attention.
Cats can also develop pica. Pica is a condition where cats chew or eat unusual things such as fabric, plastic or wool. This behavior is mostly harmless as they do not digest most of it, but can be fatal or require surgical removal if a large amount of foreign material is ingested (for example, an entire sock). It tends to occur more often in Siamese, Burmese and breeds with these two in their ancestry.
Domestic cats, especially young kittens, are known for their love of string play. Many cats cannot resist a dangling piece of string, or a piece of rope drawn randomly and enticingly across the floor. This notorious love of string is often depicted in cartoons and photographs, which show kittens or cats playing with balls of yarn. This propensity is probably related to their hunting instinct. If string is ingested, however, it can get caught in the cat’s stomach or intestines, causing illness, or in extreme cases, death. Due to the possible complications of ingesting string, string play is sometimes replaced with a red dot laser pointer. Some people also discourage the use of laser pointers for play with pets, however, because of the fear damage to the sensitive eyes and/or the possible loss of satisfaction associated with the successful capture of an object or of prey.
Because of their small size, domestic cats pose almost no danger to humans—the main hazard is the possibility of infection (e.g. cat scratch disease, or, rarely, rabies) from a cat bite or scratch, although a cat could potentially also inflict severe scratch or puncture damage to the eyes of a human - dogs have been known to have become blinded by cats in fights, with the cat targeting the eyes on purpose and with accuracy rather than at random.
Cats can be destructive to ecosystems in which they are not native and whose species did not have time to adapt to their introduction. In some cases, cats have contributed to or caused extinctions.