Dogs bear their litters roughly 58 to 68 days after fertilization,[17][71] with an average of 63 days, although the length of gestation can vary. An average litter consists of about six puppies,[72] though this number may vary widely based on the breed of dog. In general, toy dogs produce from one to four puppies in each litter, while much larger breeds may average as many as twelve.
Dogs have been described as carnivores[117][118] or omnivores.[17][119][120][121] Compared to wolves, dogs have genes involved in starch digestion that contribute to an increased ability to thrive on a starch-rich diet.[19] Based on metabolism and nutrition, many consider the dog to be an omnivore. However, the dog is not simply an omnivore. More like the cat and less like other omnivores, the dog can only produce bile acid with taurine, and it cannot produce vitamin D, which it obtains from animal flesh. Also more like the cat, the dog requires arginine to maintain its nitrogen balance. These nutritional requirements place the dog part-way between carnivores and omnivores.[122]

Dogs have been described as carnivores[117][118] or omnivores.[17][119][120][121] Compared to wolves, dogs have genes involved in starch digestion that contribute to an increased ability to thrive on a starch-rich diet.[19] Based on metabolism and nutrition, many consider the dog to be an omnivore. However, the dog is not simply an omnivore. More like the cat and less like other omnivores, the dog can only produce bile acid with taurine, and it cannot produce vitamin D, which it obtains from animal flesh. Also more like the cat, the dog requires arginine to maintain its nitrogen balance. These nutritional requirements place the dog part-way between carnivores and omnivores.[122]
The origin of the domestic dog includes the dog's evolutionary divergence from the wolf, its domestication, and its development into dog types and dog breeds. The dog is a member of the genus Canis, which forms part of the wolf-like canids, and was the first species and the only large carnivore to have been domesticated.[14][26] The dog and the extant gray wolf are sister taxa, as modern wolves are not closely related to the population of wolves that was first domesticated.[14]
The scientific evidence is mixed as to whether companionship of a dog can enhance human physical health and psychological wellbeing.[191] Studies suggesting that there are benefits to physical health and psychological wellbeing[192] have been criticised for being poorly controlled,[193] and finding that "the health of elderly people is related to their health habits and social supports but not to their ownership of, or attachment to, a companion animal." Earlier studies have shown that people who keep pet dogs or cats exhibit better mental and physical health than those who do not, making fewer visits to the doctor and being less likely to be on medication than non-guardians.[194]
It is a reflexive action for many dogs to try to pull away their back leg if it is lifted. Avoid lifting the leg to the side because this is uncomfortable for most dogs. Instead, pick up the foot and gently pull forward or backward. Don't be upset or punish the dog, just struggle through it and praise your dog when he or she is still. If you have a medium or large dog, you may be able to get away with clipping the nails on the hind feet without having to lift the foot.
Humans would also have derived enormous benefit from the dogs associated with their camps.[136] For instance, dogs would have improved sanitation by cleaning up food scraps.[136] Dogs may have provided warmth, as referred to in the Australian Aboriginal expression "three dog night" (an exceptionally cold night), and they would have alerted the camp to the presence of predators or strangers, using their acute hearing to provide an early warning.[136]
Unlike other domestic species which were primarily selected for production-related traits, dogs were initially selected for their behaviors.[96][97] In 2016, a study found that there were only 11 fixed genes that showed variation between wolves and dogs. These gene variations were unlikely to have been the result of natural evolution, and indicate selection on both morphology and behavior during dog domestication. These genes have been shown to affect the catecholamine synthesis pathway, with the majority of the genes affecting the fight-or-flight response[97][98] (i.e. selection for tameness), and emotional processing.[97] Dogs generally show reduced fear and aggression compared with wolves.[97][99] Some of these genes have been associated with aggression in some dog breeds, indicating their importance in both the initial domestication and then later in breed formation.[97] Traits of high sociability and lack of fear in dogs may include genetic modifications related to Williams-Beuren syndrome in humans, which cause hypersociability at the expense of problem solving ability.[100][101][102]
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