Dogs

Among professional breeders, dogs are only allowed to mate for a specific purpose. Sometimes dogs are bred to create puppies to sell, or sometimes to carry on an award-winning purebred line. Breeders who do this are usually experienced in this process. Dog breeders have access to records which allow them to accurately guess which characteristics will "breed true" in a particular dog. Dog breeders also have accurate information on the complexities of the reproductive process for the breed of dog that they are accustomed to handling.

Fertility

As with most domesticated species, one of the first and strongest effects seen from selective breeding is selection for cooperation with the breeding process as directed by humans. In domestic dogs, one of the behaviours that is noted is the abolition of the pair bond seen in wild canines. The ability of female domestic dog to come into estrus at any time of the year and usually twice a year is also valued. The amount of time between cycles varies greatly among different dogs, but a particular dog's cycle tends to be consistent through her life. This is also called in season or in heat. Conversely, undomesticated canine species experience estrus once a year, typically in late winter.

Menarche

Most dogs come into season for the first time between 6 and 12 months, although some larger breeds delay until as late as 2 years. Like most mammals, the age that a bitch first comes into season is mostly a function of her current body weight as a proportion of her body weight when fully mature. The different rates of maturation are responsible for the menarche, not the chronological age.

Pregnancy and litters

A general rule of thumb is that a mammal will produce half as many offspring as the number of teats on the mother. This rule is altered in domesticated animals since larger litters are often favoured for economic reasons. Dogs bear their litters roughly 9 weeks after fertilization. An average litter consists of about six puppies, though this number may vary widely based on the breed of dog. Since a mother can only provide nutrients and care to a limited number of offspring, humans must assist in the care and feeding when the litter exceeds approximately eight puppies. Some breeds have been developed to emphasize certain physical traits beyond the point at which they can safely bear litters on their own. For example, the Bulldog often requires artificial insemination and almost always requires cesarean section for giving birth.

Spaying and neutering

Spaying or neutering refers to the removal of the male's testicles or the female's ovaries and uterus, in order to eliminate the ability to procreate, and reduce sex drive.

Animal control agencies in the United States and the ASPCA advise that dogs not intended for further breeding should be spayed or neutered so that they do not have undesired puppies. In Europe, spaying or neutering of dogs is less common, and owners are usually instead advised to keep their dogs on leash and under supervision to avoid unwanted matings; drugs are used to prevent pregnancy and abort unwanted litters. [citation needed]

Because of the overpopulation of dogs in some countries, puppies born to strays or as the result of accidental breedings often end up being euthanized in animal shelters. Spaying and neutering can also decrease the risk of hormone-driven diseases such as mammary cancer, as well as undesired hormone-driven behaviors. The hormonal changes involved with sterilization are likely to somewhat change the animal's personality, however, and some object to spaying and neutering as the sterilization could be carried out without the excision of organs.

Contrary to myth, it is not required for a female dog to either experience a heat cycle or have puppies before spaying, and likewise, a male dog does not need the experience of mating before neutering; these myths are responsible for numerous unnecessary health problems and unwanted puppies. Female dogs spayed before their first heat have a vastly lower incidence of many forms of cancer than dogs which are spayed after their first heat or pregnancy. A female dog can become pregnant on her first heat cycle (which can take place as early as six months), and should be kept away from intact male dogs, including littermates, over the age of 4 months. Many veterinarians recommend that owners neuter/spay their pets around the age of 5 months.