The typical female dog becomes sexually mature by the age of one. Some females mature by 6 months, others by the age of 18 months. The typical male dog becomes sexually mature by the age of one. An adult female dog is called a bitch; an adult male dog, a dog, or, if used for breeding, a stud.
A sexually mature female dog goes into heat (enters a period of sexual receptivity) twice a year for a period of about 10 days. During this time, the female emits an odor that attracts males.
After a gestation period of 63 days, the female whelps (gives birth to a litter of puppies). A litter may consist of up to 15 puppies, depending on the breed. Some breeds, such as the Pomeranian and Chihuahua, typically give birth to small litters, generally no more than three puppies; other breeds, such as the Airedale terrier and Great Dane, generally give birth to litters of 5 to 12 puppies.
Puppies are born with their eyes closed and do not open them until they are about 10 to 14 days old. Puppies grow rapidly and are ready to be weaned about six weeks after birth. At this time, they have all their milk teeth. The full set of 42 permanent teeth (22 in the lower jaw and 20 in the upper jaw) begins to erupt when a puppy is about five months old. Dogs usually reach full growth and maturity by the age of two.
The average life span of a dog is 12 years, although some dogs live 20 years or more. As a dog ages, the hairs around its muzzle and face become white or gray. Many old dogs suffer from arthritis, cataracts, hearing loss, and kidney disorders.
There are more than 425 modern breeds of dogs, created by selectively breeding dogs with desired characteristics. Inbreeding involves breeding a dog with a close relative. Linebreeding involves breeding a dog with a more distant relative, such as a cousin. In about 10 generations, the dogs will reproduce true to type—that is, the offspring resemble their relatives. Such dogs are called purebreds and typically produce offspring with predictable traits; mongrels, or mixed breeds, do not.