The normal life span of a small or medium-size dog is about 15 years. A large dog lives only about ten years, however. On the average, a ten-month-old dog is sexually mature. Smaller bitches go into their first heat (become responsive to their first mating) at an earlier age than larger ones.

Fetal puppies grow in their mother's womb before they are whelped, or born. Whelping is usually a painless task. After each of her litter is whelped, she licks the pup as dry as she can. The newborn, hungry puppies snuggle by the teats on the bitch's underside, where she nurses them.

The puppies draw nourishing milk from their mother until they are weaned, or given food more solid than milk to eat. The time of weaning depends on the size of the litter and the amount of milk in the bitch. Sometimes it occurs as early as three weeks. Puppies should not be weaned, however, any later than their seventh week.

The puppy's eyes, which are closed at birth, open when it is between one and two weeks old. It then begins to see. Its first teeth, the puppy or milk teeth, erupt through the gums during the third to sixth week of its life. Puppy teeth are mostly incisors and canines. By the third month, the first of the permanent teeth work through, and by the seventh month they all do. By the time it reaches its first birthday a puppy is considered a dog.

Although sexually mature beforehand, a dog ordinarily does not attain full growth until its first birthday or even later. By this time, however, it is capable of a wide range of responses to its environment. When it meets another dog, its ear position indicates how interested it is in the newcomer. If its ears are erect, it is concentrating on the other. If its ears are pointing forward, it is on the alert. If the dog holds its tail high and wags it, the animal is happy and confident. If it drops its tail and remains still, the dog is apprehensive. If it pulls its tail between its legs, the dog is afraid. If on meeting a person or another dog it pulls back its lips and growls, it is making a threat. If it bares its teeth without growling, the dog is ready to attack and bite. A male dog establishes a territory by marking the boundaries with urine, scent from the anal glands, or even feces. The dog will then defend that territory against intruders. Every six or seven months a female dog goes into heat and will mate with nearly any available male within the three-week length of her heat.

When a dog reaches old age, its eyes begin to weaken. Cataracts may also form in the lenses of its eyes. The hair on its muzzle turns gray. The old dog begins to feel numerous aches and pains and might become easily irritated and snap at members of the family. Its body systems are breaking down, and it can no longer behave as it did when younger.