Hair and Skin  
The Skeleton  
The Muscular System  
The Nervous System  
The Digestive System  
The Urinary System  
The Reproductive System  

THE DOG'S REPRODUCTIVE SYSTEM

The male dog reaches sexual maturity at between 6 to 8 months. He can then mate at any time of the year and for a large part of his life. Some males seem to be more sexually active in the late winter and early spring. This could be a throwback to his wild ancestors and the modern wolves and coyotes that breed in late winter and early spring.

The reproductive organs of the male include the penis, testicles and the prostate gland. Males, to be efficient breeders, must be in good health and not overly fat. Adequate exercise is very important in keeping the male sexually potent.

Females can mate and produce offspring twice a year. Successive matings are not usually recommended. Also, a female entering her heat period or estrus cycle for the first time should not be bred. Rather, she should be permitted to reach maximum growth. When bred on the second heat period and thereafter, the bitch will produce healthier and more vigorous puppies.

The reproductive organs of the female include the vagina, ovaries and uterus. The bitch has a reproductive cycle that is divided into four phases: 1) a nine-day period in which the external genitals are enlarged, swollen and emitting a bloody discharge; 2) another nine-day period during which ovulation occurs and the bitch has a strong desire to mate (she can be mated during this stage); 3) the anestrum period lasting 2 months; and 4) the metestrum period lasting 3 months. The anestrum and metestrum periods are post-heat periods during which the female cannot be mated. The average female reaches sexual maturity or the first heat period at 6 to 8 months of age. Some individuals and breeds vary from this rule, not coming into heat for as long as 12 months.

Domestic dogs will mate with wolves and coyotes and produce hybrids. They will also breed with jackals. The reproductive similarity between the dog, wolf and jackal has received considerable study. The duration of pregnancy in the dog and wolf is the same—60 to 63 days, and the wolf-dog puppies are fertile. Coyotes are small wolves and the same breeding similarities exist between them and dogs.