The first dog evolved roughly 12,000 years ago. Today, dogs are among the most successful mammals, with more than 400 different breeds and an estimated global population of 100 million.
All breeds of pregnant dogs carry a litter for roughly 63 days. Unlike human eggs, dog eggs aren't mature when they are released from the ovaries, so they're not ready for fertilization right away. That's why the male dog's sperm will attach to the uterine wall, and wait for several days until the egg matures for fertilization. Because of the time lapse, mothers can bear puppies with two different fathers.
Puppies are born blind and unable to walk. At birth, the mother will lick the puppies clean to stimulate their breathing. She will also eat the placenta, which releases a chemical to begin the production of milk. This instinctive behavior dates back to when dogs lived in the wild: by eating the placenta, the mother would cover any evidence of puppies that might attract predators.
Photo by Pioneer Productions
