Mating Behavior
The modern dog is well removed from its wolf origins, its mating rituals are similar to those of other social animals and are still rooted in the behavior of a pack animal.
Who mates with whom is dictated by the social structure of the pack. The dominant male rarely permits other males to mate with receptive females. Males willingly fight over females for the right to mate.
Females do not necessarily choose the winner, however; they sometimes mate with a third party while the first two quarrel over rights.
Familiarity is also important; females rarely mate with unknown males. Given a ready supply of available and familiar males, females ovulate and then mate with their partners several times.
The likelihood of pregnancy is very high. By contrast, in the rather unnatural circumstances of man-made mating rituals, the dog owner often takes a female to an unfamiliar male and expects her to mate willingly - something that many females are reluctant to do.