In the past, many animal species were considered monogamous; but now ethologists distinguish between social and sexual monogamy.
In the first case ( social monogamy ) two individuals live in pairs and provide for the care of their offspring and the search for food.
Sexual monogamy implies exclusivity in mating and therefore the "certainty" of kinship. 92% of bird species are socially monogamous.
Among the mammals are bats, some canids (such as wolves and foxes ), certain primates ( gibbons and some small monkeys in the New World), the beavers of North America, a few mice and rats, the giant otter and some South American rodents, a few species of seals and some African antelope.
ADULTERERS. However, even in species considered a model of sexual monogamy, the search for extra-couple relationships is far from infrequent. The tendency to have more sexual partners would be true for males as well as for females, who are willing to "betray" their social partner with more attractive looking individuals who, in some cases, turn out to be active seductresses.
The differences between birds and mammals. Social monogamy is much more widespread in birds than in mammals, because chicks for eating depend on both parents. In mammals, on the other hand, puppies are suckled by their mother. The father is therefore no longer indispensable, and can leave to look for other females. And this is why mammals seem to prefer the creation of larger nuclei than a "traditional family".
The case of pigeons. Pigeons are strictly monogamous, and faithful for life. It takes a long time for the male pigeon to choose the companion. And he woos it for a long time. Finally the two "get engaged". In fact, mating is not talked about until the couple is considered stable: at least a week must pass. At that point the female takes the initiative: the male is near the beak. He offers her food and shoves it directly into her mouth, as is done with chicks. It means that he is able to take care of children too. After twenty days the babies are born. Mum and dad alternate with the hatching and both produce pigeon milk, a whitish secretion used to feed the chicks.
That libertine of the sea lion. There is a very simple rule to understand if an animal prefers a fixed couple or free mating: check if there is a marked sexual dimorphism . If the male is much larger, or has a very different appearance from the female, polygamy is established, but if they have similar dimensions, the relationship between the two sexes is monogamous.
And the number of partners is proportional to the difference in size between husbands and wives: the sea lion, which weighs about three times more than its partner, can have hares composed of hundreds of females.