Birds vs. mammals
In the natural world, the concept of monogamy in sexual partners remains largely with birds rather than mammals. Among the most monogamous birds are penguins, cranes, pigeons, parrots, swans, geese, doves, and albatrosses.
Experts estimate that only five percent of all mammal species (excluding humans) are believed to be monogamous, compared to nearly 90 percent of all bird species. Monogamy is defined as a mating system in which one male mates with just one female in a breeding season. Mammals believed to live in monogamous pairs include gibbons, wolves and beavers.
Gibbons, as well as orangutans and gorillas, are anthropoid apes, the primates that most closely resemble humans, physically and behaviorally. Male and female gibbons are regarded as monogamous. They pair up for life and form a family that stays together until the offspring grow up and leave home. The bond between the couple is reinforced by the hours they spend grooming each other.
Less is known about the sex life of the shy orangutan, because they seldom breed in captivity and observing them in the dense forest terrain in which they live is difficult.
A matter of necessity
"As much as we like the thought of soul-mates for life, romantic ideals of the human variety don’t really apply to wild species that just follow the laws of nature necessary to survive,” says Sybille Klenzendorf, wildlife conservation, WWF.
The amount of parental investment necessary to raise viable offspring appears to be the leading reason why some animals are monogamous. For birds, one parent is needed to incubate the eggs in the nest, while the other must gather food to bring back to the nest, a task that may require flying a great distance. By the time the baby birds have fledged, another breeding season is about to start, so the parent birds may decide to stay together and mate again simply to save the time it would take to look for a new mate.
By contrast, female mammals reproduce through an internal nurturing of an embryo, and then develop mammary glands which produce milk, a food source for newborns. With no immediate responsibility to raise the young, the male mammal is free to pursue other sexual liaisons. When mammals are monogamous, it is usually because male involvement in parenting is necessary, such as in training young wolves how to hunt.
Straying from the nest
Some birds will engage in what scientists call extra-pair copulations. In such cases, the male bird has a primary partner whom he assists in raising offspring, then takes on a secondary partner whose young receive no paternal benefit. The male bird must weigh the benefit of this secondary coupling with the risk that his primary female may mate with rivals in his absence.
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