Every February, lovers exchange cards, candy, flowers, and other gifts as tokens of their enduring devotion. Many legends and historic events contributed to the idea that the middle of February—Valentine's Day—should be a day for romance. One was the common belief in France and England during the Middle Ages that February 14th was the beginning of birds' mating season. They thought that birds paired for life and set an example as the
highest aspiration of human love. In reality, monogamy, a pair-bond between a single male and female, is comparatively rare among animals—even among birds.
Based solely on their observations, biologists once thought that more than 90 percent of bird species were monogamous. Picture an archetypal male and female robin, collaborating in nest building, then devotedly taking turns incubating the eggs and feeding their young. Those that did not pair for life, thought the researchers, were at least annually monogamous, forming new bonds each mating season. Recently, DNA analysis of nestlings nixed that idea. It turns out that many male birds are raising other males' babies! Most of our feathered brethren practice social monogamy—they live and raise young in pairs but are sexually unfaithful.
In the animal kingdom, the essential directive is to reproduce one’s kind. Individual females are limited in the number of babies they can produce. They can improve the genetic quality of their young by being promiscuous. A good provider is not necessarily the best sperm donor. So while the female songbird builds a nest with her mate, she may sneak out and go looking for a male with brighter feathers, a bigger body size, or a more glorious singing voice.
Among birds, raptors and waterfowl are most likely to pair for life. Eagles remain faithful to their mate until one dies. According to Ducks Unlimited, 44 percent of waterfowl species are truly monogamous. Most ducks are seasonally monogamous, and swans and geese are often life mates. (Remember, you can view and celebrate bald eagles, the "poster child" for avian monogamy, at Lake Pueblo State Park’s Annual Eagle Day Festival, held Saturday, February 6, and Sunday, February 7, 2010.)
Monogamy is even rarer in mammals. Less than three percent of mammalian species are thought to be monogamous. The tiny list includes beavers and a few other rodents, otters, a few bats, certain canines and felines, a few hoofed mammals, and some primates. Again, the need to continue the species is paramount. Most herd animals and many game birds practice polygyny—one male is bonded to several females at once during the breeding season. Common Colorado "polyginists" include elk, bighorn sheep, pheasants, and prairie chickens. Others species, such as bears, are completely promiscuous; there is not even a temporary pair-bond between mating partners. Mating among promiscuous animals may still be a very selective process, but it is accompanied by no other relationship.
Among mammals that are monogamous, survival of the species usually depends upon both parents raising the offspring. Usually, the young take a long time to mature and the female does not have the capacity to rear a litter without assistance from her mate. The male helps rear the young by carrying, feeding, defending or socializing them.