Monogamy

Monogamy refers to the norm or condition of a single male mating with a single female by forming a “pair bond,” with both defending territory and caring for the young. The need to care for the young necessitates many of the species to enter into monogamous relationships. For example, monogamous male parent birds care for the young by watching for danger (willow ptarmigans) or defending a territory with a nest cavity (eastern bluebirds), whereas his mate collects the food to feed the offspring. Male birds are involved further in most passerines, where they feed brooding females and/or help to feed the young. In herons, egrets, some woodpeckers, and others, males not only provide food for the young but also share in incubation. On the contrary, for species whose males contribute little or nothing to rearing the young, females are very choosy in selecting the best mates. Males of these species compete to be chosen and are not themselves choosy. Females of these species, because of their biological ability to produce only a limited number of offspring, prefer mating with only the best males. From the male point of view, mating many times and with many females increases the chances of producing surviving offspring. Therefore, males must invest their energy to make themselves more attractive and appealing to as many females as possible. For example, the magnificently tailed peacock's elaborate train does nothing to enhance survival but is necessary for attracting mates.

The monogamous “bond” may vary from a single season to a lifetime. For example, most of the birds seem to form a monogamous bond that lasts from as short a period as single nesting (house wrens) to as long as a lifetime (albatrosses, petrels, swans, geese, eagles, and some owls and parrots). In a similar fashion, many monogamous animals pair for a season, but some (e.g., gibbons) pair for a lifetime.

During recent years, many ornithologists and behavioral ecologists began to view monogamy as part of a “mixed” reproductive strategy, that is, where matings may occur outside the primary pair bond but both members of the pair still contribute substantially only to the care and feeding of the young from their own nest. Some species are viewed to be “facultatively” monogamous; that is, they are monogamous due to certain environmental constraints. Once these constraints are removed or altered, they would typically exhibit some other form of mating system such [Page 1619]as polygyny (one male mating with more than one female) or promiscuity (mating without forming pair bonds). For example, North American dabbling ducks are considered to be monogamous only due to the inability on the part of males to monopolize more than one female. These ducks breed synchronously, and their populations typically contain more males than females.

Prairie voles (Microtus ochrogaster) are among a select 5% of mammals that are habitually monogamous. Once they have mated, the males “fall in love” and stay close to their partners, protecting them, helping them raise offspring. Their close cousins, the meadow voles (Microtus pennsylvanicus), however, mate with multiple partners and pay little attention to subsequent offspring. When researchers alter the behavior of meadow voles by manipulating a single gene—vasopressin receptor— meadow voles become as monogamous as prairie voles.

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