photograph of Red-winged Blackbird by Rohan Kamath
|Polygyny, where one male mates with more than one female while each female mates with only one male, is thought to be the fundamental mating system of animals. The reason is straightforward. By definition, the sex that produces the larger reproductive cells (eggs) is the female, and the one that produces the smaller (sperm) is the male. Males therefore make a smaller investment in the embryos that result from the fusion of egg and sperm cells. The difference is especially pronounced in birds, since the sperm is microscopic and the egg (relatively) gigantic. The male thus puts proportionately little effort into any single embryo, while the female has a great stake in each one, since she can produce relatively few eggs in her lifetime. Females must therefore exercise care in choosing the fathers of their limited number of young. It would seem, in contrast, that male birds should be much less choosy and attempt to|
have as many mates as possible, since evolution
favors behavior that leads to leaving a maximum number of
offspring. A male that mates with a weak or otherwise unfit
female loses a small part of his reproductive potential; a
female making a similar mistake may sacrifice all or almost
all of hers.
Most birds, however, are monogamous. Apparently both parents must help to rear the young if the adults are to have much chance of leaving any genes to posterity. Under what circumstances, then, can polygyny occur? One idea is that polygyny is likely when males hold territories that vary greatly in the quality of resources. Females will tend to choose superior males -- by inference those that have high-quality territories. When those males already have mates, females have a choice. They can either select a male that holds an inferior territory, or they can become the second mate of one of the superior males. If the difference between high- and low-quality territories is great enough, the latter strategy will be better -- little or no aid from a male holding a resource-rich territory will yield a better chance of producing surviving offspring than the full cooperation of a male with an inferior territory. The male with a superior territory will benefit by increased reproduction, as will the second female.
Often that is precisely what is found. For example, female Marsh Wrens near Seattle, Washington, sometimes mate with already-mated males, even when bachelor males are available. The number of females mated to each male is related to the amount of emergent vegetation in the males' territories, which, in turn, is presumably an indicator of the availability of insect food. Studies of Red-winged and Yellow-headed Blackbirds, Dickcissels, Indigo Buntings, and Lark Buntings also show clear relationships between various aspects of territory quality and the likelihood that a male holding a given territory will have more than one mate.
Polygyny is not always associated with territoriality. Certain seed-eating savanna species of African weavers (relatives of House Sparrows) have superabundant resources and the males are not territorial, presumably because defending an area does not increase their access to food. The females apparently do not need help from males to raise the young, and the weavers nest in colonies, minimizing the need for a partner in nest defense. The female is thus free to choose any male to father her offspring, regardless of his other attachments. Here, as in situations where males are territorial, polygyny is related to the availability of resources -- in this case their superabundance rather than their uneven distribution.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.