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. SEE: Monogamy;
Promiscuity;
Natural
Selection;
Territoriality;
Cooperative
Breeding. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.