Sexual Selection

It was Charles Darwin who originally proposed that the so-called secondary sexual characteristics of male animals -- such as the elaborate tails of peacocks, bright plumage or expandable throat sacs in many birds, large racks in mooses, deep voices in men -- evolved because females preferred to mate with individuals that had those features. Sexual selection can be thought of as two special kinds of natural selection, as described below. Natural selection occurs when some individuals out-reproduce others, and those that have more offspring differ genetically from those that have fewer.

In one kind of sexual selection, members of one sex create a reproductive differential among themselves by competing for opportunities to mate. The winners out-reproduce the others, and natural selection occurs if the characteristics that determine winning are, at least in part, inherited. In the other kind of sexual selection, members of one sex create a reproductive differential in the other sex by preferring some individuals as mates. If the ones they prefer are genetically different from the ones they shun, then natural selection is occurring.

In birds, the first form of sexual selection occurs when males compete for territories, as is obvious when those territories are on leks (traditional mating grounds). Males that manage to acquire the best territories on a lek (the dominant males) are known to get more chances to mate with females. In some species of grouse and other such birds, this form of sexual selection combines with the second form, because once males establish their positions on the lek the females then choose among them.

That second type of sexual selection, in which one sex chooses among potential mates, appears to be the most common type among birds. As evidence that such selection is widespread, consider the reversal of normal sexual differences in the ornamentation of some polyandrous birds. There, the male must choose among females, which, in turn, must be as alluring as possible. Consequently in polyandrous species the female is ordinarily more colorful -- it is her secondary sexual characteristics that are enhanced. This fooled even Audubon, who confused the sexes when labeling his paintings of phalaropes. Female phalaropes compete for the plain-colored males, and the latter incubate the eggs and tend the young.

There is evidence that female birds of some species (e.g., Marsh Wrens, Red-winged Blackbirds) tend to choose as mates those males holding the most desirable territories. In contrast, there is surprisingly little evidence that females preferentially select males with different degrees of ornamentation. One of the most interesting studies involved Long-tailed Widowbirds living in a grassland on a plateau in Kenya. Males of this polygynous six-inch weaver (a distant relative of the House Sparrow) are black with red and buff on their shoulders and have tails about sixteen inches long. The tails are prominently exhibited as the male flies slowly in aerial display over his territory. This can be seen from more than half a mile away. The females, in contrast, have short tails and are inconspicuous.

Nine matched foursomes of territorial widowbird males were captured and randomly given the following treatments. One of each set had his tail cut about six inches from the base, and the feathers removed were then glued to the corresponding feathers of another male, thus extending that bird's tail by some ten inches. A small piece of each feather was glued back on the tail of the donor, so that the male whose tail was shortened was subjected to the same series of operations, including gluing, as the male whose tail was lengthened. A third male had his tail cut, but the feathers were then glued back so that the tail was not noticeably shortened. The fourth bird was only banded. Thus the last two birds served as experimental controls whose appearance had not been changed, but which had been subjected to capture, handling, and (in one) cutting and gluing. To test whether the manipulations had affected the behavior of the males, numbers of display flights and territorial encounters were counted for periods both before and after capture and release. No significant differences in rates of flight or encounter were found.

The mating success of the males was measured by counting the number of nests containing eggs or young in each male's territory. Before the start of the experiment the males showed no significant differences in mating success. But after the large differences in tail length were artificially created, great differentials appeared in the number of new active nests in each territory. The males whose tails were lengthened acquired the most new mates (as indicated by new nests), outnumbering those of both of the controls and the males whose tails were shortened. The latter had the smallest number of new active nests. The females, therefore, preferred to mate with the males having the longest tails.

The widowbird study required considerable manipulation of birds in a natural environment that was especially favorable for making observations. Evidence for female choice of mates has also been accumulated without such intervention in the course of a 30-year study of Parasitic Jaegers (known in Great Britain as "Arctic Skuas") on Fair Isle off the northern tip of Scotland. The jaegers are "polymorphic" -- individuals of dark, light, and intermediate color phases occur in the same populations. Detailed studies by population biologist Peter O'Donald of Cambridge University and his colleagues indicate that females prefer to mate with males of the dark and intermediate phases, and as a result those males breed earlier than light-phase males. Earlier breeders tend to be more successful breeders, so the females choices increase the fitness of the dark males. O'Donald concludes that the Fair Isle population remains polymorphic (rather than gradually becoming composed entirely of dark individuals) because light individuals are favored by selection farther north, and "light genes" are continuously brought into the population by southward migrants.

Further work, including some, we hope, on North American species, is required to determine the details of female choice in birds. The effort required will be considerable, and suitable systems may be difficult to find, but the results should cast important light on the evolutionary origin of many physical and behavioral avian characteristics.

We know remarkably little about the origins of sexual selection. Why, for example, do female widowbirds prefer long-tailed males? Possibly females choose such males because the ability to grow and display long tails reflects their overall genetic "quality" as mates -- and the females are thus choosing a superior father for their offspring. Or the choice may have no present adaptive basis, but merely be the result of an evolutionary sequence that started for another reason. For instance, perhaps the ancestors of Long-tailed Widowbirds once lived together with a population of near relatives whose males had slightly shorter tails. The somewhat longer tails of males of the "pre-Long-tailed" Widowbirds were the easiest way for females to recognize mates of their own species. Such a cue could have led to a preference for long tails that became integrated into the behavioral responses of females. Although we are inclined to think the former scenario is correct, the data in hand do not eliminate the second possibility.

SEE: Natural Selection; Polygyny; Dominance Hierarchies.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.