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.