Encyclopedia of Evolution.pdf - Online Reading Center

one, free of parasites, is likely to produce many long colorful
feathers. High levels of testosterone, which enhances
aggression in birds and mammals, also decreases the function
of the immune system. The males that manage to have
both the longest, most colorful feathers as well as the most
aggressive territorial defense may be able to leave the most
offspring. Feathers will continue to evolve to be longer and
more colorful until the feathers become too expensive or
burdensome. In many bird species, male ornamentation
consists of only a few feathers or a crest; but in some, such
as peacocks and birds of paradise, the ornamentation is so
excessive that it constitutes a significant drain of resources
and puts the male in greatly enhanced danger of attack by
predators. Researchers have experimentally augmented the
feathers of some males and found that the males with artificially
long feathers have more offspring.
• In many bird species, males congregate in leks to display
their plumage and their mating dances, and females stroll
around and choose from among them.
• Symmetry is a fitness indicator in many species, including
humans, who show a preference for faces and bodies that
are symmetrical. Poor health often results in asymmetry
between the right and left sides of a vertebrate.
Expensive fitness indicators among mammals include
antlers (which in the now extinct Irish elk could reach over
six feet, or two meters, in expanse). Natural selection can
put a stop to sexual selection, but not before sometimes bacchanalian
excesses of ornamentation are produced. The more
expensive the better, up to a point, because if the ornamentation
were cheap, less healthy males could cheat by producing
an ornament that merely made it look healthy. Aside from
being expensive, and being truthful representations of health,
fitness indicators may be arbitrary, as reflected in the tremendous
diversity of feather ornamentation and courtship dances
among the almost 9,000 bird species.
Once fitness indicators come into existence, as a result
of sexual selection, they can serve as reproductive isolating
mechanisms that help to separate one population into two.
This can lead to speciation, when males with novel traits
are chosen by females with a preference for those same novel
traits.
In many animal species, sexual selection has produced
particularly marked characteristics that facilitate competition
between males:
• Size dimorphism. In many animal species, the male is larger
than the female even though it is the females, not the
males, that have most of the costs of reproduction. Large
size gives males a competitive advantage in fighting with
one another.
• Weapons. The same antlers that serve as fitness indicators for
female choice also serve as weapons for fighting other males.
• Impulsiveness. In most species, it is the males that are more
impulsive, and sometimes more violent. This is why, as
Darwin noted in 1871, it is usually males (even among the
invertebrate animals) that are, as he said in his inimitable
Victorian prose, more “pugnacious” than females. In their
hurry to mate, males can sometimes be absurdly undiscrim-
sexual selection
inating: Male turkeys, for example, will attempt to mate
with a replica of a female turkey head.
• Interference with the reproduction of other males. There
are numerous instances in which males interfere with the
opportunity for other males to inseminate females. In some
stick insects such as Necroscia sparaxes, the male will copulate
with the female for weeks at a time, thus keeping rival
males away. Whether in flies or in primates, males may
have very large reproductive organs, which deliver a large
amount of sperm. This helps to outcompete the sperm that
a previous male may have left. Male voles produce more
sperm if there are other males in the vicinity than they do if
they are alone. The males of some species have penes with
bristles, which are believed to scrub away sperm left by
other males. In other cases, males produce secretions that
plug up the female’s reproductive tract after he has mated
with her, thus preventing the sperm of other males from
entering. However, the males have also evolved the ability
to remove the plugs inserted by other males.
• Stealth. In many cases, males have evolved to become
stealthy in their approach to females. In some salamanders
such as Plethodon jordani, the female follows the male
until the male drops a spermatophore, a structure that contains
sperm; the female then sits on the spermatophore and
receives the sperm. Sometimes a third male will intrude
between the couple, waddle along as if he is a female, then
when the first male drops his spermatophore, the second
runs off with the female. In some fishes, some males imitate
females, infiltrate themselves into the harems controlled by
dominant males, and mate with some of the females. Perhaps
the ultimate example is when certain male bats (Myotis
lucifugus) creep around amid hibernating females and
inseminate them.
• Chemical components of semen. Some male houseflies
even produce semen that contains proteins that reduce the
female’s interest in copulation—after he has finished with
her, of course. This reduces the chance that she will accept
the advances of another male. In some cases, the compounds
in the semen are even slightly poisonous. In one
experiment, in which male flies were allowed to evolve in
competition but female flies were not, the semen from the
male flies actually killed the females.
Male-male competition may in some cases have promoted
speciation. In dragonflies and damselflies, males clasp females
behind their heads to mate with them, thus keeping other males
away. Their penes scrub out the sperm of previous males. This
process can be so violent that the females get severely injured.
But the post-reproductive survival of the female does not matter
to the male. According to evolutionary biologist Ola Fincke,
the female evolutionary response to this conflict of interest may
have been new shapes and colors, which make it more difficult
for the males to recognize the females.
In animals with either monogamous or harem mating
systems, in which the male can be reasonably confident of his
exclusive access to the female, males have small reproductive
organs. Gorilla males, for example, have penes even smaller
than those of some ducks.