256 CHAPTER 10female, there are opportunities for other males to prevent his sperm from fertil-
izing her. The penis of Calopteryx damselflies has a structure that resembles a very
small rake or scoop (FIGURE 10.11). It plays no role in helping transfer sperm to
the female. Its job is to remove the sperm of any males that mated previously with
the female. By removing those sperm, the male increases the number of eggs he
will fertilize, and decreases the fitness of the other males. This increases the male’s
relative fitness, and so the structure is favored by sexual selection.
A male can also win at sperm competition by making it more difficult for later
males to fertilize his mate’s eggs. One strategy is to guard the female against
other suitors until she has laid the eggs he fertilized. Another is to deposit in her
a sperm plug, which is a physical barrier to later matings. A third strategy to win
at sperm competition is simply to make more sperm. Females of some primate
species mate with many males, leading to strong sperm competition. Males of
those species have evolved large testes to produce copious sperm. As you would
predict, species in which females mate with only one or a few males have smaller
testes (see Figure 3.20) [21]. The human testes are slightly smaller than average
for primates our size, suggesting that our ancestors may have had monogamous
tendencies.
A particularly gruesome kind of male-male competition results
in infanticide. Lions are social animals that live in groups. Occa-
sionally, the dominant male in a group will be ousted by one or
more rivals. The new males often kill all the young in the group
(FIGURE 10.12) [35]. This decreases the number of offspring left to
the next generation by previous males, which increases the rela-
tive fitness of the new males. Further, it causes the females in the
group to become fertile more quickly (since females are not recep-
tive while they are caring for their young). This again enhances the
fitness of the new males since they can now mate and have their
own offspring.
Male combat, sperm competition, and infanticide are just three
of the diverse ways by which males interfere with each other’s
reproduction. A simple theme unites them all: any mutation or
trait that increases the fitness of a male relative to other males
is favored. Sexual selection gives an evolutionary advantage to
selfish genes, even if they have negative effects on other males,
on females, or the species as a whole. Infanticide is a particularly
graphic example. We will return to these kinds of sexually antag-
onistic traits in Chapter 12.Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_10.11.ai Date 01-26-17FIGURE 10.11 A male damselfly removes (A) (B)
the sperm of a female’s previous mates
before depositing his own. (A) In this
photo of mating damselflies (Calopteryx
splendens), the blue individual is the male
and the gray individual the female. When
they mate, claspers on the end of the
male’s abdomen grasp the female behind
her head. She then curls her abdomen
so that the opening of her oviduct (at the
end of her abdomen) makes contact with
his penis (on his thorax, near the base
of his wings). They then copulate. (B) A
scoop-like structure on the male’s penis
removes the sperm of males that copu-
lated with the female earlier. A clump of
sperm adheres to the structure, at bottom.
(B courtesy of J. Waage.)FIGURE 10.12 This male lion has killed a cub after displac-
ing the cub’s father and other adult males in the group that
he recently joined. Male infanticide has also been described
in many other species of mammals.10_EVOL4E_CH10.indd 256 3/22/17 2:25 PM