254 CHAPTER 10The outcome of sexual selection is strongly influenced by the operational sex
ratio, which is the relative number of males and females available to mate at any
moment [16]. Many females in a population are unavailable because they are devel-
oping new eggs, carrying embryos to term, or caring for young. Not so for males,
who are often able to remate shortly after their last reproductive bout. Consequently,
females are often a limiting resource for males. Whenever the number of males and
females is unequal, the more common sex must compete for access to the less com-
mon sex. Traits that make males more successful in competition for mates are there-
fore favored by sexual selection, leading to spectacular evolutionary outcomes such
as the long tail of the widowbird. There is potential for sexual selection on males
even in species that appear to be monogamous. Among passerine birds that are
socially monogamous, it is not unusual for more than 10 percent of offspring to be
fathered by males that are not the female’s social partner (her apparent mate), and
some males are particularly sought after as secondary sexual partners [50].
Males are not always the sexually selected sex. The red phalarope (Phalaropus
fulicarius) shows sex role reversal: females are larger and more brightly colored
than the males (FIGURE 10.8A). After mating, a female lays her eggs in the nest of
a male, who is responsible for all of the incubation and rearing of the chicks. While
he is preoccupied with those duties, the female can mate with another male and lay
more eggs in his nest. Males, by contrast, get no fitness benefit from mating with
additional females once their nest is filled with eggs. A similar situation occurs in
seahorses. Males have a pouch in which they carry and nurture their developing
young, and males are courted by females who seek to lay their eggs in the pouches
(FIGURE 10.8B). Species with sex role reversal are the exceptions that prove the
rule about why sexual selection is more intense on one sex than the other. In these
species, Bateman’s principle is reversed: mating with more partners increases the
fitness of females but not males. Females have therefore evolved bright colors,
courtship behaviors, and other traits that increase their mating success.Sexual selection by male-male competition
The first of two modes of sexual selection identified by Darwin is male-male com-
petition, in which males interfere directly with each other. In taxa ranging from bee-
tles to whales, males have evolved horns and other structures to prevent other males
from mating. Males of the red deer (Cervus elaphus) carry a magnificent set of antlers
in the breeding season (FIGURE 10.9A). These antlers are no mere ornaments: males
use them in fights that are dangerous. About 20 percent of males sustain permanentFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_10.08.ai Date 01-25-17FIGURE 10.8 Sex role reversal. (A) (B)
(A) Two female red phalaropes
(Phalaropus fulicarius) fight over
the smaller, duller-plumaged
male on their breeding ground.
In contrast to most birds, female
phalaropes court males, which
care for the eggs and young in
their nests. (B) A male Austra-
lian seahorse (Hippocampus
breviceps) giving birth from his
pouch. Males choose which
courting females will lay eggs in
their pouches.10_EVOL4E_CH10.indd 254 3/22/17 2:25 PM