All AbouT SEx 255
injuries, for example blindness in an eye, from fighting. But the payoffs are big for
the winners. While nearly half of all males fail to reproduce in a given year, the most
successful males sire up to five offspring [12].
Fights like those in the red deer are called male combat. Male red deer follow
groups of females to prevent other males from mating with them. In other spe-
cies, such as the stag beetle (Lucanus cervus; FIGURE 10.9B), males defend critical
resources needed by females (for example, sites where they lay eggs
or feed), and then mate with females as they arrive. Ecological fac-
tors can have an important impact on the potential for sexual selec-
tion. When females need resources that are clumped in time or space,
there is opportunity for a few males to control the resources and obtain
the majority of matings, leading to strong sexual selection [16]. When
resources are dispersed, there is less opportunity for males to control
them and gain a strong sexual selection advantage.
Just as in sports, there can be more than one winning strategy in
male-male competition. This situation opens the door to the evolution
of alternative mating strategies, which are divergent ways that males of
the same species use to acquire matings [33]. Isopods are crustaceans
distantly related to crabs. Males of a species called Paracerceis sculpta
have three genetically determined morphs (FIGURE 10.10) [46]. The
largest is the alpha morph. These males dominate the other morphs
and guard several females in a harem. Males of the beta morph are
very similar in size and shape to females. By mimicking females, beta
males can slip past alpha males, gain access to the harem, and mate
with the real females. The gamma morph is by far the smallest. These
males use stealth and speed to sneak past alpha males and gain access
to females. Despite their striking differences in size, shape, and behav-
ior, the three morphs have equal reproductive success. This is an exam-
ple of a polymorphism maintained by negative frequency-dependent
selection (see Chapter 5).
A common tactic used by males to interfere with each other’s repro-
duction is sperm competition [37]. Even after a male has mated with aFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
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Evolution4e_10.09.ai Date 01-26-17(A) (B)FIGURE 10.9 Males of many animals have evolved horns and other weapons that
they use to fight with each other for reproductive access to females. (A) Male red deer
(Cervus elaphus) fighting during the breeding season. Successful males guard groups
of females with which they mate. (B) Male stag beetles (Lucanus cervus) fighting. Males
defend the sites where females lay their eggs, and mate with females when they ar-
rive to lay.Futuyma Kirkpatrick Evolution, 4e
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Troutt Visual Services
Evolution4e_10.10.ai Date 11-02-2016FemalesAlpha
maleAlpha
maleGamma male
Beta maleFIGURE 10.10 These isopods (Paracerceis sculpta) live
on and inside sponges. The large alpha male guards a
harem of females by blocking the entrance to the cavity
in which they live. A medium-sized beta male (left) and
a small gamma male (right) try to gain access to the fe-
males by tricking the alpha male. The beta male imitates
a female, while the small gamma male seeks to reach
the females by slipping past the alpha male. (From [45],
illustration by Marco Leon.)10_EVOL4E_CH10.indd 255 3/22/17 2:25 PM