All AbouT SEx 257
Sexual selection by female choice
Many extravagant male secondary sexual traits are useless for male-male compe-
tition. Instead, their function is to attract females and persuade them to mate. This
second mode of sexual selection is called female choice.
Do female animals in fact have the ability to make choices? Many species that
show evidence of sexual selection by female choice have very simple nervous sys-
tems that are not capable of higher levels of cognition. In the context of sexual
selection, the word “choice” is used much more broadly to mean any phenotype of
the female that biases the type of male that she will mate. A female’s mate choice can,
for example, be influenced by the colors to which her eye is most sensitive. Experi-
ments like that with the long-tailed widowbirds have confirmed in many species
that females do bias their mate choice, so there is no doubt that sexual selection by
female choice occurs.
Furthermore, mating preferences can evolve. Females of closely related spe-
cies have innate preferences for their own species, which shows that those pref-
erences have evolved since those species’ most recent common ancestor. A sec-
ond line of evidence comes from heritability experiments (discussed in Chapter
6) that have established that there is genetic variation for mating preferences in
many species [24].
How and why do female mating preferences evolve? When choosing a partner,
the biggest mistake a female can make is to mate with another species. Hybrid off-
spring typically suffer from low viability or fertility. This generates strong selection
for the reinforcement of female mating preferences that discriminate against males
of other species (see Chapter 9). Other evolutionary factors also cause mating pref-
erences to evolve. Here we look at four of them: direct benefits, pleiotropic effects,
good genes, and Fisher’s runaway process.
Males of many species provide their mates with direct benefits, which are
resources that increase the females’ survival and reproductive success. Direct
benefits come in many forms, including food and care for the offspring. When
crickets and katydids mate, the male inseminates the female with a large sper-
matophore made up of lipids and carbohydrates as well as sperm (FIGURE 10.13)
[19]. After insemination, the female eats the spermatophore, and those nutri-
ents increase the number of eggs she lays. Since larger
males make larger and more nutritious spermato-
phores, natural selection favors female mating prefer-
ences for large males. In some species, the spermato-
phore has become so large that females can obtain all
of their food simply by mating. In some insects and
spiders, the nutritional gifts used to seduce females
have evolved to the ultimate extreme. After copula-
tion, male redback spiders (Latrodectus hasselti) often
somersault into their female’s mouth and are eaten!
In addition to benefiting his mate, a male’s suicide
enhances his own fitness: males that sacrifice them-
selves fertilize more eggs than those that do not [2].
Whenever males provide their mates with direct ben-
efits, natural selection favors female preferences for
male traits that increase female fitness.
Direct benefits cannot, however, explain the evolu-
tion of female mating preferences for some of the most
extreme male mating displays in the animal kingdom.
In some birds, fishes, insects, and other taxa, males
congregate in leks, which are arenas in which males doFIGURE 10.13 Males of some species provide direct benefits to fe-
males. This female Mormon cricket (Anabrus simplex) will eat the large
white spermatophore her mate has placed in her genital opening.
(Courtesy of John Alcock.)10_EVOL4E_CH10.indd 257 3/22/17 2:25 PM