Chapter 7

Darwin defined sexual selection as "the advantage which certain individuals have over others of the same sex and species, in exclusive relation to reproduction."50 Today most evolutionary biologists consider sexual selection for traits that promote success in acquiring mates to be a form of natural selection. Why? Compare the conditions that cause the process of natural selection to occur with the conditions that must cause sexual selection to occur. Is the factor "differences among individuals in age at death" on your list? Does sexual selection theory have elements in common with game theory?

Male rats, sheep, cattle, rhesus monkeys, and humans that have copulated to satiation with one female are speedily rejuvenated if they gain access to a new female. This phenomenon is called the Coolidge effect, supposedly because when Mrs. Calvin Coolidge learned on a tour that roosters copulate dozens of times each day, she said, "Please tell that to the President." When the President was told, he asked his guide, "Same hen every time?" Upon learning that roosters select a new hen each time, he said, "Please tell that to Mrs. Coolidge." Provide a sexual selectionist hypothesis for the evolution of the Coolidge effect. Use your hypothesis to predict what kinds of species should lack the Coolidge effect.

Gene-centered thinking tells us that sexual reproduction is a Darwin- ian puzzle in and of itself. Why? (Consider the fitness consequences for asexual (i.e., parthenogenetic) females in competition with sexual ones in a given population in which both types produce equal numbers of offspring.) When you have dealt with this problem, you should be surprised to learn that in many aphids, females are perfectly capable of cloning themselves (Figure \(7.7),\) which occurs in one generation after another before one group of females produces a generation of both sons and daughters; these males and females then engage in sexual reproduction before their descendants resume cloning themselves. What is the Darwinian puzzle here? Read Moran and Dunbar 138 for one possible solution.

Satellite and attached male horseshoe crabs do not have the same reproductive success. Why hasn't sexual selection eliminated the low-payoff satellite option if males exercising this option do not leave as many descendants as attached males do?

Roosters compete with one another for social dominance, and not surprisingly, dominant males have greater copulatory success than subordinate males. Use sexual selection theory to account for these differences among the two categories of males: dominant males release more sperm per ejaculate than subordinates, and dominants transfer more and better (faster-moving) sperm to females with large red combs on their heads, whereas subordinates provide all their mates with sperm of the same quality (the same velocity). 43 In addition, use conditional strategy theory to predict how males should behave if two dominant males were placed together until one became subordinate.

The digger bee's "postcopulatory courtship" consists of elaborate tactile stimulation that the male provides his partner after she has accepted his sperm. Why is this behavior a Darwinian puzzle, and what might its adaptive value be?

Mate guarding should be common in species in which females retain their receptivity after mating and are likely to use the sperm of their last mating partners when fertilizing their eggs. But there are species, including some crab spiders, in which males remain with immature, unreceptive females for long periods and fight with other males that approach these females. \(^{59}\) How can "guarding" behavior of this sort be adaptive? Produce sexual selectionist hypotheses and some predictions derived from them.

Male barn swallows have thin outer tail feathers that are somewhat longer than those possessed by females. When Anders Moller analyzed the effect of tail length on male mating success in the barn swallow in Europe, he did an experiment in which he made some males' tail feathers shorter by cutting them and made other males' tail feathers longer by gluing feather sections onto their tails. \(^{137}\) But he also created a group in which he cut off parts of the males' tail feathers and then simply glued the fragments back on to produce a tail of unchanged length. What was the point of this group? And why did he randomly assign his subjects to the shortened, lengthened, and unchanged tail groups? And why did a team of Canadian biologists repeat Moller's experiment on another continent? 185 And why did yet another team of British ornithologists study the effect of the tail "streamers" on the maneuverability of male swallows, given their interest in female mate choice? \(^{29}\)

Males of the Houbara bustard (Figure 7.36 ) differ in the time spent displaying their remarkable ornamental feathers to females. The more time a male spends in displaying during the early years of adulthood, the faster the quality of male sperm declines over the bird's lifetime. 159 What does this finding say about the cost of sexually selected traits for males? What is puzzling about the discovery that males with high rates of display early in adulthood continue displaying at a relatively high rate later in life?

Richard Prum argues that researchers need a "null model" of the effects of sexual selection in order to determine whether in fact mate choices lead to adaptive outcomes, with female preferences for certain male attributes providing the choosy females with better genes or superior parents. 162 For Prum, the Lande-Kirkpatrick models provide the null in which preferences have no utilitarian adaptive effect but instead are the arbitrary products of a nonselectionist process. He argues that we need to test the predictions from the null model first, rather than focusing on predictions from adaptationist hypotheses about such things as signal honesty and fitness benefits of female preferences. Why have so many researchers employed the adaptationist approach? Do we also need a null hypothesis for behavioral traits that adaptationists have assumed (for the purposes of hypothesis development and testing) promote survival by defeating predators?

When forced copulation results in the death of the female, neither sex benefits. Explain why males may behave in ways that result in the demise of their sexual partners.

Stuart Wigby and Tracey Chapman formed three populations of fruit flies with different sex ratios (female biased, even sex ratio, and male biased). Not surprisingly, the frequency with which females mated increased from female- biased to male-biased populations. After 18 and 22 generations of selection, fresh females from the three selected lines were taken from their environments and placed in cages with equal numbers of males. The mortality rate of females from the male-biased line was less than that of females from the even-sex- ratio line, and much less than that of females from the female-biased line.210 What do these results tell us about the evolutionary consequences of sexual conflict between the sexes in this species?

Males of the parasitic isopod lchthyoxenus fushanensis (Figure 7.45 ) pair off with females in cavities they construct inside their victims, freshwater fish. At times, sexual cannibalism occurs early in the breeding season, when females eat their partners; males may eat their mates later in the season. \(^{196}\) Replacement partners of both sexes are readily available. In addition, males can transform themselves into females, a phenomenon that occurs only when males have achieved a fairly large size. The larger the female, the more fecund she is. The smaller the difference in size between male and female, the fewer the number of offspring produced by a pair. Why has conflict between the sexes reached such an extreme state in this species? What kinds of males are females expected to eat? What kinds of males are expected to eat their partners? How can you account for the difference in the timing of cannibalism by males and females?