1. Chapter 7: Sexual Selection Intersexual and Intrasexual Selection
Evolutionary Models of Mate Choice
Learning and Mate Choice
Cultural Transmission and Mate Choice
Male-Male Competition and Mate Choice

2. In 1860, Charles Darwin wrote in a letter to his friend Asa Gray that “…the sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!” Darwin was worried that the peacock’s tail couldn’t possibly help in its survival, and thus could be construed as evidence against his grand theory of evolution by natural selection.

3. Intersexual and Intrasexual Selection
FIGURE 7.1. Natural selection and gamete size. Natural selection favors large and small gametes over medium-sized gametes. (From Low, 2000)

4. Bateman’s Principle
Females are choosier because eggs are expensive to produce, thus female RS (reproductive success) is limited compared to males.
Female choosiness in mate selection should create greater variance in male RS
FIGURE 7.2a. Competition for mates. (A) Male deer battle with their antlers. (Photo credits: Manfred Danegger/Photo Researchers, Inc.)

5. Evolutionary Models of Mate Choice:
Direct benefits and mate choice
The direct benefits model of female mate choice. Here males provide a direct benefit (in this case, food), and females choose males based on how the resource affects their fitness. Prey items are shown in green. (Based on Kirkpatrick and Ryan, 1991) Scorpionfly nuptial gifts.
FIGURE 7.3. The direct benefits model of female mate choice. Here males provide a direct benefit (in this case, food), and females choose males based on how the resource affects their fitness. Prey items are shown in green. (Based on Kirkpatrick and Ryan, 1991)

6. Evolutionary Models of Mate Choice:
Direct benefits and mate choice
FIGURE 7.4. Scorpionflies and nuptial gifts. To obtain mates, male scorpionflies present females with nuptial gifts, which are prey items that the females consume during courtship or mating. The male (top) provided the female (bottom) with a blowfly (at arrow), which she eats as they copulate. (Photo credit: Randy Thornhill)
Scorpionflies and nuptial gifts. To obtain mates, male scorpionflies present females with nuptial gifts, which are prey items that the females consume during courtship or mating. The male (top) provided the female (bottom) with a blowfly (at arrow), which she eats as they copulate. (Thornhill, 1980)

7. Evolutionary Models of Mate Choice:
Direct benefits and mate choice
FIGURE 7.5. The direct benefits model and nuptial prey gifts. (A) Copulation time as a function of direct benefits. Male scorpionflies provide females with a nuptial prey gift. Up to about 19mm2, the greater the value of the gift, the longer a male is allowed to copulate with the recipient. (B) Longer copulation time leads to greater sperm transfer in scorpionflies. When males provide large nuptial gifts to females, the added copulation time translates into more sperm transferred to the female. (From Thornhill, 1976, 1980b)

8. Evolutionary Models of Mate Choice:
Good genes and mate choice
FIGURE 7.6. Indirect benefits in pronghorns. Offspring males that had large harems (green line) had higher survival rates than offspring from other males (orange line), suggesting that females were selecting males based on some measure of a male’s genetic quality. (From J. Byers and Waits, 2006, p )
Indirect benefits in pronghorns. Offspring males that had large harems (green line) had higher survival rates than offspring from other males (orange line), suggesting that females were selecting males based on some measure of a male’s genetic quality. (Byers and Waits, 2006)

9. Evolutionary Models of Mate Choice:
Parasite resistance and good genes
FIGURE 7.7. Peacock’s tail. An example of an elaborate, costly trait in males. (Photo credit: Corbis)
Hamilton and Zuk hypothesis (1982) Handicap Principle (Zahavi, 1997) and “proxy cues” (Milinski and Bakker, 1990)

10. Evolutionary Models of Mate Choice:
Parasite resistance and good genes
FIGURE 7.9. Color, parasites, and good genes. One reason stickleback females may prefer the most colorful (red) males is that color indicates resistance to parasites.
Color, parasites, and good genes. One reason stickleback females may prefer the most colorful (red) males is that color indicates resistance to parasites.

11. Evolutionary Models of Mate Choice:
MHC and good genes
FIGURE Female sticklebacks prefer males with more MHC alleles. Female sticklebacks spent more time on the side of the tank with males that had many different MHC alleles than with males that had few MHC alleles. (From Reusch et al., 2001)
Female sticklebacks prefer males with more Major Histocompatibility Complex (MHC) alleles.
Female sticklebacks spent more time on the side of the tank with males that had many different MHC alleles. (Reusch et al., 2001)

12. Evolutionary Models of Mate Choice:
MHC and good genes
FIGURE Female sticklebacks use peptides to assess MHC. Female sticklebacks were given a choice between water from a tank containing a lone male, or water from a tank with that same male plus added MHC peptides. (From Milinski et al., 2005)

13. Evolutionary Models of Mate Choice:
Runaway sexual selection
FIGURE Stalk-eyed flies and runaway selection. Male stalk-eyed flies show variation in the length of their eye stalks. Wilkinson bred lines of flies with long and short eye stalks to test the runaway model of sexual selection. (Photo credit: Mark Moffett/National Geographic Stock)

14. Evolutionary Models of Mate Choice:
Sensory bias and the emergence of mate choice
FIGURE Sensory exploitation model. In the sensory exploitation model, female preference (P+) is assumed to predate the male trait (T) that is preferred. In one lineage here (shown in orange), the male trait is immediately preferred once it appears (T+). (From M. J. Ryan and Rand, 1993, p. 189)

15. Evolutionary Models of Mate Choice:
Sensory bias and the emergence of mate choice
FIGURE Frog calls and sensory bias. Physalaemus pustulosus males (shown here) add a unique “chuck” sound to the end of their calls. The calls of Physalaemus coloradorum males lack a chuck. The calls of these two species have been used to test models of the sensory bias hypothesis. (Photo credit: Michael Ryan)

16. Learning and Mate Choice
FIGURE Pavlovian conditioning and number of sperm. Two different lines of male quail learned to pair a distinctive experimental chamber with a chance to mate with a female. Males were then placed in the chamber with a model female. Afterward, spermatozoa samples were taken. Males from both lines that had learned to pair the chamber with the chance to mate produced significantly more spermatozoa than control males (treatments 3 and 4, which consisted of males from the two lines that had not been given the chance to associate these two cues). (From Domjan et al., 1998)

17. Learning and Mate Choice:
Sexual imprinting
FIGURE Overriding an imprinted sexual preference. Male Japanese quail were raised in an environment with normal brown-colored quail and based on prior work done by researchers, these males were expected to have imprinted on brown females as future mates. These males were then put through a battery of tests in which experimenters measured the percentage of time they spent near a window. Conditioning—in which males learned that they would have the chance to mate with blond, but not brown, females— overrode sexual imprinting. B = blond, N = brown strain. (Based on Domjan, 1992, p. 53)

18. Cultural Transmission and Mate Choice:
Mate-choice copying
FIGURE Black grouse and mate-choice copying. Males try to attract females with which to mate by displaying on leks. This involves strutting, flapping their wings, jumping in the air, and hissing while females observe. (Photo credit: Jan Smit/Foto Natura/Minden Pictures)
Black grouse and mate-choice copying. Males try to attract females with which to mate by displaying on leks. This involves strutting, flapping their wings, jumping in the air, and hissing while females observe (Gibson, 1991).

19. Male-Male Competition and Sexual Selection:
Red Deer
FIGURE Approach and assessment in male red deer. When two male red deer approach within 100 m of each other during the mating season, any number of outcomes are possible. Thicker lines represent more likely outcomes. (From Clutton-Brock et al., 1979)

20. Male-Male Competition and Sexual Selection:
Elephant seals
FIGURE Elephant seal fights. Male elephant seals are much larger than females and fight for access to females during the breeding season. (Photo credit: François Gohier)

21. Male-Male Competition and Sexual Selection:
Pinnipeds: a phylogenetic perspective
FIGURE Harem size and sexual selection. In pinnipeds, there is a positive relationship between harem size and the relative difference in size between males and females. Each point represents a comparison between two species of pinnipeds. The x-axis and y-axis are adjusted independent contrast measures. (From Lindenfors et al., 2002, p. 189)
Harem size and sexual selection. In pinnipeds, there is a positive relationship between harem size and the relative difference in size between males and females. Each point represents a comparison between two species of pinnipeds (Lindenfors et al., 2002).

22. Male-Male Competition and Sexual Selection:
Male-male competition by cuckoldry
FIGURE Parental male bluegill sunfish tending his eggs. Parental male sunfish build nests and care for their eggs by fanning them and defending them against predators. (Photo credit: Dr. Bryan D. Neff)

23. Male-Male Competition and Sexual Selection:
Male-male competition by cuckoldry
FIGURE Bluegill morphs. (A) Bluegill parental male preparing a nest. (B) Sneaker males hiding behind plants awaiting a chance to quickly sweep into a parental nest. (C) A satellite male swimming over a nest containing a male and female. (D) A satellite male swimming between a parental male and a female. (E) A composite of A–D. (Based on Gross, 1982)
Bluegill morphs. (A) Bluegill parental male preparing a nest. (B) Sneaker males hiding behind plants awaiting a chance to quickly sweep into a parental nest. (C) A satellite male swimming over a nest containing a male and female. (D) A satellite male swimming between a parental male and a female. (E) A composite of A–D. (Based on Gross, 1982)

24. Male-Male Competition and Sexual Selection:
Male-male competition by cuckoldry