Sexual selection
Opposing forces
 Conflict forms basis for Darwin’s theory of natural selection
 How then does cooperation evolve?
 Group selection arguments are weak
 Conflict even found within “cooperative relationships” such as mating pairs
Levels of cooperation
 Attractive forces must outweigh repulsive and centrifugal forces for
cooperation to occur
Fitness
 Attractive and repulsive forces are measured by fitness
 Inclusive fitness of an allele is its effect on the individual that bears it (direct
fitness) and the fitness of other individuals that carry it (indirect fitness)
 Remember kin selection and Hamilton’s rule?
 More on that calculating that later
Frequency dependent selection
 Important in conspecific interactions
 Rare variant is “released” from competition
Evolutionarily stable strategies (ESS)
 A strategy such that, if all the members of a phenotype adopt it, then no
mutant strategy could invade under the influence of natural selection
 Used extensively in behavioral ecology
Hawk versus dove strategy
 Hawk – More likely to win the prize
 Dove – More likely to escape injury
 Is Dove an ESS?
Is Hawk an ESS?
 Yes, if benefit is greater than cost
 Fitness (V) is greater than cost of injury (C)
 No, if benefit is less than cost
 Optimal strategy is variable
 Mixed strategy results based on amount of fitness payoff
Hawk versus dove
 Individual will adopt hawk behavior with a probability of V/C
Sexual selection
 Differences among individuals of a sex in the number of mates they obtain
 Intersexual selection – driven by interaction between the sexes
 Intrasexual selection – driven by interaction within the sexes

Variation in mating success is generally greater for males than females
Intrasexual competition
 Sperm or pollen competition
 Multiple paternity in plants, squirrels, bears, birds, lizards, spiders, human
twins
 Large ejaculates increase probability of fertilizing the egg
 Medflies ejaculate 2.5x more sperm when mate in presence of other males
Testes size in primates
Sperm scoopers in damselflies
Intrasexual competition
 Combat or other direct competition
 Selection for armaments, large body size, tactics
Combat in staghorn beetle
Color signals and combat in blackbirds
Intrasexual competition
 Infanticide
 Male lions that take over prides of females
 Female jacanas that destroy nests incubated by males
 Leads to faster mating opportunities
Summary of intrasexual competition
 Primarily male-male competition over mates
 Females may compete over mates if access to males is limiting
 Three main types of intrasexual selection
 Combat
 Sperm competition
 Infanticide
Intersexual selection
 Female choice
 Human biases in the study of female choice
 Examples of female choice
 Theories about why females are choosy
Bias in the study of choice
 Darwin ridiculed for assertion that females have a ‘sense of beauty’
 Females thought to choose correct sex and species only
 Courtship to overcome female ‘reluctance’ or fear of predation
Female choice…
 Details later…much evidence of choice across a variety of taxa
 With DNA markers, evidence of multiple mating partners in ‘socially
monogamous’ species
Female choice example: Tungara frogs
 Males advertise with calls to females
 Two calls, simple (tun) and complex (tun gara gara)
 Females prefer complex calls
Cost of sexy calls
 Bats locate males making complex calls more easily
Female choice example: barn swallows
 Barn swallows and tail length
The famous peacock’s tail
Why be choosy?
 Good genes: advertisement a true indicator of quality
 Barn swallows and mites
Why be choosy?
 Good genes: advertisement a true indicator of quality
 Eyespot area and offspring weight in peacocks
Why be choosy?
 Good genes: advertisement a true indicator of quality
 Gray tree frogs
Why be choosy?
Why be choosy?
 Resource acquisition: nuptial gifts, or advertisement indicates future paternal
care
 Hangingfly nuptial gifts
Sensory bias
Why be choosy?
 Sensory bias: females have intrinsic preferences; advertisements are arbitrary
 Crests in grassfinches
 Supernormal stimuli
Runaway selection for sensory bias
Runaway selection for sensory bias
 Sexy son hypothesis
 Genetic correlations between choice and trait due to assortative mating
 Offspring of female will be ‘sexy’, have same advertisement favored by
females
 Original trait is arbitrary
Summary of intersexual selection
 Advertisements may or may not be informative
 Single display could have multiple meanings and benefits
Male parental care
 Kittiwakes, monogamous birds, male care, similar variation in offspring number
When male care exceeds female care
 Pipefish: female success limited by space in brood pouch of male
 Males choose females based on body size, skin markings
When male care exceeds female care
 Red phalaropes, showy females court males, which provide all care for eggs
and young
When male care exceeds female care
 “Sex-reversed” birds often have precocious young, polyandry, female-biased
sex ratios
 Nest predation is high, only one parent stays at nest to decrease visibility
 Females selected for multiple mating (replacement clutches) which apparently
decreased parental care
Why don’t all species have male care?
 Most species have NEITHER male nor female care
 Lactating mammals are an extreme
 Male care more common when paternity more certain
 For example, internal fertilization
Generally, males aren’t very choosy
(especially when polygynous)
 In some frogs & water striders, males have evolved special ‘release signal’
given to other males who have clasped them in error
 Wild turkeys will court not only stuffed females but models of female heads
suspended 40 cm above the ground
 Sockeye salmon will spawn with any red object
Male mating mistakes