Female reproductive success is largely determined by parental effort Male reproductive success is largely determined by mating effort Because females invest much time and energy in offspring, a wasted mating is costly Therefore, females are predicted to be more choosy in selecting mates than males

Female choice of mates: What criteria do females use in choosing mates (e.g. resources, ‘good genes’)? How important has female choice been in the evolution of male traits that are: - not used in male-male competition? - costly in terms of the male’s survival? (e.g. the peacock’s tail, bright color in many male birds)

How do females benefit from choosing particular males? Direct (non-genetic) benefits - Better resources - Increased parental investment Indirect (genetic) benefits - Fitter offspring

How do females benefit from choosing ornamented males? Are benefits direct? - Better resources - Increased parental investment Are benefits indirect? - Does the trait indicate good genes? - Is it an honest reflection of male quality (e.g. survival, resistance to parasites) - Is it costly (a handicap)? - Is it also used in male-male competition?

Before After Widowbirds Tail treatment

Responses of female house finches to males of different plumage color

Male fitness in house finches

Male brightness is correlated with condition. Females prefer males with brighter bellies. Males infected with parasites are duller, not chosen by females Stickleback fish

Reliable cues that cannot be faked They seem arbitrary (bright colors, elaborate song) but what they indicate is not They are costly to acquire “Condition-dependent” traits

Heritability of stripe in great tits

Mate choice and tail length in barn swallows Tail treatment

No. of mites on offspring in other nests Male tail length (mm) Male fitness in barn swallows

Test: Aggressive Playback Control: Submissive Playback Control: No Playback % of nests containing extra-pair young High-ranking males

Test: Submissive Playback Control: Aggressive Playback Control: No Playback % of nests containing extra-pair young Low-ranking males

Effects of bands on male blue- throats BO malesNon-BO males

Can female choice lead to the evolution of traits that are costly in terms of natural selection? YES: 1. Females can use an elaborate trait to assess a male’s resources, parental ability, and/or genetic quality. 2. If the trait is costly, it is an honest signal of male quality, because males in poor condition can’t cheat. 3. Although the trait may be arbitrary, what it signals is not. ** The cost component is crucial **

Stickleback fish

1.If the male trait and the female’s preference are heritable, the ‘trait’ gene and the ‘preference’ gene may become linked. 2.Female choice may drive the male trait beyond the optimum point in terms of natural selection. 3.High quality males should be able to overcome at least some of this cost

Female preference Dull Bright Male coloration No. of individuals Optimum, natural selection Optimum, female choice Costs (e.g predation pressure)

Runaway (Fisherian) sexual selection: female 1. There is a genetic correlation between female preference and the male trait. **2. Female preference for the trait pushes the trait beyond the point where it is adaptive, even for ‘high quality’ males. **3. There are no longer any fitness benefits to mating with males with exaggerated traits. **4. Females gain only by producing ‘sexy sons’. **5. When the decrease in survival counter- balances sexual attractiveness, the runaway process will halt.

Indicator (‘Good genes’/Handicap) models 1.Female choice may drive the male trait beyond the optimum point in terms of natural selection 2.High quality males should be able to overcome at least some of this cost 3.Costly male traits are preferred by females because they indicate high heritable fitness If the trait is expressed in proportion to condition, a well-developed trait should signal the carrier’s condition Females who mate with such males produce both fitter sons and fitter daughters

Good evidence for condition-dependence of secondary sex traits Some evidence that females who choose males who have an extreme version of a trait gain direct benefits (resources) Some evidence that male condition has some genetic basis. Do females ever choose males on the basis of genes only, independent of the resources they provide?

Mating success in peacocks Eye-spot number

Offspring growth in peacocks Sons Daughters

What criteria do females use in choosing mates? Good resources? Good condition? Good genes independent of resources? How important has female choice been in the evolution of male traits that are: not used in male-male competition? costly in terms of the male’s survival?

The degree of sexual dimorphism is a good indication of the intensity of sexual selection. Monomorphic: Often monogamous. Males invest in parental effort. Dimorphic: Often polygynous. Males invest in mating effort.

Why does sexual selection seem to occur more in the form of intra-sexual selection in mammals and inter-sexual selection in birds?

What is the relation between female choice and male-male competition?

Does male redwing blackbird song function to deter other males, to attract females, or both? Males sing louder, switch song types more for females than for males. BUT Territories broadcasting song are invaded less rapidly by other males

What is the relation between sexual selection and natural selection?

Female preference Dull Bright Male coloration No. of individuals Optimum, natural selection Optimum, female choice Costs (e.g predation pressure)

Predation High Medium Low Guppies Streams

Control Start of experiment Crenicichla (predator on adults) Rivulus (predator on juveniles) Guppies

Guppies in Trinidad Male ornamentation is important for female choice, but not for male-male competition. There may be a genetic correlation between female choice and male ornamentation. Ornamentation may reflect male quality: *Diet *Parasite load *Predation risk

M only M >>> F M >> F M > F M = F Singing behavior

Prairie voles : Monogamous Males and females have same-sized territories. They perform equally well in mazes The hippocampus is the same size in both sexes. Meadow voles : Polygynous One male’s territory encompasses several females’ Males out-perform females in mazes Males have a bigger hippocampus

Intra-sexual selection generally concerns male-male competition. It can affect size, weapons (e.g. antlers, canines), and brain structures Sexual selection: Summary

Inter-sexual selection generally concerns female choice Females may base their choice on: – direct examination of resources – male traits that are correlated with resources – male genetic quality independent of resources

Male ornaments are not arbitrary, but have probably evolved because females use them as reliable indicators of male resources and/or genetic quality Inter-sexual selection (female choice) does not simply reaffirm the results of intra-sexual selection (male-male competition) or natural selection. It can lead to the evolution of male traits that are costly in terms of fighting and/or survival.