Chap.06 Sexual selection

鄭先祐 (Ayo) 教授國立台南大學 環境與生態學院

生態科學與技術學系 環境生態研究所 + 生態旅遊研究所

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Sexual selection

Intersexual and intrasexual selectionEvolutionary models of mate choiceLearning and mate choiceCultural transmission and mate

choiceMale-male competition and sexual

selection

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Intersexual and intrasexual selection

Intersexual selection Individuals of one sex choose which

individuals of the other sex to take as mates.,

Intrasexual selection members of one sex compete with each

other for access to the other sex

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Bateman’s principle

Work on fruit flies Females should be the choosier sex because

eggs are expensive and because a female’s reproductive success is limited compared with that of a male, and this should translate into greater variance in the reproductive success of males.

Until about 30 years ago, much of work on mate selection focused almost exclusively on intrasexual competition, rather than mate choice, or intersexual selection.

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(A) Male deer battle with their antlers (B) male horses compete for females.

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Pulse song in fruit flies

During courtship, male fruit flies would make pulse song, and the interval between pulses appears to be critical in terms of the mate choice of female fruit flies.

Recent work suggests that the genetics of song appear to involve three loci that account for a good deal of variance in courtship song.

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Evolutionary models of mate choice

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4

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1. Direct benefits

Direct benefits and nuptial gifts in scorpionflies

Female scorpionflies choose their mates using a very basic rule Choose male that bring relatively large prey

items during the courtship process (Fig. 6.5) These nuptial gifts, which are consumed

during courtship, provide females with a direct tangible benefit, food.

There is a positive relationship between prey size and copulation time.

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2. Good genes

Females get more than direct resources such as food and shelter from their mates, they also get sperm, and with it genes that are passed on to offspring.

For example, in pronghorn antelopes, male provide female with no direct benefits. Most females end up mating with a small

subset of males in their population. Attractive males vs. nonattractive males

(Fig. 6.7)

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Parasite resistance and good genes

Honest indicators, should be generally costly to produce, The costlier the trait, the more difficult it is to

fake, and hence the more likely it is that this trait is a true indicator of good genes. (Fig. 6.8 Peacock’s tail)

If information on internal parasitization is unavailable, using some other trait that correlates well with the one of interest. One such proxy cue appears to be body

coloration (Fig. 6.9)

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One reason stickleback females may prefer the most colorful (red) males is that color indicates resistance to parasites.

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MHC and good genes

MHC = major histocompatibility complex

Animals may prefer mating with others who have a dissimilar MHC. Such a preference could arise because

offspring resulting from a mating between individuals with very different MHCs will have a new combination of MHC genes.

Females rats or mice use odors to determine if another individual is a good MHC match.

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a negative correlation a negative correlation between pleasantness between pleasantness and MHC similarity.and MHC similarity.

MHC that as dissimilar MHC that as dissimilar from their own to be the from their own to be the most pleasant.most pleasant.

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MHC allele counting hypothesis

Individuals should prefer mates with many MHC alleles, rather than choosing a mate that is dissimilar in MHC-related genes.

Using wild-caught stickleback fish from three populations. When females were given a choice

between males, some of whom had few MHC alleles and some of whom had many such alleles, they consistently preferred males with a greater number of MHC alleles (Fig. 6.13)

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Symmetry and good genes

Developmental stability, is a measure of how well an organism handles changing environment as it matures.

Symmetry is a measure of the similarity of the left and right sides of an organism. Females may select more symmetric

males. (Fig. 6.15) (Fig.6.16)

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Smell, attraction, and fertility in women

College females smelled T-shirts worn by males for two days, and then the women rated the attractiveness of the scent of forty –one T-shirts worn by the men. (Fig. 6.17) (A) women who were closed to ovulation in

their menstrual cycle preferred the scent of more symmetric males, while

(B) women at a low fertility point I their cycle showed no such preference.

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Runaway sexual selection

Assume that two genes exist: a gene that codes for a particular trait in males, and a gene that codes for mating preference in females. The two genes become associated with

each other through time. Stalk-eyed flies and runaway selection

(Fig, 6.18, Fig. 6.19)

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Sensory bias and the emergence of male choice Sensory exploitation, sensory bias, preexisting

bias model of mate choice. Suppose that, red berries are the most

nutritious food source. Females who are best able to search out and

subsequently eat red berries survive and reproduce better.

Once a preference for all things red is in place, if red feathers should suddenly arise in males of this normally blue-feathered species, birds with these red feathers may be chosen as mates because the female’s nervous system is already designed to preferentially respond to red objects.

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Sensory exploitation model

The sensory bias model leads to a clear prediction regarding phylogenetic history.

The female preference trait (P) should predate the appearance of the male trait (T) (Fig. 6.20)

範例：1. Tricolor vision, fruits, and sensory

biases in primates

2. Frogs and sensory biases

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female preference trait (P)male trait (T)

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1. Tricolor vision, fruits, and sensory biases in primates

Whereas other mammals have two types of photoreceptor cones in the eye (dichromatic vision), many, but not all, primate species have three types of cones (trichromatic vision).

Possessing a third type of cone allows some primates to detect red-orange colors, whereas many other mammals are colorblind in this spectrum. Trichromatic vision allows primates to more

easily pick out red-orange fruits and edible ripe red leaves in a complex natural environment.

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Females prefer males with red fur coloration and/or red facial skin.

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Phylogenetic tree

Whether trichromatic vision predated a preference for red fur /face coloration? If trichromatic vision appeared after red

fur/face coloration, the sensory bias hypothesis must be rejected.

If trichromatic vision appeared before red fur/face coloration, then this is consistent with the sensory bias hypothesis.

Phylogenetic analysis found that trichromatic vision evolved earlier than red coloration in males (Fig. 6.22)

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2. Frogs and sensory biases

兩種青蛙的鳴叫 high frequency “whine” Physalaemus pustulosus Physalaemus coloradorum

Pustulosus 雄蛙鳴叫聲會有” chuck” (low-frequency) ，對 females 較有吸引力。

Coloradorum 雄蛙鳴叫，沒有 ” chuck” 錄音，人工加上” chuck” ，對 females 同

樣較有吸引力。 Support the sensory bias hypothesis.

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Physolaemus pustulosus male

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Learning and mate choice

Sexual imprinting Young individuals quickly learn mating

preferences from their interaction with adults.

Imprinting is restricted to some small time window during normal development, but the length of this window varied dramatically across species.

Learning and mate choice in Japanese quail

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Experimentally examine sexual imprinting

1. Using the cross-fostering approach.2. Employing the “novel trait” approach,

in which offspring are raised by parents that have some novel trait that an experimenter has introduced.

Using the novel trail approach to studying sexual imprinting on the mannikin bird, Lonchura leucogastroides.

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Four groups

Group 1, served as a control. Group 2, offspring were raised with their

mother and father, each of whom had a red head feather attached to their forehead.

Group 3, father with a red feather Group 4, mother with a red feather.

Young mannikins imprinted on the red head feather of their parents, and expressed a preference for such adorned birds when they themselves matured.

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Learning and mate choice in Japanese quail

Classical conditioning in adults affects mate choice and that adult male Japanese quail will quickly learn to stay in areas in which they have the opportunity to mate with a female.

While earlier work had demonstrated that sexual imprinting affects Japanese quail mate choice, Nash and Domjan found that learning can also play another role in mate choice. They examined whether adult classical

conditioning might override the effects of sexual imprinting as a juvenile.

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Three phases (Fig. 6.25)

Phase 1, a brown male was allowed to see a blond female and was then given the opportunity to copulate with her

Phase 2, the same male could see a brown female quail, but was never in physical contact with her. A given male learned that, the presence of blond

( 金黃色 ) female meant a mating opportunity, but the presence of a brown( 褐色 ) female did not.

Phase 3, males were tested to see how much time they spent near brown and blond females Adult-stage learning about who was likely to be a

receptive mate overrode the effects of early sexual imprinting.

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B= exposure to a blond female

N= exposure to a normal (brown) female

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Cultural transmission and mate choice

Mate-choice copying When a female’s mate-choice

preference is affected by the preference of other females in her population.

案例： Mate-choice copying in grouse Mate-choice copying I mice

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Once some females have chosen mates, mate-choice copying will play a role in the mating decisions of other female black grouse, as they choose to mate with the males chosen by other females.

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Black grouse and mate-choice copying

A single “top male” grouse obtains about 80 percent of all the mating at an arena (lek).

Older females mated, on average, three days earlier than younger females, suggesting that mate-choice copying, if it occurred, was most common among younger females.

Using stuffed “dummy” females placed randomly on a male territory within a lek. Males courted these dummy females and even

mounted them and attempted numerous copulations.

Females were more interested in mating with a male who had copulated with other females on his territory, even if they were dummy females.

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Sage grouse

Fifty-six days of observation, lekComputer simulationSupport for the mate-copying

hypothesis, as the unanimity of female mate choice increased as more females appeared, and this increase occurred more quickly then expected by chance (Fig. 6.27). The green points are the observed matings

and the orange points are the expected matings if the females had not been copying the mate choices of other females.

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Song learning and mate choice in cowbirds

Cowbirds One population from South Dakota (SD) One from Indiana (IN)

Cowbirds display different social behaviors and sing different songs across populations.

A cross-fostering experiment (Fig. 6.28)

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Song learning and mate choice in cowbirds

Birds paired up and mated with others who came from the same rearing regime (IN or SD) in which they were raised. SD birds raised with SD adults preferred SD

birds as mates, and SD birds raised with IN birds preferred IN birds as mates– the mating preferences of the SD birds were strongly dependent on the social environment in which the birds were raised.

Males copied the songs of the adults with which they were raised.

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Male-male competition and sexual selection

Red deer roars and male-male competition

male-male competition by interferenceMale-male competition by cuckoldry

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Males in possession of harem (holders) roared much more than males not holding a harem (solitaries), as well as more often then males in the pre-rut( 發情 ) or post-rut periods.

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Male-male competition by interference

An interesting subset of male-male interactions consist of males interfering with another male while that male attempts to mate with a female. Common in amphibians and insects.

Elephant seal, a species in which males form large harems of females and in which the males are much larger than the females (Fig. 6.31). Only 8.3% of males mated, but some individual

males inseminated 121 different females.

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Not only did alpha (dominant) males mount females more often, but that the females protested their mounts less often than mounts by nondominant adults or subadults (SA3, SA4).

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Sexual size dimorphism and male-male competition from a phylogenetic perspective

The relationship between sexual size dimorphism and male-male competition in the pinnipeds (seals, walruses, and sea lions), including the elephant seals.

They hypothesized that in species in which harem size is large, and hence where male-male competition is most intense, sexual size dimorphism should be greatest. The relative size of males increased as harem size

increased. As harem size increased, female body size would

remain fairly constant.

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Male-male competition by cuckoldry

In bluegill sunfish, three male morphs– parental, sneaker, and satellite– co-exit within populations. Parental males are light-bodied in color,

build nests, and are highly territorial, chasing off any other males.

Sneaker males are smaller, less aggressive, and do not hold territories.

Satellite males tend to look like females.

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Parental male Sneaker males A satellite male A satellite male

Ayo NUTN website:http://myweb.nutn.edu.tw/~hycheng/

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