Chapter 7- Sexual selection

Question 1

Nest building in male bowerbirds

Answer 1

Males build either a teepee like nest or an avenue nest, which has tall walls running down an open path. The nests are surrounded by trinkets that have been discovered and arranged by the male. Around these nests, males perform elaborate behavioral displays to solicit mating opportunities from females

Question 2

Optical illusions

Answer 2

When a scene from the environment is projected onto the two dimensions of the retina in different way from what is actually occurring in the 3D reality of that scene in nature.

Question 3

Forced perspective illusions (bowerbirds)

Answer 3

Forced perspective illusions occur when an observer is in a stationary position, like when female bowerbirds watch males display and bring them trinkets from a set position in an avenue nest. Males arrange their trinkets in an area of the nest called the gesso for females to see. Males are able to arrange the items to create the illusion that the items are larger than they actually are. The illusion appears to increase the male’s reproductive success.

Question 4

Sexual selection

Answer 4

Depends on the advantage that certain individuals have over other individuals of the same sex and of the same species, relating to reproduction. Divided into intrasexual selection and intersexual selection.

Question 5

Intrasexual selection

Answer 5

Members of one sex compete with each other for access to the other sex- can involve two males or two females competing

Question 6

Intersexual selection

Answer 6

Individuals of one sex choose which individuals of the other sex to take as mates. Can involve males choosing females or females choosing males

Question 7

Female mate choice in cichlid fish

Answer 7

Females tend to prefer males that are more dominant and aggressive. Researchers wanted to know whether observing a fight between males triggers action in certain areas of a female’s brain. Females were placed in a tank with a male and each end, and their preference was measured by the time they spent near each male. After this, females saw a fight between these two males. Researchers measured gene expression levels in c-fos and egr-1 genes to determine how this observation affected brain activity. Seeing the preferred male win or lose had dramatic effects on the levels of gene expression in the brain

Question 8

In female cichlids, which brain areas were affected by seeing her preferred mate win or lose a fight?

Answer 8

When observing their mate win, there was increased gene expression in both c-fos and egr-1 in the preoptic area and ventromedial hypothalamus- both of these areas are linked to reproductive physiology and reproductive behavior. When their preferred male lost a fight, c-fos and egr-1 gene expression was high in the lateral septum of the brain- this area is linked to anxiety-like behaviors. This could reduce the probability of mating and reproduction.

Question 9

Why do males compete for mating opportunities in most species?

Answer 9

Females produce fewer, but larger gametes. Each egg is valuable because it’s more scarce. However, males produce a large amount of sperm using much less energy. Therefore, male reproductive success is limited by the much lower rate of gamete production of females compared with that of males. Males produce millions of sperm, so they have to compete for access to a scarce resource (eggs).

Question 10

Which size gametes does natural selection favor?

Answer 10

Natural selection favors large (eggs) and small (sperm) gametes over medium sized gametes

Question 11

Bateman’s principle (2)

Answer 11

1. Females should be the choosier sex because eggs are expensive to produce, and because a female’s reproductive success is limited compared to that of a male.
2. Females’ greater choosiness in mate selection should translate into greater variance in the reproductive success of males

Question 12

Natural selection acting on male competition for mates

Answer 12

Any male trait that confers mating and fertilization advantages and is passed down across generations will increase in frequency in a population over time, because these males will produce more offspring than their competitors. This is one component of intrasexual selection.

Question 13

Why is female mate choice more prevalent?

Answer 13

Females stand more to lose than males by making a bad mate choice. Females invest more energy in making each gamete. They also invest energy in gestation before the offspring is born. Therefore, they are under strong selection pressure to choose good mates that will produce healthy offspring.

Question 14

Epigametic (secondary) sex characteristics

Answer 14

Usually in males- traits that play a role in attracting mates, other than physical genitalia. Includes ornamental plumage, bright colors, and courtship displays

Question 15

Pulse song genetics

Answer 15

Male fruit flies sing to females during courtship by vibrating their wings. Pulse song is one form of song in fruit flies that is very conspicuous during courtship. The interval between pulses appears to affect female fruit flies’ choice of mates. Pulse song appears to be controlled by 3 genetic loci that account for a lot of the variance in courtship song

Question 16

Monogamous

Answer 16

A single male pairs up with a single female

Question 17

Polygamous

Answer 17

Some males mate with many females

Question 18

Polyandrous

Answer 18

Some females mate with numerous males

Question 19

Mating systems (3)

Answer 19

1. Monogamous
2. Polygamous
3. Polyandrous

Question 20

Sexual selection is stronger in which mating systems?

Answer 20

Polygamous and polyandrous systems. This is because some individuals will obtain many mating opportunities and others won’t obtain any. There is usually less variation in reproductive success in monogamous systems

Question 21

4 evolutionary models of mate choice

Answer 21

1. Direct benefits
2. Good genes
3. Runaway selection
4. Sensory exploitation

Question 22

How can environmental issues impact mate choice?

Answer 22

Pollution can impact mate choice- some species distinguish between males of their species by their color patterns. Pollution can make species who live in bodies of water be less able to see mates through the water. Managed animal populations (in zoos) reduces suitable models on which to sexually imprint. Females may also choose to mate with males they wouldn’t otherwise choose, and also devote less resources to their offspring. In addition, some species may interpret human disturbances as dangerous and will spend more time being vigilant and less time with their mates. This means that low quality males may have mating opportunities they wouldn’t normally obtain.

Question 23

Direct benefits model of mate choice

Answer 23

Hypothesizes that selection favors females that have a genetic predisposition to prefer mates that provide them with tangible resources (other than sperm) to increase their ability to reproduce. This includes resources like food, safe shelter, and assistance with parental care.

Question 24

How do nuptial gifts in scorpionflies benefit females?

Answer 24

This is an example of direct benefits of mate choice. Female scorpionflies choose mates that bring them relatively large prey items during the courtship process (nuptial gifts). They provide females with a direct benefit of courtship in the form of food. Females also determine how long they will mate with a male based on the size of his gift (based on the direct benefit they receive). There is a positive relationship between copulation time and the number of sperm transferred during mating. Any time shorter than seven minutes usually involves no sperm transfer. A female that chooses a mate that brings large nuptial gifts produces more eggs and has a longer lifespan.

Question 25

Sexual selection pressure on male scorpionflies

Answer 25

Nuptial prey size has a strong effect on mating opportunities and sperm transfer, so there is strong sexual selection pressure on males to bring large nuptial gifts to females. Finding prey that is large enough is time consuming and dangerous, because increased foraging time puts males at risk of predation. Males will often sample and then discard prey that is too small, while females would never discard small prey. Selection pressure has also resulted in males stealing large prey from each other in order to obtain mating opportunities.

Question 26

Good genes model of mate choice

Answer 26

Proposes that selection favors that females choose the males with genes best suited to their particular environment. Females receive indirect benefits because their offspring receive some of the good genes that caused their mother to choose a specific mate.

Question 27

How do female pronghorn antelopes choose mates?

Answer 27

Female pronghorns make a long search to find mates by visiting different males that already have harems of females. Females select males as mates based on a male’s ability to defend his harem, and they end up mating with a small subset of males. This leads to high variance in harem size among males and to males with large harems siring a disproportionate number of offspring.

Question 28

Mate choice in female pronghorns study

Answer 28

Researchers hypothesized that females were using current harem defense as an indicator of good genes in males. If so, males with large harems should have offspring that are more likely to survive. Researchers followed females during mating season and recorded which males were selected as mates. Offspring were marked when born and their survival was measured. The results were consistent with the hypothesis- offspring from attractive males had higher survival rates- suggests that females were selecting males based on some measure of a male’s genetic quality.

Question 29

Honest indicators

Answer 29

Indicators of genetic quality that can’t be manipulated by males- male pronghorns, for example, could “cheat” by pretending they have a larger harem than they actually do, so females would mate with them. Selection would actually favor males who do this.

Question 30

Parasite resistance and good genes

Answer 30

Parasite resistance could be an honest indicator because it is a costly trait that is difficult to fake. Females receive an indirect benefit by selecting males with this trait. Some parasites can’t be seen, but females can use other observable traits, such as body coloring, to determine which males are infected- infected males tend to have duller colors. Female birds and female sticklebacks tend to prefer brighter males.

Question 31

Major histocompatibility complex

Answer 31

A set of genes involved in disease resistance. Proteins produced by MHC genes guide the body in identifying self vs foreign cells. The MHC is extremely variable and very few people have the exact same MHC

Question 32

MHC and mating choice

Answer 32

Researchers predicted that animals prefer mating with others that have a dissimilar MHC. This can lead to offspring with strong immune systems. Diseases evolve so rapidly that a constantly changing set of MHC genes helps them to hit a “moving target”. Females can use odors to determine whether a male is a good MHC match

Question 33

MHC and odors in humans (Milinski and Wedekind)

Answer 33

Researchers tested undergraduate students in Switzerland, and had their blood samples tested for MHC analysis. Men were instructed to wear a cotton T-shirt for 2 nights. Women not on oral contraceptives consistently found the odors of the T-shirts from males with dissimilar MHCs more attractive. This suggests that MHC has a significant effect on female mate choice in humans.

Question 34

MHC choice in sticklebacks

Answer 34

Researchers hypothesized that to produce disease-resistant offspring, individuals should prefer mates with many MHC alleles rather than choosing a mate that is dissimilar in MHC related genes. Researchers capture fish from 3 populations that live in interconnected lakes. When females were given a choice between males- some had few MHC alleles and some had many- they consistently preferred the males with a greater number of MHC alleles.

Question 35

How do female sticklebacks assess the number of MHC alleles a male has?

Answer 35

Females use chemical cues. The greater the number of MHC alleles in an individual, the greater the number of MHC peptides (short strings of amino acids displayed on the cell surface). It was hypothesized that female sticklebacks were able to use odor to assess a male’s diversity of MHC peptides, and that MHC peptide ligands (molecules that bind to proteins) were the key underlying proximate mechanism of female mate choice. Females were exposed to two water columns- one had water that had had a male swimming in it, one had water supplemented with MHC peptide ligands. The peptide ligand diversity of both the male and female were known. It was found that when a pair had a less than optimal number of MHC peptide ligands, the addition of synthetic ligands to one side of the water column made the odor on that side more attractive. When the male and female had the optimal number of MHC peptide ligands, the addition of ligands made that water column less attractive. This suggests that the number of peptide ligands was the proximate cue used to select males with good genes.

Question 36

How do MHC peptide ligand odors affect females after they give birth?

Answer 36

After females give birth, they avidly forage and raid the nest of other sticklebacks to eat their eggs. Males guard nests from female attacks. Females are actually repelled by the MHC peptide ligand odor of their former mates, because repulsion reduces the chance of a female accidentally raiding the nest of her mate and cannibalizing her own eggs

Question 37

Runaway sexual selection model

Answer 37

Centers on the relationship between alleles at two loci. One locus houses alleles that code for female preference, and the other houses alleles associated with the male trait that females prefer. Over evolutionary time, specific alleles from the two genes become associated with each other- when one allele is present in male offspring in a clutch, the other allele is likely to be present in female offspring from that clutch. It results in a feedback loop because selection will produce exaggerated traits (like color pattern in males and preference genes for color in females) in both sexes. It has a “snowball effect” where traits change dramatically over time.

Question 38

Stalk eyed flies and runaway selection

Answer 38

Female flies prefer to mate with males who have eyes at the end of long stalks- variation in this trait is due to genetics. In one treatment of a study, males with the largest eye stalks were allowed to breed with females. In the second treatment, males with the shortest eye stalks were allowed to breed. In the long stalk treatment, average stalk length increases over generations, while in the short stalk group, average stalk length decreased. The study also found a positive link between the length of the male eye stalk and the female preference for this trait. Over time, when females from the short stalk line were given the choice of mating with males of different eye stalk lengths, they actually chose males with short eye stalks. Female preference changed due to selection pressure on males. In the long eye stalk line, females preferred long eye stalks, but no more so than control flies. Therefore, the evidence was contrary to predictions of the runaway selection model that selection on males produced significant changes in female eye stalk length preference.

Question 39

Sensory exploitation/sensory bias/preexisting bias model

Answer 39

State that when a male trait first emerges it is preferred by females because it elicits a neurobiological response that is already in place in females, and that this response is not associated with mating preferences initially. For example, if red berries are the most nutritious food source available to a bird species, females that eat this berries will be better at surviving and reproducing. Natural selection should therefore favor the neurobiological circuitry that allows females to hone in on red items in their environment. If red feather arose in males of the species, birds with red feathers might be chosen as mates because the female’s nervous system to set to respond preferentially to red.

Question 40

Sensory bias model and phylogenetic history

Answer 40

The female preference trait should predate the appearance of the male trait on a phylogenetic tree. The model describes the origin, not the maintenance, of the female preference trait.

Question 41

Sensory bias model in frogs

Answer 41

Males in two species (pustulosus and coloradorum) of frog use calls to attract females. Both species have calls beginning with a high frequency whine- females pick up the whine part using the basilar papilla of the inner ear. Pustulosus frogs add a low frequency “chuck” sound to the end of their call. Pustulosus females usually prefer to mate with males that make the chuck sound, which is detected using the amphibian papilla section of the inner ear. The hypothesis was that the preference for chucks in these females was due to a pre existing sensory bias in favor of low frequency sounds. In the analysis of phylogenetic data, it was found that the common ancestor of both frog species did not use a chuck call. However, coloradorum females prefer chuck calls if a prerecorded chuck call is played. This suggests that auditory circuitry in both species is designed to prefer low frequency sounds. The preference for chucks predated the actual appearance of the chuck trait.

Question 42

Conditioned stimuli and mating behavior

Answer 42

After exposure to a conditioned sexual stimulus, males are quicker to copulate, become better competitors than other males, display higher levels of courtship, and produce more sperm and more offspring. For example, male quails learned to pair an experimental chamber with a chance to mate. Males that had been conditioned to expect mating produced more sperm.

Question 43

Sexual imprinting

Answer 43

Young individuals imprint on the behavior and morphology of adults (usually their parents) and use these characteristics to guide their selection of mates. Imprinting usually occurs during a small window of development

Question 44

Cross fostering to examine sexual imprinting

Answer 44

Researchers can test the hypothesis that offspring raised by adoptive parents show different mating preferences than by those raised by biological parents. It can also examine whether mating preferences are linked to something about the behavior or morphology of the adoptive parents.

Question 45

Novel trait approach of examining sexual imprinting

Answer 45

Offspring are raised by parents that have some novel trait introduced by a researcher

Question 46

Novel trait approach in mannikin birds

Answer 46

Researchers added a red feather to the forehead of adult mannikins as a novel trait. They hypothesized that offspring that were raised in the presence of adults with the red head feather would display a sexual preference for individuals with red feathers when they were older. Group 1 was a control where offspring were raised with their parents who did not have a feather, in group 2 both parents had the feather, in group 3 only the father had the feather, and in group 4 only the mother had the red feather. It was found that males raised with a mother with the feather preferred females with red head feathers as mates, and females raised with a father with a red feather preferred males with the feather.

Question 47

Neurobiology of imprinting in zebra finches

Answer 47

When animals rely less on learning (like with imprinting), there is a decrease in dendritic spine density after an individual has imprinted on the phenotype of the opposite sex. Male zebra finches exhibit a decrease in dendritic spine density in the MNH area after exposure to a female. There were also fewer dendritic spines in males who had completed the process of sexual imprinting.

Question 48

Classical conditioning in the mate choice of Japanese quails

Answer 48

Researchers hypothesized that classical conditioning as an adult might override the effects of sexual imprinting as a juvenile. Each trial of the experiment contained a male, a female, and another female with lighter colored feathers. The brown males were raised with other brown quail, and had already sexually imprinted on the brown females. In the first phase, the male was allowed to mate with the blond female. In the second phase, the male could see the brown female but not mate with her, and he learned that the blond female’s presence meant an opportunity to mate while the brown female’s did not. In the last phase, time near each female was measured. It was found that blond female quails elicited a stronger response. Therefore, learning in adulthood overrode the effects of sexual imprinting.

Question 49

When does female mate choice copying occur?

Answer 49

When a male’s probability of being preferred as a mate increases as a result of having been preferred by a female in the past. If a male has an X percent chance of mating if he hasn’t recently mated and Y percent chance of mating if he has recently mated, female mate copying if found using the formula Y-X.

Question 50

Leks

Answer 50

Black grouse mating arenas that are found in Finland, Wales, and Scotland. Male black grouse gather at leks and each one defends a small territory.

Question 51

How does mate choice copying occur in black grouse?

Answer 51

Male grouse have specific behaviors used to attract females. However, a single male grouse will usually obtain 80% of all matings on a lek. Females looking for mates will often travel to leks in groups before mating, so they will synchronize their trips to male territories in an arena. Mate choice copying seems to occur among younger females, as older females tend to mate earlier than younger females.

Question 52

Mate choice copying study in black grouse

Answer 52

Researchers placed stuffed dummy females on 7 randomly chosen male territories in a lek before real females arrived. Males courted the dummy females and even attempted to mate with them. After observing the males, the real females were more interested in mating with a male they had observed mating, even if the mate wasn’t real. This suggests that mate choice copying explains the skew in male reproductive success than a specific trait or characteristic of a territory

Question 53

Mate choice copying study in sage grouse

Answer 53

Researchers studied female mate choice in two different leks. They hypothesized that the agreement in female mate choice would increase as more females mated on a specific day, because there would be more opportunities to observe and copy mate choice. Researchers observed all interactions on a lek and compared the data with a computer simulation that estimated how often females would have chosen the same male on a lek if they hadn’t been copying each other. The results supported mate choice copying, as the agreement in female mate choice increased as more females appeared. The increase occurred more quickly than would be expected by chance

Question 54

Oxytocin

Answer 54

A neurohormone secreted by the pituitary gland, plays an important role in social behaviors like mate choice, maternal bonds, and individual recognition.

Question 55

Role of oxytocin in mate choice copying in mice

Answer 55

Researchers studied mate choice copying in 3 lines of mice, two of which were normal and one of which had an OT knockout. A female was allowed to choose between two males, one of which had recently associated with an estrous female and had traces of odor in his area, and one of which hadn’t. Normal females copies the mate choice of other females. However, OT knockout females did not exhibit mate choice copying- there were no other differences in learning, so the results suggest that knocking out the gene for OT has the effect of inhibiting mate choice copying behavior in mice.

Question 56

OT knockout mice

Answer 56

OT “knockout” mice are mice that have deletions of the OT gene. These mice appear to learn normally, except in the context of social learning and mate choice.

Question 57

How do songbirds learn the songs they sing?

Answer 57

They learn the songs from others, who are referred to as tutors.

Question 58

Cultural transmission and mate choice in cowbirds

Answer 58

Juvenile and adults cowbirds were selected from two different populations (Indiana and South Dakota), which had different social behaviors and different songs between species. Researchers conducted a cross-fostering experiment where juvenile SD birds were raised with either SD adults or IN adults. When juvenile cowbirds matured, they were observed with unfamiliar birds from the SD and IN populations. The birds tended to mate with others that 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. Therefore, mating preferences were dependent on the social environment in which the birds were raised. This is likely due to copying and sexual imprinting. Males copied the songs of the adult males with which they were raised regardless of the adult’s population. Females also preferred songs that were like those of the males with which they were raised. This could be due to females copying the song preferences of adult females in their population or to females imprinting on the songs of males they were exposed to during development.

Question 59

Male-male competition in red deer

Answer 59

Male red deer form harems during mating season and fight off other males to defend their harems. Many males are injured while fighting, suggesting there is a strong selection pressure for accurately assessing an opponent’s strength in order to avoid injury.

Question 60

Sexual selection in male red deer study

Answer 60

Researchers hypothesized that the main indicator of strength and fighting ability in males was roaring- roaring was likely used to assess an opponent’s fighting ability in the context of harem holding. The more a male roared, the more likely he was to win a subsequent fight. Researchers used males of different sizes and abilities, and played tapes of other males roaring at different rates. When the tapes included sounds from a larger opponent, male red deer responded by being more attentive to the calls and increasing their own rate of roaring. Male-male competition for females has led to a social system involving fine tuned communication designed to maximize access to females and minimize injuries

Question 61

Male-male competition by interference

Answer 61

A subset of male-male interactions involve one male interfering with a second who is attempting to mate with a female. This is common in amphibians and insects- in earwigs, males may displace lighter males during mating to gain access to the female. In some cases, females may solicit males to try to remove a rival during the mating event

Question 62

Why would females incite male-male competition?

Answer 62

It may increase the probability of mating with the highest ranking male in a group

Question 63

Mating system of elephant seals

Answer 63

Male elephant seals form large harems of females where the males are much larger than the females, Harems can include up to 40 females which are defended by a large and dominant bull seal. There is a big variation in reproductive success among males. A small percentage of males will mate, but those that do will inseminate many females.

Question 64

Females inciting male-male competition in elephant seals study

Answer 64

Researchers marked estrous female elephant seals and tracked attempted matings with these females. Most attempted matings were protested by the females- protesting included many behaviors, like loud calls. Protesting was effective, because over 60% of attempted mating was interrupted by another male to some extent, opposed to 25% of mating that went unprotested. Females often protested subordinate males mounting more than they did dominant males, which led to an increased probability of mating between high ranking males and protesting females

Question 65

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

Answer 65

Researchers studied the relationship between sexual size dimorphism and male-male competition in pinnipeds (seals, walruses, and sea lions). They hypothesized that sexual size dimorphism (males larger than females) would be greatest when harem size is larger and therefore when there is more male-male competition. Sexual selection for large body size in pinnipeds should act more strongly on males than females, so researchers predicted that as harem size increased, female body size would remain constant. To test these hypotheses, researchers gathered data for 38 species of pinnipeds. There was a positive correlation between harem size and sexual size dimorphism- the larger the harem size, the larger the relative size of males compared to females. The relative size of females remained constant

Question 66

3 reproductive types (morphs) of bluegill sunfish

Answer 66

Parental, sneaker, and satellite- they can coexist within populations.

Question 67

Parental morphs of bluegill sunfish

Answer 67

These males are light bodied, build nests, and are highly territorial to chase off males that come near their territory. In the days before their eggs hatch, males fan the eggs to oxygenate them and defend the nest against predators

Question 68

Sneaker males bluegill sunfish

Answer 68

Sneaker males are smaller, less aggressive, and don’t hold territories. They have hiding places near a parental male and swim into a territory while the parental male and shed their sperm. Therefore, sneakers are trying to outcompete other males by sneaking into the parental male’s territory and cuckolding the parental male. Sneaker males invest most heavily in sperm production due to their mating strategy, and they have the highest ratio of testes size to body size

Question 69

Satellite males bluegill sunfish

Answer 69

Satellite males look like females and position themselves between a mating parental male and female. The parental male may treat a satellite male and a female and attempt to mate with both of them- at this point the satellite male will release its own sperm

Question 70

Which bluegill sunfish morph produces more sperm?

Answer 70

Sneaker males produce more sperm per ejaculate. This is because sneaker sperm is always competing with parental sperm, but not all parentals are cuckolded. Sneakers also experience a shorter lifespan than parental fish as a cost of producing so much sperm. Sneaker sperm was found to be less likely to fertilize eggs than parental sperm.

Question 71

Difference in sperm between sneaker and parental bluegill sunfish

Answer 71

Sneakers invest in producing many short lived, lower quality sperm, parentals invest in producing fewer but higher quality sperm.

Question 72

Pre-copulatory sperm competition in males

Answer 72

Studies have found that males can adjust the number and quality of sperm they produce in response to competition to other males. There is a confound- males adjust sperm quality and number in response to the females they are attempting to mate with. Ideally, research should be conducted without direct male-female interactions. This occurs in some insect species

Question 73

Sperm transfer in insects

Answer 73

In some species, sperm transfer is disassociated with male-female interactions. Males deposit a capsule of sperm and proteins at the tip of a stalk, and females can pick up a spermatophore to fertilize her eggs

Question 74

Sperm transfer in springtails study

Answer 74

Females in this species pick up only one spermatophore and receive indirect benefits from discriminating between spermatophores. Researchers tested whether males adjusted the size and quality of spermatophores in response to male competitors, and if this impacted female choice of spermatophores. Treatment 1 was a lone male control, treatment 2 placed a male with another male who was producing spermatophores, treatment 3 placed a male with another male who was not producing sperm. It was found that males in treatment 1 produced the most spermatophores of the 3 treatments. In the presence of other males, males trade off quantity vs attractiveness of spermatophore production as a result of indirect competition from others