Sexual selection

Question 1

What is the main principle of sexual selection?

Answer 1

Reproductive success is the key. It is how many offspring an individual has that survive and reproduce themselves.

Question 2

What are secondary sexual characteristics?

Answer 2

These are male/female distinctions appearing at puberty.

• size
• coloration
• weaponry
• shape
• ornamentation

Question 3

What kind of morphological and behavioural traits are hard to explain using natural selection, mostly in males. Those traits and behaviours that hinder and do not promote survival?

Answer 3

These are things that might attract predators or lead to a slow escape. Examples are the plumage on birds of paradise, a peacocks tail and an elks enormous antlers.

Question 4

Why was another mechanism needed to explain these features not explained by natural selection?

Answer 4

1. Not easily explained by natural selection. They are not apparently beneficial to survival and sometime detrimental to survival.
2. Secondary sexual characteristics don’t appear until adulthood.

Question 5

What re the two mechanisms proposed by Darwin leading to the evolution of secondary characteristics?

Answer 5

Male-male competition

or

Female mate-choice.

However, these are not discrete categories, some sexual systems can have male-male competition with female mate choice as well.

Question 6

What are the benefits of secondary characteristics in male-male competition for intra-sexual selection?

Answer 6

Secondary characteristics give males a reproductive advantage when in competition with other males

Question 7

What are three types of male-male competition?

Answer 7

1. Contests - aggressive fighting
2. Scrambles - coming into contact and by chance you’re in competition.
3. Endurance rivalry - outlasting the other males

Question 8

There is variation in competition between animals. Give an example species that shows extreme male competition and the effects this has on female mate choice.

Answer 8

Elephant seals endure weeks of fighting for dominance. The females arrive after the battles are sequestered by dominant males which constrains female choice because females often can’t exercise choice if males are much larger than them. Especially if the baby is with them.

Question 9

There is variation in competition between animals. Give an example species that shows extreme female choice.

Answer 9

Bowerbirds

Males construct and decorate elaborate structures.

Females then visit several bowers where males court females with vocals and visual displays.

Female then chooses mate(s), then leaves to lay eggs and rear chicks alone.

Question 10

Outline the differences in the key aspects of natural and sexual selection

Answer 10

NS emphasises survival where SS emphasises reproduction.

NS emphasises getting to adulthood. Once in adulthood then SS kicks in.

NS natural environment is key although social environment can have an impact. But SS the social environment key.

NS operates on all individuals where SS operates more on males (in primates)

Question 11

What external physical traits can indicate intrinsic qualities that are a product of “good genes”?

Answer 11

Body/ canine size or colour intensity.

These physical traits can indicate;

• testosterone level (Higher levels of testosterone has been linked to lower immunity. So despite this, if he has this bright red coloration and he is still healthy shows he is a strong male.)
• parasite load
• fighting ability
• age
• tenure stage

Question 12

What are the two types of “good genes” model?

Answer 12

1. “Handicap” models.
   This is where ornamentation reduces average male survival. However females can assess a male’s ability to survive despite his burdensome ornament because the expensive signal = possession of excess energy.
2. “Viability indicator” models. This is where females can assess male quality by using characters that indicate a less apparent trait which can determine male fitness.

Question 13

What are some typical male traits that may be viability indicators?

Answer 13

• behavioural displays
• acoustic displays
• ornaments
• weapons
• pelage/plumage
• skin colouration
• chemical signals

Question 14

Give some examples of species showing these viability indicators

Answer 14

1. Orangutans without fatty cheek (unflanged) pads are seen as subhordinate males. Unflanged males can rape females.

Flanges will lose fattiness once he cannot hold down his territory.

2. Birds: male plumage brightness and song quality is reduced by chronic parasite infections.
3. Red skin primates often represent testosterone level or male health. E.g Gelada males: red patch on chest fades when they lose rank.

Redhead Uakari monkeys fade when they are ill.

Dominant Mandrills have brightest colors and most fatted rumps. Females prefer to mate with most brightly colored males.

Question 15

Why would females care about male testosterone levels?

Answer 15

Because it’s related to male success in agnostic competition.

Testosterone suppresses immune response so the males are superior because they’ve still survived even with a lower immune response.

Strength of immune function may be heritable and the females offspring could benefit.

Question 16

What features does male-male competition result in?

Answer 16

weapons or size of body/canines

Question 17

What features does female mate choice result in?

Answer 17

ornaments or displays

Question 18

What was Darwin’s contradictory thoughts on the subject of female choice?

Answer 18

“The female… with the rarest exception, is less eager to copulate than the male… she generally requires to be courted; she is coy, and may often be seen endeavouring for a long time to escape from the males.

Traditionally the observation of females running were seen as playing hard to get. Not that the females were testing the male.

Coyness was believed to be part of a universal female strategy to ensure she mated with the one, best male. This theory remained unchallenged for 100 years until research on female choice languished until 1970’s.

Question 19

What was some of the evidence found to prove female choice?

Answer 19

Many studies showed that females could discriminate very small difference between males.

Researchers found that females favoured male widow birds with longer tails.

Mation Petrie (1994) showed that females preferred peacock males with more eye-spots on their feathers after she manipulated them. The offspring of peacocks that had more eye-spots weighed more.

Question 20

What does female coyness mean?

Answer 20

Coy means that females need to be impressed by male but won’t actively test the male.

In most species, females seek out multiple matings with different males.

In some species there is sperm competition within the female after multi mating.

Question 21

Why would a female solicit multiple matings?

Answer 21

To manipulate information on paternity. High levels of infanticide (males killing other babies that aren’t theirs) by confusing paternity, it reduces infanticide