Sexual selection

Question 1

What was Darwin’s problem with peacocks?

Answer 1

• The peacock’s tail
• Increases risk of predation so must have a benefit
• Sexual selection

Question 2

What selection is male-male competition?

Answer 2

• Intrasexual selection
• Competition between members of the same sex

Question 3

What selection is female choice?

Answer 3

• Intersexual selection
• Choice (by one sex) of mates of the other sex

Question 4

What are secondary sexual characters?

Answer 4

• Size
• Fighting ability
• Weapons
• Body colouration etc

Question 5

What is the underlying basis for sexual selection?

Answer 5

• Male and female gamete size difference
• Anisogamy
• Sexual selection works differently on the sexes

Question 6

Male-male competition examples: Elephant seals

Answer 6

• Male elephant seals - dominant beachmasters have higher mating success - selection pressures to grow large.

Question 7

Male-male competition examples: Common toad

Answer 7

• Weight / large size of male has more advantages because they can displace smaller males and mate with female.

Question 8

Body size and Harem size in pinnipeds: How does males / female size relate to harem size?

Answer 8

• In monogomous species, Male / female body size do not differ
• Body size dimporphism increases as harem size increases - sexual dimorphism

Question 9

Antler length and mating system in deer

Answer 9

Polygnous males have longer antlers than monogamous males

Question 10

Sexual selection doesnt have to lead to sexual dimorphism: Example?

Answer 10

Crested auklets

Question 11

How can natural selection lead to sexual dimorphism?

Answer 11

• Sexual dimorphism can arise if the sexes occupy different niches
• E.g. Huias birds with differing bill morphologies

Question 12

Female choice example

Answer 12

• Widow bird (long tails)
• Females preferred long tailed males
• Female choice has lead to runaway selection

Question 13

Choice in great crested auklets: Mutual sexual selection

Answer 13

• Long crest length increased courtship
• Both males and females performed more courtship behaviours to long crested models (of the opposite sex).
• Male-male interactions are won by long crested birds
• Female-female interactions are won by long crested females
• Both sexes perform courtship displays

Question 14

Example of Sex role reversal

Answer 14

• Pipefish: males get pregnant
• Males are the limiting sex
• Female bias
• Males choose females that are large and produce more eggs

Question 15

Sex role reversal: Mormon Crickets

Answer 15

• Produce a edible spermatophore that is 25% of their body weight
• Can only produce once in life
• Females have greater reproductive output
• Males are choosy

Question 16

Sex role reversal in Katydids
(Nuptial gifts)

Answer 16

• When food is scarce males are more choosy
• When food is abundant males are less choosy and compete more
• Females compete when food is scarce and
• the environment is affecting reproductive behaviour

Question 17

Conclusions of lecture

Answer 17

• Any pattern of choice and competition can occur (E.g. female or male choice)
• Each sex can play both roles
• The environment can influence the pattern of sexual selection