Sexual selection

Question 1

What is intrasexual selection?

Answer 1

Male-male competition

Question 2

What is intersexual selection?

Answer 2

Female choice

Question 3

Describe how anisogamy leads to sexual selection

Answer 3

= Differentiation between gametes (e.g females produce 1 large egg, males produce many small sperm)
- Means that males can fertilise eggs at a faster rate than females can produce them (higher potential reproductive rate)
- Males compete, females choose
= sexual dimorphism, sexual selection

Question 4

Describe intrasexual competition in Elephant seals (LeBoeuf, 1974)

Answer 4

• Males fight viciously, winner is dom (beachmaster) and mates with multiple females (other males get few/no muff)
• Dominance correlates with mating success, which is linked to paternity

Question 5

Describe the takeaways from Davies & Hallidays (1978) study with toads

Answer 5

• Larger males have more reproductive success
• Relationship between size and repro success is stronger for males than females

Question 6

Will monogamous or polygynous species have more sexual dimoprhism?

Answer 6

Polygynous (male mates with multiple females) has more pronounced sexual dimporpism because competition for females is more intense

Question 7

Describe how antler length/ body size changes in Deer with different mating systems

Answer 7

• Polygynous males have longer antlers than monogamous males after correcting for body size
• Antlers used as weapons for male-male competiton

Question 8

In what siutation would natural selection lead to sexual dimorphism?

Answer 8

If the sexes occupy different niches
E.g Huia birds:
Males bill adapted to boring into rotten wood to find bugs while females long bills used into crevices of solid wood

Question 9

Describe Andersson (1982) widowbird study

Answer 9

They mate on leks (males display, females choose).
Anderson wanted to know whether males long tails made them better at competeing for females, or females prefered long tails?

H: Do female widowbirds prefer long-tailed males? (first study to look critically at female choice)

R: Anderson concluded that female choice was to prefer long tails

Question 10

Describe how investment in offspring leads to competition/choice

Answer 10

Question 11

Describe Jones & Hunter (1993) study with Crested Auklets

Answer 11

• Both sexes invest in offspring
• Both have crests (secondary sexual character)
• Prediction: mutal sexual selection
• Used models to manipulate crest length (on both sexes)
  Results:
• Both sexes performed more courtship displays to long crested models
• More intrasexual competition interactions were won by long-crested
  = mutal mate choice & competition

Question 12

Give an example of sex role reversal

Answer 12

Male Pipefish can get ‘pregnant’
- Females can produce egg pouches to fill 2 male pouches
- Operational sex ratio (OSR) is female biased = male pouch space is limited
- Males choose large females that produce more eggs than smaller ones

Question 13

Why would male orthopterians want to be choosy?

Answer 13

Spermatophores / nuptial gifts are ‘expensive’ so they are choosy

Question 14

Describe the environmental sex role reversal in Katydids

Answer 14

• Food supply of katydid varies greatly throughout the breeding season
• P: Males should be more choosy when the resources needed to make the nuptial gift are scarce
• R: Males were choosy and females compete for males only when the food needed to make nuptial gift is scarce
• R: When food is abundant, males compete more and females choose

Question 15

Describe environmentally determined sex role reversal in Mormon crickets

Answer 15

• Males transfer huge spermatophore (25% of its body mass) to females when they mate = so can probably only mate once in lifetime!
• While, females can produce several clutches
• OSR is female biased, so male choice
• High density groups form in breeding season, females compete for males (larger, more feccund preferred)