Sexual Selection

Question 1

Darwin: Natural Selection

Answer 1

the fittest individuals will survive and reproduce

succeeding generations will become adapted to their environment

Question 2

Darwin’s Problem

Answer 2

The problem with the theory of natural selection is that it can’t explain costly traits that lower an individual’s survival

Question 3

Darwin: Sexual Selection

Answer 3

“a struggle between the individuals of one sex, usually the males, for possession of the other sex”

Question 4

Sexual Selection

Answer 4

Male-male Competition (intrasexual selection)
Competition between members of the same sex

Female Choice (intersexual selection)
Choice, by one sex, of mates of the other sex

Question 5

Secondary Sexual Characters

Answer 5

Wilson’s bird of paradise (colourful feathers, blue skin cap); Antlers on moose, Hooked jaw in sock-eye salmon

Question 6

What’s the basis for sexual selection?

Answer 6

Anisogamy: sexual reproduction by the fusion of dissimilar gametes.

Question 7

Potential reproductive rates

Answer 7

Bateman (1948) Drosophila

Males have a higher potential reproductive rate than females
So males compete and females choose
This can lead to sexual dimorphism

Question 8

Sexual Dimorphism

Answer 8

Male and female Mallard ducks

Question 9

Male-male competition

Answer 9

Most basic form is direct physical contests among the members of one sex, over access to mates

Male elephant seals can weigh 2000kg and they fight vigorously
The winner (beachmaster) has a harem of dozens of females.
Other males get few or no matings.

Males fight to become dominant and dominance is correlated with mating success.
DNA analysis shows that mating success is linked to paternity.

Le Boeuf (1974)

Question 10

Male-male competition in toads (Bufo bufo)

Answer 10

Davies & Halliday (1978)

Larger males are not dislodged and finish copulation whereas smaller males are dislodged some of the time (13/23).

Question 11

Body Size and Harem Size in Pinnipeds

Answer 11

As body size dimorphism increases harem size increases

Question 12

Antler Length and Mating System in Deer

Answer 12

Polygynous ( in which one male lives and mates with multiple females, but each female only mates with a single male) males have longer antlers than monogamous males after correcting for body size

Question 13

Two Important Points

Answer 13

Sexual Selection doesn’t have to lead
to Sexual Dimorphism

Sexual Dimorphism can arise from processes other than Sexual Selection

Question 14

Female choice

Answer 14

Often difficult to distinguish between male-male competition and female choice

Need to carry out experimental manipulations to show female choice with any degree of certainty

Do female widowbirds prefer long-tailed males? Andersson (1982). Shortened, control (cut then re-stuck) & elongated. Tail treatment vs. number of nests per male.
Female long-tailed widowbirds prefer males with long tails

Question 15

Mutual Competition and Choice

Answer 15

Low investment in offspring leads to competition

High investment in offspring leads to choice

Equal investment in offspring means both sexes compete and choose

Question 16

Competition in Crested auklets

Answer 16

Both sexes invest in the offspring
Both sexes have crests

We’d predict mutual sexual selection
Jones & Hunter (1993)

Proportion of responses involving courtship is higher in those with a longer crest in both males and females.

Both sexes perform more courtship displays to long crested models leading to mutual mate choice

In both sexes long crested individuals are dominant leading to mutual competition

Question 17

Sex Role Reversal

Answer 17

Observation: In some species males invest more in reproduction than females

Prediction: There will be sex role reversal in species in which males invest a lot in offspring.
Females will compete and males will be choosy.

Question 18

Pipefish: Males That Get ‘Pregnant’

Answer 18

Males get “pregnant” and provide oxygen and nutrients to a clutch of eggs in an egg pouch.

During male pregnancy, females can produce eggs to fill 2 male pouches. Given an even sex ratio, male pouch space is limited.

Males choose large females that produce more eggs than small ones.

Question 19

Nutritious Spermatophores

Answer 19

Mating pair of crickets with female (left) eating nuptial gift of male (right).

Question 20

Sex Role Reversal in Katydids

Answer 20

Males should be more choosy when the resources needed to make the nuptial gift are scarce.
Australian katydid Kawanaphila

Food supply varies greatly through the breeding season.

Australian katydid Kawanaphila males are choosy and females compete for males only when the food (pollen) needed to make a nuptial gift is scarce.

Question 21

Sex Role Reversal: Mormon Crickets

Answer 21

Mormon crickets, large, flightless orthoptera.

Males transfer a huge (25% body mass) edible spermatophore to females when they mate so they can only mate once.
Females can produce several clutches

Females can mate more often than males so the Operational Sex Ratio (OSR) is female biased.

Mating aggregations occur, where females compete for males and males are choosy

Being choosy is beneficial for males of the Mormon cricket. They mate with larger, more fecund, females.

Question 22

Sexual Selection

Answer 22

Any pattern of choice and competition can occur

Each sex can play both roles

The environment can influence the pattern
of sexual selection