Sexual Selection Flashcards
Darwin: Natural Selection
the fittest individuals will survive and reproduce
succeeding generations will become adapted to their environment
Darwin’s Problem
The problem with the theory of natural selection is that it can’t explain costly traits that lower an individual’s survival
Darwin: Sexual Selection
“a struggle between the individuals of one sex, usually the males, for possession of the other sex”
Sexual Selection
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
Secondary Sexual Characters
Wilson’s bird of paradise (colourful feathers, blue skin cap); Antlers on moose, Hooked jaw in sock-eye salmon
What’s the basis for sexual selection?
Anisogamy: sexual reproduction by the fusion of dissimilar gametes.
Potential reproductive rates
Bateman (1948) Drosophila
Males have a higher potential reproductive rate than females
So males compete and females choose
This can lead to sexual dimorphism
Sexual Dimorphism
Male and female Mallard ducks
Male-male competition
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)
Male-male competition in toads (Bufo bufo)
Davies & Halliday (1978)
Larger males are not dislodged and finish copulation whereas smaller males are dislodged some of the time (13/23).
Body Size and Harem Size in Pinnipeds
As body size dimorphism increases harem size increases
Antler Length and Mating System in Deer
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
Two Important Points
Sexual Selection doesn’t have to lead
to Sexual Dimorphism
Sexual Dimorphism can arise from processes other than Sexual Selection
Female choice
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
Mutual Competition and Choice
Low investment in offspring leads to competition
High investment in offspring leads to choice
Equal investment in offspring means both sexes compete and choose
Competition in Crested auklets
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
Sex Role Reversal
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.
Pipefish: Males That Get ‘Pregnant’
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.
Nutritious Spermatophores
Mating pair of crickets with female (left) eating nuptial gift of male (right).
Sex Role Reversal in Katydids
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.
Sex Role Reversal: Mormon Crickets
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.
Sexual Selection
Any pattern of choice and competition can occur
Each sex can play both roles
The environment can influence the pattern
of sexual selection
