Lecture 6: Sexual Selection Flashcards
Natural selection:
DARWIN
-The fittest individual will survive and reproduce
-succeeding generations will become adapted adapted to their environments
BUT this cannot explain costly traits that lower an individuals survival (e.g. peacock’s tail)
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:
- not characters obviously used for reproduction (they’re primary)
- traits like size, fighting ability, weapons (salmon, moose, Wilson’s Bird of Paradise)
whats the basis for sexual selection?
- Anisogamy (different gametes)
(-females eggs large and costly, limited)
–> male can generally fertilise eggs at a faster rate than females can produce them
Bateman 1948 Drosophila
-males number of copulations is directly related to number of offspring produced
-females little difference whether 1 cop / 5
MALES HAVE A HIGHER POTENTIAL REPRODUCTIVE RATE THAN FEMALES
MALES ___ FEMALES __
Males compete, females choose –> this can lead to sexual dimorphism
sexual dimorphism:
Male and Female Mallard ducks
-different morphologies between sexes
Male -male competition most basic form and example (Le Boeuf 1974)
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 (beach master) & have a harem of dozens of females
- other males get few/no mating
- males fight to be dominant, dominance is correlated with mating success, which is linked to paternity
Male-male competition in toads (Bufo bufo)
- Davies and Halliday 1978
- males of all sizes L and S will try and mate females
- 23 S M and female, in 10 Large male replaced female, 13 small finished copulation, but if initial male was large, small male never replaced him
predict that in polygynous species we’d expect more or less sexual dimorphism than in monogamous
MORE
body size & Harem size in Pinnipeds
as body size dimorphism increases harem size increases
Antler Length and Mating System in Deer:
polygynous males have longer antlers than monogamous males after correcting for body size
2 important points about sexual dimorphism:
- sexual selection doesnt HAVE to lead to sexual dimorphism (M&F crested auklets)
- sexual dimorphism can arise from processes other than SS (NS may result in SD so M&F don’t have to compete with each other)
problem with identifying Female choice
is it M-M competition or female choice???
-You must carry out experimental manipulations to show female choice with any degree of certainty (must show female rejecting male! to confirm it)
Female choice: Andersson 1982
- Widowbirds African species, males present jumping displays fluttering their tails
- M with long tails tended to mate more than those with short
- -M-M comp/F choice?
- 3 options for F, Shortened, control and elongated
- -> FEMALES chose longer tailed males! SO FEMALE CHOICE
Mutual competition and Choice: possible? is determined by ?
YES
- Low investment in offspring (M) –> COMPETITION
- High investment in offspring (F) –> CHOICE
- EQUAL investment by both (monogamous) CHOICE AND COMPETITION
Crested Auklets Jones & Hunter 1993
- Both sexes invest in the offspring & both sexes have crests
- both M & F performed long courtship displays to long crests
- significantly more fighting interactions (F-F / M-M) were won by long crested birds
=both sexes choose & both fight
-Mutual mate choice and mutual competition = MUTUAL SEXUAL SELECTION
Sexual role reversal:
-Males invest more in reproduction than females (nuptial gift/care to young)
SO M should be choosey, F compete
e.g. of sexual role reversal;
PIPEFISH (seahorse);
- 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 / nuptial gift and example
=female to eat during mating, because its costly, MALE is choosey about what to mate with
- males should be more choosy when resources to make nuptial gets are scarce
- e.g. KATYDIDS (cricket)
environment affecting reproductive behaviour: Mormon crickets
- large flightless orthoptera
- M transfer huge (25%body mass) [1 spermatophore in lifetime) nuptial gift to females
- F can produce several clutches of eggs
- OPERATIONAL SEX RATIO (OSR) is F bias
- F-F competition for M
- M demonstrate choice, F accepted are heavier & produce more eggs
can the environment influence the pattern of SS
YES
