Lecture 6: Sexual Selection

Question 1

Natural selection:

Answer 1

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)

Question 2

sexual selection:

Answer 2

• 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 3

secondary sexual characters:

Answer 3

• not characters obviously used for reproduction (they’re primary)
• traits like size, fighting ability, weapons (salmon, moose, Wilson’s Bird of Paradise)

Question 4

whats the basis for sexual selection?

Answer 4

• Anisogamy (different gametes)
  (-females eggs large and costly, limited)
  –> male can generally fertilise eggs at a faster rate than females can produce them

Question 5

Bateman 1948 Drosophila

Answer 5

-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

Question 6

MALES ___ FEMALES __

Answer 6

Males compete, females choose –> this can lead to sexual dimorphism

Question 7

sexual dimorphism:

Answer 7

Male and Female Mallard ducks

-different morphologies between sexes

Question 8

Male -male competition most basic form and example (Le Boeuf 1974)

Answer 8

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

Question 9

Male-male competition in toads (Bufo bufo)

Answer 9

• 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

Question 10

predict that in polygynous species we’d expect more or less sexual dimorphism than in monogamous

Answer 10

MORE

Question 11

body size & 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 males have longer antlers than monogamous males after correcting for body size

Question 13

2 important points about sexual dimorphism:

Answer 13

• 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)

Question 14

problem with identifying Female choice

Answer 14

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)

Question 15

Female choice: Andersson 1982

Answer 15

• 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

Question 16

Mutual competition and Choice: possible? is determined by ?

Answer 16

YES

• Low investment in offspring (M) –> COMPETITION
• High investment in offspring (F) –> CHOICE
• EQUAL investment by both (monogamous) CHOICE AND COMPETITION

Question 17

Crested Auklets Jones & Hunter 1993

Answer 17

• 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

Question 18

Sexual role reversal:

Answer 18

-Males invest more in reproduction than females (nuptial gift/care to young)
SO M should be choosey, F compete

Question 19

e.g. of sexual role reversal;

Answer 19

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.

Question 20

nutritious spermatophores / nuptial gift and example

Answer 20

=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)

Question 21

environment affecting reproductive behaviour: Mormon crickets

Answer 21

• 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

Question 22

can the environment influence the pattern of SS

Answer 22

YES