Sexual Selection & Secondary Characters
Potential for Sexual Selection:
Ratio Variance in Reproductive Success,
Lesser PI Sex / Greater PI Sex
Monogamy vs Polygyny (Polyandry)
Sexual Selection & Secondary Characters
• J. L. Hoogland, D. W. Foltz (1982) Variance in Male and
Female Reproductive Success in Harem-Polygynous
Mammal, the Black-Tailed Prairie Dog (Sciuridae:
Cynomys ludovicianus) Behav. Ecol. Socio. 11, 155-163.
• Colonial rodent, coterie (harem)
• Operational sex ratio < 1
• Blood-protein polymorphism, electrophoresis: parentage
Males: Greater Variance in Reproduction
Same pattern, 4 of 5 years studied
Black-Tailed Prairie Dog
Males disperse from natal colony;
Females philopatric: male-male
competition, access to coterie
Female choice: “Ranking adult male”
(RAM) in multi-male coteries
Sexual Selection & Secondary Characters
V[Male Rep Success] >
V[Female Rep Success]
Sexual Selection More Intense in Males:
Benefit/Cost of
Secondary Sexual Characters in Males
Secondary Sexual Characters
Females: Likely Choose Males with Greater PI, Higher
Quality Resources, …
But when Male PI lower
Bright Colors, Ornaments,
Elaborate Displays of Males Common
Sexual Selection: Advance Mating Success &
Enhance Fitness, Despite Cost
Breden, F., Stoner, G. 1987. Nature 329:831.
Trinidad guppy (Poecilia reticulata):
dichromatism, males brightly colored
Breden, F., Storer, G. 1987. Nature 329:831.
Most streams:
Females prefer brighter males as mates,
AND
Brighter males suffer more predation.
Exception:
Intense predation (cost); male coloration reduced
“Fits;” but why do females prefer brighter males?
Secondary Sexual Characters
R.A. Fisher: runaway process of sexual selection
IF secondary sexual characteristic M expressed in
one sex genetically correlated with preference
for this trait in the other sex, mate choice for M
also selects for strong preference.
Self-reinforcing (runaway) until another force
balances sexual selection.
Runaway process of sexual selection
One Allele: bright color in male
genome
preference for bright color in
female genome
Fisher’s “genetic correlation”
Balance: mortality?
Secondary Sexual Characters & Female Choice
“Sexy Son” Hypothesis:
Given Female Preference,
Female Chooses Male Genes Likely
to Render Her Sons Attractive
Doesn’t Answer Question ?
Secondary Sexual Characters & female Choice
“Good Genes” Hypothesis:
Females Choose Mates with Genes
Increasing Mean Fitness of her
Offspring
Circular? Correlates of good genes?