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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.

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sexual selection secondary characters
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 characters1
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
Males: Greater Variance in Reproduction

Same pattern, 4 of 5 years studied

black tailed prairie dog
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 characters2
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

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
Breden, F., Stoner, G. 1987. Nature 329:831.

Trinidad guppy (Poecilia reticulata): dichromatism, males brightly colored

breden f storer g 1987 nature 329 831
Breden, F., Storer, G. 1987. Nature 329:831.

Most streams:

Females prefer brighter males as mates,


Brighter males suffer more predation.


Intense predation (cost); male coloration reduced

“Fits;” but why do females prefer brighter males?

secondary sexual characters1
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
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

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 choice1

Secondary Sexual Characters & female Choice

“Good Genes” Hypothesis:

Females Choose Mates with Genes

Increasing Mean Fitness of her Offspring

Circular? Correlates of good genes?