Transcript 21_FemaleChoice

Announcements
EXAM II next Wednesday (March 14th)
Review Monday (March 12th)
No Class Friday (March 16th)
Why did ansiogamy evolve?
Can lead to conflict among sexes (which will invest more).
Bigger gametes (more parental investment - be choosy
when selecting mates)
More gametes (less parental investment - mate as many
times as possible)
Sexual Selection occurs in two forms
Intrasexual: Male-male competition
Intersexual: Female choice
Sexual selection: “the result of a non-random
association between a (secondary sexual) character and
a component of mating success."
Møller (1994)
secondary sexual character:
physical or behavioral characteristics that appear in
sexually mature animals
differentiate between the sexes without having a direct
reproductive function
(e.g, horns, color patterns, facial hair, breasts)
Some exaggerated ornaments or displays do not give a
competitive advantage (in competition for mates) but are directly
selected for by females.
Many may even incur a cost to the male with the display!
Long tailed widowbirds
Pied flycatchers
Sexual selection through female choice…
… what benefits do females get?
Evolution of female preferences for male traits:
The Fisher Process - “runaway sexual selection”
Indicator Mechanisms - of heath, good genes
Sensory Bias - pre-existing perceptual mechanism
Species Recognition
Evolution of female preferences for male traits:
The Fisher Process - two step process
1. Trait starts by conferring an advantage (slightly longer tail =
better flight)
2. Preference for trait that confers advantage evolves
Females that mate with males with long tails produce offspring
with advantage (longer tail) and with preference for trait!
This leads to a “runaway process” in which the preference
continues even though the exaggerated trait no longer
confers a fitness advantage.
Evolution of female preferences for male traits:
Indicator Mechanisms - of heath, good genes
Handicap Principle (Zahavi)
signals are expensive to produce, can be costly,
are therefore accurately convey information
about quality
Geoffrey Hill and colleagues
showed that house finch color
determined by carotenoids
gotten from diet. Better foragers
= brighter red.
evidence for healthy mate hypothesis
evidence for good genes
Marion Petrie 1994
The “Sexy-Son” Hypothesis - Weatherhead and
Robertson (1979)
Females choose males with particular traits because
their male offspring will later be preferred
(either because they out compete
other males in a dominance hierarchy
or are more attractive to females).
Evolution of female preferences for male traits:
Sensory Bias - pre-existing perceptual mechanism
Current signal (context A) exploits a sensory system that
evolved in a different context (for example foraging).
Evidence - preference may precede evolution of a signal!
“chase-away sexual selection”
What is preference surpasses the trait’s range?
Use the Environment.
Bower birds
What is preference surpasses the trait’s range?
Bring in a “wing man”.
March 7, 2007
In some animal societies, cooperation occurs in close-knit family
groups and kin selection explains apparently selfless behavior. Not
so for the lance-tailed manakin. Males of this little tropical bird
cooperate in spectacular courtship displays with unrelated partners,
and the benefits of lending a helping wing may only come years
down the line.
Additional reading:
Sexual Selection
Malte Andersson
The Red Queen: Sex and the Evolution of Human Nature
Matt Ridley
Why is sex fun? The evolution of human sexuality
Jared Diamnod