How does sexual selection influence animal behavior?
A (male) sage grouse
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6/10/08: Reproduction: Sexual Selection
Lecture objectives:
1.
Be able to define sexual selection and use the
theory to generate hypotheses and
predictions
2.
Understand why and how male-male
competition can select for reproductiverelated behaviors and traits
3.
Understand why and how female choice can
select for reproductive-related behaviors and
traits
2
Darwin recognized that some individuals have
a reproductive advantage over others
Sexual selection:
a form of natural selection that occurs
when individuals
relative to others of the same sex
Sexually selected traits:
1.
2.
(males compete; females choose)
3
Sexually selected traits/behaviors arise from…
Male-male competition
(Intrasexual selection)
- Aggression
- Distinct/Alternative mating strategies
- Sperm competition
- Mate guarding
Female choice
(Intersexual selection)
- Parental effort
- Nuptial gifts
- Ornaments & courtship that
don’t include material benefits
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Male-male competition can select for traits related to
aggression over mating
5
Male-male competition can select for alternative mating strategies
Marine isopod males
alpha
beta
gamma
# females
6
Male-male competition can select for mate guarding
Hypothesis: Males millipedes “ride”
females as a form of mate guarding
Time riding
Benefits of guarding a female (post-mating)_________
as the risk that she will mate again increases
Millipede with pair
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Male-male competition can select for sperm competition
Females in many species will mate
with ______________
and use their sperm to fertilize her eggs
Outcome might be determined by:
-
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Elaborate male genitalia:
Females have the
opportunity to mate
with >1 male
Females have short
periods of receptivity
Elaborate male genitalia:
Females are mostly
monogamous
Females have longer
periods of receptivity
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Female choice can select for males
that advertise paternal effort (“I’ll be a good dad!”)
signal:
Body shaking as an honest signal of
paternal effort in 15-spined sticklebacks
signal:
Song as an honest signal of paternal effort
in sedge warblers
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Female choice can select for nuptial gift-giving
A
transferred by a male to a female
just prior to or during mating
Gift of a ____: Male hangingflies
* Males that give larger gifts will have
a higher reproductive success…
because the female leaves as soon as
she’s done eating!
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Nuptial gifts (continued)…
Gift of __________ (as a meal):
Sexual cannibalism in male redback spiders
- Eaten males fertilize more eggs
than uneaten males
(full female is less likely to mate again soon)
- Males have a 20% chance of finding a mate
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Nuptial gifts (continued)…
Gift of _______________:
Transfer of alkaloids in spermatophores of moths
Hypothesis:
PA (-) females will develop
after mating with a PA (+) male
Utetheisa ornatrix
I like a man…er,
moth, with high
levels of PA!
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Nuptial gifts (continued)…
Gift of ___________
Sex is thirst-quenching
for female Bruchid beetles
Gift of … __________________
Spermatophores: ~10% of male body weight
Dehydrated females were more likely to mate
Once hydrated, a female was less likely to mate
with another male
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Female choice can select for
male ornaments and courtship
that provide no material benefits
Widowbird
Jumping spider courtship
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1. Healthy mate theory:
females choose males who are likely to have good health
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2. Good genes theory:
females choose males who are likely to have good genes
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3. Sexy sons theory:
females choose males who are likely to have sexy sons
Son: Time before mating
Example:
Attractiveness in Drosophila males is __________
(sexy males had sexy sons)
What might make a trait “sexy”?
Male: Time before mating
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Intra and Intersexual selection often go hand-in-hand
Function of display?
Done in presence of males and females
Elephant seals
Amazonian river dolphins
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Sexual conflict arises when males and females
have different interests
__________________ in orangutans
________________ in Panorpa scorpionflies
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