Models of mate choice
Good resources – direct benefits
Good resources
courtship feeding
Good genes – indirect benefits
“Sexy son” hypothesis (Fisher)
“Handicap hypothesis” (Zahavi) Good
genes for sons, daughters
parental care, protection
fanning eggs
good nesting site, territory
Resources from males can tip the
scales of relative investment
Good resources
Nuptial gifts and copulation time
Mormon cricket males give over 25%
body weight in their spermatophore
Nuptial gifts
Scorpion fly
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“Ultimate” nuptial gift
Why does a spider female kill
and eat her mate?
Why does a praying mantis
eat her mate’s head?
Survival is just one strategy to
transmit genes. When she eats
him, she makes more eggs bearing
his genes. His future reproductive
chances are low
“Sexy son” hypothesis
Stalk-eyed flies (also stickleback ex. in text)
2 genetic strains were developed: males with short vs.
long eye stalks. Selection on males.
“Sexy son” hypothesis
Fisher 1930 – A strictly genetic theory
 Females with preexisting sensory bias
 Males w/ this trait are selected, females with
the preference are selected
 Trait might provide nothing beyond mating
success
The parents
Trait A
Preference A
Trait A
Their offspring
Preference A
“Sexy son” hypothesis
This hypothesis predicts a ‘runaway’ effect
When does it stop?
 Females of either strain preferred males from their own
strain.
Stalk-eyed fly
2
Handicap hypothesis
Display can be indicator of health (choice
based on ‘honest signal’) (Zahavi 1975)
Handicap hypothesis
External features, behavior can indicate
general health
Comb and tail length changes with
infection
Guppy females choose males
who closely inspect predators
Handicap hypothesis
Peacock chick growth and survival is
linked to ‘eyespot’ size.
Handicap hypothesis
When females are allowed to choose, their
offspring have higher fitness
fruit flies, ducks, mice, others
However, females care more for their young
when mated with preferred male
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Mammals: where is the flashiness?
Females prefer:
 Larger harems (ex. seals, ungulates)
 Dominant, older males w high T (olfac)
 Good caregivers (in spp w male care)
 What about horns, antlers? Their size
correlates w/ many variables
Increased male investment
changes sexual selection
 Monogamy and equal care
 Sex-role reversals occur when males
invest more
Equal investment both are choosy
Crested auklet
Reversed sex roles: water bugs
Male back space is limiting resource
Females court males
Sea dragons, sea horses, pipefish

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After mating: competition goes on
If females mate w/ more than one male
 Sperm competition
 Mate guarding
 Cryptic female choice
Cryptic female choice
Strategic allocation / ejection of sperm
Behavior of dominant vs. subordinate males
Cloacal contractions
Mate guarding: benefits and costs
 Benefits and costs depend on opportunities
Benefits to females: multiple mates
Extra-pair copulations in ‘monogamous’
species
 10-40% of offspring of song birds are from
‘adulterous’ matings
 indirect benefits
 Ex: Brown antechinus – polygamous females
produced a greater portion of surviving young
than monogamous females
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Sexual conflict
Sexual conflict
When interests of males and females do
not coincide
...what’s
so stinky?
Drosophila male
accessory gland protein
hanging flies
heh heh
heh….
paternity uncertainty
higher # carriers
female EPC’s
less care by male
Who wins the sexual arms race?
Chase-away sexual selection
 Drosphila selection lines –
show manipulation of arms
race
 Male develops trait that happens to
exploit a sensory bias that females already
have
 Penis/vagina shape in ducks
with forced copulations
 Females get ‘taken in’ by males that may
actually be sub-par – selection to ‘resist’
 Also predicts ‘runaway’ effect on male
traits
Duck vagina (left)
and penis (right)
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Chase-away sexual selection
 Preexisting female biases may drive
Chase-away SS
Platys and swordtails
Basic types of mating systems
Mating systems
Monogamy
Polygamy (polygyny, polyandry)
Promiscuity or polygynandry
Describes both social and
sexual relationships
Patterns and constraints in
parental care
Parental care occurs in a minority of animals
What is the tradeoff?
Costs for females vs. males
Physiology, ecology, mating opportunities
Patterns and constraints in
parental care
Ectotherms vs. Endotherms
Internal vs. External fertilization
Mammals vs. Birds
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Level of parental care in ectotherms
vs endotherms
Endotherms – young have high energy
demands and require warming and
feeding.
Ectotherms
Parental care is rare among ectotherms
“r-selection” instead
Most investment costs are from mating or eggs
Examples of rare care - guarding
Constraints on mammals vs. birds
One or two parents?
Demands of young - compare RS of uni- and
biparental care.
Opportunity to find a mate – if you desert, what
are the chances to find mate, territory?
Opportunity to desert
Constraints on mammals vs. birds
Birds - 90% of spp. are monogamous
biparental care
Mammals - 95% spp. are polygynous
Constraints on care
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Why do some ectotherms care?
Energy and opportunity costs are still similar to noncaring spp., but RS benefits of care are much higher.
As soon as one parent is caring, benefits for other to
care are reduced.
What gender should care?
Ectotherm uniparental care
Internal vs. External fertilization
External fertilization
Male guarding eggs
Why is male care associated w/external
fertilization?
1. Paternity certainty
2. Order of gamete release
3. ‘Association’ with eggs
Internal fertilization
Female octopus guarding eggs
Polygyny and male care along reefs
Resource defense polygyny in reef fish
(external f), cuttlefish (internal f)
males defend multiple nest sites, guard
eggs, have multiple mates
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