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Conflict between the sexes
In order to have sexual reproduction, you need 2 sexes
In many organisms, males and females are in conflict over
their investment of reproductive effort
- males invest in mating opportunities
- females invest directly in offspring
This conflict manifests in many traits associated with sex
- Males and females often express very different traits
to maximize their respective success at reproduction
Asymmetrical investment in offspring
In animals with internal fertilization (pregnancies) and/or
parental care for offspring, females nearly always invest
more time and energy per offspring than males
- males invest 15 minutes in copulation
- females invest 8 months in pregnancy, 3 years in nursing,
and 8 years in protecting and nurturing her young
Asymmetrical investment in offspring
Males need only contribute one haploid genome (their genes)
to their offspring
Females generally invest far more energy per offspring
Some animals release sperm and eggs directly into the
environment, and provide no parental care
- in these organisms, eggs are larger and contain
energetically expensive yolk
- females can make fewer gametes (egg cells) than males
Asymmetrical investment in offspring
Because of this asymmetry in reproductive investment, males
and females face different challenges in maximizing their
fitness (making the most offspring)
Once a female’s eggs have been fertilized, she generally cannot
mate again until she’s done caring for that batch of offspring
A male can mate as often as he can find a receptive female
Asymmetrical investment in offspring
Consider a population w/ one male fish and 10 females:
one male can mate
with, and fertilize,
all 10 females
Male reproductive success is limited by access to mates
Asymmetrical investment in offspring
Now, consider a population w/ 10 male fish and one female:
Only one male can
ultimately father her
Female will (1) choose
her mate, or (2) mate with
the male who can fend off
all his competitors
Female reproductive success is not limited by access to mates
Sexual dimorphism
In many species, males and females are extremely different in
their appearance or behavior: sexual dimorphism
Fiddler crabs
Sexual dimorphism
Darwin’s “sequel,” The Descent of Man and Selection in
Relation to Sex, was dedicated to this phenomenon:
How could traits involved in male display or competition
persist in the face of natural selection?
- shouldn’t natural selection tend to weed out brightly colored
individuals (by predation), or those wasting energy on big
tails or hard-to-build body structures?
Sexual selection
Darwin’s answer: a distinct form of selective pressure exists that
tends to maximize reproductive success: sexual selection
Whereas natural selection makes a population more adapted
to its environment, sexual selection does not
- it makes one sex more successful in mating with the other
- increases fitness, by increasing reproductive success
Intrasexual selection
Males compete for territory, control of females, or opportunities
to fertilize; females just go with the winner
(1) Combat is a common form of pre-mating competition,
resulting in evolution of crazybig male features for fighting
- antlers of deer
- body size: in iguanas & elephant seals, big males larger
than the ideal size don’t survive for long, but farther many
children due to holding better & larger territories
Intrasexual selection
Males compete for territory, control of females, or opportunities
to fertilize; females just go with the winner
(2) Sperm competition is a form of post-mating competition
- males can make larger ejaculates when there’s competition
for females
- can be triggered by scent of nearby competitor males
- can be modified depending on male’s assessment of
a female’s promiscuity
- multiple kinds of sperm: sprinters, blockers and killers
- extreme case: testicular hijacking in bugs
Intersexual selection
When males cannot monopolize territory, females may be able
to “choose” among the males (or are they?..)
This introduces a novel form of evolutionary pressure:
males versus females in a continual struggle, each seeking
to maximize their different reproductive goals
Many aspects of this conflict are not a matter of “choice,” but
depend on a continuous back-and-forth evolutionary tug of war
Sexual selection and female “choice”
male, w/
Fiddler crabs
Males have traits important in mating displays (color, tail, claws)
whereas females lack such ornamentation
Sexual selection and female “choice”
Moller (1988) studied female choice in barn swallows
- males have long tails
- hypothesis: females prefer males with longer tails
- hence, sexual selection has driven the evolution of this
male display trait
Sexual selection and female “choice”
Data from Moller (1988):
distribution of tail length in males
Sexual selection and female “choice”
manipulate tail length of 44 males, divided into 4 groups of 11
 Shortened tail: clip 2 cm out of tail feathers
 Control 1: clipped 2 cm off, glued back on (control for gluing)
 Control 2: captured and banded birds, didn’t touch feathers
 Enlongated tail: glued the 2 cm cut off of shortened tails
onto the end of the tail of these birds
% second
Result: males with elongated tails:
(1) got mates the fastest
(2) frequently produced 2nd clutches
(3) had the most # of offspring
# of
% second
# of
Conclusion: females choose males based on tail length,
which is presumed to indicate the relative fitness of a male
Shortened tail treatment:
clip 2 cm out of tail feathers
What is a potential flaw in
this experiment?
Data from Andersson (1982):
female choice for tail length in the
Experiment: alter tail length by 25 cm
Result: elongated males had more
nests on their territories;
shortened males had fewer nests
Conclusion: male ornaments are
favored by female choice, and may
have evolved because of it
Male display vs. female resistance
Female Choice theory: males with big tails are advertising their
good genes/health, are therefore attractive to females
- may be genetic (carry good alleles)
- may be phenotypic (conditional: no parasites, healthy)
Run-away selection theory: male alleles super-stimulate
females with dramatic mating displays and exaggerated features
 females counter-adapt by reducing their interest
in the exaggerated trait
Run-away evolution of male display
Female attraction to a male trait that is absent
(pre-existing bias)
Mutation produces a rudimentary male display trait
Run-away evolution of male display
Female attraction to a male trait that is absent
(pre-existing bias)
Mutation produces a rudimentary male display trait
Female fitness
Male attraction
Run-away evolution of male display
Female fitness
Exaggeration of
male display
Male attraction
Male display vs. female resistance
Run-away evolution of male display features, to overwhelm
female disinterest (which increases every generation)
Eventually, males end up stuck with huge display features
that do nothing, but are necessary to get any attention at all
- females appear to win this evolutionary contest
Receiver bias in female choice
Evidence for pre-existing sensory bias (“receiver bias”):
(1) foraging cues:
- female stickleback fish prefer red objects when foraging,
also prefer males with red throats + bellies
- female bowerbirds are attracted to food objects that match
the color of objects males use in their displays
(2) female sailfin mollies prefer males with more surface area
(3) predator avoidance: female wax moths avoid bat sonar, but
are attracted to higher frequency vibrations from male moths
(4) general attraction towards symmetry
Female choice based on fitness cues
Presumption of many studies: all exaggerated traits somehow
convey to females which males are the most fit
- may or may not: some traits overwhelm female sensory
systems, promoting bad mating decisions and not
indicating male fitness
- other traits may be honest indicators of male fitness, and
females that prefer these traits will produce fitter offspring
- combination: flaws in big display traits may indicate a
male’s lack of resistance to parasites, so may tell a female
something useful
Female choice based on fitness cues 1
Female grey tree frogs prefer males who produce longer, faster
mating calls: Do these males have higher fitness?
Experiment: fertilize eggs with sperm from long-calling vs.
short-calling males
- fitness of offspring was then judged by 5 criteria:
how fast offspring grew and matured, their size, survival
Results: longer-calling males fathered offspring that were the
same or more fit than kids of short-calling males
Fitness cues 2: male choice of females
Female baboons produce sexual swellings during ovulation
- also advertize her reproductive value, as predicted by theory
of “honest signaling”
- females with larger swellings attained reproductive maturity at
at younger age, had more offspring, and more surviving kids
Result: males expend more effort fighting over, and spend
more time grooming, females with larger swellings
Domb & Pagel 2001, Nature 410:204-206
Female choice can operate after mating
- female birds can be coerced into mating by “inferior” (socially
less dominant) males
- however, when this happens, they can lure dominant
competitors to displace sperm of their rivals
- can also selectively eject sperm of inferior males
- female ducks lay larger eggs after mating with preferred
- no effects of paternity on offspring condition