Sexual Selection
• Sex and the Marine Environment
What is sexual selection?
Natural Selection:
Differential reproductive success due to
variation among individuals in survival and
reproduction.
What is sexual selection?
Darwin (1871) defined sexual selection as:
Differential reproductive success due to
variation among individuals in success at
getting mates.
Sexual selection is non-random
variance in reproductive success.
 Two forms of sexual selection:
 Intrasexual selection: direct competition for
mates between members of the same sex,
usually male-male competition.
 Intersexual selection: differences in
attractiveness to the opposite sex, usually
non-random mate choice by females.
Mating systems
Monogamy One male mates exclusively with one female
Polygamy Individuals mate with more than one partner
Polygyny Some males mate with more than one female
Polyandry Some females mate with more than one male
Promiscuity Males mate with more than 1 female and vice versa
Monogamy
Polygyny
Promiscuity
Mating systems
Mating systems influence sexual selection
Strength of sexual selection
Variance in mating success
All males have
same mating
success = 1 mate
Some males = 0 mates
Some males = 1 mate
Some males = 2 mates
Most males = 0 mates
One male = 8 mates
Monogamy
Moderate Polygyny
Strong Polygyny
Harem polygyny
Elephant seals (Mirounga) - breeding females cluster together on beaches
- allows males to defend a harem of many females at once
Male elephant seals weigh up to 3x more than females!
Male reproductive success is highly variable:
8 individual males inseminated 348 females in one study!
Bull male elephant seals engage
in violent, bloody fights over
females – large size confers an
advantage in male combat
Polyandry
When is polyandry favored?
When males become limiting resource for reproduction
This often occurs when low offspring survival requires
male parental care, so that males have the greater
Reproductive effort and lower reproductive rate
What is the consequence?
Sexual selection is stronger on females than males
Saddleback Tamarins,
Spotted Sandpipers,
and Red-necked
Phalaropes are
examples of polyandrous
species
The form of Sexual Selection is directly related to
the relative investment in offspring production.
Bateman’s principle: The sex that invests more in
offspring production has fewer reproductive
opportunities.
Thus, the sex that has a higher investment
 Should be more discriminating (choosier).
 Become a limiting resource for the opposite sex.
 This is usually the female sex
ANISOGAMY
FEMALES: Sex that produces relatively
few, large well-provisioned gametes
(eggs)
MALES: Sex that produces many,
“cheap” gametes (sperm)
Females are more invested
• Females also often invest more than
males in other forms of care
– Females bare the burden of pregnancy
– Care for offspring
• Only in fish is male care more common than
female care
– Males guard the eggs in their territory so they can also
secure multiple mates
Why don’t males care for
offspring more often?
• Females can sometimes mate multiply
(polyandry)
– Therefore, not all offspring may be sired by
the male
Why don’t males care for
offspring more often?
• Females can sometimes mate multiply
(polyandry)
– Therefore, not all offspring may be sired by
the male (paternity sharing)
• Opportunity costs
– A male’s mating success is more driven by
how many pairings he can achieve
• Especially because females are choosy
Why do females invest more in
offspring care?
• If females invest more prior to mating, there is
competition amongst males for female
investment
• This causes variability amongst male success
• As long as females continue to provide care, it is
less likely males will contribute (a positive
feedback loop)
The form of Sexual Selection is directly related to
the relative investment in offspring production.
Bateman’s principle: The sex that invests more in
offspring production has fewer reproductive
opportunities.
Thus, the sex that has a higher investment
 Should be more discriminating (choosier).
 Become a limiting resource for the opposite sex.
 Evolution of gender and sex resources
 This is usually the female sex
The asymmetric nature of sexual selection
often leads to dramatic sexual dimorphism in
characters directly related to male-male
competition and/or female choice.
Peacock
Peahen
Sexual dimorphism
Selection for combat in males
• Natural selection selects for male
morphologies that make them more
competitive
– Larger body size
– Greater strength
– Weaponry
Selection in Males: Body Size
• Larger male copepods
can carry larger
spermatophores
• Mating with large males
allows females to fertilize
more eggs and reduces
the need for frequent
matings
Weaponry
Fiddlers!
Weaponry
• Narwhals!
And in terrestrial birds
• Some really awesome courtship behaviors
• Marine birds
 Sexual selection can be
very strong and often
opposes natural
selection.
 This can lead to
exaggerated and
sometimes maladaptive
development of male
traits.
Irish Elk
(Megaloceros giganteus)
Sexual vs natural selection
Guppies (Poecilia) - sexual selection can favor traits that reduce survival
- laboratory selection studies by John Endler
Natural selection (predation) favors
spot patterns that match background
Sexual selection (mate attraction) favors
male patterns that contrast background
Intra- and inter-sexual selection
Intrasexual selection
All of the examples thus far have involved interactions within a sex
Darwin’s theory of intra-sexual selection was readily accepted, even in his time
Intersexual selection
Intersexual selection has always been controversial
We know mate choice occurs because we can observe
it directly
However, it is debated if and how some sexually
dimorphic traits evolve in response to mate choice
Are females really choosy?
• Females can prefer
elaborate
ornaments
– Long tailed
widowbirds
Why are females choosy?
• Two broad benefits:
• Good resources
– Competitive males often secure the best
territories
• Good genes
– Body size and other competitive characteristics
will be inherited by the females offspring
– Humback whale size and mating calls
But what are the genetic benefits?
• 1) Fisher’s hypothesis: gaining attractive sons
– Females want sexy sons to attract more mates in
the future
– Often produces runaway sexual selection
– Usually begins with a trait that confers some
adaptive benefit and then is further exaggerated
because it is selected for by females
Fisher’s runaway model
Survival Selection
Sexual Selection
Total male fitness
(survival + mating)
Fitness
Female choice
adaptive for survival
Fitness due to survival
Tail length
But what are the genetic benefits?
• 1) Fisher’s hypothesis: gaining attractive sons
– Females want sexy sons to attract more mates in
the future
– Often produces runaway sexual selection
– Usually begins with a trait that confers some
adaptive benefit and then is further exaggerated
because it is selected for by females
– 2) Handicap hypothesis
GOOD GENES MODEL
ELABORATED MALE TRAITS MAY BE
INDICATORS OF HERITABLE GENETIC
QUALITY (I.E. FITNESS).
The Handicap Principle (Zahavi 1975)
 Some males may have a heritable trait that reduces
viability.
 Only males with “Good Genes” can survive despite the
handicap.
 Females that mate with these males will have offspring
with higher fitness.
GOOD GENES MODEL
ELABORATED MALE TRAITS MAY BE
INDICATORS OF HERITABLE GENETIC
QUALITY (I.E. FITNESS).
Hamilton and Zuk added to this hypothesis:
- Sexual displays are reliable indicators for genetic
resistance to disease or parasites
- Advertisements of healthy animals
Evidence of females choosing
for sexy sons
• Females prefer
brighter red colors
– And the intensity of
red in sons is
correlated with the
red in his father
• But is the only benefit
of this trait increased
matings?
Males with bright red color
should not show increased
viability, just increased
mating success
Evidence of females choosing
for good genes
• Reddish color is derived
from carotenoids in a fish
diet
– Better color, better foraging,
• Even subordinate males
express this color early in
the mating season, but only
more fit males are able to
maintain it though the
breeding season
Healthier male fish are more red
• Red coloration
declines in males
infected with
parasites
• Healthy red males
were also likely to
have a strong MHC
profile
Genetic Benefits
• 1) Sexy sons and daughters
– Only the sexy are selected
• 2) Good genes as fitness indicators
– Only good quality males that can afford elaborate
ornaments can survive
• But then how is variation maintained in a
population?
• Mutations
• Traits are quantitative (many genes are responsible)
• Variation in choice over time
Female behavior can also help to
maintain genetic variation
• Female copying- females imitate the
choices of other females
– 3 morphed isopod
• Female isopods prefer to mate with other females
and can copy matings of other females
• This may account for the maintenance of the 3
male morphotypes
Sex Role Reversal
• When males make large contributions to
parental investment, males may become the
choosy ones
• Seahorse mating
Sex Role Reversal
• When males make large contributions to
parental investment, males may become the
choosy ones
Sexual Conflict Over Mating
• If males need to maximize matings and
females need to choose the best partner
– Then females need to resist mating and males
need to be aggressive
• Results in a sexual selection arms race
• Males competing for a female sea turtle
Sexual Conflict Over Mating
Male Traits
Enforced Copulation
Female Traits
Resistance
Intromittant organs which
enhance mating success
Elaborate reproductive tracts
that are obstacle courses for
sperm
Mate guarding, frequent
copulation
Remove or displace sperm of
rival males
Seek extra pair copulation
Sperm Ejection
Copulatory plugs and ant—
Sperm choice
aphrodisiacs
Accessory glands to manipulate Chemical defense
females
Sexual Conflict Over Mating
• The male guppy grows claws on
its gonopodium (anal fin) to
counteract sexually-selective females.
Chase Away Sexual Selection
• This sexual conflict can drive antagonistic coevolution between the sexes
• Females will evolve resistance to male ploys
because acquiescence is costly
Sexual Conflict
• Selection has favored
Littorina saxatilis female
snails that do not signal their
gender in their slime trails