What defines males and females?
Gamete size.
Males produce many small gametes
(sperm).
Females produce a few large gametes
(eggs).
How did dichotomy come about?
In distant ancestor gametes probably
were equal sized.
Shed into water and joined together to form
zygote.
Any small fast gamete would have had a
selective advantage in seeking out others.
Expect sperm to evolve.
Once sperm evolved, large egg is only other
logical strategy. Small or medium-sized
eggs provide too little nutrient for zygote
When fertilized by a tiny sperm.
Hence, get bimodal distribution of very
small and very large gametes.
Hamster egg
and sperm
Because of the difference in parental
investment of two sexes, females (in
general) are the choosy sex.
Parental investment: any effort (nutrients,
time, energy) invested in producing and
rearing young.
Females can produce only a few
offspring because they must invest
heavily in each.
This fundamental difference dictates basic
patterns of male and female reproductive
behavior.
In general, females increase reproductive
success by investing heavily in the offspring
and by mating with the best quality males.
Kiwi and
egg.
Female ruff assessing male
Males increase their reproductive success
by increasing their number of matings by
competing for access to females.
The greater the difference in parental
investment by the two sexes, the more
intense the competition among members of
the sex that invests the least (generally
males) for access to the opposite sex.
In many species, males
contribute little or
nothing to rearing the
young.
In these cases, females
choose best male
genes or allow males
to fight for access.
Male Raggiana Bird
of Paradise displaying.
In such high-variance systems, a few males
obtain almost all matings and most
males get none.
Male and female Sage Grouse
When both sexes invest more evenly in rearing
offspring, male and female reproductive
success is more evenly balanced.
In some cases, males are the choosy sex.
In these cases, males invest more or have the
lower reproductive potential.
Male pipefish carry eggs and young in a
brood pouch and provide nutrients and oxygen
for several weeks.
In that time female can produce enough eggs
for 2 males.
Male brood space is limited and hence
males are the choosy sex.
Males select females based on their size
and the size of their ventral fold.
Mormon crickets
A male Mormon cricket (a type of katydid)
transfers a large edible spermatophore to a
female when he mates.
Spermatophore is 25% of males body mass.
Hence, males can probably only mate once, but
females can mate more often because energy
from spermatophores used to make eggs.
Spermatophore
Female Mormon cricket eating spermatophore
When Mormon crickets swarm, males have
access to multiple females and are choosy.
Select heavier females because these
can produce more eggs.
A 3.2 gram female produces 50% fewer eggs
than a 3.5g female.
Mean sizes
of rejected
and accepted
female
Mormon
crickets
Which sex is the choosier can vary seasonally.
In an Australian katydid pollen supplies vary
seasonally.
When pollen scarce spermatophores are hard
to make and males are choosy.
When pollen is abundant, females become
the choosy sex.
Sexual Selection
The process that determines whose genes
get into the next generation is:
Natural Selection.
Many factors affect an individuals fitness
i.e., the number of copies of its genes.
it gets into the next generation.
Include: foraging ability, ability
to avoid predators, and ability to obtain
mates
Darwin proposed that elaborate display
structures (e.g. colorful plumage) and
weapons (e.g. antlers), exist only because
they contribute to this last component of
natural selection.
They increase an individual’s ability to
obtain mates.
Component of natural selection
exclusively concerned with obtaining
mates is called sexual selection.
Elaborate plumage increases reproductive
success, but also increases mortality.
There is, therefore, a trade-off between
survival and reproductive success.
Two components of sexual selection
Male-male Competition (not always
male:male, but non-choosy sex :
non-choosy sex competition
doesn’t trip off the tongue)
Female Choice (preference
by females for certain physical
characteristics in males).
Male-male Competition
Contests between males to hold harems
are common in mammals e.g. deer,
lions, antelope, elephant seals.
Males that dominate other males can
secure harems of females and obtain
exclusive mating access to them.
Bull elk and harem
Strong relationship between fighting success
and reproductive success.
Southern Elephant Seals
Northern Elephant Seals
Competition between males has led to
extreme sexual dimorphism when males
can potentially control large harems.
Male and
female fur
seals
In seals there is a strong relationship
between harem size and relative sizes
of males and females.
In harbor seals, harems are small and
sexes similar in size.
In elephant seals, harems are large and
males much larger than females.
Male-male competition among
marine iguanas.

Natural selection acts strongly on body size of
male marine iguanas on Galapagos Islands.
 Intermediate size males survive better than
larger or smaller males.
 Reason is that a large body is expensive to
maintain and obtaining enough food can be
difficult, even though large iguanas can harvest
more food.
Male-male competition among
marine iguanas.
 Maximum
male body size consistently
exceeds the body weight that can be
sustained, but female body weights do not.
Asterisks indicate maximum body sizes that iguanas could
maintain successfully in each of two study years.
Male-male competition among
marine iguanas.
 Why
is male body size larger than we
would predict based on maximizing
survival?
Male-male competition among
marine iguanas.
 Female
iguanas lay one clutch of eggs per
year and mate only once.
 Females
invest as much as 20% of their
body mass in a clutch, so they invest
much more than males. Males compete to
fertilize females.
Male-male competition among
marine iguanas.
 Male
iguanas stake out territories on rocks
where females bask between feeding
bouts and fight other males to defend their
territories.
 Territory holding males much more
attractive to females.
 Male mating success strongly related to
his ability to hold and defend a territory
that females like to use.
Territories of numbered male marine iguanas.
Male-male competition among
marine iguanas.
 Territories
held by males 65 and 59 were
strongly preferred by females for basking.
 Male 59 was the largest male in the colony
and to claim the territory had to eject 4
other males.
Male-male competition among
marine iguanas.
 Male
59 had more than four times as
many copulations as any other male in the
colony.
 For
the colony as a whole mean body size
of males who got to copulate was
significantly larger than mean body size of
all males who tried to copulate.
Male-male competition among
marine iguanas.
 Because
body size is heritable and
confers such a huge advantage in mating,
male marine iguanas male marine iguanas
have been sexually selected to have large
body size despite the survival costs.
Other strategies besides
dominance
 Not
every male can be a dominant
male, so frequently males must use
other approaches to obtain mates.
Subordinate strategies among
baboons
 Baboon
troops have dominance
hierarchy, but subordinate males form
friendships with females and protect
the female and her offspring.
 Females
often mate with these males
rather than the dominant male.
Low ranking male baboons also may form
alliances against higher ranking males to
take over oestrus females.
Satellite males

Some males (satellites) attempt to
intercept females moving towards
males that are calling or defending a
resource valuable to females.
Great Plains Toad
and Bighorn Sheep
satellite males.
Conditional mating strategies in Scorpionflies.
Conditional mating strategies in
Scorpionflies
Male Scorpionflies use one of three mating
strategies.
I.
Defend dead insects as food resources
for females.
II. Offer saliva as food gift to females.
III. Intercept females and force copulations.
Conditional mating strategies in
Scorpionflies
 Randy
Thornhill placed 10 males and 10
female Scorpionflies in a cage with 2 dead
crickets.
Conditional mating strategies in
Scorpionflies
 Two
males controlled crickets: averaged
6 copulations.
 Saliva
gift males got about 2 copulations
each.
 Forced
copulation males (couldn’t make
gifts) males achieved about 1 copulation
each.
Conditional mating strategies in
Scorpionflies
 If
different strategies are genetically
determined, expect less successful to
disappear.
 Why
are they all retained in population?
Conditional mating strategies in
Scorpionflies
 Because
Scorpionflies play strategies
conditionally.
 They
do.
play the role that is the best they can
Conditional mating strategies in
Scorpionflies
 When
the two dominant males were
removed from the cage, other males
switched their strategies.
 Two
 Two
saliva gift males took over crickets.
interceptors took over abandoned
saliva gifts.
Pure mating strategies in
crustaceans
 Paracercis
sculpta lives in intertidal
sponges.
 Three
 Each
distinct male types exist.
has a different mating strategy.
Pure mating strategies in
crustaceans
 Alpha:
Large. Exclude other males from
sponge.
 Only
one alpha per sponge.
Pure mating strategies in
crustaceans
 Beta:
Medium sized. When encountered
by alpha pretends to be a female and
mates. Sneaks matings from female.
 Gamma:
Tiny. Alphas throw gammas out
of sponge. Sneak matings from females.
Paracercis sculpta
males
Pure mating strategies in
crustaceans
 Are
 If
strategies pure or conditional?
pure: should have (i) genetic basis and
(ii) mean reproductive success of strategies
should be equal.
Pure mating strategies in
crustaceans
 If
strategies conditional: (i) strategies
should be environmentally induced.
 (ii)
mean reproductive success of
strategies need not be equal.
Pure mating strategies in
crustaceans
 Payoffs
to the different male types appear
to be equal which suggests the strategies
are pure rather than conditional.
Sperm Competition
Females commonly mate with more then
one male.
Males attempt to maximize their chances
of fertilizing eggs through sperm
competition.
Sperm competition in damselflies.
Male damselflies defend territories in which
females lay eggs.
Female lays eggs in several territories and
mates with each resident male.
Male
Female
Mating damselflies
Female swings abdomen under male and
male copulates.
Male has highly specialized penis that he
uses like a scrubbing brush to remove
sperm from previous males.
Spines on penis and lateral horns that are
covered with spiny-like hairs snag sperm
and remove them.
When previous males sperm removed
(up to 90-100%) male inserts own sperm.
Damselfly penis showing lateral horn
Spines
Lateral horn close up
Sperm caught on
spiny hairs.
Other sperm competition strategies are
widespread.
A shark’s penis has two tubes, one of
which can spray seawater and may
flush sperm out of female.
Male Dunnocks peck a females cloaca
before mating to make her release sperm
Mate Guarding
Males also attempt to ensure their paternity
by preventing other males from mating
with a female.

Mate Guarding
in dunnocks

Benefits of guarding must be weighed
against cost of lost mating opportunities
for the male.
Under what conditions should you expect
guarding behavior to pay off?
Expect mate guarding when:
Female is likely to mate again and use
sperm of most recent partner to fertilize
eggs.
When females are scarce, so chance of
finding another female is low.
Mate guarding should be most intense when
female can be fertilized.
In white-fronted bee-eaters females can be
fertilized only in 4-5 day period during egg
laying.
During this period males mate frequently
With their females and guard them almost
continuously.
Other males also harass females and try to
copulate with them.
Timing of
bee-eater
mate
guarding.
Female choice
Female choice based on
(i) Material benefits obtained from male
(ii) Male attractiveness and display
Material Benefits
Many females require a nuptial gift from
males before they will mate.
The nuptial gift enhances the females
reproductive success by e.g.
enhancing her ability to make eggs.
Spermatophore benefits to female katydids.
Upper line with spermatophore.
Male Gluphisia moths provide
Spermatophores that are high in
sodium ions.
Sodium stored in eggs.
Sodium is valuable to larvae which
eat low sodium food.
In many species females allow males to
copulate only while she is still consuming
the gift.
Gift size is, thus, very important to male
reproductive success
Ultimate nuptial gift some males provide
is allowing themselves to be eaten by female
Occurs in praying mantises and spiders.
Female choice based on male attractiveness
Long-tailed
widowbirds are
African
savannah birds
have tails that are up
to 0.5 m long,
which they display
to females.
Malte Andersson: Tail length crucial factor
in attractiveness of male widowbirds to
females.
Altered tail lengths of males. Observed
effects on mating success.
Four treatments:
Tail shortened
Tail lengthened
Controls
Tail cut and re-glued same length
Tail not cut.
Success in attracting mates before treatments
Success in attracting mates after treatments
Extrinsic displays of bowerbirds.
Bowerbirds are large songbirds that
occur in New Guinea and Australia.
Males construct elaborate display
structures called bowers.
McGregor’s Bowerbird
Bowers are made of grass or sticks
and decorated with brightly colored
objects.
Two major styles of bower:
Maypole bowers and
Avenue bowers.
Builders of each bower type more closely
related to each other than to builders of
other type.
McGregor’s
bowerbird and
its simple
maypole bower
McGregor’s
Bowerbird
maypole bower
Satin bowerbird at its avenue bower
In bowerbirds the stimulus value of
bowers appears to have somehow
replaced the stimulus of bright plumage.
In general, bowerbirds
with most elaborate
plumage build least
elaborate bowers.
Males maintain their bowers and
continually replace wilted or damaged
material.
Males also frequently steal items from
each other.
The bower is used only for mating.
It is not used as a nest.
Satin Bowerbird
Studies by Gerald Borgia.
Male Satin Bowerbird, which has bright
blue eyes decorates its bower with
anything blue it can find.
Items include: trash, glass, paper,
feathers, pottery, etc.
Male Satin bowerbird at bower.
Male Satin Bowerbirds destroy other
males bowers and steal items whenever
they can.
Dominant males are better able to defend
their bowers and have more time to
destroy other males bowers.
Ability and status of male directly
related to quality of his bower.
Females prefer males with well-made,
well-decorated bowers.
Females visit an average of 3.6 bowers
before mating.
Satin Bowerbird
In Borgia’s studies 5 of 22 males
accounted for 56% of 21 copulations.
These males had the most blue parrot
feathers, shells, and leaves as decorations.
Also had best built bowers in terms
of symmetry, stick size, and construction.
In experiments, males whose decorations
were removed obtained fewer matings.
Explanations for female choice
Good Genes versus Fashion Icons
Good genes ideas propose that
exaggerated male plumage signals
genetic or physiological superiority.
Zahavi’s handicap principle: Elaborate
plumage handicaps a male, for example
by being expensive to maintain or by
making him more obvious to predators.
A males’ ability to
tolerate such a
handicap
demonstrates
his quality to
potential mates.
An alternative good genes idea is the
Hamilton and Zuk hypothesis that
ornamental plumage signals resistance
to diseases and parasites.
Females are able to evaluate resistance
by assessing a male’s coloration and
display.
W.D. Hamilton and Marlene Zuk carried
out experiments with red junglefowl.
Hens prefer males with large fleshy combs.
Combs affected by testosterone which is
related to physical condition.
Males artificially infected with parasitic
nematodes have smaller combs and are
less favored by hens.
Testing the Hamilton-Zuk hypothesis
It is not enough to show only that females
prefer males with lower parasites burdens.
There are 4 key assumptions of the
hypothesis.
Assumptions of the Hamilton-Zuk
hypothesis
(i) Parasites reduce host fitness.
(ii) Parasite resistance is genetic.
(iii) Parasite resistance is signalled by
elaboration of sexual ornaments.
(iv) Females prefer males with the most
elaborate signals.
Anders Pape Møller tested the
Hamilton-Zuk hypothesis with
experiments on Barn Swallows.
Barn Swallows are
monogamous. Males
have elongated outer
tail feathers which
they display to
females
Møller first showed that females prefer
males with longer tails.
Males with artificially lengthened tails
were more attractive to females and
obtained more EPC’s and were able to
obtain mates quicker.
Bloodsucking mites are common nest
parasites of swallows.
Nestlings in nests with high parasite loads
are lighter and have lower survival rates.
There is a large amount of variation in
the level of resistance that chicks show
to mites.
Møller switched chicks between nests.
Nestling resistance to parasites was
correlated with that of its parents even
when the chick was reared in another nest.
Therefore, parasite resistance is partially
genetically determined.
Parents with longer tails had young with
lower parasite burdens regardless of
the nest the chicks were reared in.
Strongest relationship between male tail
length and offspring parasite load.
Conclusion: tail length indicates resistance
to parasites and this resistance is heritable.
Fashion Icon Hypotheses
These hypotheses suggest that the elaborate
plumage of male birds is a result of runaway
preferences by females for particular colors or
plumage patterns.
The idea behind runaway preferences is
that females may have innate preferences
for certain colors or plumages.
There need be no “reason” for this.
If such preferences are genetically based they
can be selected for.
A preference may spread at first passively in
the population, but then spread more rapidly as
it becomes more common and if there is a
linkage between the genes for the female
preference and the male character being
selected for.
Nancy Burley has shown that Zebra Finches
have innate preferences for certain colors.
Male Zebra finches given red leg bands are
more attractive to females than males with
green leg bands.
Also, males given artificial “hats” that are red
or white become more attractive, but males
given yellow, blue, or green hats become less
attractive.
Male Zebra finches also make choices and
prefer females with black leg bands and
reject females with red bands.
Thus, mating is assortative because the
“best” males mate with the “best” females.
The good genes and fashion icons
hypotheses may both be relevant to
the evolution of brightly colored
male plumage and the effects may
reinforce each other.