Chapter 10: Sexual selection.
 Males
and females often are strikingly
different in size and appearance (sexual
dimorphism).
 E.g
long-tailed widowbirds male is black
with long tail, female is dull brown.
Sexual dimorphism
 Less
extreme sexual dimorphism occurs in
humans.
 Males
about 10% taller on average.
Sexual dimorphism
 Why
does sexual dimorphism occur?
 Many
of the traits seen in the showier sex
seem likely to reduce prospects of
survival.
 Evolution
by natural selection cannot
explain showy traits.
Sexual dimorphism
 Charles
Darwin suggested sex provided a
solution.
 If
traits increase mating opportunities then
this could more than compensate for
reduced survival.
Sexual selection
 Sexual
selection: differential reproductive
success due to variation among
individuals in obtaining mates.
Amount of parental investment
 Differences
in amount of parental
investment by members of each sex are
key in determining which sex will be the
choosier.
 Parental investment: energy and time
expended on offspring.
Amount of parental investment
 In
general, mothers invest more heavily in
offspring than fathers.
 In
90% of mammals, females provide
substantial parental care and males little
or none.
Amount of parental investment
 In
general, because of difference between
sexes in investment, a female’s lifetime
reproductive success will be limited by the
number of young she can rear.
 In
contrast, a males will be limited by the
number of matings he can obtain.
Amount of parental investment
 This
disparity suggests sexual selection
likely to be a more powerful influence on
evolution of males than on females.
Examples of investment
differences
 Rough-skinned
newts: males compete to
mate with females at ponds. Females visit
pond, mate then leave.
 Jones
et al. (2002) sampled all males and
females and used molecular analyses to
assign paternity to all offspring.
Note scales on y-axes are not the same for males and females
Rough-skinned newts
 Most
males failed to mate and there was
much greater variation in male mating
success. In contrast, all females mated at
least once.
 Most
males fathered no young and a few
males fathered almost all of them. All
females reproduced.
Broad-nosed pipefish
 In
pipefish and their close relatives the
seahorses males provide all the parental
care.
 Male
has a brood pouch in which females
lays eggs. Male tends eggs until they
hatch.
Broad-nosed pipefish
 In
this species females compete for
access to males and access to their
pouches.
 As
a result, males have less variance in
mating success than females
Note scales on y-axes are not the same for males and females
Broad-nosed pipefish
 Based
on the preceding data we can
conclude that the sex that invests more
should be the choosy sex.
 Conversely, in the sex that invests less
there should be intense competition to
mate.
Forms of sexual selection
 Two
ways in which process of sexual
selection may develop.
 Males may fight among themselves to
control a resource important to females or
to control a group of females. Male-male
competition.
 Males may advertise for females by
displaying or singing: Female choice.
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.
Sperm competition
 Male-male
competition may continue
even after mating is over.
 Fertilization,
 In
not mating is the goal.
many animals (including humans, but
rarely) a female may produce a brood
fathered by more than one male.
Sperm competition
 What
factors influence success in sperm
competition?
 Number
of sperm produced. (lottery
analogy).
Sperm competition
 Gage
(1991) tested idea that males might
adjust number of sperm adjusted
depending on risk of sperm competition.
 Experimental
male Mediterranean fruit
flies reared either alone or with another
male. Then allowed experimental male to
mate with a female.
Sperm competition
 Males
mating in presence of another male
produced 2.5X as many sperm as males
reared alone and mating in absence of
potential competitor.
Sperm competition
 Other
male strategies for success include:
mate guarding in which males deter other
males from copulating.
 Blocking female genital opening with a
plug.
 Removing other males’ sperm from female
(male damselflies use hooked penis to
scoop out sperm).
Figure 10.27 from Animal Behavior text
Damselfly penis (note spines for extracting sperm).
Infanticide
 In
some animals infanticide is practiced as
a way to enhance mating success.
 In
lions males that take over a pride kill all
the cubs to bring females back into estrus.
Otherwise they would have to wait to mate
and males usually hold prides for only a
couple of years.
Infanticide
 Female
jacanas (a long-toed bird that can
walk on water lilies) also practice
infanticide. Female jacanas defend
territories and lay eggs for multiple males.
 If a female loses her territory, new female
kills any young or destroys eggs to free up
males to tend her young.
Female Choice
 In
many species males cannot monopolize
females and males must advertise for
mates.
 Females
inspect multiple males before
choosing one.
Female Choice
 Females
evaluate large number of
possible male traits: display, song
repertoire, physical appearance.
Female choice in Barn
Swallows
 Male
Barn Swallows have longer tail
streamers than females.
 Males display their tails from small display
territories and while flying.
 Female visits several males before
choosing one to nest with. Male assists
with feeding of young
Female choice in Barn
Swallows
 If
males and females cooperate in rearing
young why would there be sexual
selection?
 Males
might be able to secure extra-pair
copulations (EPCs).
 Anders Pape Moller tested hypothesis that
tail length is a sexually selected trait and
that females prefer males with longer tails.
Female choice in Barn
Swallows
 Moller
monitored male mating success
after manipulating tail length.
 Four
•
•
•
•
groups in experiment.
Shortened tail feathers
Tail cut, but glued back (control I)
Tail uncut (control II)
Elongated tail feathers
Female choice in Barn
Swallows
 Elongated
males attracted mates more
quickly and had greater mating success.
 Females
mated to short-tailed males were
significantly more likely to seek extra-pair
copulations from males than females
mated to males with longer tails.
Mate choice in Gray Tree Frogs
 Males
call to attract females. Males differ
in the length of their call (number of trills
given).
 Gerhardt
et al. tested hypothesis that
females prefer longer calls.
Mate choice in Gray Tree Frogs
 Playback
experiment. Different length
calls given from pair of loudspeakers.
 Female
frogs allowed to choose between
speakers.
Mate choice in Gray Tree Frogs
 75%
of females preferred long calls
regardless of volume.
 72%
of females also went past closer
speaker giving short call to approach
distant speaker giving longer call.
Mate choice in Gray Tree Frogs
 Also
quantified female preferences for
different calls precisely by comparing them
to a standard length call.
 Females
strongly discriminated against
short calls. Consistently preferred
increasingly long calls.
(Y-axis should read phonotaxis score not phototaxis score)
Female choice
 Both
female barn swallows and gray tree
frogs actively choose mates.
 Why
do they do so?
Females get better genes
 One
explanation is that choosy females
mate with males with better genes.
 Welch
et al. examined genetic superiority
hypothesis.
Females get better genes
 Collected
eggs from gray tree frogs and
fertilized half with sperm from long-calling
and half with sperm from short-calling
males.
 Reared
half of young on generous diet,
others on restricted diet.
Females get better genes
 Measured
five aspects of offspring
performance:





larval growth
time to metamorphosis
mass at metamorphosis
larval survival
post-metamorphic growth
Females get better genes
 In
18 comparisons over two years of study
offspring of long-calling males performed
better in all 6 cases where there was a
statistically significant difference.
 Offspring
better.
of short calling males never did
Choosy females acquire
resources
 In
many species males provide resources
that benefit female and her young.
 Females
that can choose better providers
reap a direct benefit in food provided.
Hangingflies
 Randy
Thornhill studied hangingflies.
 When
a male hangingfly catches an insect
it releases a pheromone to attract a
female.
 Male
offers prey to female. If she accepts,
they mate while she feeds.
Hangingflies
 The
larger the prey the longer the female
eats and the more sperm the male
transfers. After 20 minutes male has
transferred the maximal quantity of sperm.
Hangingflies
 Male
must offer a meal that lasts at least 5
minutes or no sperm are transferred.
 If
meal lasts less than 20 minutes female
breaks off copulation.
 At
20 minutes male breaks off copulation
and seeks another female to offer the
remains of the meal to.
(female rejected the food item)
Hangingflies
 Female
preference for males providing
large meals benefits her in two ways.
 1. Provides nutrients and energy that
allows her to lay more eggs.
 2. Saves her from having to hunt for
herself. Hunting is dangerous. (Males
twice as likely to be caught in spider
webs.)
Choosy females may have
preexisting sensory biases.
 Females
use sense organs for tasks other
than mate selection.
 Males
may exploit sensory biases of
females that make them particularly
responsive by incorporating these stimuli
in their displays.
Choosy females may have
preexisting sensory biases.
 Proctor
(1991,1992) work on water mites.
 Tiny freshwater animals that catch
copepods. Very poor vision. Depend on
smell and touch.
 Females hunt copepods by standing on
four hind legs with four forelegs spread in
net-like fashion.
Water mites
 Female
mite stands in position until she
detects vibrations then turns towards them
and clutches.
 Mating
does not involve copulation.
Instead male deposits a spermatophore
and tries to induce female to accept it.
Water mites
 When
male smells a female he
approaches female while lifting and
vibrating his front legs. Frequency of
vibrations similar to copepod frequency.
 Female turns towards male and clutches.
Male then fans legs which carries
pheromone towards female who may then
pick up the spermatophore.
Diversity in sex roles.
 In
a few organisms males invest more in
offspring than females do.
 In
pipefish male brood eggs in pouch.
Limiting resource is time because females
can produce eggs faster than males can
rear them.
Diversity in sex roles.
 Because
males invest more, expect they
should be choosy.
 In pipefish, Nerophis ophidion, females are
larger than males and have larger
skinfolds.
 In
paired-choice tests females did not
discriminate between different sized
males.
Diversity in sex roles.
 However,
males are choosy and prefer
larger females and those with larger skin
folds
Diversity in sex roles.
 In
another pipefish, Syngnathus typhle,
males and females are similar is size and
appearance.
 Females
can change color to emphasize
zig-zag pattern on sides.
Diversity in sex roles.
 Females
competing over males display
their dark colors.
 Females initiate courtship and mate more
readily than males.
 Males
are choosy. Males prefer females
showing fewer black spots (which indicate
parasitic infection)
Diversity in sex roles.
 Other






“sex-role reversed” species include:
jacanas
moorhens
phalaropes
spotted sandpipers
giant waterbugs
some katydids
Sexual selection in plants
 Male
flowers produce pollen. To fertilize a
female flower pollen must be transported
to it.
 Males
must attract pollinators to transport
their pollen to female flowers.
Sexual selection in plants
 Expect
male flowers to be more attractive
to pollinators than female flowers.
Sexual selection in plants
 Study
of wild radish plants found that
males with bright yellow flowers more
successful at attracting pollinators than
males with white flowers.
 Flower color did not affect female
reproductive success.
Sexual selection in plants
 In
herb Wurmbea dioica males make
larger flowers than females.
 Pollinated
by bees and Vaughton and
Ramsey (1998) found larger flowers were
visited more often than smaller flowers.