Chapter 11.2: 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.
 Peacock male is brightly c0lored with an enormous tail
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.
 The goal of natural selection is to maximize
reproductive success not life span. An individual that
dies young but produces many babies is more
successful (from the point of view of natural selection)
than one who lives a long time but produces fewer
offspring.
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 is the key factor in determining
which sex will be the choosier.
 Parental investment: includes all of the energy and
time expended on rearing an offspring.
 Parents also take on a lot of risk e.g. in defending
offspring from predators.
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.
 Because females invest more in the offspring they are
picky about which males they mate with.
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.
 Because they must compete for matings males will as
we shall see be selected to either fight to control
females (male-male competition) or to be very
attractive to them (female choice).
Examples of investment
differences
 Rough-skinned newts: males compete to mate with
females at ponds. Females visit pond, mate then leave.
 Females expend a lot of energy in producing eggs and
so are picky about which males they choose to mate
with. Males only contribute sperm and will mate with
any female given the opportunity.
 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.
 This pattern is true in many species. Males have
highly variable reproductive success. Most males fail to
mate, but a few are very successful.
 Females have less variance in reproductive success.
Essentially all females are able to mate, but there is not
much difference between females in how many babies
they have.
 For example, in bitterling fish (next slide)
 However, it is not always males that have the greatest
variation in reproductive success.
 For example, in seahorses and pipefish males have less
variance in reproductive success because they care for
the babies and females have to compete for a limited
supply of childcare.
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 and higher
variance in reproductive success [some individuals
secure many mates, most males obtain few or none.]
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. Females choose only the
most attractive males to mate with and reject most
males.
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.
The bigger stronger males have higher rank and they mate more often.
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.
The bigger the prize (in terms of mating opportunities) the more
strongly selection has favored the evolution of bigger males.
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 (seaweed is a relatively low quality food),
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.
 Bigger males are common even though their life
expectancy is reduced.
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.
 Remember fertilization, not mating is the ultimate
goal.
 In many animals (including even humans, but very,
very rarely) a female may give birth at one time to
multiple offspring and these may have different
fathers.
Sperm competition
 What factors influence success in sperm competition?
 Number of sperm produced. (lottery analogy).
 The more tickets (sperm) that an individual has in the
lottery the better chance of winning (i.e. fertilizing the
egg)
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).
 Increasing swimming speed of sperm. Faster sperm
can get to the egg quicker and sometimes sperm
collaborate to swim faster.
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.
They 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 of short calling males never did better.
 Thus, females who chose males who gave longer calls
received better quality sperm and produced higher
quality offspring.
Choosy females acquire
resources
 In many species males provide resources that benefit
female and her young. Resources can take many forms.
It could be food or it could access to a territory, which
contains areas to feed, and safety from predators or
safe nesting nests
 Females that can choose better providers reap a direct
benefit in resources 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.)
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