Sexual Selection
Males and females often are strikingly different in size and appearance (this is called sexual
dimorphism). E.g in long-tailed widowbirds the male is black with a long tail, while the female is dull
brown.
A peafowl male (peacock) is brightly colored with an enormous tail, whereas the female is dull in
appearance.
Less extreme sexual dimorphism occurs in humans. Human males are about 10% taller on average.
Why does sexual dimorphism occur?
Many of the traits seen in the showier sex seem likely to reduce prospects of survival. Thus, evolution
by natural selection apparently cannot explain such showy traits.
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: 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 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.
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. 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 male will be limited by the number of
times he can mate.
This disparity suggests sexual selection likely to be a more powerful influence on the 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
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 (below)
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.
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.
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.
Data for reproductive success in broad-nosed pipefish (below). Note scales on y-axes are not the same
for males and females
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.
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. There is a strong relationship between fighting success and reproductive success. The
bigger stronger males have higher rank and they mate more often.
Above data on mating success and rank in the Northern Elephant Seal
Competition between males has led to extreme sexual dimorphism when males can potentially control
large harems. 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. The reason for this 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.
Maximum male body size consistently exceeds the body weight that can be sustained, but female body
weights do not. Clearly big males are common even though their life expectancy is reduced.
Why is male body size larger than we would predict based on maximizing survival? Males aren’t
maximizing survival, they are maximizing their chances to reproduce.
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. Consequently, males compete to
fertilize females.
Asterisks indicate maximum body sizes that iguanas could maintain successfully in each of two study
years.
Male iguanas stake out territories on rocks where females bask between feeding bouts and fight other
males to defend their territories. Territory holding males are much more attractive to females than
other males. Male mating success strongly related to his ability to hold and defend a territory that
females like to use.
Diagram above shows territories of numbered male marine iguanas.
The 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 59 had more than
four times as many copulations as any other male in the colony.
For the colony as a whole, the mean body size of males who got to copulate was significantly larger than
mean body size of all males who tried to copulate.
Because body size is heritable and confers such a huge advantage in mating, male marine iguanas have
been sexually selected to have large body size despite the survival costs.
Sperm Competition
One might think that once a male has mated with a female he could stop worrying aboput competition,
but that is not true. In fact, male-male competition may continue even after mating is over.
Remember fertilization of eggs, 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.
What factors influence success in sperm competition? Produce more sperm. Number of sperm
produced. (lottery analogy). The more tickets (sperm) that an individual has in the lottery the better his
chance of winning (i.e. fertilizing the egg).
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 he allowed the experimental male to mate with a female. Males mating in the presence of
another male produced 2.5X as many sperm as males reared alone and mating in absence of potential
competitor.
Other male strategies for success in sperm competition 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.
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.
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, the 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. 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. 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 the hypothesis that tail length is a sexually selected trait and that females
prefer males with longer tails. 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
Males with elongated tails 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 frogs call to attract females. Males differ in the length of their call (number of trills given).
Gerhardt et al. tested the hypothesis that females prefer longer calls.
Carried out a playback experiment. Different length calls given from pair of loudspeakers. Female frogs
were allowed to choose between speakers.
75% of females preferred long calls regardless of volume.
72% of females also went past a closer speaker that was giving a short call to approach distant speaker
giving longer call.
Gerhard et al. 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)
Why are females choosy?
Two major reasons females are choosy –they obtain better genes and/or they obtain other resources
from males
Choosy females acquire better genes
Both female barn swallows and gray tree frogs actively choose mates. Why do they do so? One
explanation is that choosy females mate with males with better genes.
Testing the better genes hypothesis
Welch et al. examined the genetic superiority hypothesis. They collected eggs from gray tree frogs and
fertilized half of the eggs with sperm taken from long-calling and half with sperm taken from shortcalling males.
They then reared half of the young on a generous diet and the others on a restricted diet. Then they
measured five aspects of offspring performance:

larval growth

time to metamorphosis

mass at metamorphosis

larval survival

post-metamorphic growth
In 18 comparisons over two years of study the offspring of long-calling males performed better in all 6
cases where there was a statistically significant difference. The 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.
Experimental design for Welch et al. experiment.
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. Randy Thornhill
studied hangingflies.
When a male hangingfly catches an insect it releases a pheromone to attract a female. The male offers
prey to female. If she accepts, they mate while she feeds.
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.
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 (females aren’t romantic!).
At 20 minutes the male breaks off copulation and seeks another female to offer the remains of the meal
to (males aren’t romantic either!).
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 are 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, the male brood eggs in pouch. Limiting resource is time because females can produce eggs
faster than males can rear them. Because males invest more, we expect they should be choosy.
In the pipefish, Nerophis ophidion, females are larger than males and have larger skinfolds. In pairedchoice tests females did not discriminate between different sized males.
However, males are choosy and prefer larger females and those with larger skin folds
In another pipefish, Syngnathus typhle, males and females are similar in size and appearance. Females
can change color to emphasize zig-zag pattern on sides. Females competing over males display their
dark colors.
Females initiate courtship and mate more readily than males. Males, however, are choosy. Males prefer
females showing fewer black spots (which indicate parasitic infection)

Other “sex-role reversed” species include:
Birds: Jacanas, moorhens, phalaropes, spotted sandpipers
Insects: giant waterbugs , some katydids