Chapter 10: Sexual selection.

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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.

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Sexual dimorphism

• Less extreme sexual dimorphism occurs in humans.
• Males about 10 taller on average.

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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.

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Sexual dimorphism

• Charles Darwin suggested sex provided a solution.
• If traits increase mating opportunities then this
  could more than compensate for reduced survival.

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Sexual selection

• Sexual selection differential reproductive
  success due to variation among individuals in
  obtaining mates.

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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.

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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.

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Amount of parental investment

• In general, because of difference between sexes
  in investment, a females 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.

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Amount of parental investment

• This disparity suggests sexual selection likely
  to be a more powerful influence on evolution of
  males than on females.

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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.

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Note scales on y-axes are not the same for males
and females
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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.

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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.

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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

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Note scales on y-axes are not the same for males
and females
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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.

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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.

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Contests between males to hold harems are common
in mammals e.g. deer, lions, antelope, elephant
seals.
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Males that dominate other males can secure harems
of females and obtain exclusive mating access to
them.
Bull elk and harem
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Strong relationship between fighting success and
reproductive success.
Southern Elephant Seals
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Northern Elephant Seals
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Competition between males has led to extreme
sexual dimorphism when males can potentially
control large harems.
Male and female fur seals
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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.
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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.

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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.

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Asterisks indicate maximum body sizes that
iguanas could maintain successfully in each of
two study years.
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Male-male competition among marine iguanas.

• Why is male body size larger than we would
  predict based on maximizing survival?

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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.

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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.

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Territories of numbered male marine iguanas.
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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.

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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.

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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.

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Sperm competition

• Male-male competition may continue even after
  mating is over.
• Fertilization, not mating is the goal.
• In many animals (including humans, but rarely) a
  female may produce a brood fathered by more than
  one male.

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Sperm competition

• What factors influence success in sperm
  competition?
• Number of sperm produced. (lottery analogy).

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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.

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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.

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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).

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Figure 10.27 from Animal Behavior text
Damselfly penis (note spines for extracting
sperm).
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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.

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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.

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Female Choice

• In many species males cannot monopolize females
  and males must advertise for mates.
• Females inspect multiple males before choosing
  one.

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Female Choice

• Females evaluate large number of possible male
  traits display, song repertoire, physical
  appearance.

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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

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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.

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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

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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.

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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.

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Mate choice in Gray Tree Frogs

• Playback experiment. Different length calls
  given from pair of loudspeakers.
• Female frogs allowed to choose between speakers.

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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.

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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.

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(Y-axis should read phonotaxis score not
phototaxis score)
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Female choice

• Both female barn swallows and gray tree frogs
  actively choose mates.
• Why do they do so?

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Females get better genes

• One explanation is that choosy females mate with
  males with better genes.
• Welch et al. examined genetic superiority
  hypothesis.

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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.

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Females get better genes

• Measured five aspects of offspring performance
• larval growth
• time to metamorphosis
• mass at metamorphosis
• larval survival
• post-metamorphic growth

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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.

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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.

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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.

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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.

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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.

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(female rejected the food item)
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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.)

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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.

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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.

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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.

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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.

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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.

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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.

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Diversity in sex roles.

• However, males are choosy and prefer larger
  females and those with larger skin folds

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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.

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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)

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Diversity in sex roles.

• Other sex-role reversed species include
• jacanas
• moorhens
• phalaropes
• spotted sandpipers
• giant waterbugs
• some katydids

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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.

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Sexual selection in plants

• Expect male flowers to be more attractive to
  pollinators than female flowers.

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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.

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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.