Sexual Selection

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Sexual Selection
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Sexual Dimorphism

• Males and Females of a species look different \
• Does not make sense in light of natural selection
  
• Natural selection pressures apply equally to male
  and female so
• Why arent both sexes selected for in the same
  way

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Sexual Selection

• Darwin recognized that individuals differ in
  their success in obtaining a successful mating.
  He called this sexual selection
• Sexual selection is differential reproductive
  success due to variation among individuals in
  success at mating.
• Evolutionarily speaking failure to mate and leave
  offspring is equivalent to dying young

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Differences in selection pressures between males
and females

• Females typically make a larger parental
  investment.
• It takes more effort to produce eggs or
  pregnancies than to produce sperm
• in 90 of mammals the female provides significant
  parental care while the male provides none
• In most species neither parent cares for the
  young but
• Females make a much larger investment in
  constructing the young

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This leads to a large difference in factors that
effect the reproductive success of males vs.
females
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REPRODUCTIVE SUCCESS IN FEMALES

• determined by number of eggs she can make or
  pregnancies she can carry
• Not determined so much by the number of males she
  can convince to mate with her

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REPRODUCTIVE SUCCESS IN MALES

• more likely to be limited by the numbers of
  females he can convince to mate with him
• Not as much by the number of sperm he can provide

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THEREFORE

• Access to females will be limiting factor to the
  success of the male but access to males will not
  be limiting to females success (example of the
  rough skinned newts in text page 405-406)
• Leads to an asymmetry in the fitness limits of
  the two sexes .
• which leads to different mating behaviors by the
  two sexes.

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Behavioral consequences of this asymmetry of
fitness limits

• Males should be competitive with other males
• Females should be choosy in the males they select
  
• These differences are not inherent to maleness
  and femaleness so much as they are dictated by
  survival and fitness.

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Types of male/male competition

• Intrasexual where males can directly monopolize
  access to females
• males will fight with each other.
• Females mate with the winners
• Intersexual Where males cannot control access to
  females
• the males advertise for mates
• the female chooses

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Three types of malemale competition / Intrasexual

• Combat
• Sperm Competition
• Infanticide

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Intrasexual - COMBAT

• large body size
• weaponry
• armor

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EXAMPLE Of MARINE IGUANAS Galapagos

• Females make a large investment in producing and
  raising young (20 of body mass into the eggs
  themselves) digs nest, buries eggs and guards a
  few days
• Males do nothing
• In Iguanas, males can monopolize females by
  staking out territories in which the females
  graze and sun themselves
• Females are not choosy

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Territories are only a few square meters.
Numbers identify the males which own the
territory. 59 and 65 were the most successful
malesPrime territories are near the water
Males fight over territories, winner takes all
females in the territory
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The bigger the male iguana is the more likely he
is to win a good territory

• Although large body size is disadvantageous in
  and of itself on the islands because it takes so
  much energy to maintain a larger body size
• In fact selective forces are always working
  against larger body size
• Male iguanas get big because bigger males get
  more mates and pass on their big-male genes
• Sexual selection actually works in opposition to
  natural selection in order to choose for
  characteristics which give males a chance to mate
  successfully

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Intrasexual- SPERM COMPETITION

• Occurs with internal fertilization where one
  female mates with more than one male within a
  short period of time
• Sperm are in a race to the egg
• Animals in this situation typically have larger
  ejaculates with many more sperm than males that
  are not in this type of competition ( Example of
  the medfly in the book)

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Other strategies developed due to sperm
competition

• prolonged copulation
• copulatory plugs
• application of pheromones to the female to reduce
  her attractiveness to other males
• Scoop out the sperm left by a previous suitor

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Intra sexual - INFANTICIDE

• Animals that live in groups. Such as a pride of
  lions
• Pride composed of a group of closely related
  females with 2 or 3 adult males related to each
  other but unrelated to the females
• average time a male holds a pride is a little
  over 2 years

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If females are nursing cubs, they will not return
to breeding condition until the cub is weaned

• When a new male moves in to a new pride, he will
  usually kill any young who are still nursing.
  25 of cub deaths in the first year are due to
  infanticide. (10 of all deaths)
• Spontaneous abortions also happen often when new
  males move in. Female cuts her losses because cub
  would be killed shortly after birth anyway

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Intersexual Selection
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Male - Male Competition / Intersexual

• Based on Female Choice
• Female choice leads to elaborate displays by
  males
• Occurs when males cannot monopolize the access to
  females
• Females are highly selective in these situations

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BARN SWALLOW EXAMPLE

• male and female build nest together and both feed
  the chicks
• since they both care for the young seems to
  equalize parental investment but other factors
  suggest they do exhibit sexual selection
• not a monogamous species
• sexual dimorphism suggests that there is female
  choice involved
• The quality of the potential parents may vary

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Studies show that males use tail length to
attract females.

• He displays his tail while perching and flying to
  the females after he sets up a territory and
  builds a nest
• Males with elongated tails attracted more mates
  and mated more quickly. The longer the tail the
  bigger the difference.
• Also showed that if the female mated with a
  shorter tailed male they were more likely to seek
  other males to mate with as well
• Also the longer tailed males were more likely to
  convince more than one female to mate with them

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GRAY TREE FROGS

• Males advertise with calls both the length and
  speed of calls seem to be relevant to the female
  choice
• When males hear other males calling nearby they
  increase both the speed and length of their calls
  
• Researchers have observed females actually going
  to the more distant caller suggesting selection
  was occurring

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Experimental results

• Used loud speakers to present calls to frogs
• 75 of females preferred long calls to short
  calls, even when short calls were louder
• Also 72 of females went past the closer speaker
  to the more distant speaker which was giving
  longer calls

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What is the motive for female choice? What
benefit is gained?
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Possibility 1 Choosy females may get better
genes for their offspring

• Perhaps males that give better displays are
  genetically superior . (This is also called
  honest advertising)
• This was tested in an experiment with the Gray
  Tree frogs

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Compared tadpoles from the same mother which were
sired by either long-calling males or by short
calling males

• Frogs from the two groups were maternal
  half siblings
• Five aspects of fitness related performance were
  monitored
• larval growth rate
• time to metamorphosis
• mass at metamorphosis
• larval survival
• post-metamorphic growth

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Table 11.2 results, Long calling males have
significantly higher fitness
Table 11.2
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Possibility 2 Choosy females may benefit through
acquisition of resources

• Hangingflies example
• males present food to females
• if she accepts the food they mate while she is
  eating the food

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The larger the food is the longer they copulate
and the more sperm that are deposited

• If not enough food she breaks off and goes
  looking for another male bearing gifts
• What benefits are there for the female when she
  chooses males bearing large gifts?
• Provides her with more nutrients, so she can lay
  more eggs
• Saves her from having to hunt herself, hunting is
  dangerous. Males die in spider webs at more than
  twice the rate of females.

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Possibility 3 Choosy females may have
pre-existing sensory biases

• Possible that selection on another trait may make
  a female more responsive to certain cues from
  males
• In these cases the female preferences evolve
  first and then the male mating display follows
  and takes advantage of female biases already in
  place.

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Example of water mites

• have poor vision rely mostly on smell and touch
• Hunt by doing a net-stance where mite stands on
  a plant by its hind legs puts its front legs out
  to form a net and waits

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When pray swim close by they cause vibrations in
the water and the mite turns towards the
vibration and clutches at the vibration

• Male water mites mimic the pray in order to find
  females.
• The male gets the females attention by mimicking
  the vibration of the prey species. If the female
  turns and clutches at him he deposits the
  spermatophore.

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• Males induce females to accept their sperm sacks
  by fanning the water across the spermatophore
  towards the female
• Moving water carries the pheromones towards the
  female

The female will then detect the pheromone and
might pick up the sperm packet
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Other explanations for female choice

• It is possible female choice is simply arbitrary
  no real selection going on for any apparent
  reason.

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R.A. Fisher proposed that there is a genetic
basis for female preferences based on some aspect
of the male appearance....

• Once a particular male advertisement is favored
  by a majority of the females, selection by
  females will automatically reinforce selection
  for that trait in male offspring (sons)
• AND......
• Daughters (female offspring) will also inherit
  the preference for that same trait passed on to
  them. Must show that preference is genetically
  based to prove this hypothesis.
• This is sometimes called the sexy sons or
  runaway selection hypothesis.

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Why sexy sons do you suppose?

• Females choosing more fashionable males will
  produce more fashionable sons and therefore more
  grandchildren than females choosing unfashionable
  mates
• Example in book of stalk-eyed flies neatly shows
  that
• Selection by females for either long-eye stalks
  or short eye stalks in males can produce an
  evolutionary response in the females which
  selects for the corresponding eyestalk length in
  future generations.

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Which is the most likely reason why females are
choosy.

• There is no one hypothesis that is better than
  another.
• Individual species use different strategies
• all of these explanations are mutually compatible
  and in many species more than one strategy may be
  involved
• This is a very active area of evolutionary
  research today.

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Diversity in Sex Roles

• Sex role reversal

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In some species it is the male who provides all
the parental care

• sea horses and pipefish
• In these organisms the female lays her eggs
  directly into the male brood pouch and the male
  supplies the eggs with oxygen and nutrients
  until they hatch
• In these cases
• Males invest a greater amount of energy.
• access to male brood pouches limits the
  reproductive success of females
• females should compete with each other and males
  should be choosy

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Pipefish example

• In one species of pipefish the females are larger
  and have dark blue stripes and skin folds on
  their bellies which appear to be advertisements
  for males
• Skin folds are only present during mating season
• In captivity females only develop skin folds when
  males are present

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Studies show that males appear to prefer larger
females with larger skin folds
Number of males tested

• Females show no tendency to discriminate between
  males
• Same sex-reversed effects are seen in other
  species as well

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Sexual Selection in plants

• Plants are often sexually dimorphic
• The seed parent (female) makes a much larger
  reproductive investment than the pollen donor.
• Mating involves successful pollination
• Mating success access to pollinators
• Principles of sexual selection we have been
  studying would predict that the success of pollen
  donors is more limited by access to pollinators
  than is that of the seed parent(female)

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Wild radish example

• self-incompatible
• white and yellow flowers white shows simple
  dominance over yellow
• study population of 8 WW (white) and 8 ww
  (yellow) plants
• Monitored pollinator visits to each color of
  flower. ¾ of the pollinator visits were to
  yellow flowers.
• Measured reproductive success through both male
  and female function

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Results Female success

• For females simply count the number of plants
  that produced fruits containing seed.
• Essentially all plants produced seed.

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Results - Male success

• The measure of the males success is a bit more
  tricky
• Could not do it by individual plant
• Had to raise the seeds produced by the yellow
  seed parent and determining how many white and
  how many yellow flowers
• WHY?
• Because if the yellow parent (recessive) had been
  pollinated by a yellow pollen it would produce
  yellow plants but if by a white pollen it would
  produce white plants

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• Remember, Yellow flowered plants got ¾ of the
  pollinator visits

• If male reproductive success is limited by
  pollinator visits then male pollen from
  yellow-flowered plants should have gotten ¾ of
  reproductive success, since they received ¾ of
  the visits.
• ¾ of the seeds did produce yellow flowered
  plants. So male success was directly related to
  the access to pollinators and where they
  delivered the pollen.
• but seed parents (females) had equal success in
  producing seed and it did not matter which male
  was the pollen provider
• Thus, reproductive success of males is more
  limited by access to pollinators than is the
  females

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If all of this is true then.

• we would predict that dioecious plants would have
  dimorphic flowers and that ??? flowers
  would be showier?
• Males do whatever they can to attract
  pollinators and assure that their pollen will be
  successful.

male

• Many studies bear this out

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In wind pollinated species .

• the size of flower parts is simply designed to
  protect the reproductive parts, the larger the
  reproductive parts, the larger the perianth
  (sepals petals)
• In animal pollinated, however, we see a variety
  of strategies ..

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Males often have larger and more showy perianths
even though their reproductive structures are
much smaller than the females

• Males also seem to have stronger odors and more
  flowers per inflorescence
• Not all flowers are dioecious. In this case
  larger flowers are usually more successful.
• Larger flowers are visited preferentially by bees
  and butterflies.
• Larger flowers will be visited first and their
  pollen, once delivered to the female, will have a
  head start on forming a pollen tube in the
  pistil of the flower it pollinates.
• Meanwhile, females typically receive 4 times the
  amount of pollen needed to produce seed
  successfully

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The devious orchid

• Male flowers actually train male bees not to
  visit other males
• The orchid also assures that no other pollen
  coming to the same female flower at a later time
  will be able to successfully pollinate
• How do they do all of this?

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Catasetum orchids are dramatically dimorphic

• Males produce their pollen in a pollinarium which
  is held by a triggered mechanism
• When a bee trips the trigger the pollinarium
  shoots at the bee and sticks to the bees back
• After being shot the bees avoid going to other
  male flowers
• When the bee visits the female flower the flower
  is smaller and rubs the pollinarium off of the
  bee
• and it lodges on the receptive stigma
• In response the stigma swells and shuts out any
  further pollination

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The end of Chapter
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