Mating Systems and Sexual Selection

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Mating Systems and Sexual Selection
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What is the meaning of life?What is the purpose
of life?

• As far as I can tell, life has no meaning. It is
  simply an artifact of the laws of physics and
  chemistry.

• The purpose of life is simple Do it again.

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A key to understanding mating systems, is
understanding secondary sexual characters.

• These characters generally increase the males
  chance of obtaining copulations, and thus, his
  chances of reproductive success.
• They may reduce his probability of survival.

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Dimorphic traits are a consequence of sexual
selection.

• There are 2 components
• There may be competition amongst males for access
  to females.
• Female Choice! (This idea, that females chose
  males, was controversial until recently).

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Mating systems and sexual selection.

• Sexual selection is a form of directional
  selection.
• Sexual selection acts more strongly on males than
  on females. Why?
• Relative energetic cost of reproduction for males
  and females.
• Difference in the advantage of multiple matings
  by males and females. (a males reproductive
  output is a direct consequence of number of
  matings)

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Male Female differences

• Females invest a lot of energy in eggs and care
  for young.
• Females have relatively limited reproductive
  opportunities.
• Sperm are relatively inexpensive, and males
  usually invest little in offspring.
• Males are limited only by access to females.

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Male Female differences

• Variance of reproductive output for females is
  low.
• Variance of reproductive output for males is
  high.
• Overall mean for both sexes is probably
  equivalent.
• Males are more likely to by polygynous /
  promiscuous.

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Mating systems

• Usually polygynous
• Short breeding season and large aggregations of
  females.
• Results in explosive mating assemblages and
  scramble competition among males for females.

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Mating systems

• Breeding season long females dispersed.
• A) find female and guard her from other males
  until mating.
• B) call in females via advertisement (eg leks)
• C) if necessary resource is limited, establish
  territory

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Explosive mating aggregations

• Common amongst species that breed in ephemeral
  ponds Scaphiopus.
• Favored by short life of the pond.
• Also favored by possibility of cannibalism by
  older individuals.
• Males usually arrive at pond first.
• It is a mad scramble for anything that moves.

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Arrival of wood frogs at a temporary pond.
Notice in both years, males arrive first and in
greater numbers than females.
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Spotted and mole salamanders produce
spermatophores, that are then picked up by the
females. Males produce multiple spermatophores,
even on top of those of other males.
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Snakes exhibit mating aggregations as well.

• Female garter snakes produce a pheromone that
  attracts males.
• Males form huge clusters, and actually ambush
  females as the emerge from hibernacula. If it
  moves, mate with it.
• Males deposit a waxy plug in the cloaca that
  makes the female undesirable to other males.
• Is this female choice or male-male competition?
  How does male and female success differ?

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Mate searching

• If the species is dispersed throughout the
  habitat, mate searching over large areas is
  required.
• If the male finds a female, it is essential to
  protect the resource from other males.

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Mate Searching

• When snakes emerge from hibernacula, the disperse
  to forage, and are for the most part solitary.
• How then to find a mate when breeding season
  starts?
• Males have larger home ranges than females,
  increasing their chances of encountering females.

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Mate Searching

• Males initiate a search for females. How?
• Random movements?
• Straight line?
• Among prairie rattle snakes, almost all females
  will mate in a given season.
• Few males will manage to mate.
• They are long lived, and have multiple
  opportunities to mate within a life time.

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Mate Searching

• The search strategy depends on the distribution
  of females.
• If females are congregated in some way (food
  resource etc.) than males can have smaller home
  range and spend more time on courtship displays.
• If females are dispersed, then males spend more
  time searching and less oncourtship etc.

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Mate Guarding

• Amplexus is a form of mate guarding.
• Atelopus (Bufonidae) males amplex weeks before
  the breeding season starts in an effort to secure
  a female. Costs for male? Costs for female?
• Chase other males away.
• Push the female away from other males.

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Mate guarding

• Size matters!
• Males will fight with intruding males, and the
  bigger animal usually wins.
• Male tortoises often attempt to flip other males
  over.
• Males will stay with the female for extended
  periods, even though this may reduce possible
  encounters with other females. Why?

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Mate Guarding

• When females are rare, best to guard the female
  you have.
• When females are abundant, best to find another
  one.
• Some mate guarding occurs even after mating,
  because females can store sperm and or have
  multiple paternity within a single clutch.
  (bummer for the males)

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Mate Guarding

• If you are going to guard a female, guard a large
  one. Why?
• Large females produce larger clutches of eggs
  than small females. Thus, your return on
  investment is greater.
• Male lizzards tend to select large females over
  small females, when they have a choice.

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Mate Guarding

• Occurs in snakes as well.
• Prolonged copulation (up to several days) is a
  form of mate guarding.

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The bigger snake almost always wins!Size
matters.
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Lek Systems

• Males aggregate, often defending small
  territories, in an effort to obtain females.
• The territories are not always near a definable
  resource, and are maintained only for the
  duration of breeding.
• Amphibian leks differ from avian leks because
  mating must take place in association with water.

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Leks

• Since territories are not based on resources,
  females must choose males based on other male
  qualities, like display posture, coloration,
  vocalization etc.
• Triturus sp. Males form leks, and are alos
  amongst the most dimorphic of all salamanders.
  Males display not only w/ color, but also with
  posture.

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Leks

• Many anurans form leks. The males form choruses
  near water or in water, but not necessarily by
  oviposition site.
• Female choice is based on call qualities.
• When chosen, the male enters amplexus with the
  female, and she carries the male to the
  oviposition site.

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Leks

• Nile crocodiles form leks, American alligators do
  not (mate searching).
• Herbivorous Iguanids generally do not form leks.
  Why?
• Low energy food source, widely dispersed, and not
  defensible. Quality of territory probably
  conveys little information about the male.

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Leks

• However, marine Iguanas do form leks. Why?
• There is a defendable resource. Namely, basking
  sites where the feeding females emerge from the
  water.

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ABOVE distribution of male marine iquanas. Open
circles are territorial males, closed circles are
peripheral males.BELOWmale reproductive
success vs. lek size for marine iguanas.
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Leks

• Large marine iguana males are selected over
  smaller males. What does large size indicate
  about the male?
• Even when population densities are low, males
  form leks.

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Leks

• Salamanders are most likely to form leks, because
  they usually have internal fertilization, and
  thus oviposition may be some distance from
  copulation site.
• Exceptions Andrias japonicus, Plethodon cinereus
  (both use resource defense). In the case of
  Plethodon, females choose male territories on
  basis of diet (they determine whether males are
  eating ants or termites based on their scat).

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Resource Defense

• Green frog and Bull frog both exhibit resource
  defense.
• They have extended breeding seasons, making leks
  difficult.
• They maintain territories for up to 2 mo.
• Territories include oviposition sites.
• Quality of oviposition site is more important
  than male quality/size, but male quality/size is
  correlated with territory quality.
• Site quality is based partly on temperature.

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Above Green frog combat for access to
females.Below Green frog amplexus.
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Yes, size matters.
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Resource Defense

• Insectivorous sit-and-wait lizards are
  territorial during breeding season.
• Display, courtship, and site defense.
• Basking sites, display perches, food, and
  retreats are critical.
• A bigger territory means more access to females,
  and potential polygyny (depending on female home
  range size).
• (difference between home range and territory?)

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Above female density vs. female home range
sizeduh.Below home range ratio (male/female)
relative to female home range size for
territorial (solid) and non-territorial (open)
lizards. Why decrease? Cost of defense.
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The Big Picture (sexual selection-wise)

• If males have many opportunities in a season to
  reproduce, some will and some wont. The variance
  in reproductive success will be large. If they
  reproduce only once, variance will be smaller.
• When the variance is large, sexual selection will
  be stronger.

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The big picture

• Compare scramble competition for mates with leks,
  or say, compare prairie rattle snakes with
  red-sided garter snakes.
• Use the following figure to understand the role
  of sexual selection.

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Males!

• A determinant of male mating success is ability
  to persist. Reproduction is expensive (that is,
  calling, looking, handling etc) and when you do
  that, you dont feed, and consequently lose mass.
• Blow your wad on one night A. maculatum and A.
  texanum produce many spermatophores even though
  only 1 or 2 are used but what choice do you
  have?
• Spread your wads out produce one now and then
  like plethodontids and have almost everyone count.

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The cost of being a male marine iguana. Mating
occurs in early January.
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Size matters

• In explosive breeding animals, male-male combat
  is important, and thus male size is critical.
• Large males have higher reproductive success.
• This also results in dimorphism and allometric
  differences in the components that matter.

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Components critical to fighting other males
notice the hypertrophied pectoral appendage on
the male.
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Sexual dimorphism related to fighting compare
males and females for these 2 species.
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Female Choice

• Resources controlled by male.
• Choice based on behavioral or morphological
  traits is more difficult.
• Once the trait arises, it can be maintained and
  increased through runaway selection.
• Trait is an indicator of male quality.
• Sensory exploitation model.

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Female Choice

• In playback experiments, it has been shown that
  female anurans prefer longer, louder, more
  complex calls (they require more energy to
  produce).
• It turns out, there have been relatively few
  studies of female choice in herps.

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Alternative Mating Strategies

• Sneak matings
• Includes forced copulations.
• Satellite males
• Female mimicry
• Sexual interference

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Behavior of satellite males of the naterjack toad.
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Sexual interference
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Sperm Competition

• Multiple matings by females.
• Multiple paternity.
• Few observations of simultaneous multiple
  matings, but some species have sperm storage.

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Arboreal egg mass is fertilized by multiple
males. There is strong selection for increased
sperm production by males, and in fact, the
testes of these and some rhacophorids are very
large.
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Sexual Size Dimorphism

• Often a consequence of sexual selection.
• Can also be reverse sexual size dimorphism.

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