Ecology Lecture 10

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Ecology Lecture 10

• Life History Patterns 1

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Topics covered (both Life History Lectures)

• Sexual selection
• What criteria do individuals use to choose mates?
• Mating systems
• How many mates does an individual have
• What factors determine this?
• Parental care/parental investment in offspring
• Allocation of resources
• Quick reproduction vs. growth long life (r and
  K strategists, etc)

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Relevance to Population Ecology

• Life-history characteristics determine
• Organization of individuals in space and time
• Examples
• How the population perpetuates itself
• discretely vs. continuously rapidly vs. slowly,
  etc
• Which characteristics will be selected for and
  persist within the population (due to sexual
  selection)
• Conversely, selective pressures (abiotic factors,
  distribution of resources, etc..) will affect the
  life-history patterns observed

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Sexual selection

• Defined
• Selection for characteristics/behaviors that
  maximize chances of mating and producing the
  most, and highest quality, offspring.
• A category within natural selection
• General pattern Male-male competition and
  female choice
• Common pattern with many exceptions!

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Example Satin bowerbird mating behavior

• Each male build an elaborate bower where he
  conducts his courtship display
• Bowers located near each other
• Each female visits several times, finally chooses
  a mate

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Bowerbird males of mates

• Some males much more successful than others

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Bowerbird females of mates

• Only 1/3 of females have 1 mate

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Why do males usually compete, while females
choose?

• Hypothesis 1, A.J. Bateman Eggs are expensive,
  sperm is cheap!

• Amount of energy invested in a single gamete is
  much greater for females (eggs) than for males
  (sperm)
• Female bird may invest up to 30 of body weight
  in eggs.

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Eggs are expensive, sperm is cheap.

• Difference in investment per gamete
• Example Fairy wrens
• Males have 8 billion sperm in testes at once
• Females lay six eggs maximum per clutch
• Female limited by egg production,
• Male limited by number of mates only (presumably
  unlimited sperm)
• Operational sex ratio skewed toward males

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Is sperm really cheap?

• It often takes a large number of sperm to
  fertilize a single egg, due to
• Hostile environment within female
• Acid
• Attacks by the immune system

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Is sperm really cheap?

• Sperm competition among males
• Occurs when females have multiple mates
• Possibly the predominant situation
• Fitness advantages for female (will explore in
  next lecture)
• Some males may actually run out of sperm
• Garter snakes, zebra finch, blue crabs, rams

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What if there is no sperm competition?

• In sea horses, eggs are deposited into pouches,
  and there is not sperm competition.
• Why not?
• Male sea horses have relatively low sperm counts!

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Is sperm really cheap?

• Drosophila bifurca one sperm with long tail
• Sperm tail is 20x length of his body
• His testes make up 11 of his body mass.

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Why do males usually compete, while females
choose?

• Hypothesis 2 (R. Trivers) Competition vs.
  choice is based on individual with the most total
  parental investment
• Often the female (example mammals)
• But in some species, male makes a greater total
  investment

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Gulf pipefish

• While male cares for a single brood, a female can
  produce two clutches of eggs ? male has greater
  total parental investment
• Operational sex ratio skewed toward females.
• Males choose large, ( ornamented females over
  small, drab ones.

Female
Male
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Types of sexual selection

• Intrasexual selection maintenance of traits that
  assist in competition within the gender ?
  successful mating
• Intersexual selection maintenance of traits that
  are attractive to the opposite gender

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Intrasexual selection 1 Adaptations to gain
access to females

• Dominance behavior characteristics
• Example 1 sexual dimorphism in elephant seals

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Male-male competition and sexual dimorphism
(seals)
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Intrasexual selection 1 Adaptations to gain
access to females
Male dung beetle, Phanaeus vindex (Rattlebox
photography)

• Weaponry for fighting with other males.

• Example dung beetle!

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Intrasexual selection 1 Adaptations to gain
access to females

• Sneaker strategies
• Example 1 Plainfin midshipmen
• Dominant male features and behavior
• Nest building, singing, guarding
• Sneaker male features and behavior
• No nest, no singing, just sex
• Small fish, big balls!

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Dung beetles two morphs

• Behavioral and morphologial differences similar
  to midshipmen
• Large, dominant males with horns defend burrows
• Small, hornless males with big balls sneak
• Midshipmen
• Genetically-based differences
• Dung beetles
• Nutritionally-based differences

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Intrasexual selection 2 Adaptations favoring
the use of ones sperm

• Displacing or inactivating rival sperm
• Damselfly scooper penis

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Intrasexual selection 2 Adaptations favoring
the use of ones sperm

• Displacing or inactivating rival sperm
• Example Chemical sperm inactivation in fruit
  flies

Photo San Francisco Exploratorium
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Intrasexual selection 2 Adaptations favoring
the use of ones sperm

• Mechanisms to avoid sperm displacement
• Mate guarding (Example many crab species)

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Intrasexual selection 2 Adaptations favoring
the use of ones sperm

• Mechanisms to avoid sperm displacement
• Prolonged mating and cannibalism (example
  redback spider)

• Female less likely to mate with another if she
  eats him
• He has low likelihood of finding a new mate (high
  predation)
• Andrade, 1996

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Intrasexual selection 2 Adaptations favoring
the use of ones sperm

• Mechanisms to avoid sperm displacement
• Anti-aphrodisiac (Example Heliconius erato)

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Intersexual selectionFocus on female choice

• Material benefits Nutrition Ex hangflies)
• Length of mating time depends on quality and size
  of courtship gift

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Intersexual selectionFocus on female choice

• Material benefits Anti-predator substances
• Defensive compounds in arctiid moths

Photo butterfly-conservation.org
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Intersexual selectionFocus on female choice

• Ability of males to provide sufficient sperm
• Female fruit flies (some species) choose virgin
  males

Photo San Francisco Exploratorium
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Intersexual selectionFocus on female choice

• Parental ability
• Cannot assess directly
• May be correlated with other features of the male
• Example 1 Redwing blackbird
• Epaulettes correlated with nest defense
• Courtship effort correlated with feeding effort

Photo Vancouverislandbirds.com
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Intersexual selectionFocus on female choice

• Parental ability (Ex sedge warbler)
• Size of song repertoire correlates with chick
  weight at fledging

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Sedge warbler (cont.)

• Female sedge warblers choose on the basis of
  repertoire size.
• Thus they choose the most fit males

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Intersexual selectionFocus on female choice

• Health/Genetic quality
• Example song repertoire in great reed warbler
• Females chose males with larger repertoires
• This was correlated with greater offspring
  survival (unrelated to parental care)

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Intersexual selectionFocus on female choice

• Health/Genetic quality
• Example Bright coloration of sticklebacks
  negatively correlated with low parasite loads

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Intersexual selectionFocus on female choice

• Health/Genetic quality (bright color negatively
  correlated with parasite load)
• Advantages to females choosing these males
• Avoid getting parasites while mating ?
• Avoid transferring parasites to young
• Are choosing healthier males their health status
  may be related to genetic quality

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Satin Bowerbirds multiple signals of health and
fitness (and good genes?)
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Origin/maintenance of mate choice for
exaggerated characteristics

• In some species, why do males develop what appear
  to be extreme traits that actually can hamper
  their survival?
• Example Peacocks tail

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Origin/maintenance of mate choice for
exaggerated characteristics

• Hypothesis 1 Runaway selection (R.A. Fisher)
• Directional that takes on a life of its own
• Starts as an honest signal ? more extreme.
• Mechanism Females choose males with large
  tails, multiple eyespots. ? the next generation
  has a higher proportion of these males.
• Will work even if his traits are not honest
  signals of quality. Why?
• Evidence of arbitrary choices by females (bird
  band example)

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Origin/maintenance of mate choice for
exaggerated characteristics

• Hypothesis 2 Handicap or good genes
  hypothesis (R.A. Fisher)
• Exaggerated trait might decrease chance of
  survival, only males with superior genes can
  survive despite the handicap
• Example peacock tail as a handicap.
• In this case, a female choosing a male with these
  traits would be improving her fitness. (His
  signal is an honest signal of fitness.)

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Distinguishing between Runaway Selection and
Good Genes (Petrie)

• Methods
• Males of different ornamentation/tail length and
  randomly bred them with females
• Why random?
• Young raised under identical conditions and then
  released
• Results
• Offspring of the attractive males weighed more
  at day 84.
• Offspring of the attractive males were more
  likely to be alive after two years
• Which hypothesis is supported by this data?

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Petries peacock data
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Origin/maintenance of mate choice for
exaggerated characteristics

• How extreme can a characteristic become? Under
  what conditions will directional selection stop?
  (Think about costs vs. benefits)