Conditional sex allocation I

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Conditional sex allocation I

• Basic scenarios

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Trivers Willard

• Environmental conditions differentially influence
  fitness of males and females, then selection
  favours conditional sex allocation.
• Sex ratio adjustment
• Environmental Sex Determination
• Sex change

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Trivers Willard

• Assumptions mammal population
• better female condition ?higher offspring quality
• higher offspring quality ?higher adult quality
• sons greater fitness benefit from resources than
  daughters

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Trivers Willard

• Applied to wide range of organisms
• Sex ratio adjustment
• host size in parasitoids
• maternal condition in ungulates
• mate quality in birds
• ESD in shrimps fish
• Sex change in reef fish shrimps

host size maternal condition mate
quality age etc. etc.
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Condition dependent sex allocation

• Environmental variable ? variation in offspring
  fitness
• Fitness consequences differ between sexes
• Selection favours offspring sex varies with
  environment

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Parasitoid wasps host size

• Solitary parasitoid wasps
• Host size variation ? offspring fitness variation
• Increase in body size ? more benefit to females
• Females should produce sons in relatively small
  hosts, daughters in large hosts

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Parasitoids host size sex ratio

• Females do produce sons in small hosts, daughters
  in
• large hosts

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Parasitoids host size sex ratio

• Females adjust their offspring sex ratio in
  response to the relative host size

no perfect fit, not only relative size ? not
entirely flexible behaviour
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Parasitoids host size sex ratio

• Not always flexible behaviour, but fixed rules
• Not always sex ratio response
• host size doesnt influence wasp size
• females not able to asses host size
• host size not reliable indicator of
• resources ? koinobionts
• Host quality

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Parasitoids body size fitness

• Much less evidence
• Some lab evidence for greater female fitness
  benefit
• Field studies scarce, especially for males

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Ungulates maternal quality

• Red deer
• sex ratio ? rank of mother
• high rank females ? better condition ? more
  heavier young
• high quality young ? high quality adults
• sons greater benefit from
• resources than daughters

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Ungulates maternal quality

• Other species mixed results (within species?)
• Theory can predict opposite pattern ? maternal
  transmission of condition (rank/territory)
• Reproductive success/value different
• Overall support for TW in ungulates

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Ungulates maternal quality

• Species variation data quality
• Behavioural pre-conception measures ? strong
  response
• Morphological post-conception measures ? weak
  response

Behavioural vs. morphological
Pre- vs. post-conception
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Ungulates maternal quality

• Species variation selective forces
• sexual dimorphism
• maternal inheritance of condition
• nutritional stress

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Non-ungulates maternal quality

• Also in other species (birds, marsupials,
  insects, seals, whales, primates, humans
  plants)
• Also other factors (see chapter)
• No clear a priori predictions
• Not always adaptive sex allocation
• Need to know fitness consequences!

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Birds mate attractiveness

• Females should produce more sons when mated to
  attractive or higher quality male
• High quality mates ? high quality offspring
• Sons benefit more than daughters
• Empirical evidence in many bird species
• e.g. blue tits sex ratio ? male UV correlation

mate attractiveness
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Environmental Sex Determination

• ESD
• sex determined by embryonic environment
• Environment different fitness consequences
• for males females ? TW

environmental quality
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ESD shrimp example

• Gammarus duebeni
• ESD ? photoperiod
• long day ? males
• short day ? females
• Budle Bay (north)
• reproduction April-August
• males ? early in season ? growth ? bigger
• females ? late in season ? no growth ? smaller
• big males ? more mating success
• greater fitness consequences for males

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ESD shrimp example

• Totton Marsh (south)
• reproduction year round
• ESD? 2 cues photoperiod temperature
• better adjustment to wider range of variation
  during breeding season
• autumn ? females ? no growth ? small, mature this
  season
• winter ? males ? growth ? big, next season
• spring ? females ? small, this season
• overlapping generations

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Sex change

• Reproductive value varies with age
• Relationship different for males females
• Indeterminate growth (fish, invertebrates,
  plants)
• Protogynous sex change
• large males more mating success than large
  females
• Protandrous sex change
• large females more mating success than large
  males

age
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Sex change when?

• Fixed rules?
• Mainly in response to local conditions
• removal of dominant male
• exact cues unknown
• Reproductive value males females changes
  differently with size
• Patterns can be more complicated

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Conclusions

• TW conditional sex allocation in response to
  environmental conditions, if conditions affect
  fitness males and females differentially
• 1. sex allocation in response to relative
  environmental conditions
• 2. extent of sex ratio adjustment depends upon
  selection pressure environmental predictability
• 3. TW often applied too simplistic ? real
  organisms more complex ? difficult to make a
  priori predictions

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Future

• Estimate fitness consequences
• Meta-analyses
• Neglected taxa
• Quantitative tests of theory