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Conditional sex allocation I. Basic scenarios. 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. son. daughter.

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Conditional sex allocation I


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trivers willard
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
trivers willard3

son

daughter

fitness offspring

maternal quality

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
trivers willard4
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.

condition dependent sex allocation

A

B

fitness

τ

environmental quality

Condition dependent sex allocation
  • Environmental variable  variation in offspring fitness
  • Fitness consequences differ between sexes
  • Selection favours offspring sex varies with environment
parasitoid wasps host size
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
parasitoids host size sex ratio
Parasitoids: host size & sex ratio

Females do produce sons in small hosts, daughters in

large hosts

parasitoids host size sex ratio8
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

parasitoids host size sex ratio9
Parasitoids: host size & sex ratio

Not always flexible behaviour, but fixed rules

Not always sex ratio response:

  • host size doesn’t influence wasp size
  • females not able to asses host size
  • host size not reliable indicator of

resources  koinobionts

Host quality

parasitoids body size fitness
Parasitoids: body size & fitness

Much less evidence

Some lab evidence for greater female fitness benefit

Field studies scarce, especially for males

ungulates maternal quality

males

females

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

ungulates maternal quality12
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

ungulates maternal quality13
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

ungulates maternal quality14
Ungulates: maternal quality

Species variation – selective forces

  • sexual dimorphism
  • maternal inheritance of condition
  • nutritional stress
non ungulates maternal quality
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!

birds mate attractiveness
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

environmental sex determination
Environmental Sex Determination

ESD

sex determined by embryonic environment

Environment different fitness consequences

for males & females  TW

environmental quality

esd shrimp example
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

esd shrimp example19
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

sex change
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

sex change when
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

conclusions
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

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