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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
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
maternal quality
son
daughter
fitne
ss o
ffspr
ing
Trivers & Willard
Applied to wide range of organisms:
1. 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
son
daughter
fitne
ss o
ffsp
ring
son
daughter
fitne
ss o
ffsp
ring
host sizematernal condition
mate qualityage
etc. etc.
Condition dependent sex allocation
1. Environmental variable variation in offspring fitness
2. Fitness consequences differ between sexes
3. Selection favours offspring sex varies with environment
environmental quality
A
B
fitne
ss
τ
Parasitoid wasps & host size
Solitary parasitoid wasps:
1. Host size variation offspring fitness variation
2. Increase in body size more benefit to females
3. Females should produce sons in relatively small hosts, daughters in large hosts
Parasitoids: host size & sex ratio
Females do produce sons in small hosts, daughters in
large hosts
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 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: host size & sex ratio
Much less evidence
Some lab evidence for greater female fitness benefit
Field studies scarce, especially for males
Parasitoids: body size & fitness
Ungulates: maternal quality
Red deer
sex ratio rank of mother:
1. high rank females better condition more & heavier young
2. high quality young high quality adults
3. sons greater benefit from
resources than daughtersmales
females
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 quality
Species variation – data quality
Behavioural & pre-conception measures strong response
Morphological & post-conception measures weak response
Ungulates: maternal quality
Behavioural vs. morphological Pre- vs. post-conception
Species variation – selective forces• sexual dimorphism• maternal inheritance of condition• nutritional stress
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!
Non-ungulates: maternal quality
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
Birds: mate attractiveness
son
daughter
fitne
ss o
ffsp
ring
son
daughter
fitne
ss o
ffsp
ring
mate attractiveness
ESD
sex determined by embryonic environment
Environment different fitness consequences
for males & females TW
Environmental Sex Determination
son
daughter
fitne
ss o
ffsp
ring
son
daughter
fitne
ss o
ffsp
ring
environmental quality
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 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
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
son
daughter
fitne
ss o
ffsp
ring
son
daughter
fitne
ss o
ffsp
ring
age
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
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
son
daughter
fitne
ss o
ffsp
ring
son
daughter
fitne
ss o
ffsp
ring
Future
• Estimate fitness consequences
• Meta-analyses
• Neglected taxa
• Quantitative tests of theory