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
fitness offspring
son
daughter
maternal quality
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
fitness offspring
•
•
•
son
daughter
host size
maternal condition
mate quality
age
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
fitness
A
B
τ
environmental quality
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
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 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
Much less evidence
Some lab evidence for greater female fitness benefit
Field studies scarce, especially for males
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
males
resources than daughters
females
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 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 quality
Species variation – selective forces
• sexual dimorphism
• maternal inheritance of condition
• nutritional stress
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
Females should produce more sons when mated to attractive
or higher quality male:
son
fitness offspring
High quality mates  high quality offspring
Sons benefit more than daughters
daughter
mate attractiveness
Empirical evidence in many bird species
e.g. blue tits: sex ratio  male UV correlation
Environmental Sex Determination
ESD
sex determined by embryonic environment
Environment different fitness consequences
for males & females  TW
fitness offspring
son
daughter
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
fitness offspring
Sex change
son
Reproductive value varies with age
Relationship different for males & females
daughter
age
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
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
fitness offspring
son
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
daughter
Future
• Estimate fitness consequences
• Meta-analyses
• Neglected taxa
• Quantitative tests of theory