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