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 fitness offspring

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 host size maternal condition mate quality age 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 fitness 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 daughters males 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 mate attractiveness

ESD sex determined by embryonic environment Environment different fitness consequences for males & females  TW Environmental Sex Determination 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 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

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