Conditional sex allocation I Trivers & Willard Trivers & Willard Trivers & Willard Condition dependent sex allocation
Pickering, Aaron, Meteorologist has reference to this Academic Journal, PHwiki organized this Journal Conditional sex allocation I Basic scenarios Trivers & Willard Environmental conditions differentially influence fitness of males in addition to 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
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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 Environmental variable variation in offspring fitness Fitness consequences differ between sexes Selection favours offspring sex varies with environment 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 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 doesnt 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 as long as greater female fitness benefit Field studies scarce, especially as long as males 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 quality Other species: mixed results (within species) Theory can predict opposite pattern maternal transmission of condition (rank/territory) Reproductive success/value different Overall support as long as 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 as long as ces 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: 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 ESD sex determined by embryonic environment Environment different fitness consequences as long as males & females TW 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 as long as 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 as long as males & females Indeterminate growth (fish, invertebrates, plants) Protogynous sex change large males more mating success than large females Prot in addition to rous 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 in addition to 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
Pickering, Aaron Meteorologist
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