Sex ratios Flashcards
What was Fishers theory 1930, regarding sex ratios?
It pays to produce the sex with the higher mating success.
Mating success per male =
Total mating success/ No. males
Same for females.
Always individually advantageous to produce the rarer sex as they have higher mating success.
It is anESS argument, before ESS term was invented by Maynard Smith and before economists invented game theory.
describe some evidence for fishers theory
Basolo, 1994 - Platy fish have 3 factor sex determination: Males: YY, XY
Females: WX, XX, WY
therefore 6 possible combinations.
experimented with different groupings of known genotypes and found that pops initially produce more of the rarer sex, and usually end up as 50/50.
to have an ES sex ratio,…
S - success
C- cost
Smales/Cmales = Sfemales/Cfemales.
Because S=T/N
NmalesxCmales = Nfemales/Cfemales
This ratio must be equal for there to be an equal investment in each sex.
Usually, sex ratio is inversely proportional to costs of producing each sex.
example of beh that shows level of investment into each sex.
wasps - the volume of food a mother packs larval cells with, indicates the level of investment.
in species which give more food to f than m, SR is male biased.
in species where amount of food for f and m is equal, SR is equal.
Example of unequal investment into sexes in mammals.
Polygynous mammals, Males larger than females. eg red deer.
M have a Greater growth rate in utero, More milk till weaning, Mother in poorer condition after bearing son than after daughter.
However, the SR remains equal - maybe due to harder to influence SR, maybe females cost more later.
example of male biased SR due to local resource competition.
sons eventually disperse and daughters stay on home territory. end up competing with the mother.
therefore there is a male biased SR.
Silk and brown 2008 found similar thing in primates, and a small bias in favour of the dispersing sex.
define parental investment
Trivers: the reduction in future expected reproductive success due to investment in the current offspring
example of local resource enhancement
helpers.
when absent, mothers should bias SR towards helping sex.
eg African wild dog - M help more, a new pack has a M biased SR. also in alpine marmot.
Hamiltons theory of local mate comp in figwasps
Fig wasps lay eggs inside figs where the flowers are. seed feeds larvae, symbiosis. larvae fight inside by m severing others heads off with mandilbes. surviving males mate with sisters. females get covered in pollen and leave. gives a f biased sex ratio.
eg of local mate competition, not eh whole pop. resources that went into males could have gone into females.
example of local mate comp in mites
Female-biased sex ratio
Mate inside the mother
Leave when mother bursts
not interacting with the whole pop.
give an example of where fisher and hamiltons theories may combine
Fig wasps. if >1 females enter the fig, the ratio moves towards 50:50. example of Fishers theory, as it eventually ends up as 50:50
example of hamilton’s because there in local mate comp.
where do fisher and hamiltons theories combine regarding superparasitism?
Werren - Nasonia vitripennis
the first female laying an egg on the host is only one and offspring all male, so there is local mate comp, and also will be a female biased SR
The 2nd female detects parasitism on host already, and biased the SR accordingly. in small clutch - male bias. if large clutch - f bias.
where do fisher and hamiltons theories combine regarding selection in the lab?
Macke et al. (2011), Spider mite Tetranychus urticae 3 levels of LMC 1 foundress 0.23 male 10 foundress 0.45 >100 foundress (Fisher) 0.5
When is there conflict over the sex ratio?
Trivers and Hare, 1976.
Conflict between reproductives in social hymenoptera.
It’s not just the mother who controls allocation of resources
Relatedness between interested parties may differ
Queen equally related to sons and daughters, so equal investment. Workers however are 3x more related to sisters so invest 3x more in them than brothers.
why did Trivers and hare suggest tat workers have control over brood SR
argued by Trivers and Hare that workers would have control over SR. In monogynous ants, expected SR 3:1. however found 0.63 investment in females rather than 0.75 - perhaps some queen control?
