Sex allocation
[1][page needed] Sex allocation theory tries to explain why many species produce equal number of males and females.
The idea of equal allocation fails to explain these expected ratios because it assume that relatives do not interact with one another, and that the environment has no effect.
This led to a great deal of research on whether competition or cooperation between relatives results in differential sex ratios that do not support Fisher's principle.
She argued that the African bushbaby (Otolemur crassicaudatus) demonstrated a male-biased sex ratio because daughters associated with mothers for longer periods of time than did sons.
[6] Clark predicted that the effect of the LRC on sex allocation resulted in a mother investing preferentially in male offspring to reduce competition between daughters and herself.
Local resource competition has been hypothesized to be the reason that passerine birds are more likely to be female, while ducks and geese are more likely to have male offspring.
[10] In such a case, mothers would preferentially adjust the sex ratio to be female-biased, as only a few males are needed in order to fertilize all of the females.
It is also predicted that the strength of the selection upon the mothers to adjust the sex ratio of their offspring depends upon the magnitude of the benefits they gain from their helpers.
These predictions were found to be true in African wild dogs, where females disperse more rapidly than males from their natal packs.
[14] Evidence for LRE leading to sex ratios biased in favor of helpers has also been found in a number of other animals, including the Seychelles warbler[15] (Acrocephalus sechellensis) and various primates.
Trivers and Willard (1973) originally proposed a model that predicted individuals would skew the sex ratio of males to females in response to certain parental conditions, which was supported by evidence from mammals.
[16] While the Trivers-Willard hypothesis applied specifically to instances where preferentially having female offspring as maternal condition deteriorates was more advantageous, it spurred a great deal of further research on how environmental conditions can differentially affect sex ratios, and there are now a number of empirical studies that have found individuals adjust their ratio of male and female offspring.
[17] In tawny owls, a female-biased sex ratio was observed in breeding territories where there was an abundance of prey (field voles).
Wiebe and Bortolotti (1992) observed sex ratio adjustment in a sexually dimorphic (by size) population of American kestrels (Falco sparverius).
[18] These findings modify the Trivers-Willard hypothesis by suggesting sex ratio allocation can be biased by sexual size dimorphism as well as parental conditions.
A study by Clutton-Brock (1984) on red deer (Cervus elaphus), a polygynous species, examined the effects of dominance rank and maternal quality on female breeding success and sex ratios of offspring.
[19] These findings support the Trivers-Willard hypothesis, as parental quality affected the sex of their offspring, in such a way as to maximize their reproductive investment.
Similar to the idea behind the Trivers-Willard hypothesis, studies show that mate attractiveness and quality may also explain differences in sex ratios and offspring fitness.
Weatherhead and Robertson (1979) predicted that females bias the sex ratio of their offspring in favor of sons if they are mated to more attractive and better quality males.
Fawcett (2007) predicted that it is adaptive for females to adjust their sex ratio to favor sons in response to attractive males.
[1][24] Consistent with this hypothesis, as selfing in wild rice (Oryza perennis) increases, the plants allocate more resources to the female function than to male.
[25] It has been difficult to measure exactly how much fitness individual plants are able to gain from investing in one or both sexual functions,[3][26] and further empirical research is needed to support this model.
[31] Higher corticosterone levels in breeding female Japanese quails were associated with female-biased sex ratios at laying.
