Sex allocation

Biology theory
Sex allocation is the allocation of resources to male versus female reproduction in sexual species. Sex allocation theory tries to explain why many species produce equal number of males and females. In dioecious species, where individuals are either male or female for their entire lifetimes, the allocation decision lies between producing male or female offspring. In sequential hermaphrodites, where individuals function as one sex early in life and then switch to the other, the allocation decisions lie in what sex to be first and when to change sex. Animals may be dioecious or sequential hermaphrodites. Sex allocation theory also applies to flowering plants, which can be dioecious, simultaneous hermaphrodites, have unisexual plants and hermaphroditic plants in the same population, have unisexual flowers and hermaphroditic flowers on the same plant or to have only hermaphroditic flowers.

R. A. Fisher developed an explanation, known as Fisher's principle, of why sex ratios in many animals are 1:1. If there were 10 times more females in a population than males, a male would on average be able to mate with more partners than a female would. Parents who preferentially invested in producing male offspring would have a fitness advantage over those who preferentially produced females. This strategy would result in increasing numbers of males in the population, thus eliminating the original advantage of males. The same would occur if there were originally more males than females in a population. The evolutionarily stable strategy (ESS) in this case would be for parents to produce a 1:1 ratio of males and females. This explanation assumed that males and females are equally costly for parents to produce. However, if one sex were more costly than the other, parents would allot their resources to their offspring differentially. If parents could have two daughters for the same cost as one male because males took twice the energy to rear, parents would preferentially invest in daughters. Females would increase in the population until the sex ratio was 2 females: 1 male, meaning that a male could have twice the offspring a female could. As a result, males will be twice as costly while producing twice as many offspring, so that males and females provide the same proportion of offspring in proportion to the investment the parent allotted, resulting in an ESS. Therefore, parents allot equal investment of effort in both sexes. More generally, the expected sex ratio is the ratio of the allotted investment between the sexes, and is sometimes referred to as Fisherian sex ratios. However, there are many examples of organisms that do not demonstrate the expected 1:1 ratio or the equivalent investment ratio. 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.

W. D. Hamilton hypothesized that non Fisherian sex ratios can result when relatives interact with one another. He argued that if relatives experienced competition for resources, or benefited from the presence of other relatives, then sex ratios would become skewed. 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.

Local resource competition (LRC) was first hypothesized by Anne Clark. 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. Since sons disperse further from the maternal territory than do daughters, they do not remain on the territories and do not act as competitors with mothers for resources. Passerine birds demonstrate largely daughter based dispersal, while ducks and geese demonstrate mainly male based dispersal. 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. Other studies have hypothesized that LRC is likely to influence sex ratios in roe deer, as well as primates. Consistent with these hypotheses, the sex ratios in roe deer and several primates have been found to be skewed towards the sex that does not compete with mothers. 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. Species with wingless males that can only mate with sisters were predicted to have higher rates of female biased sex ratios, while species with winged males that can travel to other figs to fertilize non related females were predicted to have less biased sex ratios. and ants, because they often face strong LMC. Other animals that often disperse from natal groups are much less likely to experience LMC.

Local resource enhancement (LRE) occurs when relatives help one another instead of competing with one another in LRC or LMC. In cooperative breeders, mothers are assisted by their previous offspring in raising new offspring. In animals with these systems, females are predicted to preferentially have offspring that are the helping sex if there are not enough helpers. However, if there are already enough helpers, it is predicted that females would invest in offspring of the other sex, as this would allow them to increase their own fitness by having dispersing offspring with a greater rate of reproduction than the helpers. 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. Males are therefore more helpful towards their mothers, as they remain in the same pack as her and help provide food for her and her new offspring. The LRE the males provide is predicted to result in a male biased sex ratio, which is the pattern observed in nature. Consistent with predictions of LRE influencing sex ratios, African wild dog mothers living in smaller packs were seen to produce more male biased sex ratios than mothers in a larger pack, since they had fewer helpers and would benefit more from additional helpers than mothers living in larger packs. (Acrocephalus sechellensis) and various primates. Though individuals may not consciously decide to have fewer or more offspring of the same sex, their model suggested that individuals could be selected to adjust the sex ratio of offspring produced based on their ability to invest in offspring, if fitness returns for male and female offspring differ based on these conditions.

In many species, the abundance of food in a given habitat dictates the level of parental care and investment in offspring. This, in turn, influences the development and viability of the offspring. If food availability has differential effects on the fitness of male and female offspring, then selection should shift offspring sex ratios based on specific conditions of food availability. Appleby (1997) proposed evidence for conditional sex allocation in a study done on tawny owls (Strix aluco). This is related to Fisher's “sexy son” hypothesis, which suggests a causal link between male attractiveness and the quality of sons based on the inheritance of “good genes” that should improve the reproductive success of sons. Fawcett (2007) predicted that it is adaptive for females to adjust their sex ratio to favor sons in response to attractive males. Based on a computer model, he proposed that if sexual selection favors costly male traits, i. e. ornamentation, and costly female preferences, females should produce more male offspring when they mate with an attractive male compared to an unattractive male. For example, in the bluehead wrasse, the largest males have 40 times the mating success of smaller ones. Thus, as individuals age, they can maximize their mating success by changing from female to male. Removal of the largest males on a reef results in the largest females changing sex to male, supporting the hypothesis that competition for mating success drives sex change.

A great deal of research has focused on sex allocation in plants to predict when plants would be dioecious, simultaneous hermaphrodites, or demonstrate both in the same population or plant. Research has also examined how outcrossing, which occurs when individual plants can fertilize and be fertilized by other individuals or selfing (self pollination) affect sex allocation. Selfing in simultaneous hermaphrodites has been predicted to favor allocating fewer resources to the male function, as it is hypothesized to be more advantageous for hermaphrodites to invest in female functions, so long as they have enough males to fertilize themselves. Consistent with this hypothesis, as selfing in wild rice (Oryza perennis) increases, the plants allocate more resources to the female function than to male. The model predicted that dioecy evolves if investing in one sexual function has accelerating fitness benefits than investing in both sexual functions, while hermaphroditism evolves if investing in one sexual function had decreasingly lower fitness benefits. and further empirical research is needed to support this model.

Depending on the mechanism of sex determination for a species, decisions about sex allocation may be carried out in different ways. In haplodiploid species, like bees and wasps, females control the sex of offspring by deciding whether or not to fertilize each egg. If she fertilizes the egg, it will become diploid and develop as a female. If she does not fertilize the egg, it will remain haploid and develop as a male. In an elegant experiment, researchers showed that female N. vitripennis parasitoid wasps altered the sex ratio in their offspring in response to the environmental cue of eggs laid by other females. Historically, many theorists have argued that the Mendelian nature of chromosomal sex determination limits opportunities for parental control of offspring sex ratio. However, adaptive adjustment of sex ratio has been found among many animals, including primates, The exact mechanism of such allocation is unknown, but several studies indicate that hormonal, pre ovulatory control may be responsible. For example, higher levels of follicular testosterone in mothers, signifying maternal dominance, correlated with a higher chance of forming a male embryo in cows. In species that have environmental sex determination, like turtles and crocodiles, the sex of an offspring is determined by environmental features such as temperature and day length. The direction of bias differs between species. For example, in turtles with ESD, males are produced at lower temperatures, but in many alligators, males are produced at higher temperatures.