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. Charlesworth and Charlesworth (1981) applied similar logic to both outcrossing and selfing species, and created a model that predicted when dioecy would be favored over hermaphroditism, and vice versa. 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. It has been difficult to measure exactly how much fitness individual plants are able to gain from investing in one or both sexual functions, and further empirical research is needed to support this model. Mechanisms of sex allocation decisions 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, red deer, and birds. 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. Higher corticosterone levels in breeding female Japanese quails were associated with female-biased sex ratios at laying. In species that have environmental sex determination, like turtles