Gender change means that the sex of an individual organism changes during its lifetime. Also, mainly for humans, it is also called sex change to bring the appearance and body shape closer to those of other sexes by medical treatment.
Sex change in biology
In biology, sex change means that the sex of an individual changes from male to female and vice versa. It is one of the hermaphrodite styles, and is sometimes called a hermaphrodite adjacent to a simultaneous hermaphrodite. Transsexuals from males to females are called male premature ripening, and vice versa. There are also organisms that can change sex in both directions.
Evolution of sex change
As a theory that explains the ultimate factor of sex change, there is a theory called body length advantage theory. According to this model, sex reversal is evolutionarily advantageous when the relationship between body size or age and reproductive success differs between males and females. For example, when considering organisms that randomly marry regardless of body size or monogamous organisms, males produce a large number of sperms, even if they are small, and achieve reasonable breeding success, whereas female breeding success is Since it is determined by the number of eggs laid, it strongly depends on the body size. As a result, it will be advantageous to breed as a female when it is small, and as a female after it has grown to be able to produce many eggs, that is, male precociousness. Conversely, in organisms with a mating system in which large males monopolize a large number of females, small males can hardly breed and large males can achieve very high breeding success. In this case, it is considered that the female breeds as a female while it is small, and becomes a male after it grows to a size that can monopolize the female, that is, female precociousness is advantageous.
On the other hand, when both males and females tend to mate with the opposite sex of a size close to their own, that is, in the case of size-harmonious mating, the relationship between breeding success and size is equal between males and females, so sex change is not advantageous. .. It would be rather disadvantageous as it would cost some money to change gender.
As explained above, from this model, it is predicted that whether or not to change sex, and if so, in which direction the sex is changed depends on the mating system of the organism. This prediction is strongly supported, especially from fish studies.
Social regulation of sex change
The relationship between size and successful breeding is not always determined. In a harem-type polygamy, such as the Bluestreak cleaner wrasse, where the most dominant male in the group can monopolize the female, the relative success of breeding as a male is greater or lesser than other individuals in the same spouse group. Depends on size. Therefore, when the male disappears, the largest and most dominant female among the remaining females changes sex to the male. In this way, the induction or suppression of sex change by the social conditions of the individual is called social regulation of sex change.
The length advantage theory was first proposed by Michael Geslin (also known as Gizelin), and then developed into a more rigorous form by Robert Warner, Eric Charnov and others. Later, a model was created when there was a gender difference in growth rate and mortality rate due to Iwasa. With Warner, Roldan Munoz, along with Warner, can increase the number of female eggs laid very rapidly depending on body size, and the success of male breeding can be reduced by sperm competition.