In some animal species, the biological sex is not predetermined by the genes. That is, it WILL NOT be determined by the XX and XY chromosomes. In fish and reptiles in particular, gender is more flexible. Environmental factors such as the social system or the breeding temperature in crocodiles and turtles can influence sex. Some species of fish are even able to change sex over the course of their lives.

This dichotomy, hermaphroditism, can be observed in 27 families of countless species of bony fish. The trait is widespread among the fish species. During the transformation, the behavior of the fish changes, as well as their appearance and anatomy. The process is completely completed within a few days or weeks.

The direction of the change depends primarily on the social structure of the fish. So far, science has explained the change with economies of scale. In some situations it is better for the animal to be smaller or larger.

When females become males and males become females

In the case of the anthias, dominant and aggressive males in particular claim territories and smaller ones are excluded from reproduction. Therefore, the change from females to males makes sense. Those animals that begin life as females reproduce until the dominant male dies. Then the females transform and take over the role of the male.

However, there are also reverse examples: The monogamous clownfish. The female here tends to be larger as this allows her to lay more eggs. Also, the largest member of the group is a female and the second largest is a dominant male. All others are male only. If the female dies, her mate takes over her role. The largest male then moves up.

Coral gobies choose their sex freely

The sex change in coral gobies is more complex. Because of many predators and the sedentary nature, both females and males are able to change sex. Two fish that meet each other can always form a heterosexual pair. (Id.)