Adaptive Value

Again, the question of why the bluehead wrasse changes sex has to be asked. What is the advantage of such a behavior?

The most accepted model of sequential hermaphroditism is the size-advantage hypothesis In this model, it is proposed that if reproductive success increases with size or age more rapidly for one sex than the other, then it is to the organism’s advantage that it changes sex in the appropriate direction (Ghiselin, 1969).

It has been reported that the proportion of IP males correlates with the population size of a given area (Munday et al., 2006). In large reefs, it is harder for TP males to defend their harems and IP males are able to experience high breeding success, thus allowing the proportion of IP males to reach as high as 50% (Perry and Grober, 2003). Likewise, in areas with a small to medium population, the reverse is true; there are fewer IP males and more TP males. This uneven distribution of IP and TP males is adaptive for the latter since they are able to control the favored breeding sites and monopolize the mating opportunities. Since the second situation is more common, the readiness with which a large female fish takes over in the absence of the dominant male plays an important role in ensuring reproductive success. Achievement of the territorial terminal phase status means that there is nearly a 30-fold increase in daily mating success (Warner, 1984). Therefore, it also advantageous for there to be small IP females as it is with them the newly sex-changed male mates with, thereby ensuring reproductive success and effort.

One TP male wrasse swimming with harem of females.
Image from

Key Terms

Adaptive Value
The overall fitness or advantages an organism has due to a particular trait.