The ability to change sex from male to female or vice versa (what is known as sequential hermaphroditism) is surprisingly common in the animal kingdom, especially amongst certain species of fish, jellyfish, and gastropods. Many reef fish such as wrasses, for instance, that are born female are able change sex to male (protogyny) during their lifetime. This is usually driven by the loss of the dominant male, allowing the largest (formerly) female to then take control of the harem.
The reverse, when a male becomes female, rarely occurs amongst these fish species, and the infrequency of these events makes it difficult for scientists to understand why this might ever happen. So in order to get to the bottom of this, a group of researchers from Japan set up a field experiment with the bluestreak cleaner wrasse (Labroides dimidiatus).
The largest males of this coral reef species normally have complete control over mating rights with a group of females (polygyny). As a result, males are seldom left without a mate. But the researchers wanted to see what would happen if these males did become widowed. So they removed most of the females from the study site and carefully observed the behaviour of the remaining males.
If foreign individuals were allowed to move into the study site, then the widowed males would always pair with the new arrivals. They did this regardless of whether the new individual was female, juvenile, or even another widowed male. So long as this type of immigration continued to occur, then the widowed males never attempted to move out of their territory.
It was only when this immigration ceased that the surviving males actually moved out to search for a new mate. They paired with the nearest single individual that they could find, again, whether they be male or female. And it was in this type of situation, when two males formed a pair, that the smaller fish then changed sex to become female. The pair would begin a courtship ritual, finalising in an upward swimming spawning behaviour with the smaller taking the female position.
So it seems that these unusual reverse male-to-female sex changes evolved out of a need to find new mates in catastrophic situations where the local population is massively reduced. Even in such dire circumstances, it is imperative for the surviving fish to avoid any extra movement that might expose them to more predators. As a result, they simply pair with the nearest individual possible, and even change sex if necessary.
Reference: Kuwamura T, Kadota T, & Suzuki S (2014). Testing the low-density hypothesis for reversed sex change in polygynous fish: experiments in Labroides dimidiatus. Scientific Reports, 4 PMID: 24621782
Image: Nautilus Scuba