“The truth about Finding Nemo unveiled!” and similar headlines have been circulating the internet for a while now. If you don’t know the story have a little read here but to sum it up, scientists have pointed out that if Nemo’s mother died in real life, his father would actually turn into a female to replace his mother, and then Nemo would become his (newly-female) father’s sexual partner. Not the story I think Disney originally wanted to tell!
In fact, all clownfish are born as male and at any point in their lifetime can change their gender from male to female to ensure that there are always enough females in the population to mate with.
But clownfish are not the only species on the planet that can change gender. Organisms that change sex during their lifetime are known as sequential hermaphrodites, and there are many examples of these across the animal kingdom. Sequential hermaphroditism is common in plants, snails, slugs, worms and fish. Species of wrasse fish for example, can do the opposite to Nemo, with females at the end of their lifetime showing the ability to change to male.
As biologists we love to give everything a name so let me fill you in on the lingo: when males turn into females (like the clownfish) this process is known as protandry, and when females turn into males (like the wrasse fish), we call this protogyny.
Going back to the movies theme, you may recall another example of sequential hermaphroditism hiding in Jurassic Park when Dr. Grant discovers that the supposedly all female dinosaurs in the park are mysteriously breeding. The explanation given in the film is that the scientists used frog DNA to fill in the missing gaps of the recovered dinosaur genomes (the complete genetic material needed to create an individual). The scientists in the movie used DNA from the common frog (Rana temporaria) and in the wild the females of this frog species have been shown to spontaneously change gender to male. And so in the movie the frog DNA incorporated into the dinosaurs gives them the ability to change to a male gender, and eureka the mystery is solved!
But why do all these organisms change gender in the first place? Scientists believe this phenomenon first evolved due to the way in which these species chose their sexual partners. In protogyny hermarphrodite species the largest males are the most attractive to females. But it takes time to grow and become large and attractive. Individuals that are born female can therefore mate and reproduce until they grow big enough that they would attract themselves given the chance and switch gender. This means that during their lifetime they can see the experience of mating from both sides of the partnership. Pretty weird huh?
In protandry hermaphrodite species, as females age the number of eggs they can produce in one sitting increases. By being able to change gender during their lifetime, in both protogyny and protoandry species it allows individuals to get the best of both worlds; to be a male or a female when the time is right to maximise their reproductive ability. And so this is another fine example of evolution, which works to maximise your ability to pass on your genes to the next generation. Greater reproductive ability = more offspring (and so more of your genes left in the world when you die).
So sequential hermaphroditism; a pretty wacky, but incredibly clever way of getting more action.
Let me know what you think about this fascinating phenomenon in the comments below!
If you found this interesting, this article by National Geographic is a really great piece on a topic very similar; how organisms can ‘dress up’ as the opposite sex. Take a look!