Science Questions

How does temperature determine sex in some species?

Sun, 29th Nov 2009

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Question

Joel Hillman asked:

How does temperature determine sex in some species?

Answer

Helen -   That’s a great question.  It’s a wonderful phenomenon that happens in lots of reptiles, and it also happens in some fish as well.  Instead of having genes or chromosomes, that determine whether or not an embryo turns into a male or female, it depends on temperature.  So for example, alligators: they will lay their eggs in a nest, and if it’s incubated at around 30 degrees, they will all turn into females.  If they’re incubated at 33 degrees, they will all turn into males.  And if you get temperatures in between those two, you get a varying mixture of males and females in a different ratio.

So, clearly, the environment is telling the animal whether to turn into a male or a female.  And it’s a wonderful question to think about how this happens, and also why this happens.  Why should an animal benefit from letting the environment determine whether or not its offspring turns into male or female? 

Some of these questions were answered by a study in Nature last year by Daniel Warner and Rick Shine from the University of Sydney. They were working with these wonderful creatures called Jacky Dragon Lizards from Australia and they look wonderful, and they sound wonderful.  And they have this temperature-based sex determination.

But another good thing about them is that they've got quite short life spans, because the big problem with studying this type of sex determination in things like crocodiles and turtles, which also do this, is that they live an awfully long time.  So to really to get the grips of what’s going on they’re not an ideal subject. 

But these little lizards only live for about three or four years and what they’ve shown is that a key event in the sex determination of these lizards is the conversion of testosterone into oestradiol, a form of oestrogen.  This is brought about by an enzyme called aromatase, happens at very low temperatures and tells the developing dragon to become a female.

What they did in the paper was to override the enzyme, and by blocking it they could artificially turn males into females even if they were being incubated at a female temperature.  What that showed was that over a number of breeding seasons males actually have more babies if they were hatched at a normal temperature than if they were hatched at a female temperature.  So really what they’re showing, and they’ve done this the other way around as well, is that if you’re forced to be the wrong sex, you’re not as good at having babies.  

So that reveals a bit about why this has evolved in the first place, and it gives us an idea of how it happens as well.  But it doesn’t really explain in nature why it should occur.  There has to be a benefit.  It has evolved.  We see it in many different species. So there have to be reasons why, at different temperatures, males do better than females because that’s what we see in all these different animals.

This also leads us to think about maybe an ongoing problem we might have to face with climate change: that if in these animals their sex is being determined by a temperature and if that temperature is going up, especially of things like fish and the oceans, which we know may increase in temperature, it could start causing all sorts of trouble.  So we will see.  But it’s all rather wonderful and fascinating.

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Joel Hillman asked the Naked Scientists: Naked Scientists, I had been taught that some animals genders can be decided by the temperatures and some other factors, for example, a warmer temperature will induce a male. But now with my further studies (good old medicine!), in humans at least, gender is decided at conception, by combinations of X and Y chromosomes. I would have assumed that these these would have been the same with humans and animals. Could you explain this please? Joel Johnny was a chemist, Johnny is no more, What Johnny thought was H2O was H2SO4 What do you think? Joel Hillman , Wed, 27th May 2009


It is advantageous for male crocodiles to become as big as possible, this can only happen when conditions are warm producing the maximum vegetation and consequently the maximum amount of prey.

Also warm conditions enable the cold-blooded croc to be most active and able to catch more prey and become big as possible. RD, Wed, 27th May 2009

But how does it actually affect it? the advantages you stated are clear, but what is the physiological effect?
Thanks Shadec, Fri, 12th Jun 2009

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