Sex Determination in Reptiles

Is it a boy or a girl? It’s a simple enough question to answer you’d suppose, but if you’re talking about reptiles, you might need to think again!

A mammal’s sex is usually defined by a fairly straightforward matter of chromosomes – the familiar XY for male and XX for female. Birds follow the same general idea – but a little confusingly, they use different sex chromosomes and swap things around, ZW this time being a female and ZZ the male. Some reptiles share the mammal approach, while others are bird-like and since the whole thing is based on genetics, it’s called genotypic sex determination (GSD).

The fun really starts, however, when you start looking at some of the oldest members of the reptile family tree – the crocodilians, turtles and New Zealand’s unique Tuatara – all of which use temperature-dependent sex determination (TSD). For these animals, the temperature of the eggs during a critical period of incubation decides whether the embryo inside develops into a male or a female – and often it’s only a few degrees that makes all the difference.

It’s Cool To Be A Girl – Well, Sometimes!

For tortoises and turtles, for instance, a warm nest will see all – or nearly all – of the hatchlings emerge as female, while a clutch incubated a little cooler turn out male. Many species of crocodilians reverse this with females hatching from cooler nests, while others seem to produce females at both higher and lower temperatures, with maleness determined by being incubated somewhere in between.

When it comes to the Tuatara, however, it’s definitely cool to be a girl! According to research at Victoria University, eggs incubated at 22 degrees C (72F), 80 per cent will be male; at 20 degrees C (68F), that entirely reverses – and drop that to 18 degrees (64F) and they’ll all be female.

Simples? No, Not Really!

Just when you think you’ve got all this GSD-TSD stuff nicely sorted, reptiles have another little trick up their scaly sleeves – and it’s an absolute blinder! In some species, sex determination can involve both chromosomes and temperature, with warm or cold incubation able to change the route originally mapped out for the developing embryo by genetics.

The investigation of this aspect of reptilian biology is fairly new, so just how the mechanism behind it all works isn’t fully understood right now, but there is some evidence to suggest that it could be more widespread than you might think. One Australian skink, for instance, normally uses the X and Y chromosomes to determine sex, but it has been shown that a low incubation temperature can effectively turn genetic females into males, leaving them able to function reproductively in their new “wrong” role. For this little lizard, it means – somewhat bafflingly – that males may be XY or XX, and XX can be male or female! If that isn’t confusing enough, there’s another Australian lizard in which high incubation temperatures can make a chromosomally male animal into a female. This species uses the bird-like Z and W sex chromosomes – so for it, females can be either the “normal” ZW or a “changed” ZZ, though any male has to be ZZ .

Why So Complicated?

One idea that has been put forward to explain why some animals use both GSD and TSD systems is that they are on a sort of evolutionary journey, changing from using one system completely, and beginning to adopt the other. It’s certainly an intriguing suggestion but for the moment it is nothing more than speculation. The truth is, nobody knows why some species go about things in such an apparently complicated way – and it’s not just reptile that do it; there are good examples of the same mechanisms in some kinds of fish and various amphibians too.

What advantage allowing temperature to determine sex could have has also been the subject of much debate – but little real progress. Obviously this could, in theory, enable a mother to adjust the sex ratio of her offspring by deliberately manipulating the temperature of incubation, but this seems a pretty far-fetched idea for reptiles. Parental care is unusual amongst these animals and of all the varieties that do make use of TSD, only the crocodilians really go in for nest tending; to date there is no evidence to suggest that they deliberately warm things up or cool them down to favour one sex or the other!

Sex determination in reptiles is a fascinating subject and for the moment at least, it seems it is destined to retain much of its mystery.

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