Skink Sex Selection

And now my much awaited post on sex selection in skinks.

With mammalian arrogance we think of sex selection as something that happens genetically. Two XX chromosomes makes a female, and X and a Y makes a male. Occasional variations exist, such as people who are XXY, but usually that’s how it works.

However, many reptiles do it differently. For them, sex selection is often linked to temperature (TSD). Turtle eggs incubated at higher temperatures are female, at lower temperature male. There is a cross overpoint where the numbers come out about even. Tuatara do the reverse – higher temperatures mean more males.*

Ok, so that’s a bit weird when you first encounter the idea, but you ain’t seen nothing yet. Tasmanian snow skinks use Temperature-dependent sex selection when living at sea level, but determine their sex genetically at altitude. This might not seem that big a deal until you realise we are not talking about two different species here. (However, the behaviour of an individual does not vary with altitude, it’s a pattern developed over many generations). It’s one species, perfectly capable of interbreeding, although the University of Tasmania scientists who discovered this have yet to take the obvious step and see what happens if you breed a male from the high country with a sea-level female or vice versa.

Unlike most species using TSD skinks give birth to live young. Sex depends on how long the mother gets to sun herself on warm rocks while pregnant.

The discovery offers extraordinary opportunities to investigate the mechanisms of the two ways of selecting the sex of offspring, including the much debated question of whether parents actually exercise a choice – do egg laying species place the eggs in warmer conditions when circumstances favour one sex, or do they lay the eggs the same way but produce more of one sort of offspring in warm years. The question is important as we head for an warmer environment, which may see huge predominance of reptiles of one sex unless the parents can adapt.

There is an evolutionary reason for the skink behaviour, but I’m not going to explain it unless someone bothers to ask.

*Reading Wikipedia I’ve discovered there is another form of TSD, where you get males in the middle and females at each end, which is pretty whacky in itself.


About Stephen Luntz

I am a science journalist, specialising in Australian and New Zealand research across all fields of science. My book, Forensics, Fossils and Fruitbats: A Field Guide to Australian Scientists is out now through CSIRO Publishing. I am also a professional returning officer for non-government organisations. I'm very politically active, but generally try to restrict this blog to scientific matters.
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8 Responses to Skink Sex Selection

  1. So in these temperature-based sex determining animals, both genders have the same chromosomes then?

    This confuses my tiny mammalian brain, because obviously different characteristics are being exhibited … so would it mean that different genes are turned on and off by the temperature differences?

    How robust is it – I mean if you get males at (*pulling number out of hat*) at an average of over 25C and females at under 25C then what happens if you hit exactly on 25C? Or is it more statistical, so that the higher the temperature the higher the percentage of male offspring sort of thing? Same question with the altitude I guess, what happens to the Tasmanian Snow Skinks that live halfway up the mountains instead of all the way up?

    And OK, how could TSD give you an evolutionary advantage? My immediate thought is that skinks are cold blooded and the ones living at altitude are probably in an environment that’s more difficult for them so in colder years (which are harder for them) they need more females to give them a better chance of perpetuating the species perhaps? You didn’t actually say which temperature provides which outcome though, so I may be totally off the mark here! Alternatively it could be food-based, that at altitude there’s less food in years which have more snow? Or … at this point I’m pretty sure I could argue myself into almost anything so you’d better put me out of my misery!

    I really love reading this information – it’s invariably stuff that I have nooooo idea about! THANK YOU!!!

  2. Yes, somehow (and I’ll admit I don’t understand how) particular characteristics are turned on or off by the temperature of incubation.

    I think it is statistical. Certainly there is a cross over zone (I think usually about 2 degrees) where you get some of each, with more males at one end and more females at the other. In terms of the skinks from halfway up the mountain, at this stage they don’t know. It’s a new discovery which has opened a lot of questions, and of course they’d love to get money to look into as many of these as possible.

    In terms of the evolutionary advantage, it goes like this. It seems that when mating time comes larger females produce more offspring. However, unlike in so many other species, larger males don’t seem to – they haven’t been observed fighting over mates and whatever makes female snow skinks choose one mate over another*, it doesn’t seem to be size.

    That being the case, it’s an advantage for a mother to give birth to females early in the season, and males later on. The females get a few crucial extra weeks growing time before it gets to mating season, making them larger and more likely to pass on the genes. The males can be born any time. With breeding done in autumn, TSD is an easy way of ensuring more females are born early, and more males born late.

    This raises the obvious question – if there is an advantage for TSD, why don’t the high altitude skinks use it? Well we have a partial answer to that. At high altitude the skinks grow more slowly, and are not ready to mate the same year they’re born. By the time the following mating season comes around they’ve been growing for almost two years, rather than less than one – the benefits conferred on a female from being born a few weeks early have been washed out by that time, so TSD doesn’t help.

    What I don’t understand, and couldn’t get a clear answer on, is why determining sex genetically is better at high altitudes. Presumably it is, because if TSD has advantages lower down the mountain, and is neutral higher up, one would expect it to spread through gradual mating of skinks near the cross-over point. Both by logic and from the conversation with the scientist it seems clear there is an advantage to genetic selection which causes it to triumph when TSD’s benefits don’t outweigh, but I’m not sure what that is. My guess is that it avoids the danger that if you have a few hot years in a row you end up with no males in the population or (much worse) a few cold years and there are hardly any females. However, that’s just a hunch.

    Generally speaking species where the heat produces more females should be able to cope with climate change ok – they’ll have a predominance of females, but the surviving males will just have a great time and breeding will not be too impaired. Eventually they’ll probably adapt their temperature settings so that the threshold is a bit warmer and they’ll continue on as before.

    Tuatara’s and their ilk, are in deep trouble (well frankly Tuatara’s are in deep trouble generally, this is just one more problem for them, but even more common species may be threatened). Lots of males and few females means few offspring, almost inevitably sending populations into downward spirals.

  3. Wow! It’s so very cool (no pun intended!) and does make sense now that you explain it. Makes sense that larger young females are better partners too – they (erm, we?) have to give so much physically to the offspring!

    I am definitely curious why the TSD hasn’t spread, too. Perhaps non-chromosomal gender (is that the right term?) has other disadvantages in the longer term? Or perhaps the change is evolutionarily recent and just has not had time to spread – if it were not advantageous in the higher altitude populations it would not spread very fast… I could not even guess at a timeframe, not being a biology geek in the slightest.

    I am constantly amazed by how much we don’t know about biology – by it’s unfolding complexity and beauty and how much everything affects everything else… just the fact that my own body can be so very broken and still function seems miraculous to me! And it’s so sad that so much of this is being lost before we ever have a chance to discover it.

  4. It seems clear that TSD is reasonably old and has had time to spread.
    Given that Tuatara, the last surviving dinosaurs, use it, it’s a fair bet that at least some dinosaur species did too.

    Moreover it has definitely evolved multiple times. The fact that in some species high temperatures mean males and in others high temperatures mean females is an indication of independent evolution. I’m not aware of it existing in mammals (or birds) so it may not be possible with us for some reason, but it exists in many reptiles and some fish. However, certainly not all of either use it, which suggests there is some downside, but the only one I can think of is having a hot or cold season produce too many offspring of one sex for a healthy population.

    I agree about how amazing biology is, and of course what a tragedy that we might be losing species as fascinating as this without ever knowing about it. Fortunately snow skinks do not seem to be endangered, but who knows what other species out there are similarly astonishing, but will become extinct before we learn about them.

  5. I’m never certain what that “the last surviving dinosaurs” stuff means, honestly. It’s like there’s an implication that they stopped evolving, which makes no sense. If we already know that TSD evolved multiple times in multiple species, why couldn’t it have evolved recently (evolutionarily recently, at least) in the Tuatura? It’s not the sort of thing that’d leave a fossil record, after all…

  6. Last surviving dinosaurs just means that they are the only species that is close enough to dinosaurs to be grouped with them, rather than with lizards, or birds or whatever. Of course these groupings are a bit arbitrary, and biologists sometimes talk about them as if there was a definite right answer, and it was just a matter of working that out, rather than a series of options none of which are really right. Stephen J Gould wrote some marvelous articles on this – one I remember was on trying to work out whether the three species of zebra really are a group, or if one actually belongs somewhere else.

    It’s certainly possible that TSD evolved recently in Tuatara – as you note it doesn’t show in the fossil record. Nevertheless, they are a species that has evolved very slowly. My understanding (and I’ll stress I’m no biologist) is that when a species evolves slowly in a lot of ways it doesn’t tend to evolve rapidly in others, but even if I’m right on that, a general trend doesn’t mean it hasn’t happened in this case, so yes I was jumping the gun a bit there to say it was likely some dinosaurs used it – but it is at least possible.

  7. Hmmm. I really wish I knew more about biology! That just makes me wonder why some animals evolve “slowly” compared to others with similar expected lifetimes …

    (Yeah, I was that pain in the ass in high school who always asked the teachers questions they couldn’t answer!)

  8. I’ve wondered that myself. Part of the answer seems to be the rate of chance of environmental conditions, but it seems that’s not the whole story. I’m not sure how well the rest of the explanation is known – I’ve tried to ask the question of zoologists on occasion and not got a clear answer, but I’m not sure if that was because they actually didn’t know, did but couldn’t explain it, or they had a reasonable explanation that I just couldn’t follow.

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