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.