(27 December 2017) The Eastern Water Dragon is one of a relatively small group of reptiles whose sex is not determined by the chromosomes alone, but by the breeding temperature of the eggs.
Of many creatures we saw at our nightwalk in Joalah NP the eastern water dragon (top photo) stood out; not that it is that rare…. they’re quite common even in many gardens and parks with creeks …. but one normally sees them only during the day. That is when they are awake and visible. The last time I had seen them in the forest at night was in 2005 (second photo).
So, what is this high temperatures females issue? With mammals, birds, even many insects and most reptiles the sex of the offspring is determined by the genes. For instance human men have a couple of chromosomes called XY, while females have XX. After fertilisation of the egg, nothing will change that fact.
With some reptiles, particularly crocodiles and dragons, this is more complicated. Genetics do play a minor role, but sex is mainly determined by the temperature the eggs experiences when in the nest site. If the temperature is relatively low, then the young will be females. At a higher temperature they will become males, but if the temperature is even higher, than the offspring will hatch as females again: the high temperature women are born (not to be mixed up with highly temperamental women!).
Interestingly, the females in this latter group are genetically males: they have male chromosomes, but will live a female life and are able to procreate like females. The mechanisms of this strange phenomena are not fully understood.
What would be the evolutionary advantage? One reason is that – for some smaller dragon species – at low temperatures, early in the season, the number of females will be high, allowing them a longer period to mature and therefore to reproduce before the winter kicks in. It also seems that in a nest with only males OR females, instead of males AND females, the health of the offspring is higher: more young survive their first year on the planet.
On the other hand, the major downside currently is that climate change may have a devastating effect: the rapid change in temperature will outpace the ability of these creatures to adapt. At some stage they might only create high temperature females, which will lead to extinction. It is one of the theories that explains the extinction of the dinosaurs 65 million years ago; because of global cooling after a meteorite impact they only produced low temperature female offspring. With no males around, that was it. Our ‘male’ role in reproduction might be small, but is crucial nevertheless.
f/6.3, 1.3 seconds, tripod, no flash, only torch light. By setting the white balance to match the temperature of the torch light, the real colours show up. Isn’t he (she?) pretty? Total size of the subject including the tail: app. 70 cm.