Snapping turtles, part II: hyperexcitability, supercooling, and the inevitable recolonisation of Europe in the Anthropocene
More on the fantastic snapping turtles, this time focusing on how cold tolerant they are, and what this might mean for their distrubution. Recall that the term ‘snapper’ refers specifically to Chelydra serpentina, and is never used for alligator snapper Macroclemys temminckii.
It turns out that there are questions, not just about how big alligator snappers get (see previous entry), but also how far north they might occur. While, ‘officially’, they don’t get further north than Iowa, there are reports of alligator snappers from Canada (Anon. 2003). You might think that these were snappers (as these are definitely present that far north), but witnesses say that the animals were just too big for that (you’d also think they’d report the distinctive eggbox-like carapace knobs, but no details on that). The northerly occurrence of snappers is interesting, as not only are they cold-tolerant, they’re really cold tolerant, and able to tolerate literally freezing conditions. They hibernate under water (Ultsch 1989), of course, but they’ll also remain active at temperatures as low as -2 to -4ºC. They can tolerate supercooling down to -6ºC, and specimens cooled to as low as -9ºC have remained alive (Costanzo et al. 1999).
Clearly, they are adapted for dealing with low temperatures. Like other cold-adapted reptiles and other ectothermic vertebrates, they exhibit behavioural hyperexcitability when chilled (that is, they suddenly become apparently alert and vigorous), and (counter-intuitively) increase the number of movements they make when exposed to really low temperatures. Apparently, hyperexcitability and an increase in rate of movement ‘may stem from failure of inhibitory synapses, which are more thermally labile than excitatory ones’ (Costanzo et al. 1999, p. 165; Prosser & Nelson 1981). This may be advantageous as it encourages the animal to move away from a potentially lethal environment. To restate that in more simple terms: extreme cold makes the animals become twitchy and restless, and this perhaps encourages them to move elsewhere. This behaviour is thought to be adaptive as it doesn’t occur in species than don’t have to deal with extreme cold, nor does it occur in species that are even more cold tolerant than snappers (e.g. Painted turtles Chrysemys picta).
So this brings us on to something else: given that they’re kept as pets a lot in Europe, and given that – like all pet turtles - they’re good at getting themselves released (i.e., there’s something about turtles that makes people want to go chucking them out into the wild), will we end up having them as ferals in Britain and elsewhere? The answer is yes, and… they’re already here, at least judging by the number of individuals that get reported (Beebee & Griffiths 2000).
Already there is much concern over the many feral Red-eared sliders Trachemys scripta we have in Britain. Though it might seem ‘nice’ to have sliders in the wild, they’re presumably eating lots of our already beleaguered fauna, and preliminary studies indicate that they have a heavy impact on lissamphibians. The ecological impact of snappers - bigger, nastier, hungrier - would be greater. At the Zoologica 1997 Convention I spoke to a group of ecologists who were studying the impact that feral freshwater turtles are having on British natives. I took the free paperwork they provided, but have since lost it. They confirmed that there are now indeed enough feral snappers for us to start worrying, though I don’t think they had an accurate head count or anything. But you can verify that they’re there by just keeping an eye on the press: in July 2003 a snapper was captured near Walsall in the West Midlands, for example.
But if they are here, could they maintain populations, or breed? Alderton (1988) thought that this was unlikely, stating that the climate here is too cool to allow enough successful breeding to occur. However, snappers can maintain viable populations ‘where the growing season has at least 100 frost-free days per year’ (Tarduno et al. 1998). Most of Britain has less than 100 frost-free days per year, so that criterion is fulfilled. More generally, Tarduno et al. (1998) wrote that viable populations ‘do not occur in areas with a warm-month average maximum temperature of less than 25ºC. This measure corresponds to a warm-month mean temperature of 17.5ºC and a mean annual temperature of 2.5ºC’ (p. 2241). After some internet research on the British climate, I found that England has a warm-month average maximum temperature of about 26ºC, a warm-month mean temperature of 15.6ºC, and a mean annual temperature of between 8.5 and 11ºC. The mean warm-month temperature is thus a little too low for snappers, but only a bit too low. It’s therefore dangerously close to likely that snappers could survive here for ever, without trouble (well, until the next glacial cycle kicks in, or until the North Atlantic gyre switches off), and as our mean temperature is set to rise, this is going to be even more true. Add to this the fact that snappers live for decades: say there are feral individuals here now, able to survive fine but not to successfully breed… well, given that they’ll still be here in a few decades time, they have the potential to stock our future waterways with their hatchlings.
This brings us on to something else. Though restricted to the Americas today, chelydrids once ruled the world, with multiple fossil species inhabiting Europe, Asia and possibly Africa (Gaffney & Schleich 1994). Why did they become extinct in these areas? To my knowledge, such questions have only been asked for the European fauna. Here, chelydrids are present right up to the middle Pliocene (Broin 1977) at least… in fact, Delfino et al. (2003) put a question mark for the presence of the group [their Fig. 1] in the late Pliocene, implying that there might be late Pliocene European fossils of the group, and Gaffney & Schleich (1994) stated that Chelydropsis extended into the Pleistocene. In asking ‘what happened to the herpetofauna?’, Delfino et al. (2003) suggested that the loss of chelydrids (and other taxa, including cryptobranchid salamanders, varanids and elapids) from Pliocene Europe might have been triggered by the climatic cooling that kicked in at this time. While living snappers are cold tolerant, they cannot persist where mean annual temperatures are too low, as we saw above.
So glacial conditions appear to have caused their extinction in Europe, and presumably in Asia too. Why didn’t they persist in southern Asia, the Middle East, or Africa, where they also occur as fossils? Good question.
Still more to come on chelydrids in a future post (now available here). And for the latest news on Tetrapod Zoology do go here.
The excellent alligator snapper photo above is from cyberlizard's excellent site.
Refs - -
Alderton, D. 1988. Turtles & Tortoises of the World. Blandford, London.
Anon. 2003. Turtl-ey amazing. Animals & Men 31, 12-13.
Beebee, T. & Griffiths, R. 2000. Amphibians and Reptiles. HarperCollins, London.
Broin, F. De 1977. Contribution à l’étude des chéloniens. Chéloniens continentaux du Crétacé et du Tertiaire de France. Mémoires du Muséum national d’Histoire Naturelle C 38, 1-366.
Costanzo, J. P., J. D. Litzgus and R. E. Lee. 1999. Behavioral responses of hatchling painted turtles (Chrysemys picta) and snapping turtles (Chelydra serpentina) at subzero temperatures. Journal of Thermal Biology 24, 161-166.
Delfino, M., Rage, J. C. & Rook, L. 2003. Tertiary mammal turnover phenomena: what happened to the herpetofauna? Deinsea 10, 153-161.
Gaffney, E. & Schleich, H. H. 1994. New reptile material from the German Tertiary. 16. On Chelydropsis murchisoni (Bell, 1892) from the Middle Miocene locality of Unterwohlbach/South Germany. Courier Forschungsinstitut Senckenberg 173, 197-213.
Prosser, C. L. & Nelson, D. O. 1981. The role of nervous systems in temperature adaptation of poikilotherms. Annual Review of Physiology 43, 281-300.
Tarduno, J. A., Brinkman, D. B., Renne, P. R., Cottrell, R. D., Scher, H. & Castillo, P. 1998. Evidence for extreme climatic warmth from Late Cretaceous Arctic vertebrates. Science 282, 2241-2244.
Ultsch, G. R. 1989. Ecology and physiology of hibernation and overwintering among freshwater fishes, turtles, and snakes. Biological Reviews 4, 435-516.