I was thinking about this the other day in relation to Suzi’s population of garden slow-worms.

Temperate bat species have the ability to self regulate body temperature to within 1 or 2⁰C of ambient and will typically select cool to mild (2-10⁰C) sites with high humidity during winter torpor. Bats can also knowingly self rouse at any time no matter what the environmental conditions may be; however, this does result in a significant demand on the animal’s fat reserves, potentially impacting on the ability of the animal to survive the winter. Despite this, bats do regularly self rouse in winter to make physiological adjustments, respond to dangers, alter roosts, drink, sometimes eat/mate and probably also to facilitate a healthy immune system.

Obviously cold blooded animals in temperate climates cannot do this, which makes me wonder about the areas they do select for hibernation. I wonder a) what are the minimum active body temperatures required by our native reptiles? b) how important or finite are fat reserves in hibernating reptiles for brain function/aiding mobility in cooler conditions etc and c) what temperature/humidity conditions do optimum hibernation sites offer?

If I had to speculate, I would think ideal conditions for a hibernating reptile might include high humidity with temperatures often, or occasionally, above the ambient and hovering near the animal’s minimum active temperature. This would aid the reduced rate of metabolism typically required in hibernation, as well as providing minimum temperatures suitable for minor and occasional mobility. These must be important considerations for any animal that is torpid for extended periods of time (to avoid danger, make spatial adjustments, drink etc). Are there any studies/publications on hibernation requirements in the UK? Caleb, Will? It is an area I would like to explore further.

I wonder are Suzi’s slow-worms exploiting the artificial thermal properties of their surroundings and entraining their period of daily/circannual activity accordingly. There is clearly a physiological need for all non homeotherms to hibernate but i wonder what constitutes a minimum period of successful hibernation in reptiles, with clear physiological benefits come spring?  

There are a number of interesting questions raised in this post. Just some thoughts about one or two of them.

Starting at the beginning we perhaps need a slightly better definition of "immersed in water". For example, common lizards living in marshy/flow country habitats have been recorded as hibernating well below water level in air pockets trapped in the root structure. It is far from unknown for various reptiles to emerge after flooding so presumably they too are taking advantage of such air pockets.

The question of ideal temperatures for hibernating is far more complex. At one end of the scale we have animals such as the common lizard again that can withstand temperatures at or close to freezing. Not quite so extreme are terrapins such as European Pond Tortoises which can be seen to be active below the ice in ponds during winter - and that temperature cannot be far above freezing.

Humidity is another variable. Certainly a degree of humidity is desirable but it can be excessive. Looking at Lacertids, for example, from experience I know that sand lizards do not tolerate excess humidity. Green lizards and Schreibers Green Lizard are actually slightly more tolerant and common lizards extremely so. The Wall Lizard family are about as tolerant as sand lizards while animals such as the Balkan and Caucasian Green Lizard are far less so.

Although these statement move outside the UK species they do serve to illustrate the differences bearing in mind that they are all capable of living naturally in the UK and, indeed, occur in temperate climate ranges.

Incidentally, with the genus Lacerta, all that I know of appear to derive adequate physiological benefits from hibernation periods as short as two months. (based on hibernation commencing mid December and ending mid-February followed by normal reproductive activity/success) 

Hopefully this helps - although in practise I have no doubt that it will simply confuse further!

All the best

Chris

Thanks Chris. All these hibernation sites you mention must have at least one thing in common; they simply can't maintain temperatures below -1 on a regular basis. No living animal I can think of will tolerate freezing of the cell interior, although I believe a small number of frogs and turtles have evolved to produce a type of natural antifreeze.

Also since water reaches its greatest density at around 4⁰C and sinks, bottom hibernating turtles, terrapins, amphibians and possibly air bubble residing lizards!? do have the advantage of a stable and relatively mild environment that is unaffected and unaltered by the changing weather conditions (provided some form of respiration is possible). In this scenario the animal can perhaps a) maintain necessary minor activity functions, to promote health etc b) guarantee that it will remain defrosted throughout the winter c) and experience minimum loss of, or need to find, water.

I just can’t see any advantage in a cold blooded animal (or homeotherm) selecting anything other than a warmer than average winter site, (enabling some mobility to access different thermal gradients/elevations within the hibernaculum) or a site that is guaranteed to provide a safe retreat, with temperatures at the minimum activity range with water availability (e.g. lake bottom).

I still think humidity or access to water must play a massively important part in winter site selection (particularly in amphibians) since the need for regular water intake/avoidance of its loss is so intrinsic to the function of life. In our (at times) mild maritime winter climate it would be advantageous for an animal to respond to these changes in order to make physical adjustments and/or to exploit the necessary resources.

Cheers 

P.S. Thanks for yet another useful paper Caleb I will have a look in Appleby when i get home, please don't trouble yourself any more.

That is interesting, thanks Rob. The point I was making was that there has to be some form of antifreeze mechanism for cell 'freezing' to occur; meaning there is no animal that can technically be frozen, thawed and expected to function afterwards. 

I really didn't know the use of glycerol as antifreeze occurred in our native species, which is very interesting. Does this occur in many UK species (snakes etc) do you think? Presumably any hibernating reptile with fat reserves has the ability to produce it but how much of a drain on these reserves is glycerol production in say a Common Lizard and, as a result, how long can tissue be successfully 'frozen' for? I wonder is this a partial reason why Common Lizards retain such large fat reserves in the tail; to adapt them to more hostile, wetter and more prone to freezing environments. Presumably a lizard with a lost tail would not be at an advantage in a frozen hibernaculum! 

Species that do not use glycerol in cell protection would surely benefit from a warmer than ambient hibernaculum above the minimum activity temperature range, although it does seem logical to me now that all temperate evolved species of reptile would have this ability to some degree. I’m sure it’s all very complex between species!

A really interesting and current topic, given the recent weather, and thanks again for taking the time to reply Rob, Chris, Caleb....