Scottish reptiles, and particularly adders, continue to amaze me with their ability to cope with the cool and often inclement weather which they face on a regular basis.
On the 16th April 2008, I was surprised to find a female adder basking in 13°C and overcast conditions.  Since that time I have found them active in even lower temperatures, but on 2nd May this year I was absolutely stunned to find two adders and six slowworms basking in overcast and showery conditions in air temperatures of only 6-7°C.

I hadn’t found them before in such low temperatures because in normal circumstances I would not have looked; I would have cancelled the reptile search on the basis that reptiles would surely be sheltering unseen from the cold and impossible to find.
On this occasion however, the event was an organised guided walk through prime habitat in Glen Esk, Angus, and as people were enrolled and expected to turn up hoping to see our native reptiles, it seemed only fair to at least go through the motions and see what we could find.  As a fall-back, we had some artificial refugia to check so perhaps all was not lost. 
But incredibly, within minutes of starting the walk, we found the first female adder at around 10:15am, laying on dead bracken in classic basking posture.
After a few initial photos, the adder began to move off, perhaps a little more slowly than it might than if it had been fully warmed as it would be in sunny weather.  As a few more pictures were taken, including the one shown, a shower came over, which fell as hail!

Nearby, several slowworms were also found in the tops of dead bracken piles.  They were so cold, it seemed as if they could barely flick out their tongues, and put up no resistance at all when handled.  However they must have been there by choice, as the previous night had been very cold, as low as 2°C in places so they would surely have been under cover before daybreak, only venturing out in search of whatever warmth was available.

Further along the walk a few more slowworms were found out in the open and a few more hail showers fell, one of which turned to snow for a few minutes.  It was bitterly cold in the slight breeze; I could barely feel my fingers as I clutched the metal pole of the monopod attached to my video camera!  The day seemed to be getting colder rather than warmer; in reality the temperature remained the same throughout the day, no better than 7°C ambient.  We checked the temperature at ground level (I always carry a digital thermometer during reptile and amphibian surveys) and found that it was no different; not surprising as there was no sunlight strong enough to warm the ground, and also the rain and hail were probably having a cooling effect.

We stopped for a picnic at the far-most point of the walk, which was halfway along the banks of Loch Lee.  I regretted that I had chosen to carry only water for the walk, rather than a nice hot flask of coffee!
After the picnic we continued searching the banks of the loch and found the second adder.  This individual was also “basking”, and tolerated our presence for a few minutes before moving off into a clump of heather, hissing as she went.

I had thought we were very lucky to see the first adder.  Two adders displaying the same behaviour couldn’t be coincidence though, could it?  There must be more to this.
One of the party suggested that the basking adders would be benefiting from UV radiation even if they were not thermoregulating.  I would concur with this and it is possible that the adders we found had recently eaten and were trying to promote the digestion of their meal.  However, this would surely require a certain calculation of risk on the part of the individual between the benefit of basking against the likelihood of predation and reduced flight-response time due to low body temperature?

To keep things in context, adders are common in the area we were searching although still not easy to spot.  On a good day, we would expect to count adders into low double-figures.  So to find two on a day when you might expect to find none does seem exceptional, and whilst it is only two individuals, it does demonstrate that these animals are quite capable of operating in very low temperatures.

Other observations lead me to ponder over the traits of northern reptiles compared to their southern cousins.  Earlier this year, on the 12th March, I had found common lizards basking in only 9°C ambient air temperature.  On this occasion however, there was bright sunshine so the temperature on the ground would certainly have been warmer, although the easterly breeze was creating a chill-factor, and this is the earliest date I have ever found common lizards in Angus.
I have wondered for some time now if reptiles in the north of the country have adapted to cope with the cooler climate.  For example, in common with much of the highlands, in the Angus Glens the winter can be six months long, meaning a very long hibernation period for reptiles.  Perhaps to build themselves up for such a long period, they need to be active as much as possible during the spring and summer, including days when the weather would otherwise be considered “unsuitable”.

The two adders photographed are at 260m altitude.  Reptiles can be found at much higher altitudes than this where the climate can be much more extreme.  I have found common lizards at 650m on the edge of the Cairngorms, where snow can lie from October to April.  How do they manage to survive such conditions?
I have also observed that reptiles in the north seem to grow slightly larger than field guides would suggest, particularly common lizards.  I have seen and photographed individuals which are at least 100mm S/V length.  This must give them an advantage when basking and maintaining body temperature, and allow slightly larger prey items to be consumed, thus increasing the range of food available to them.

Probably the most amazing thing about our native reptiles is the ability of the neonates to survive their first winter.  All three species, common lizard, slowworm and adder, give birth to live young (an adaption to the cooler climate which is not conducive to egg incubation).  The young are fully formed and independent from day one, but these creatures are tiny, particularly common lizards at no more than 18-20mm S/V when born.  They barely have time to build themselves up in the few short months before their first hibernation, yet still they emerge in spring after spending months almost frozen in a state of torpor.

Adders and common lizards are the most northern of all reptile species, even occurring in the Arctic Circle, so one would expect that they are tolerant of cold weather to some extent.  Common lizards also exhibit one obvious adaptation to climate across their range; in the south around the Mediterranean the females – taxonomically the same species as those in the UK – lay eggs during May/June rather than carrying the burden of young in their bodies during summer.  If these animals are capable of such differences in response to their environment, then it seems reasonable that more subtle adaptations must be possible over smaller geographic boundaries.
What our native reptiles lack in diversity they certainly make up for in the many facets of their behaviour.  I always look forward to their emergence each year and there are always fascinating new observations to make.

Trevor Rose
Friends of Angus Herpetofauna
secretary@thebhs.org