We spent an unpleasant few hours the other week reading the opinions of 'experts' on various internet tortoise forums and on social media. Talk about 'facts' that are totally divorced from reality. So many opinions stated as firm and accurate facts with an incredible amount of certainty and confidence! Yet, so many of these opinions, and especially the advice based upon them, are complete nonsense. Untrue and completely baseless.
We will revisit this subject again as there are so many aspects to it that it is hard to know where to begin, however, diving in at random, let's look at a relatively simple one, and a very easy one to answer evidentially.
What is a safe minimum temperature for a Mediterranean tortoise (Testudo graeca) overnight?
A simple enough question.
Yet, the misinformation begins immediately. The first thing that comes up is what the age of the tortoise is. Some examples quoted from these sites:
"Adults are OK at 20 Celsius, but juveniles need at least 30 Celsius continuously"
"You can let adults go as low as 15 Celsius but you have to keep the babies at a minimum of 25 Celsius"
"If you let either adults or juveniles fall below 12 Celsius the adults will go into hibernation and the babies will get sick and die"
"If the temperature falls below 37F by any chance the tortoises will freeze to death"
This stuff is routinely presented as FACT.
It is nothing of the kind.
Here's the reality.
Adults and juveniles experience, in every practical sense, the exact same temperatures as adults living in the same habitat. There is no difference at all. This should really be obvious if you think about it, but thinking rationally appears to be an ability in very short supply on these platforms! What do they think happens? That the juveniles book a flight to a warmer climate when it gets chilly? Or, that somehow they have secret microclimates available that stay warm while the surroundings get cold? Of course not. They do not migrate to a warmer place and there are no 'hotspots' available that only juveniles can access.
They live side-by side, in the exact same habitat, experiencing the exact same temperatures. Unbelievably, one of these 'experts' was confidently assuring people that "juveniles go to the top of a hill where it is warmer, while adults stay in the cooler valley below". No. They don't. Not only is this nonsense in the sense that no such age-related migration of that kind takes place, but generally speaking, the higher you go in altitude the colder it gets, not warmer.
As the sun rises in the morning, provided that it is not an overcast day (and even on overcast days, some solar infra-red still reaches the surface, and tortoises are highly efficient at taking advantage of this), the carapace is the first part of the tortoise to warm up. It is worth noting that at the beginning and end of the activity season, there are very often many successive days where it is cold, wet, and even with some frost or hailstones. Torrential, cold rain is also quite common. Overnight ambient (air) temperatures will sometimes fall to -2 Celsius, or exceptionally to as low as -5 Celsius. Tortoises respond to this by digging deeper into their protective scrapes. Even a small depth of soil cover provides really good insulation and protection. Even where the surface temperature is at or below freezing, just a few Cm underground the temperature will typically maintain a minimum of 4-5 Celsius.
In the above case, the surface temperature was -1 Celsius, yet at a depth of only 7cm +5.4 Celsius was recorded. Tortoises are instinctively aware of this, and use it to great effect.
Again, to stress, both adults and juveniles are exposed to identical conditions and respond in the same way.
Depending upon the precise prevailing conditions, various parts of the tortoise can reach quite surprisingly low temperatures. For example, -0.8 Celsius or -1.7 Celsius. In theory, this is potentially dangerous as extended exposure of the tissues, especially the eyes, to such temperatures could result in damage. Again, however, tortoises handle this quite well by tending to angle the head downwards when buried and by pressing deeper into protective scrapes.
In the above case, as the tortoise emerged on a cold morning, the plastron temperature measured only 1.6 degrees Celsius (34.88F). By basking, however, it would soon raise the carapace temperature and this would permeate through the entire body rapidly. In this context it is important to know that radiant solar warmth is far more efficient at deep-tissue penetration than the infra-red wavelengths emitted by typical artificial heat sources.
Tortoises can also be surprisingly active in cold and wet weather. In this example we found this male Testudo graeca graeca active in early December in Murcia. The ground was very wet, and very cold, By basking in what little sunshine there was, however, he subsequently managed to raise his body temperature to 24 Celsius.
A tortoise prioritises protection of the head in an overnight scrape in cold, wet weather.
This juvenile emerged after a night where ambient temperatures fell to -0.5 Celsius but by basking soon elevated its core body temperature to +25.3 Celsius. One area where juveniles and adults do differ is that the smaller body mass of juveniles means that they respond more rapidly to basking than do larger adults due to thermal inertia.
All of the tortoises shown here are healthy, wild Testudo graeca graeca measured in their natural habitat. These temperatures are not exceptional. At certain points in the year they are entirely typical.
It is also worth noting that of all the 'Testudo' group, Testudo g. graeca is reputed to be by far the most 'delicate' or fragile. Certainly the temperatures and conditions experienced by T. ibera in parts of its range are far, far more severe than those experienced by Testudo g. graeca here in Spain or North Africa.
So, to conclude. No, they do not require a minimum of 30, 25, 15 or any similar double-digit overnight temperature measured in degrees Celsius to survive or be safe. There is zero basis for such claims, and any 'expert' who vehemently insists that they do is no 'expert'.
Obviously, we are in no way suggesting that you should deliberately subject tortoises to freezing conditions or extended periods of cold, miserable weather. What we are saying however, is that some of the 'facts' that you will read on the internet or on social media are demonstrably untrue and are grossly misleading. The thermodynamics of basking reptiles, and their overall biology and ecology is much more subtle and complex than many believe. We do them no good service by propagating false information or by spreading baseless myths.