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The Tortoise Myth-Busters: Episode 3

Updated: Mar 18

Can you rely on general climate information to know what your tortoise needs?

This is a fairly quick and simple one to deal with. We often see people in tortoise groups using weather and climate data (maximum, average or minimum temperatures, or humidity and rainfall) taken from weather maps or even travel brochures, often citing entire countries or regions, to decide what conditions a particular species of tortoise experiences in the wild, and then going on from that to try to provide those conditions in captivity. At first glance, this makes sense. But does it really?

In fact, it can be incredibly misleading. There are a number of reasons for this.

The first is that meteorological data from weather stations is collected in a very specific way. Temperatures are recorded using a very precise thermometer that is inside a white, reflective box with slatted sides so as not to impede airflow. This box is based upon a design by the Scottish meteorologist Thomas Stevenson back in the 19th C. This box is also double-roofed to prevent the thermometer being heated by acquired solar radiation from the roofing material and producing incorrect readings. That is also why the box (which is still known as a 'Stevenson Screen') is white, not some other colour. The idea is to reflect as much infra-red radiation as possible, as if it it did become warm, that would also influence the readings. The objective here is to record the FREE AIR TEMPERATURE accurately, and to achieve that the thermometer must not only be housed correctly, it must also be away from buildings, pavements, or other surfaces that directly add to the heating of the surrounding area if exposed to sunlight. This box is typically positioned from 1.5 to 2 metres above ground level, again to reduce surface influences and also so as not to obstruct airflow.

That is also, incidentally, why those electronic temperature display signs that you often see outside of pharmacies in places such as Spain are so grossly inaccurate. They are usually painted black and are attached to a wall. Therefore, they acquire a lot of heat in direct sunlight and this 'misleads' the temperature sensor inside. Very often they read as much as +15 Celsius too high on a summer day. Remember that the next time you see 'selfies' of tourists under such signs showing absurdly high temperatures.... they are, in fact, only reasonably truthful on cloudy days or at night.

At the risk of pointing out the glaringly obvious, tortoises, of course, do not live between 1.5 to 2 metres above the ground. They live ON the ground, and there is a truly massive difference between the thermal environment at ground level as opposed to the free-air temperatures far above it. This alone is sufficient to mean that any such meteorological temperature readings cannot be interpreted as being indicative of what the tortoises living in that region actually experience.

Here is a simple experiment that we conducted just the other day. We took a high accuracy, professional grade and recently calibrated Fluke 471 hand-held temperature and humidity meter and took our first reading at 1.5M above ground level. That gives a nice accurate reading of the ambient air temperature and the ambient humidity at that height.

This reveals an ambient (air) temperature of 29.2 Celsius and a RH of 33%.

We then immediately took a second reading at ground level, which gives a much more accurate reading reflecting what a tortoise would actually experience under exactly the same prevailing weather conditions. The difference is very substantial indeed.

Now we have an ambient air temperature of 34.3 Celsius (the kind of level where Testudo species will seek shade) and the RH has fallen from 33% to 26,1%

For good measure, we also used a non-contact infra-red thermometer to record the actual surface temperature in the same location and at the exact same time. That reading tells a different story yet again and is one that is often overlooked. The temperature recorded on this occasion is so high that tortoises would not easily tolerate it - despite the initial ambient reading suggesting that conditions would be conducive to activity.

So what is happening here? It is really quite simple. Solar radiation very quickly heats up the surface layer of the substrate well above that of prevailing ambient (air) temperatures, and far above that recorded on the 'official' figures you see reported by weather maps and climate charts. Remember, those are reporting free air temperatures above ground level. That heated substrate also very effectively heats the air immediately above it (at tortoise-level). This, in turn, also affects the ambient relative humidity, which is temperature correlated. Science demonstrates that warm air can retain more water vapour (moisture) than cold air, so with the exact same amount of absolute/specific humidity, the air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer.

That is exactly what is shown in our readings and is exactly the result that science would predict.

One further, and slightly more complex facet of this is that all of that direct sunlight hitting the substrate, making it hot to the touch, has the effect of driving moisture from it. Something very similar happens with tortoises directly under artificial heat lamps, too (the effect is much less with natural sunlight, however). Think of placing a damp piece of sponge upon a hot pavement in direct sun. It will dry out very rapidly indeed. This drying effect is further aided by dry air passing over and around it. The precise effect used by millions when 'hanging out the washing'! It is also why, even after quite heavy rain in semi-arid environments, just a day later (or even a few hours later, in some cases) the substrate can be very dry indeed again to the point that it is hard to believe that it ever rained at all.

This is why, even in conditions of moderate to fairly high relative humidity in the air, we can still have substrates that are very low in moisture content, and again why it can be highly misleading to take a weather station humidity reading and assume that this is what tortoises are living in, or on.

Another common assumption often made by tortoise keepers discussing habitat and environments in groups is that humidity at ground level is usually higher than that of the air a few feet or meters above it. As shown here, though, the very opposite is likely to be true.

All of this is very easy (and highly educational) to explore for yourself. You just need a reasonably accurate air temperature thermometer and a hygrometer. A non-contact surface temperature thermometer completes the basic kit for beginning to understand just how localised environmental conditions really are, and just how misleading general 'weather forecasts' are when it comes to understanding what animals that dwell at ground level truly experience.

ALTITUDE is yet another massive factor in a larger sense, Generally, higher altitude habitats will. be cooler than those at lower altitudes. Even within a relatively small geographical distance, these effects can be so great that they have profound effects upon tortoise behaviours. This can be so extreme as to have one higher altitude population brumating, while a nearby population at a much lower altitude is fully active. The same applies to estivation. We can have a low altitude population estivating because of extreme heat, while just a short distance away on a map, but at a much higher altitude, the same species is active because the temperatures are so much lower. We have seen that in numerous locations. Again, a good illustration of why generalisations can be highly misleading and that what really matters to tortoises is the MICROCLIMATE in a specific location rather than the MACROCLIMATE.

A very good example of just how altitude profoundly affects temperatures even a relatively short distance apart in geographical terms (Souss Valley, Morocco)

Although somewhat outside the scope of this article, other highly local geographical features such as the presence of a stream or river can have profound effects upon the microclimate of a particular area. In many cases this influence really is limited to a very narrow zone, and it may be that this is the zone where a particular species lives - but again you will not find these conditions reflected in a generalised climate chart or map.

A river flowing through desert habitat in Morocco. This creates a completely different microclimate in the immediate vicinity.

So the next time you see people interpreting general climate charts or maps of whole countries or even regions as being representative of what tortoises living there experience, you will be able to tell them why it is no such thing.

To receive updates and notifications of new articles, videos and other important news please consider 'Subscibing' to the Tortoise Trust. This supports our work and allows us to make this information available. The Tortoise Trust also offers a comprehensive online course that covers all aspects of behaviour, ecology and captive care.

All text and images copyright A. C. Highfield and Tortoise Trust 2023

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