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The Tortoise Myth-Busters: Episode 6

Updated: Apr 11

Do semi-arid habitat tortoises use 'Humid hides' in the wild? Are they necessary in captivity? Does the 'closed chamber' method have any rational basis?

A wild Testudo graeca graeca @ 10 weeks after hatching. Is it true that these spend most of their time in 'humid' microclimates?

This is a rather long article, but contains a lot of necessary detail on on a highly controversial topic. We have tried to keep it as accessible as possible to the general keeper but if we fail to provide adequate detail, or fail to cite sources, we would undoubtedly face criticism for this, so we have attempted to strike a reasonable balance. As usual, let's start with some examples of 'information' and advice seen regularly in online tortoise groups. Most of this misinformation arises from people who's sole experience is of keeping tortoises as 'pets' or as a 'hobby', often thousands of miles away from where those animals occur naturally, and who very, very rarely have any experience whatsoever of that species in the wild, or of the habitats involved. If they do read about the natural habitat and the conditions there, they frequently completely misinterpret and distort the data and fail to take account of the realities on the ground (see our other recent article on this topic). Then, we'll take a look at some verifiable facts, and logical deductions. Here's some actual examples of this type of misinformation being presented as established facts:

  • "Hatchlings need very high humidity, you want to be pushing 80% at all times and even consider offering a humid hide or sweat box that has closer to 100% humidity. Moisture is how you prevent pyramiding, and the more pyramiding you prevent from an early age, the easier it is to care for the tortoise later in life"

  • "You need constant humidity levels, day and night, of at least 70% (85% is better) and do not EVER let the temps, day and night, drop below 80 degrees as this can result in an ailing tort" (question related to Testudo hermanni)

  • "In the wild, Mediterranean tortoises spend most of their lives in hidden deep in humid vegetation where the relative humidity rarely drops below 60%. Often it is 80% or higher. They only emerge from these hides briefly to graze or reproduce"

  • "Keep the main enclosure at humidity levels of at least 45% and in addition provide a semi-enclosed 'sweat box' where the humidity is 90% or more. Use damp sphagnum moss in an upturned plastic box to achieve this. These are the conditions that Mediterranean tortoises, Russian tortoises, Sulcatas and Leopard tortoises all experience in the wild".

  • "Even if temperatures and humidity levels in the wild are sometimes lower than 80 degrees (26.67 Celsius) or 80% RH, these are sub-optimal and we can provide them with better in captivity. Why should we subject them to less-than-ideal conditions? Most die in the wild, anyway".

  • “faster growing hatchlings live under the cover of grass... providing a higher level of ambient humidity”.

  • "Babies hatch during the start of the rainy season. It is hot, very humid, rainy and marshy. There are puddles and lush green growing food everywhere. In some areas there is a dry season, but the hot monsoon season is when babies hatch, and babies find humid microclimates to hide in during drier times" (Concerning Centrochelys sulcata).

  • "You cannot raise perfectly smooth juveniles without using a humid hide with at least 80% relative humidity available at all times" and "Everything that I read shows that the hatchling needs to be raised in a closed chamber enclosure so that the humidity levels do not drop below 80%" (concerning Leopard tortoises).

Before addressing these points directly, it is important to define some terms used:

Relative humidity (RH) is usually expressed as a percentage, it measures water vapour relative to the temperature of the air. In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapour that can exist in the air at its current temperature. Warm air can hold more water vapour (moisture) than cold air, so with the same amount of absolute or specific humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer.

The dew point is the temperature the air needs to be cooled to (at a constant pressure) in order to achieve a relative humidity (RH) of 100%. At this point the air cannot hold more water in gaseous form and precipitation, either in the forms of rain, mists or fog will occur. This is why we see ground mists very early in the morning, just before the heat of the rising sun warms the air again, thereby allowing it to retain more water vapour once more.

This is what typically meant by "humid hides" for arid habitat species (Q) "Just wondering what you think the temperature inside a 90% humid hide (Tupperware container, moss and soil) should be?" (A) "Never lower than 80-83 (28.3 Celsius)...up to the 90 mark (32.2 Celsius) is fine"- you can calculate the amount of water vapour these animals are being surrounded by from that. It's 24.54 g of water per kg of air. If we do the same for some actual real-life peak RH readings from Testudo habitat (68% at 7 Celsius). Now you get 4.42 g/kg. An absolutely vast difference. Remember that the 'relative' in 'relative humidity' means exactly that. Relative to temperature. Hopefully that clarifies a) The difference and b) What is really going on here. You could think of this as how 'wet' the environment is, which is a simple but quite effective way of looking at it.

Briefly, here are the true facts concerning the claims made above. All based upon direct measurements and observations in the field, and upon accepted science.

  • Hatchlings do not experience significantly different temperatures or relative humidity levels from adults, They live side-by-side in the exact same habitats and environments. Their behaviours are almost identical. There are very slight differences due to physical size (hatchlings gain and lose heat more rapidly than a large adult) but such differences are really quite minor and it is completely wrong to suggest that somewhow they have access to hides with very high levels of humidity at all times. They do not. This assumption is entirely untrue. For an accurate overview of how moisture levels do affect keratin, however, see this article.

  • Both humidity and temperature conditions in the wild are absolutely NOT 'constant' for any temperate species. They vary hourly, daily and seasonally. Neither do these species experience minimum temperatures of 80 degrees F (26.67 Celsius). In fact, really low temperatures are experienced routinely, even in the warmest parts of Testudo graeca ranges (Morocco, Tunisia, Algeria, Libya and Southern Spain). Given the undeniable reality that tortoises have thrived in these locations for countless millenia, it is therefore absolutely ridiculous to claim that such temperatures cause tortoises to become ill. Unfortunately this is yet another example of the illogical, misleading nonsense that is regularly presented as 'fact' on tortoise keeper groups and on social media. For specific examples of temperatures actually experienced by wild Testudo graeca see here

  • Tortoises do not spend "most of their lives" hidden deep in "humid" vegetation or grasses. A few days spent in the field observing them will prove that beyond doubt. Sadly, sitting in front of a keyboard thousands of miles away, and without any direct experience of such natural habitats, or of wild tortoises, is no substitute. In fact, in many Testudo habitats there is little or no grass present. In areas where it is present in certain localities, for example in Southern Spain and parts of North Africa where Esparto grass (Stipa tenacissima or Lygeum spartum) is an important feature of Testudo graeca graeca habitats measurements recorded over a 12 month cycle using a series of SL52TH temperature and relative humidity data loggers revealed that the typical relative humidity within clumps of grass occupied by tortoises very rarely exceeded 50% during daytimes and was typically below 40%. Levels of <30% were common. The only instances of very high humidity recorded invariably coincided directly with rainfall, the dew point being reached in very cold air just before dawn, or immediately following episodes of precipitation. In total, more than 18,000 data points were recorded and in only 2% of these did relative humidity exceed 80%. There is a rise in moisture overnight, usually at approximately 3-4 am as dew begins to form. This persists for a very short time, and usually peaks at around 60% RH. It rapidly burns off as the sun rises. See our recent video for examples of typical habitat.

  • The species mentioned are all semi-arid habitat species (Mediterranean tortoises, Russian tortoises, Sulcatas and Leopard tortoises). The latter differ substantially, however, in that much more acute seasonal 'dry' and 'rainy' seasons are experienced. However, this still does not equate to access to 80% + RH on a constant basis. In fact, typical conditions are again around 20% humidity in the open air (except during rainy periods), and daytime temperatures during activity periods are commonly 24-35 degrees Celsius. Humidity levels in vegetation or retreats are typically 30-50%. The correct environment for any species should be established by reference to the natural habitat. As a very approximate guide, drier substrates (<60% RH) should be avoided for most forest dwelling species, and damper substrates (>50% RH) should be avoided for arid habitat species. Extensive studies on burrow habitat micro-climates in both North America and North Africa suggest that for Desert tortoises (Gopherus agassizzii) and Spur-Thighed tortoises (Testudo graeca) a burrow micro-climate offering circa 40-45% RH would closely approximate that found in nature (Bulova, 2002 and Highfield and Bayley, 1996). Excessively damp substrates or retreats can have serious implications for the health of arid habitat species, resulting in an increased incidence of respiratory disease, and an increase in both fungal and bacterial shell and skin infections (pers. obs). As a general recommendation for Testudo and semi-arid savannah or mountain species, such as Stigmochelys pardalis, air humidity within an enclosure should normally be in the region of 50% or slightly less.

For the record, THIS is some typical habitat of Leopard tortoises in South Africa, though they are also found in far more arid areas than shown here. Ambient humidity levels recorded in this locality, taken directly next to active tortoises, ranged from 23% to 38%. Some years ago we produced a full-length video filmed at various locations throughout South Africa and which shows these habitats in detail (NB: although no longer available separately this video is included as part of the support materials used in the Tortoise Trust Course). We guarantee - there were no 'humid hides' with 80%+ constant humidity to be found! We actively searched for them, in vain. The only areas that we encountered with truly high levels of ambient humidity were in forested Kinixys (Hingeback tortoise) habitats where Stigmochelys pardalis did not occur (the typical habitat of Hingeback species is best described as consisting of subtropical or tropical moist lowland forests, swamps, plantations and similar areas of high humidity). If we refer to a genuinely useful, rational and reliable text on Leopard tortoise natural habitats ('Leopard and African Spurred Tortoises' by Holger Vetter, 2005) this makes it very clear that Leopard tortoise primary habitat consists of "dry and hot biotypes that may sometimes also be rather rocky" and also that it "thrives in semidesert bushland that normally receives less than 100mm of preciptiation per annum". It can also, however, be found in areas with far higher levels of precipitation (up to 1,400mm in the Amatola Mountain range) and sometimes alongside river banks in otherwise very arid areas indeed (Western Namibia). The key point here is that none of these locations, even the 'wettest' of them, offer "constant access to 80-100% humid microclimates". They are fundamentally semi-arid habitats (McMaster, 2001). It is quite true that there are seasonal variations, specifically a 'dry season' and a 'rainy season', and of course, during the rainy season you can indeed find wet substrates, damp vegetation and far higher levels of RH than at other times of year. We return to the point, however, that is not constant, and for the rest of the time conditions are very arid indeed, and as Vetter states " not a speck of green is to be found at the height of the dry season".

The entire concept that these "dry and hot biotypes" somehow manage to provide constant access to areas with 80% to 100% relative humidity throughout the year is quite simply beyond ridiculous - yet this is exactly what many choose to believe. This absurdity is repeated ad infinitum as 'fact' in these online 'tortoise keeper' communities and 'followers' are urged to create such conditions in captivity.

  • 'Optimal' conditions are those found in the natural habitat where these species have lived very successfully for countless thousands of years.... not in a glass tank, a vivarium, a 'closed chamber' or pen far removed from the climates and diets that they evolved in. We'll leave it at that.

  • Juveniles do not "live under cover of grass". Extensive observations were made of wild juvenile and hatchling Testudo graeca graeca activity at various sites in Murcia and Almeria, Southern Spain, and miniature data loggers were temporarily affixed to their carapaces to record changes in both carapace surface temperature and relative humidity every 30 seconds. The results consistently indicate that juveniles are frequently exposed to ambient relative humidities as low as 20% for extended periods during browsing and basking. Tortoises were monitored over a 12 week period and at no time were they exposed to levels of relative humidity approaching 100% except during or immediately following rare episodes of precipitation as a consequence of thunderstorms. This data clearly demonstrates that while it is true that clumps of Esparto grass do indeed provide a 'higher' level of relative humidity microclimate that is exploited by tortoises, it is typically peaking at around 60% RH deep within the grass clump, and then only for a short period overnight as the temperatures are at their lowest, compared to 28% to 30% in free air, which is a very long way from the levels of 80% to 100% RH suggested by many proponents of these peculiar theories. During activity periods even deep within the scrape/hide the RH is typically 35-50%. This graph is taken from a miniature humidity logger directly attached to a juvenile (3cm) Testudo graeca graeca as it emerged from it's overnight scrape to begin foraging. Peak RH was circa 58% in the scrape and very rapidly fell to between 40% and just under 30% as it foraged in lush green vegetation in partial shade. The sudden peaks and falls in temperature are the result of moving between various levels of shade from plants to full sunshine.

Graph of temperatures and RH taken recently from a data-logger positioned in a wild juvenile Testudo graeca graeca overnight retreat under Esparto grass. Maximum RH was 63.8% for a brief period just before sunrise, and the minimum was 22.9% at 1.00pm. Temperatures ranged from 24.7C to an overnight minimum of 13.1C.

  • Regarding a more realistic overview of the African Spurred tortoise and conditions in the natural habitat, the very first thing to point out is that there is no 'monsoon' season. The term relates exclusively to the South and SE Asia and cannot be applied to Africa. Taking just one key area of distribution of Centrochelys sulcata, Mali, we find that the Malian climate is characterised by three seasons: a dry season in March to June, a rainy or wintering season from June to September and an off-season or cold season from October to February with a drying Saharan wind called the harmattan. As reported by the late M.R.K. Lambert who studied sulcata there in the field, "Growth depends on nutrient intake and follows the seasonal appearance of green vegetation with the onset of rains in June. It may be interrupted or curtailed if a mid-season rain failure results in a shortage of green vegetation. Soon after the start of the Sahelian dry season in October, the vegetation becomes increasingly dry, brown and sparse. Growth slows down as refuge is sought, and eventually ceases during the seven-month period of aestivation" (Lambert, 1993). He describes the general biotype as "woodland savannah, grass-carpeted during June-October rains, but sparsely vegetated and livestock-trampled within six weeks of the start of the Sahelian dry season. Aestivation takes place", also ""wadis (dry river beds) and desert dunes with perennial vegetation". There are several points that can be made about this. The first is that mean annual rainfall throughout the bulk of the range varies from 140 to 502mm. This is extremely low. Only in a very few areas does it rise to over 1,000mm. This is, it must be stressed a very hot and arid area indeed. Rain, when it does occur is of relatively short duration before either cold or hot dry seasons reoccur. The suggestion that during these extreme arid and dry periods there exist extremely “humid microclimates” totally defies scientific reality. These are scrubby, arid desert type locations with ambient temperatures peaking at over 40 Celsius, and with over 3,000 sunshine hours per year. The tortoises do make use of deep burrows, which they retreat to as ambient temperatures surpass 32-34 celsius. In those burrows relative humidity has been recorded as peaking at a little over 60%, and mostly between 35-50% (Lambert, pers. comm). There are no super-humid microclimates with almost year-round RH levels of 80-100% available to C. sulcata, either adult or juvenile. They simply do not exist. The assumption that they do is pure fantasy. Should anyone doubt this, the always well-researched and rational conclusions of Holger Vetter (2005, op. cit.) provide further confirmation stating that Spurred tortoises occur in: “a wide range of basically arid habitats…to semi-deserts and the margins of true deserts. The climate is usually very hot and dry (our italics) with temperatures regularly topping 40 celsius during the day and often decreasing to less than 15C at night”. He goes on to state that: “Summery rainy seasons are short and less expressed in the west of the distribution range than in the eastern or central parts and sometimes several years go by without any rainy season at all”. We are tempted to add, “good luck finding any examples of sustained 80%+ humid microclimates in that! When you do find them, be sure to let us know".

  • Finally, the much-repeated claim that "You cannot raise perfectly smooth juveniles without using a humid hide with at least 80% relative humidity available at all times" is demonstrably untrue. Sometimes, even when admitting that the claims regarding 'humid hides' in natural habitats are factually incorrect, this is the fall-back position. That in captivity it is different, and we therefore still need these 'humid' hides! No. Absolutely not. Here are several examples captive bred and raised by us over the years and where a) No humid hides of any kind were ever provided and b) Where RH levels approximated that of wild conditions for that species typically 40-50% or lower daytimes, with 60% overnight for a brief period, (provided by by a light misting with water to replicate ground mist or dew). The diets and general care were as we recommend: low digestibility, very high fibre, calcium-rich and as close to the natural graze as possible, with care taken to avoid over-feeding and also to minimise the use of artificial heat sources as much as possible. We would hope that you would agree that these are indeed quite 'smooth' - what is more, they have excellent bone density and have now survived in perfect health for up to 25 years. It is really important that the completely false and misleading advice seen in some of these forums and social media groups is countered with objective evidence and verifiable facts.

Incontrovertible proof that perfectly well formed tortoises can indeed be captive bred and raised in captivity without recourse to the use of extreme and unnatural 'closed chambers'.

It is worth spelling out in detail what these 'humid hide' and 'closed chamber' methods actually consist of. Here is a description from one breeder who uses the system:

"I take a large upturned plastic storage container with transparent sides and cut just enough of a doorway for the tortoises to enter and leave, though there is also a piece I can slide across that to keep them sealed in to ensure they get enough humidity. I only let them out briefly for extra excercise during the first year or so. They can go out more when they are much larger. The humidity in the chamber is always 90% to 100%, and I know it is high enough when I see moisture droplets running down the inside walls. To keep it that high I have a heat mat under the area and I use wet sphagnum moss over damp orchid bark as the main substrate. I also have an ultrasonic fogger feeding into the chamber by a PVC pipe. In addition I have a ceramic heater inside the chamber towards the top to make sure that the air temperature stays more or less constant at about 86F (30 Celsius), and a 24" T5 UV-B tube secured to one side. I do not put any holes or vents in as there is plenty of air inside and with daily feeding there is enough air exchange at that time. I get beautiful smooth Leopard, Greek and Sulcata tortoises from this with no PGS (Pyramid Growth Syndrome). Proof that this is what they need"

Sadly. this mind-boggling stuff is actually practiced by many keepers and accepted as 'fact'. 'like the natural habitat' and 'neccesary'. Let's try to apply some logic:

  • Where do these people believe that such species are accessing such conditions in the wild? Where is the orchid bark and wet sphagnum in arid Leopard, Sulcata and Testudo habitats?

  • If they are not using such conditions in the wild, why do they think they should be subjected to them in captivity? How do they explain that in the wild tortoises develop and grow perfectly well without them?

  • Does it not strike anyone that restricting animals in 'closed chambers' so far removed from the outdoors, sufficient space, air and natural sunlight is verging on inhumane?

  • We would urge all keepers to exercise extreme caution in relying upon generalised 'average' figures for relative humidity in tortoise habitats obtained from climate sites and maps. These can create a very misleading impression indeed. The only meaningful, reliable data is that collected at the level occupied by tortoises (not several metres above ground) and in the precise microclimates used. There are massive differences in temperature and humidity at various altitudes, and also conditions inland can vary substantially from those closer to the coast. Tortoises tend to occupy very narrow, very specific biotypes. This needs to be understood when considering climatic data. Data based on an 'average' of all habitats within a region or country is likely to be completely misleading and inappropriate. We have reported upon this separately.

  • Why do they believe tortoises need constant 30C temperatures when even species such as Stigmochelys pardalis (Leopard tortoises) sometimes experience temperatures that can, on ocassion, fall to freezing point in their habitats (though minimums of 2-5 Celsius are more typical of much of their range), and Centrochelys sulcata also experience temperatures as low 15-20 Celsius in their burrows amid much colder conditions above ground (2-5 C, though sometimes into sub-zero territory)?. Relative humidity levels in these burrows, are, as noted above, typically 35-50%, and are nowhere near the saturated levels (80%+) claimed on these groups.

It is worth considering some potentially serious side effects of keeping tortoises at these extreme levels of humidity. To do that we need to consider keratin again. It is present on tortoises as a protective material. It is tough, resistant and helps them to cope in very arid, thorny and rocky environments. Unless we saturate it with excessive moisture.

The carapace of a chelonian is comprised of a bony dermal-endoskeleton overlaid by an external corneous epidermis subdivided into rows of shields, or scutes (Zangerl, 1969). The scutes of tortoises consist of both alpha-keratin and beta-keratin cells with the latter dominating (Alibardi, 2005). Keratin has many interesting and valuable properties. One of these is that it is hygroscopic and takes up water in equilibrium with atmospheric humidity (Spearman, 1973). Keratin becomes notably pliable and less resistant in the presence of high humidity and high temperatures (Shelley, 1954).  This property was routinely exploited in the working of objects made from tortoiseshell, obtained from the carapace of the critically endangered Hawksbill turtle (Eretmochelys imbricata). The same property will also be familiar to anyone who takes a hot bath: human toe and fingernails respond in a similar manner. Similarly it is well established that factors which encourage fungal invasion (of the keratin) include increased environmental humidity. Veterinary problems associated with 'wet foot' are also common in equines, where the keratin of the hoof may become over soft and vulnerable to trauma and infection if the animal is maintained on an excessively wet substrate (Reca, 2005). Does this affect tortoises? Yes. Absolutely. There are many accounts in the veterinary literature of fungal shell infections affecting arid habitat tortoises maintained in excessively moist or damp conditions. Here is one example, a Testudo horsfieldii (Russian tortoise).

Similar issues have been identified with the beta-keratin of Ostrich claws when subjected to varying levels of ambient humidity (Bonser, 2000). Studies conducted on a wide range of keratins suggest that at relative humidity levels above 80% and below 20% profound changes in both the molecular structure and mechanical properties of keratins occur (Leeder and Watt, 1965, Duer, 2003).  At levels of relative humidity above 80% absorption of water molecules by keratin is considerable (Leeder and Watt, 1965), and this has a very significant effect upon the mechanical properties of the scute, resulting in a major degree of softening and reduction in stiffness and resistance. That keratin is resistant for a reason. It is 'designed' to function correctly at the levels of humidity found in the natural habitat.

It is very important to point out here that we do not dispute that you can raise 'smooth' tortoises without 'pyramiding' using such methods. You can. However, it is critical to realise WHY you can.... and what is REALLY happening. All is not quite what it appears. Strangely, if you peruse the kind of groups where these methods are promoted you will not find any rational, scientifically valid explanations for the process. Merely that "it works, and that is good enough for me"!

This is a very complex topic and we have written lengthy explanations giving an immense amount of background technical detail on the subject going right back to 2010. These cover the structural and physical properties of keratin (scutes), how keratin responds to varying degrees of humidity, how living bone is 'plastic' and responds to a variety of physical stresses, the changing 'strengths' and 'hardness' of keratin in response to humidity (Young's Modulous'), how keratin exerts a 'pulling power' on the bones directly beneath it, and how different modes of keratin cell proliferation differ between terrestrial chelonia and aquatic chelonia. This is a further important feature of keratin that affects physical stress on the skeleton and the modes by which it proliferates in chelonia. There are two primary methods. Tortoises exclusively rely upon a mode of cell proliferation that deposits new material at the edges of the scute, resulting in the well-known 'tree ring' effect. This generates vertical plane growth. Aquatic turtles mostly (not all) rely upon a mode where new growth occurs in a horizontal plane only, with new cells growing at an even rate beneath the older material. Many species of aquatic turtle lack 'historical growth rings' for this reason. Eventually, the older scute is shed (usually entirely) to be replaced with the new, larger scute. In terrestrial tortoises, this pattern of shedding does not occur. The keratin builds up, in a vertical mode, continually. Natural wear and tear in nature also helps to control overgrowth and excess thickening.

This vertical and expansive mode of cell proliferation of itself creates an upwards force on the skeleton. Where there is concurrent MBD of any degree, the effect will be substantial. The bone will conform to the pattern of growth of the scute. This is the primary mechanism involved in 'pyramid' growth syndrome in tortoises and is the main reason why it is not observed in those aquatic species that shed whole scutes.

These previous presentations are all available and do not need repeating here. Instead, let's try a few simple analogies that put these complex processes into easily understood terms. We'll cite from a previous article that looks at bone development in tortoises:

You could think of this as a physical, ongoing 'battle of strength' between the bones of the carapace and the layer of keratin which surrounds it. If the bones are weak and soft due to combinations of osteomalacia and ostodystrophy (i.e., various forms of Metabolic Bone Disease, most of which are caused by dietary deficiencies or excesses) then the continually applied physical stresses of the keratin (which is a very strong material indeed) will deform it easily. If, on the other hand, the bones are strong and dense, they resist these physical forces much more readily. Humidity plays a role because, as we know, keratin is incredibly hygroscopic. It is stiff when dry, but soft and pliable when wet and warm (as proved by taking a hot bath). So, if we over-dry it (under intense heat lamps) for example, we increase these stresses. The growing keratin also effectively 'pulls' the underlying bones upwards due to the pattern of cell proliferation, or 'growth rings' that are a unique feature of tortoise anatomy. Incidentally, aquatic turtles have a different method of keratin cell proliferation than terrestrial tortoises, so this so-called 'pyramiding' issue is much less evident. They also shed whole scutes, which additionally helps to prevent very thick layers accumulating. Very thick keratin exerts incredibly powerful physical 'pulling power' on the underlying bones, so very often the worst cases of 'pyramiding' feature both weakened, defective bone tissue and unnaturally thickened keratin.

Both Alpha and Beta keratins have been studied extensively, and we know quite a lot about how they perform at differing levels of moisture content and in different levels of ambient humidity. One very important feature is the way in which they gain and lose stiffness as they respond to external humidity. These changes are dramatic. They can be measured and quantified. At levels of humidity above 80% scute keratin possess only a fraction of the strength, resistance and durability that it does at 50% RH. Such changes can be measured and quantified using criteria such as Young’s Modulus. At sustained levels of 90-100% it is essentially saturated as it accumulates water molecules rapidly. It becomes extremely soft and pliable, exerting almost no stress on the underlying skeleton. At the opposite extreme of low humidity, below approximately 25% it loses water molecules and becomes very stiff and resistant. At such levels it is exerting a very substantial physical force on the bone beneath. We know from earlier tests we conducted with tortoise vivarium design, that many of these create extremely dry conditions, with sustained relative humidities as low as 12%. More recent tests have also shown that directly under basking (heat) lamps very low, very localised conditions of humidity well below 20% can occur immediately adjacent to the surface of the scutes of the carapace. This has a profound drying effect, increasing keratin stiffness, driving out water molecules, and at the same time greatly increasing stress forces upon the underlying bony skeleton.

In this graph ambient room humidity level was 70%. Within seconds, directly under a basking lamp the levels fell to 30%, then to below 20%.

This is very probably one reason why the very worst cases of so-called 'pyramiding' that are encountered are typically those where where a) There are advanced nutritional problems present resulting in various manifestations of metabolic bone disease and b) The tortoise was maintained in unsuitable enclosures with heavy reliance upon basking lamps.

It is during phases of growth that bone is most vulnerable to such effects. It is at its most plastic. The more rapid the growth is, the greater the potential for absolute or relative deficiencies to occur. This is a well known physiological relationship and affects all animals, and also humans alike. It can be extremely challenging to obtain good bone density in captive situations on artificial diets. Herbivores are especially sensitive. On greatly accelerated growth regimes achieving normal, healthy bone density is extraordinarily difficult.

Another useful analogy is a child with 'rickets', or 'soft bone disease'. This is also discussed in the linked article above. The cause is typically a combination of calcium and vitamin D-3 deficiencies. This should sound familiar to tortoise keepers.

In a human child with rickets, the effects are bone pain and soft, weak bones that can lead to bone deformities, typically in the longer bones of the legs caused by gravitational ('weight') downward pressures and by the 'pull' of muscles (causing the classic 'bowed legs' of the condition).

In tortoises, however, the cause is essentially the same, but the effects are different. It is not 'gravitational' pressures on the legs that cause deformity here, however, but the stresses caused by KERATIN that entirely surrounds the carapace and also by the powerful limb muscle attachments internally.

One of the old 'treatments' for a child with rickets involved the use of iron 'leg supports'. Examples of these devices can now be found in museums. Interestingly some veterinarians have recently attempted to use similar 'supports' to assist tortoises presenting with severe metabolic bone diseases.

What is vitally important to understand here is that such supports did not, and do not, address in any way the underlying problems such as lack of calcium and vitamin-D. They only relieve the symptoms while - hopefully - the base causes are corrected through changes to diet and environment.

How does this talk of 'leg irons' and 'rickets' relate to tortoises and humidity? It is, in fact, highly relevant and quite simple. To summarise what we have already covered:


Just as in the case of the child with rickets, however, this does nothing - absolutely zero - to deal with the underlying causes. What it does do very effectively, however, is to suppress, hide, and conceal the symptoms. Masking and concealing symptoms is never a good idea, as symptoms exist to warn us that something is going wrong.

We know that in nature semi-arid habitat tortoises are not accessing environments that in any way remotely resemble these 'closed chambers' and 'humid hides' as used by these keepers. We hope that we have more than adequately demonstrated this. If there are still those who refuse to accept this then we respectfully suggest that they invest in a professional grade hygrometer and some temperature-humidity data-loggers (plus an airline ticket) and visit some natural habitats to show us all where they are.

  • The simple fact is that they do not exist in the forms claimed.

  • it is also a simple fact that tortoises can develop smooth shells with excellent bone density both in the wild and in captivity without them. They do it all the time. They have been doing so for countless millenia.

  • If a proposed husbandry method requires us to resort to totally unnatural conditions (e.g., sustained exposures to 80%+ RH for arid habitat tortoises) to solve a problem created by other totally unnatural conditions that we subject the animals to (inadequate and unsuitable housing, exposure to damaging heat sources, poor nutrition and excess growth rates resulting in poor bone density) then we would suggest that something is very seriously wrong. It is surely a far better strategy to address the fundamental causal issues and not to merely concentrate on trying suppress the most obvious symptoms, which is the sole result of such methods.

  • To ensure that captive environments provide a suitable range of both humidity and temperatures, a safe and appropriate range should be established by reference to reliable data from the natural habitat of the species in question, not by wild guesswork, baseless speculation, wild imaginings or by reference to inappropriate and frequently misleading general climatic averages.

Further Reading:

Alibardi, L. (2005). Proliferation in the Epidermis of Chelonians and Growth of the Horny Scutes J. Morph. 265: 52-69

Bonser, R.H. C. (2002) Hydration sensitivity of ostrich claw keratin. J. Materials Sc. (21):1563-1564.

Bulova, S. (2002). How temperature, Humidity and Burrow Selection affect evaporative water loss in desert tortoises. J. Thermal Biology. (27): 175-189

Duer, M. J., McDougal, N. and Murray, R. (2003). A solid-state NMR study of the structure and molecular mobility of keratins. Phys. Chem. (5):2894-2899.

Highfield, A. C. and Bayley, J. R (1996) Observations on ecological changes threatening a population of Testudo graeca graeca in the Souss Valley, southern Morocco. Chel. Cons. Biol. 2(1):36-42

Lambert, M. R. K. (1993) On Growth, Sexual Dimorphism, and the General Ecology of the African Spurred Tortoise, Geochelone sulcata, in Mali. Chelonian Conservation and Biology, 1 (1): 37-46.

Leeder, J. D. and Watt, I. C. (1965) The role of amino groups in water absorption by keratin. J. Phys. Chem. 69 (10):3280-3284

McMaster, M.K. 2001. The status and ecology of the leopard tortoise (Geochelone pardalis) on farmland in the Nama-Karoo. M.Sc. thesis, University of Natal, Pietermaritzburg.

Reca, M. (2005) Managing Wet Feet. The Horse Magazine, May 2005.

Shelley, W. B. (1954) Dermatological Research. NEJM (250): 246-251.

Spearman, R. I. C. (1973) The Integument: A textbook of skin biology. Cambridge University Press.

Vetter, Holger, 2005, Leopard and African Spurred Tortoise Stigmochelys pardalis and Centrochelys sulcata. Edition Chimaera.

Zangerl, R. (1969). The Turtle Shell. In: Biology of the Reptilia 1(6): 311-369. (ed. Gans). Academic Press.

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(c) 2024 A.C. Highfield / Tortoise Trust

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