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The Tortoise Myth-Busters: Episode 4

"Too much calcium causes bladder stones".

We'll get straight to the point and give you an answer: No. Certainly not under normal, typical circumstances.

There are a few 'special cases' where there just might be some involvement, and where the use of calcium products that contain additional vitamins or other additives might possibly be a factor, or where there are other metabolic diseases present, but analysis after analysis of bladder calculi actually removed during surgery for this condition confirms beyond reasonable doubt that the overwhelming majority are caused by a build-up of URATES in the bladder, and are not (as many assume based upon their appearance) comprised of calcium.

Let's summarise some of those reports and surveys:

  • In one large study of tortoises suffering from urolithiasis (101 tortoises in total), detailed laboratory analysis was carried out on 66 removed 'stones' and 100% were comprised of ammonium acid urate. None were calcium-based, or even mixed calcium and urate based. It is also notable that this study identified a very high percentage of cases arising in tortoises kept indoors.

  • Another study of 40 desert tortoises who required treatment for uroliths reported that 100% of those subjected to in-depth laboratory analysis were comprised of urate salts, and not of calcium compounds.

  • In one study of Green iguanas (which are incredibly similar to tortoises in this regard) stones from 132 iguanas were analysed, and all were composed of 100% uric acid salts. None were calcium-based.

This topic does frequently causes concern among keepers, however. It is understandable why, as with many species this can indeed be a contributing factor to the formation of what are also known as 'uroliths' or 'bladder calculi'. For example, in dogs high blood calcium levels are indeed associated (with a number of additional co-factors) with the formation of calcium oxalate 'stones' in both the kidneys and the bladder. Knowing this, tortoise keepers are often worried that calcium supplements and high-calcium content diets as advised for tortoises could lead to the same problem. Hence, we see such questions arise on groups and forums regularly. For example:

  • Could all of these bladder stone problems be not only because of a dry environment but also because we are giving them too much calcium?

  • My tortoise is leaving gritty white deposits when it urinates. Is this a result of too much calcium?

  • I was advised to use a calcium supplement regularly, but I am concerned that it might cause bladder stones.

Tortoises and herbivorous lizards (Green iguanas and Uromastyx) however, have a completely different urinary system physiology and 'chemistry'. Tortoises cannot concentrate urine in the same way that mammals can, so they cannot eliminate dissolved metabolic wastes such as ammonia and urea without losing a considerable volume of water. This is not important to aquatic or rain forest species, who can easily replace the fluid lost, but it is a critical limiting factor to desert and arid habitat tortoises where water is scarce and often completely unavailable for extended periods.

The physiology and 'chemistry' of tortoises makes them extremely tolerant to high levels of wastes from the protein metabolism, with the result that they do not have to urinate very often and are therefore able to retain precious moisture. The normal waste product for protein metabolism in most animals is ammonia. Tortoises, however, convert ammonia to uric acid. Wastes from arid habitat species are excreted primarily in the form of uric acid and urate salts as this is a highly efficient strategy in a water-scarce environment. Aquatic and semi-aquatic turtles have free access to water, however, so are able to employ an ammonia-based chemistry instead. The relationship between the proportions of ammonia to uric acid generated by individual species closely mirrors the environments they come from. As habitats become moister, the shift from water-conserving uric acid elimination to water-intensive ammonia elimination is quite marked. The urethra conveys waste products from the kidney to the bladder, which empties directly into the cloaca. Tortoises also have the capacity to reabsorb water from the bladder thus conserving valuable water - a potential 'downside' of this is that if there are high levels of urates currently inside the bladder when that occurs, removing yet more fluid from the bladder is likely to greatly increase the risk that they will form a mass and solidify.

X-ray images and a post-mortem image of a young tortoise with a fatal bladder calculii that entirely filled the bladder. This tortoise had been kept in a small vivarium and had also had an unsuitable diet that was excessively high in protein.

We do know that in certain species these urate-based calculi, or 'uroliths' can occur even in wild tortoises. Perhaps the most studied case is that of Desert tortoises (Gopherus agassizii) from the southwestern United States. There could be several contributing factors to this, including habitat disturbance, physical stress or disturbance to a tortoise that causes them to void irreplaceable fluids, or other aetiologies and pathologies including bacterial or viral diseases. It is very likely multi-factorial.

In captive tortoises (as in captive Green iguanas, that are extremely similar), the key causes of bladder calculi can be summarised as follows:

Diet: Diets that are high in protein content, and especially so in the case of animal protein consumption, will cause elevated urate production (Holz, P. H, 2020). Uric acid is the end product of the purine metabolism, and therefore feed items rich in purines contribute directly to elevated uric acid levels. A high-protein diet typically contains large quantities of purines, and this may include vegetable items with very high purine levels too: beans, peas, spinach, broccoli, cauliflower, mushrooms, and soy-based items such soy meal or fresh beansprouts. None of these are appropriate feeds for arid-habitat terrestrial tortoises, but a surprising number of manufactured, packed 'tortoise foods' do include some of them or their derivatives. While this may not be the main or primary cause of bladder stones in tortoises, such diets could certainly be an important contributory factor.

Dehydration: Multiple studies indicate that chronic dehydration is the single most likely cause of bladder stone formation in tortoises. The 'background biology' also supports this view. While tortoises do live in some very arid environments, they make extensive use of specific microclimates and behaviours to preserve body fluids and to avoid dehydration. If these behaviours are disrupted, for example by not providing them with a deep enough substrate to bury in (depriving them of an important microclimate), or by subjecting them to unnatural and extremely drying heat sources, then a state of dehydration will occur and be maintained over very long periods of time. These are the exact conditions that are most likely to cause urolithiasis and additionally, to create further renal stress.

If there are dietary failures in addition, then these effects will be greatly amplified with potentially tragic results.

Tortoises kept indoors, in vivariums, tortoise tables or pens are invariably more likely to be subjected to extended periods of sub-optimal levels of humidity and to intensely drying heat sources than those maintained largely outdoors. Anecdotal reports, and our own experience over very many years, also suggests that those kept in greenhouse type units, or plastic agricultural 'tunnel' type units have similarly low levels of urolith formation to those kept fully outdoors. This is quite possibly helped by the fact that both such units typically feature deep, natural substrates that tortoises create scrapes and burrows in - something rarely achievable with indoor units, the fact that an infinite mass of external air (with 'natural' humidity content) is available and flows through the units, and a much lower usage of heat sources that are known to be intensely drying. Instead, basking is achieved largely from solar sources. With specialist plastics available now that effectively transmit UV-B, such housing systems have much to recommend them.

Although this juvenile Testudo graeca cyranaica had always been provided with a suitable substrate for burying into, a suitable diet, and fresh water continually available at just 12 months of age it passed the bladder calculii (shown next to the head for scale). It had, however, spent most of its life in an indoor unit beneath basking lamps for 8-12 hours per day. It should also be noted that some early stage carapace deformity 'pyramiding' is evident.

We have touched upon this in a previous extensive article ('The Effect of Basking Lamps on the Health of Captive Tortoises and other Reptiles') but it is worth highlighting one very specific aspect of that again here: the extremely dehydrating effects of typical artificial heat sources, and especially of basking lamps used in close proximity to captive reptiles. If we measure the relative humidity of a basking zone with the basking source off, then on again, these figures are startling.

Dramatic reductions in relative humidity recorded in the basking zone of even a modestly powered (40w)basking lamp. Under more powerful lamps (100w) the effect is even more profound.

For a tortoise to be subjected to this for many hours a day, as is typical, it would hardly be surprising for a state of chronic dehydration to occur. It is not entirely clear to precisely what extent this can be 'offset' by providing drinking water, 'misting' or periodic spraying. The fact remains that under such lamps levels of humidity occur that are below that even of arid deserts, and in the vast majority of cases the tortoise has no respite as the opportunity to create effective burrows does not exist. It is notable that in several studies, burrowing species such as Gopherus agassizii and Centrochelys sulcata appear to be among the most susceptible species to this problem. We do not feel that this is likely to be coincidental.

Burrowing species rely upon the microclimates in their burrows to preserve body fluids and to prevent dehydration in harsh environments. In captivity they are usually deprived of them.

Some suggested preventative measures:

  • Ensure that fresh drinking water is available at all times, especially for tortoises housed in indoor environments, whether part-time or full-time.

  • Try to minimise the use of artificial basking lamps and similar heat sources.

  • Try to keep tortoises in suitable outdoor environments to the maximum extent possible.

  • Ensure that herbivorous tortoises are provided with appropriate diets: high in fibre, low in protein and purines, and as close as possible to that species' intake in nature.

  • If at all possible, allow burrowing species the opportunity to do so.

Further reading:

Cerreta AJ, Keller KA, Gardhouse SM, Lulich JP, Guzman DS. Clinicopathologic findings and urolith composition for green iguanas (Iguana iguana) with urolithiasis: 21 cases and 132 stones (1996-2020). J Am Vet Med Assoc. 2022 May 11;260(10):1216-1221. doi: 10.2460/javma.22.01.0035. PMID: 35544419.

Highfield, A. C. (2015) The Effect of Basking Lamps on the Health of Captive Tortoises and other Reptiles. Old Tortoise Trust Website

Holz PH. Anatomy and physiology of the reptile renal system. Vet Clin North Am Exot Anim Pract 2020;23(1):103–114.

Keller, K. A., Hawkins, M. G., Weber, E. P. S., III, Ruby, A. L., Guzman, D. S., & Westropp, J. L. (2015). Diagnosis and treatment of urolithiasis in client-owned chelonians: 40 cases (1987–2012). Journal of the American Veterinary Medical Association, 247(6), 650-658.

Takami Y, Koieyama H, Sasaki N, Iwai T, Takaki Y, Watanabe T, Miwa Y. Survey of tortoises with urolithiasis in Japan. J Vet Med Sci. 2021 Mar 11;83(3):435-440. doi: 10.1292/jvms.20-0315. Epub 2021 Jan 21. PMID: 33473048; PMCID: PMC8025420.

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