top of page

Calcium levels in substrates, graze plants, and UV-B distribution in Mediterranean tortoise habitats

A brief introduction to some field measurements in mid-March, taken in Almeria, Southern Spain. In this video, we also briefly discuss the idea that Mediterranean tortoises make extensive use of "humid hides" where humidity levels are 80%, 90% or even 100%. As will be seen, this is completely false, although the belief that they do is commonly accepted without question as a 'fact' among many amateur tortoise keepers. As that is really a separate and complex issue of itself, look out for the next episode (Episode Six of our 'Tortoise Myth-Busters' series, coming soon that goes into much more detail on this controversial subject).

For now, let's take a look at some mapping data that provides some background for the statements made in the video relating to calcium levels in both soils and common wild graze plants, and also how the UV-B availability throughout tortoise areas of natural distribution aligns very closely indeed with high calcium availabilty, producing a near-perfect combination for sustaining healthy bone growth. The typical graze plants are also incredibly high in fibre content, low in carbohydrates, and have a very good range of additional essential minerals and vitamins.

This is a soil-chemistry map covering most of Mediterranean Europe. In this case, the darker green areas indicate high calcium carbonate levels in the soil, and consequently also, in the vegetation (though exact take-up levels and Ca:P ratios do vary with plant species). Compare this to areas with Testudo populations. This is why wild tortoises have no problems at all in sourcing lots (and lots) of calcium in their diets - and if you add in the UV-B levels in these same areas, why metabolic bone diseases in wild tortoises are practically unheard of.

As mentioned above, different plant species will have somewhat different uptake levels of calcium and other minerals, and this will also vary according to their precise situation, as soil chemistry properties can vary quite a lot even over short distances, however, in general it is clear that calcium carbonate levels throughout the entire zone of interest (those areas where there are native Testudo populations) are very high indeed.

If we take one representative group of plants that are known to feature as a prominent staple feed item for Testudo graeca graeca and Testudo hermanni hermanni, for example, Plantago species, particularly Plantago major, P. lanceolata and P. media we find that detailed analysis reveals the following:

  • Calcium levels can be quite variable, and can (as noted) also vary according to the specific site or time of year that they are measured, however analysed levels indicate that levels in the 260 mg range (P. lanceolata) and 310 mg for P. major would not be atypical. To compare to a baseline, spinach and beet average 106 and 110 mg respectively.

  • Phosphorus content was low, ranging from 23.4 mg in P. major to 34.9 mg in P. media and is significantly lower than spinach (51 mg) and similar to beet (29 mg). The measured calcium-to-phosphorus ratio in Plantago was excellent, and all would meet the nutritional requirements of tortoises in that respect.

  • Fibre amounts are also very high, ranging from 2.25 g (P. media) to 3.88 g (P. major). These results are much higher than data from vegetables like beet (1.0 g) or spinach (0.6 g).

  • Vitamin-C content from analyses of Plantago species indicate levels up to 92.0 mg compared to 34.0 mg (beet) or 51.0 mg (spinach).

  • Available carbohydrates were low and ranged from 1.99 g (P. major) to 2.81 g (P. lanceolata), this compares to beet (5.6 g) or spinach (4.3 g), and very, very significantly lower than fruits and many other commercially available 'salad items' commonly included in captive tortoise diets.

  • Oxalic acid is an antinutritional component that could potentially reduce calcium bioavailability in the diet, though the precise impact of this continues to be debated. In these species, however, the amounts ranged from 33.5 mg (P. media) to 88.2 mg (P. lanceolata), which can be considered as low to moderate levels compared to 460 mg (spinach) and 690 mg (beet).

  • Magnesium content ranged from 49.3 mg (P. lanceolata) to 95.3 (P. major). This may be compared to 65 mg (beet) and 62 mg (spinach). In nutritional terms these are all favourable levels.

  • Pottasium quantities ranged from 318 mg (P. major) to 440 mg (P. media). Again, comparing with our baseline species, beet has levels of (550 mg) and spinach has (662 mg).

Although other plant species will display varying individual readings for all of these criteria, the plants that form the major component of the wild diets of tortoises throughout the Mediterranean may collectively be summarised as:

  • Rich in calcium

  • Favourable in calcium-phosphorus ratio

  • Very high in coarse fibre content

  • Very low in available carbohydrates

In order for hebivorous reptiles to use calcium to build healthy skeletons (most visible as the carapace, of course) they also require access to both UV-B and suitable basking heat. If we look at a general map of UV-B intensity covering the same areas as the calcium map, the coincidence is striking. This map is based on readings in June, but the same general pattern occurs in earlier months, though with reduced intensity.

It should be noted that levels vary throughout the day, hour-by-hour and also seasonally. Very high levels will be recorded in say, July-August, but throughout much of their range Mediterranean tortoises will be entirely innactive and aestivating during this period due to excessive temperatures. They are typically most active March-May, and again though November (though again this varies with location and altitude). The key point, however, is that UV-B levels vary constantly - they are not 'fixed' as in a typical artificial terrarium installation - and most basking occurs early to mid mornings, not at mid-day. As the season advances tortoises typically cease activity earlier and earlier in the morning period, again due to high temperatures. By 10.30am in June, in many parts of their range they will have ceased all activity, possibly emerging once more in the late afternoon or early evening as temperatures once again reach tolerable levels. By July, many populations will be fully aestivating, not emerging again until mid-late September.

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.

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

696 views0 comments

Recent Posts

See All


bottom of page