Notes on Dietary Constituents for Herbivorous Terrestrial Chelonians and Their Effect on Growth and Development
©1996 Andrew C. Highfield, British Tortoise Trust
A practical solution is to botanically screen all dietary items carefully and exclude items which are strongly Ca:P negative from regular consumption. The remaining neutral or positive Ca:P constituents can then be supplemented with a multi-vitamin and mineral powder boosted if necessary by the addition of raw calcium lactate until a true ratio of at least 5:1 is achieved. In practice, hatchling T. graeca, Geochelone pardalis and Chelonoidis carbonaria raised on such a regime do not display the deformed carapaces and raised vertebral shields so typical of nutritional secondary osteodystrophy fibrosa which were remarked upon by Lambert (1986) and which results from an excess growth of keratin combined with the sub-optimum development of the underlying bony plates.
Vitamin D and
An Approach to
a Suspected Calcium Deficiency
The high phytic acid content of legumes which mobilizes Ca is also cause for concern. Each of these items has in the past been recommended as a suitable substitute item for inclusion in the diet of hatching chelonians. Lettuce, which is frequently condemned as a dietary constituent, is in fact relatively neutral in the Ca:P sense, ranging from 20mg Ca to 22mgP/100g in the case of Iceberg varieties, to 68mg Ca to 25mg P/100g for Romaine varieties. Although inadequate alone, it is useful neutral base for further controlled artificial supplementation. Another common tortoise food item, dandelion, is of extremely high quality comprising 187mg Ca to 66mgP/100g and combining this with 14,000 i.u- Vitamin-A, 1. 6g fibre, 0 - 19mg thiaminee and 2.7g protein. Banana fruit and leaves which are frequently utilized in tropical captive breeding projects as a readily obtainable staple dietary item are strongly Ca:P negative and liable to induce a relative Ca deficiency in rapidly growing animals (typically 8mg Ca to 30mgP/100g fresh, 32mg Ca to 104mgP/100g dried).
Some food items which appear initially to represent good sources of dietary calcium are not as attractive on closer examination. Beet greens, kale, spinach and members of the Goosefoot family Chenopodiaceae contain relatively high Ca levels but this is bound up with oxalic acid which reacts with calcium to form insoluble calcium oxalate. Opuntia cacti, a typical food item of arid habitat tortoises including Gopherus agassizi and Chelonoidis elephantopus offers by comparison 1.89% calcium to 0.02% phosphorous (Rosskopf, 1982). Other self-selected native food items of Gopherus agassizi reveal similarly positive Ca:P ratios.
The dietary preferences of T. hermanni and T. graeca in the wild have seldom been recorded, notable exceptions being the work of Swingland (1984) and Stubbs who noted that 25% of the diet of T. hermanni consisted of Rubiaceae (Bedstraw family), 22% of Leguminosae (Peaflower family), 10% of Compositae (Daisy family) and 801o Ranunculaceae (Buttercup family). Comparative figures for T. graeca include 30% Plantago (Plantain family), 26% Compositae and 10% Rubiaceae. An average Ca:P ratio for the above is 3.5:1, and a typical protein content is 2.75% (Highfield, unpublished notes).
A practical solution is to botanically screen all dietary items carefully and exclude items which are strongly Ca:P negative from regular consumption. The remaining neutral or positive Ca:P constituents can then be supplemented with a multivitamin and mineral powder boosted if necessary by the addition of raw calcium lactate until a true ratio of at least 5:1 is achieved. In practice, hatchling T. graeca, Geochelone pardalis and Chelonoidis carbonaria raised on such a regime do not display the deformed carapaces and raised vertebral shields so typical of nutritional secondary osteodystrophy fibrosa which were remarked upon by Lambert (1986) and which results from an excess growth of keratin combined with the sub-optimum development of the underlying bony plates.
In addition to stimulating excessive growth, both generally and of the keratin in particular thus increasing the calcium demand yet further, the intake of high levels of protein has two further effects a) there is a direct effect upon calcium absorption ability (Margen, 1974) and b) high protein results in high levels of blood urea and consequently increases the amount of nitrogenous waste to be processed via the renal system. Unfortunately, reliable data on blood urea nitrogen and creatinine levels of wild populations is not readily available and most studies so far published have relied upon data extracted from captive specimens, mostly pet animals belonging to members of the public (e.g. see Rosskopf, 1982). Although useful, this data is unlikely to represent a true picture of the normal blood chemistry of specimens in the native habitat. It is not at all unusual for herbivorous chelonians which have been maintained on unnaturally high protein diets to exhibit symptoms of renal dysfunction. Dehydrated animals are obviously most at risk, as the accumulated uric acid becomes deposited not only in the renal tubules but invasively throughout the pericardium, liver and other organs (Frye, 1974; Wallach, 1971). Specific conditions recorded include interstitial nephrosis and glomerulosclerosis (Rosskopf, 1981). Once again, processed tinned pet foods of animal origin are particularly dangerous as not only are they high in protein (typically 17%) and saturated fats, rich in phosphorous and relatively low in calcium but most are also very rich in other nitrates. Cheese, another item which has seriously been suggested as an appropriate dietary component for T. graeca is similarly adverse in effect (protein content typically 25%). Legumes are also spectacularly high in protein for a vegetable source (typically 10%), and should be avoided for the same reasons.
The protein requirement for most reptiles have not been studied in sufficient detail and no specific figures have been established for herbivorous terrestrial chelonians. Analysis of the native diet of Gopherus agassizi, which in many respects is typical of and habitat chelonian herbivores, suggests that the protein content of the food intake ranges from 1% (Opuntia sp. ) with grasses at a median content of 5% constituting a major part of the dietary intake (Rosskopf, 1982; Hansen et al, 1976). A safe upper protein limit for items which are regularly included in the diet would seem to be 7% as this is about as high as is ever attained in the wild by most species, even during peak periods of food availability. An average intake level of 4% would represent a close approximation of that experienced in the natural habitat.
Despite the lack of detailed information on protein demand, it is certain that the figure is very much lower Kg for Kg than mammals where 0.5g of usable protein per Kg would be a typical daily requirement. It seems probable that the daily requirement of a growing tortoise is in the approximate region of 0.20g of usable protein per Kg, although this may well vary considerably according to species and metabolic rate. Against this it should be noted that even such a low quality food item as lettuce contains an average of Ig per 100g and most legumes contain well in excess of 7g/100g.
In the wild tortoises consume not only plant leaf material, but also seeds, fruits, flowers, roots, bark and grasses. See Samour, Spratt, Hart, Savage and Hawkey (1987) for an excellent survey which conveys the scale of the dietary eclecticism of Megalochelys (Geochelone) gigantea, for example. The principal importance of this varied intake is not merely that it provides a relatively wide range of vitamins, minerals and fibre, but that by combining leaf, bark grass and seed sources an improved range of essential amino-acids is made available in a complementary process. This has the effect of increasing the potential Net Protein Utilization (NPU) factor of the diet. Where tortoises are provided with a suitable intake of essential amino-acids derived from the correct balance of vegetable matter they can exist perfectly satisfactorily on diets which are surprisingly low (in percentage terms) of protein content. Artificial amino-acid supplementation is not necessary provided a reasonable range of suitable food items are available although this has sometimes been adopted (bac,-)n, 1980 [sic]). In most captive situations pathology results consistently indicate that it is protein excess rather than deficiency which is the principal danger. Meat based products have an extremely high NPU as they represent a complete amino acid source. 100g of dog food containing 15% protein is, therefore, utilized at a greater rate than a vegetable containing an equivalent percentage of raw protein. In some circumstances and with some reptiles this may be of benefit, in the case of herbivores it is entirely inappropriate. The diet of a wild tortoise typically contains between 2%-6% plant protein which is utilized at an approximate rate of 55%. The high protein regimes adopted by some herpetologists for herbivorous chelonians include up to 20% protein which, because it is derived from amino-acid complete sources is utilized at a typical rate of 70%.
It is necessary to comment upon the claim often made that terrestrial chelonians from and habitats receive significant additional protein in the wild as a result of consuming carrion, arthropods and other insects. This is not supported by faecal pellet analysis (Dearden, Hansen and Steinhorst, 1974) which indicates that such intake is so low as to be of virtually nil dietary significance. The levels paralleled that of other miscellaneous detritus also consumed, including small rocks, sand, bird feathers, lizard skin casts and mammal hairs (Hansen et al, 1976). Most tortoises will consume anything which is presented to them whether palatable and nutritious or not. Until separated from the public by barriers, giant tortoises at San Diego zoo were known to consume popcorn, balloons, yogurt, film wrappers, chewing gum and red-painted toenails (Bacon, 1980). Habitat analysis suggests that animals from dry areas would not frequently encounter available sources of animal protein, but that animals from more humid habitats may have greater opportunities in this respect. Certainly, some T. hermanni may occasionally display an inclination to consume a passing slug or worm, but they make no effort to actively seek out such delicacies. In experiments conducted by the author, T. graeca, T. g. zarundnyi, T. g. ibera, T. g. terrestris, T. marginata and T. kleinmanni showed no interest whatsoever in slugs, worms or insects presented and indeed demonstrated active avoidance behaviour in many instances.
The Role of Symbiotic
Need to update a veterinary or herp society/rescue listing?
Can't find a vet on my site? Check out these other sites.
|Clean/Disinfect||Green Iguanas & Cyclura||Kids||Prey||Veterinarians|
|Home||About Melissa Kaplan||CND||Lyme Disease||Zoonoses|
|Help Support This Site||Emergency Preparedness|
© 1994-2014 Melissa Kaplan or as otherwise noted by other authors of articles on this site