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High growth rate diets and vitamin D3 - a response

A. C. Highfield

A recent article in Tortuga Gazette, 'How to raise baby tortoises indoors' by Scott Solar, discussed a method of rearing which results in accelerated growth rates and which condemns the use of supplements containing vitamin D3. The article goes on to infer that 'pyramiding' of scutes is directly attributable to D3 supplementation and not to consumption of excess protein. This article raises a number of points which need to be addressed:

1. Solar uses a calcium supplement (although chicken eggshells are not by any means the best source of this, being comprised not just of calcium but miscellaneous grit and other material);

2. Mr Solar recommends the use of UVB tubes for vitamin D synthesis. This approach can work, provided one uses quality tubes and changes them regularly. We trialed it here 5 or 6 years ago. Our view was, and remains, that yes, it does work but it works no better than oral supplementation and many people fail to maintain fresh tubes - when this occurs, deficiency follows. This method may be suitable for the fastidious keeper who is prepared to spend the money on high quality tubes and pay due regard to maintenance, but is very risky otherwise. For that reason, we do not recommend this system to the general public. We do recognise that 'natural' synthesis is perfectly workable, and indeed, when raising tortoises out of doors within their natural bioclimatic range we do not feel that oral D3 supplements are essential. Solar implies that tubes are intrinsically superior to oral supplements. On that we do not agree. They are simply different, with different advantages and drawbacks. Some tubes are defective and fail to emit the UVB the specification demands, for example. Other tubes fail prematurely. We are certainly aware of several cases where this has occurred with serious consequences. We have over 20 years experience with raising hatchlings of very many species, and our view remains that moderate and sensible use of a well-formulated combined calcium/D3 supplement is both safe and effective. Over-enthusiastic use of D3 compounds is certainly dangerous, and all users of such products do need to be made aware of this fact. We also endorse the use of UVB lighting, but we are not entirely satisfied that it sufficiently reliable for use in isolation.

3. Protein content. What is important is the base content of the diet averaged over a long period. They type of protein is also critical, not because, as Solar implies, "all of the high protein foods previously frowned upon for their protein content are mammalian diets", but because of the usability of different types of protein as a consequence of their amino-acid make-up. This has nothing whatever to do with D3 content, and Mr Solar is seriously mistaken here. As an example, it is possible to raise disastrously pyramided juveniles by feeding large quantities of purely vegetable-origin high protein foods: beans, peas, bean-shoots etc. Tofu, an easily assimilated purely vegetable protein source is also a frequent culprit. The amount of D3 offered or not offered, or the availability or otherwise of calcium has very little impact upon the outcome. This was tested experimentally and is repeatable. It is true that dog or cat food of animal origin is by far the worst offender, but this is because is a "complete" protein and it utilised at a high rate of efficiency as a result. In ascribing the well-known problems associated with it simply to its D3 content, Mr Solar is drawing the wrong conclusion.

It is true that the protein content of alfalfa may rise to 22% at certain points in its life-cycle. But alfalfa is not a 'complete' protein and it is utilised relatively inefficiently. Its very high fibre content is also a limiting factor in how much protein is actually consumed and utilised. I normally suggest an average protein intake of about 4% for desert and arid habitat species in captivity. I realise that this figure is lower than some others suggest. This is attained, however, using some components which are indeed far above this in protein content (clover, for example, and yes, alfalfa - which we grow at the Tortoise Trust for feeding purposes). We use a fairly low protein content because our aim is to develop tortoises which are absolutely indistinguishable from wild specimens. To achieve this, we aim for a relatively slow rate of growth. This in itself can be achieved in several ways: 1) By providing 'normal' estivation/hibernation cycles and periodic food shortages as experienced under natural conditions, and by ensuring that the tortoises have to 'work' to get their food, by taking adequate 'exercise' (just as they would when browsing naturally). In these circumstances, higher protein/calorie content foods may be provided without ill effects. Some desert species, T. horsfieldi and T. kleinmanni, for example, experience very profound seasonal variations in food availability and protein content under natural conditions. They are 'programmed' to eat as much as possible, and to grow as quickly as possible, while food is available, because, in a few weeks there will be none. Their protein intake at this period is actually quite high. However, it is counter-balanced by the 'starvation' cycle which follows. 2) By providing a regular intake of lower protein content foods. This is easier to achieve under normal captive conditions and works very well indeed. This is the approach we generally recommend, although a combination of both methods is often the most successful of all.

I do not doubt that it possible to raise 'smooth' hatchlings using the techniques Solar outlines - because he is in fact doing nothing which others have not already done before. Anyone getting lumpy or pyramided juveniles is doing something very wrong indeed as this is so very easy to avoid. Mr Solar is, however, mistaken in ascribing this defect to overdosing or underdosing on vitamin D3 and his own success in this respect to the non-use of oral D3 supplementation. He is avoiding 'lumpy' shells because he is providing an adequate supply of calcium, an adequate source of D3, and a generally well-balanced diet, even though it is higher in protein than that which would be experienced under natural conditions. D3 overdosing does not in any event result in 'lumpy' tortoises but in the mineralisation of soft tissues and in renal damage. There is absolutely nothing in the D3 mechanism which is capable of directly or indirectly causing 'lumpy' or 'pyramided' carapaces. The biochemistry of this process is well understood. In any good management program, overdosing should, in any event, never occur. Cases of simple D3 deficiency also do not manifest as 'lumpy' shells, but as soft or fragile bone tissue or osteomalacia, e,g., classic 'rickets'. Again, this process is very well documented. Most cases where D3 deficiency is encountered are also suffering from multiple dietary deficiencies and are the result of gross dietary mismanagement over a long period. Lumpy carapaces are often seen in such cases, but it would be a mistake to attribute this merely to the deficiency of vitamin D3.

The achievement of unnaturally high rates of growth is also very well documented and is closely associated with the provision of diets high in protein content. These diets may be based upon either vegetable or animal protein sources. The desirability of attaining unnaturally accelerated growth rates is extremely questionable, however. To put this in strictly colloquial terms, these animals are 'on overdrive' and are 'processing' far more protein than would naturally be consumed within a given period of time. In these circumstances, our own experience suggests that there may well be long-term damage caused to the renal system, a high incidence of urate concretions in the bladder, and also some deleterious effects upon the liver. This damage may take years to manifest. In addition, there are real concerns about the status of bone tissue formed under these accelerated growth regimes. In other animals subjected to artificially high rates of growth for commercial purposes numerous problems have been encountered. Few of these animals are intended to live for as long as a typical tortoise. The long-term effects of accelerated growth cycles in tortoises are absolutely unknown, and it will be many years before adequate data on this topic is available.

Mr Solar refers to 'pyramided shells' which are found in nature, and specifically mentions leopard tortoises (G. pardalis). Better examples would be tent tortoises (Psammobates tentorius) or Indian star tortoises (G. elegans). The appearance of these species' carapaces is primarily due to an inherited, genetic property. This has precious little to do with diet, and nothing to do with D3 supplementation. This 'pyramiding' is structurally entirely different than the dietary induced artefacts referred to previously.

A. C. Highfield 1989-1999.