High growth rate diets and vitamin D3 - a response

A. C. Highfield

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.

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.