In praise of high fibre diets
Dr. Clive Cummins
How many readers of this article would put their children on a diet of red meat and vitamin pills in order to maximise their rate of growth? Not many, I hope.
But I believe this is a fair analogy with the way many people treat their frogs and tadpoles. As most of us know from experience, Dendrobatids and other anurans in captivity can suffer a variety of disorders. Spindly-leg syndrome is well-known, but I have seen other limb mal-formations including foreshortened, splayed and stiff hind limbs in Mantella and Rana tadpoles. Under certain conditions, frogs may suffer spasms and paralysis. It is generally accepted that some of these disorders reflect a deficiency in the animals' nutrition, but what do we mean by the term "deficiency"?
I suggest that we should distinguish carefully between "not good enough" and "not enough". A typical, and understandable, response of the frog-keeper whose animals are afflicted is to supplement the diet with vitamins and minerals. This may help, and in the absence of proper, controlled experiments to identify the nature of the deficiency I regard it as prudent -up to a point. We need to bear in mind, though, that an animal's capacity to take up and use the various vitamins and minerals is limited, and that doubling the dose will not necessarily double the intake. Furthermore, there may be antagonism among different components of the diet, so that increasing the concentration of one substance in the diet reduces the uptake of another. In other words, it is important to achieve a balanced diet.
So what constitutes a balanced diet? I suggest that there's more to it than simply defining a ratio of, say, protein to fat to Vit. D3 or whatever.
Let's imagine, purely for the sake of argument, that an animal uses two substances - A and B - in the ratio of 20:1.
Let’s also assume that it can take up 100 units of A per day, but only 3 units of B per day.
Now suppose that we give a specially-formulated "ideal" diet containing A and B in the required ratio of 20:1. At rations containing up to 60 units of A per day the animal has sufficient B. Increasing the ration allows the animal to take up more A - up to 100 units per day - but its uptake of B remains limited to 3 per day, so as the ration increases, the deficit of B increases. No amount of supplementation of the diet with B will help, because the animal can't increase its uptake of this substance. This is all very hypothetical, but I believe it provides a reasonable theoretical basis for moving away from the pursuit of rapid growth, particularly through the use of high-protein diets and hyper-supplementation. If animals in the wild are able to achieve rapid growth - and evidence of this is often lacking - I suspect it is because environmental conditions allow what we might regard as a more efficient use of food resources. Suppose that, in the example above, uptake of B was dependant on a co-factor whose production or function is stimulated by light of particular wavelengths. Increasing light in that wave-band could thus help to alleviate the animal's deficiency of B, provided B was present in sufficient quantity in the diet. Another environmental factor that can be manipulated readily is temperature. In tadpoles it is generally the case that development is more sensitive to temperature than growth, so keeping your tadpoles at a lower temperature (within reason) will, all other things being equal, lead to longer larval period and larger size at metamorphosis. Apart from the obvious advantages of large size at metamorphosis, such as easier feeding, it is worth considering that slower development may allow the need for trace elements, vitamins, etc. to be fulfilled even if the diet and other aspects of the environment are less than ideal, simply because the demand is spread over a longer period.
Most of us want the best for our animals and it's easy to assume that more is better. However, I would encourage anyone whose frogs are having problems to at least consider ways of reducing the average calorific value and protein content of the animals* diet. Perhaps one of the best things any of us can do is to make use of harvested foods such as aphids, bugs, caterpillars and soil arthropods whenever possible.
If you can reduce the animals’ requirements and increase their efficiency of use of their food, so much the better.