Termoregulation in Amphibians and Reptiles

Amphibians and reptiles from two separate classes of the vertebrates, or back-boned animals. The other classes of vertebrates are the fish, which are more primitive than the amphibians and reptiles, and the birds and mammals, which are more advanced. Collectively, amphibians and reptiles number almost 10,000 known species, about the same number as there are species of birds. Amphibians evolved from fish about 350 million years ago and were the first vertebrates to colonise the land, although they were still dependent on water for breeding purposes. This disadvantage was overcome 50 million years later with the development of shelled egg. At about the same time, the animal's dependence on water was further diminished by the production of a hard, scaly skin - which meant the first reptiles had appeared. From these primitive beginnings, amphibians and reptiles have diversified to fill many niches in the order of living things. Many, such as dinosaurs, have been superseded by later, better adapted forms, but in spite of several limitations almost every part of the world has its share of amphibians and reptiles.

The most important of these limiting factors is temperature. Like all animals, amphibians and reptiles operate most efficiently at certain optimum temperatures which may vary from one species to another; unlike birds and mammals, however, they are not able to produce heat internally, but are dependent on outside sources for it, and are thus labeled "ectoterms" or "poikiloterms." For this reason, the greatest concentrations, both of individuals ad of species. are to found in the tropics, with the numbers dropping off quite rapidly as the poles are approached.

In general, amphibians tolerate and prefer lower temperatures than reptiles, and are usually found to be at about the same temperature as their surroundings. Certain reptiles, however, have one or two methods by which they can elevate their body temperature above that of their surroundings. These species bask for part of each day in order to absorb solar heat; and some of them increase their efficiency by developing dark colors when cool and becoming paler when the correct temperature is reached, or by flattening and aligning their bodies to the direction of the sun's rays early in the day in order to warm up, and thereby becoming active more quickly. Once the desired body temperature has been achieved, the animal may move into the shade to prevent over-heating. Thus emerges a pattern of basing, followed by activity, followed by more basking.

This routine is easily observed if a small group of diurnal lizards is kept in a cage with a single light bulb at one end. Shortly after the light goes on in the morning there will be a jostling for the prime basking sites, followed by the gradual dispersal of the group as each individual reaches its preferred temperature.

Radiated tortoise
Radiated Tortoise
Photo by Larysa Johnston

The time taken will depend on (among other things) the size of the individual - large bodies take longer to warm up than small ones. Other activities, such as feeding, drinking and courtship, are then pursued, with periodic returns to the basking site in order to keep the body temperature "topped up." Shortly after the light is extinguished in the evening, all the lizards will again be found around the basking site, this time pressing their limbs and bodies against rocks and logs in order to soak up the last few remaining degrees of warmth retained there. It follows from this that species which habitually bask, mainly certain snakes, lizards and turtles, should have some form of radiant heat source and conveniently-placed basking "plaforms" on to which they can climb to absorb the heat.

No-basking forms, such as nocturnal and fossorial (burrowing) species, are less able to termoregulate and it is to be expected that their captive quarters will have to be more closely adjusted to their preferred ranges. Nevertheless, burrowing species probably move through the substrate in order to find the most satisfactory place to rest, and nocturnal species may adjust their period of activity slightly according to the prevailing temperature. For example, they may emerge from their retreats during the early part of the night while the substrate is still warm. Unfortunately, the preferred temperature ranges for many species have not been established, and conditions in captivity must sometimes be the result of guesswork. On the other hand, species which come from tropical regions do not necessarily require high temperatures, especially if they hail from regions of high altitude or from cool micro-habitats (for instance, in the spray of a waterfall or in an area of permanent shade). Some species have ranges which stretch from the tropics to well within temperate regions.

All of these factors have to be considered when deciding which animals are to be accommodated and how their temperature requirements are best met. It does not mean that the majority of species are difficult or demanding to cater for in this respect. Amphibians and reptiles are, on the whole, a remarkably tolerant and adaptable group of animals, and the parameters within which they live are to some extent flexible. The behavior of the animals is a pointer of their well-being: if they spend all of their time under the heat source they are being kept too cool, but if they retreat into the corner furthest from the source soon after it has been switched on it is too hot. The power-rating of the heat source, the amount of time it is kept on and the insulation of the cage must be attended to accordingly.

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