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THE REPTILE WORLD

Reptiles are described as being cold-blood- ed, but this is not very accurate. Their body temperature is largely controlled by their sur- roundings, but there are many things they can do to regulate this temperature and, if required, keep it at a higher level. Reptiles regularly bask in the sun, absorbing its heat through their skins when they need to raise their body temperature. They usually retreat to the shade when they start to overheat and wish to lower their temperature. Some spe- cies may be able to generate heat internally in their tissues and retain this to keep their core temperature up. The largest reptiles are able to maintain a more constant temperature than the smallest species, as their bulkier bodies contain more heat and have thicker skin and fatty layers. Cold-bloodedness has some advantages over warm-bloodedness. Mammals must maintain their body temperature at a constant level,

within very narrow limits. Even minor fluctuations are potentially serious. As a result, mammals constantly need food to provide the energy to produce body heat. Reptiles, on the other hand, can cope quite well with a drop in body tempera- ture; their range of temperature tolerance is far wider than those of birds or mam- mals. Very large reptiles do

This desert spiny lizard from New Mexico lives in an extremely arid environment and feeds mainly on other lizards. It seldom strays far from a secure bolt hole, such as a rocky outcrop or burrow.

Like other lizards, marine iguanas are cold-blooded and need to sunbathe each morning to warm their bodies. Some mature males develop the red coloration seen here.

A group of marine iguanas hauled out on a lava beach on the Galápagos Islands enjoys the equatorial sun. This is the only truly marine species of lizard, and these remote islands in the Pacific are the only place where they occur.

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