What are reptiles?
Reptiles are scaly, ectothermic, egg-laying, vertebrates.
Whilst this definition encompasses most snakes and lizards, turtles, crocodiles and tuataras (an ancient group of reptiles that survive only in New Zealand), it fails to recognise the full diversity of this group.
Blue-tongue Lizard (Tiliqua scincoides)(To fully appreciate what reptiles are, we need to understand the relationships amongst both modern and past forms. To be meaningful, a taxonomic group of animals, such as reptiles, should include all descendants of a single common ancestor. However, because of the close relationship between birds, crocodiles and dinosaurs, it is not possible to do this with the living animals we normally think of as reptiles without including birds as well. This concept is easy to sell to an evolutionary biologist but doesn’t gel with the way most of us think.
From a more conventional viewpoint, it’s easier to consider reptiles as the non-amphibian animals covered by the study of herpetology –
- the Crocodilia (crocodiles, alligators, gharials etc),
- the Testudines (turtles and tortoises),
- the Squamata (snakes and lizards) and
- the Rhynchocephalia (tuataras).
Birds are so different from the scaled, non-flying reptiles that they are usually placed in their own class – Aves. Strictly speaking, ornithologists (bird-workers) also work on reptiles, but only the modern, feathered ones!
Ectothermic is more accurate than ‘cold-blooded’, a term frequently used in association with reptiles. Ectotherms are animals that maintain their body temperatures using external heat sources (primarily the sun). This contrasts with endotherms (birds and mammals) that generate their own body heat through metabolic processes. Ectothermic reptiles are able to control their body temperatures within narrow limits. Most maintain a body temperature similar to that of mammals by simply moving between sources of heat (basking in the sun or lying on warm rocks) and cooler areas (seeking shade).
We should be mindful that it is difficult to narrowly define such a diverse group as reptiles and there will always be exceptions. For instance, many snakes and lizards don’t lay eggs but give birth to live young. Leatherback Turtles aren’t strictly ectothermic but have physiological adaptations for maintaining body temperatures that are higher than ambient sea temperatures.
Saltwater Crocodile (Crocodylus porosus)
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