29.4B: Characteristics of Reptiles
- Summarize the key adaptations of reptiles
Characteristics of Reptiles
Reptiles are tetrapods. Limbless reptiles (snakes and other squamates) have vestigial limbs and, as with caecilians, are classified as tetrapods because they are descended from four-limbed ancestors. Reptiles lay on land eggs enclosed in shells. Even aquatic reptiles return to the land to lay eggs. They usually reproduce sexually with internal fertilization. Some species are ovoviviparous, with the eggs remaining in the mother’s body until they are ready to hatch. Other species are viviparous, with the offspring born alive.
One of the key adaptations that permitted reptiles to live on land was the development of their scaly skin which contains the protein keratin and waxy lipids, reducing water loss from the skin. Due to this occlusive skin, reptiles cannot use their skin for respiration, as do amphibians; all breathe with lungs.
Reptiles are ectotherms: animals whose main source of body heat comes from the environment. This is in contrast to endotherms, which use heat produced by metabolism to regulate body temperature. In addition to being ectothermic, reptiles are categorized as poikilotherms: animals whose body temperatures vary rather than remain stable. Reptiles have behavioral adaptations to help regulate body temperature, such as basking in sunny places to warm up and finding shady spots or going underground to cool down. The advantage of ectothermy is that metabolic energy from food is not required to heat the body; therefore, reptiles can survive on about 10 percent of the calories required by a similarly-sized endotherm. In cold weather, some reptiles, such as the garter snake, brumate. Brumation is similar to hibernation in that the animal becomes less active and can go for long periods without eating, but differs from hibernation in that brumating reptiles are not asleep or living off fat reserves. Rather, their metabolism is slowed in response to cold temperatures; the animal becomes very sluggish.
Key Points
- All reptiles, including aquatic ones, lay their eggs on land.
- Reptiles reproduce sexually through internal fertilization; some species are ovoviviparous (lay eggs) and others are viviparous (live birth).
- Because of the development of impermeable, scaly skin, reptiles were able to move onto land since their skin could not be used for respiration in water.
- Reptiles are ectotherms: they depend on their surrounding environment to control their body temperature; this leads to advantages, such as not being dependent on metabolic energy from food for body heat.
- Reptiles are also poikilotherms: animals whose body temperatures vary rather than remain stable.
- Some reptiles go into brumation: a long period during cold weather that consists of no eating and a decreased metabolism.
Key Terms
- viviparous : being born alive, as are most mammals, some reptiles, and a few fish (as opposed to being laid as an egg)
- ovoviviparous : a mode of reproduction in animals in which embryos develop inside eggs that are retained within the mother’s body until they are ready to hatch
- ectotherm : a cold-blooded animal that regulates its body temperature by exchanging heat with its surroundings