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Staff member Mark Schick stands in Shedd's special exhibit, Underwater Beauty. Mark Schick, Senior Director of Animal Operations and Habitats

Amphibian or Reptile? Here's the Difference

Amphibians and reptiles might seem similar—low to the ground, often in water, and not warm or fuzzy—but these two distinct groups exhibit striking differences in the three Bs: body, breeding and behavior.

First, let’s get how they are alike out of the way: Amphibians and reptiles are vertebrates—animals with backbones. The majority of species have four legs, but there are a lot of exceptions in both groups. They derive their body heat from their environment rather than from a high metabolism. You’ll find amphibians and reptiles on every continent except Antarctica, but only reptiles have seagoing species.

A tiny froglet, about the size of the leaf it sits on, peers up at the camera.

This fringe leaf froglet is an amphibian, its skin moist with mucus.

An emerald tree boa curls its sinuous body around a branch at Shedd Aquarium.

This emerald tree boa is a reptile, and has scales and dry skin.

Which brings us to one major difference that will tell you right away if that long, legless animal you’re looking at is a snake or a caecilian: Skin.

Reptiles have scales, and their skin is dry. Amphibians do not, and their skin is often moist with mucus, which keeps them from drying up. While many amphibians, including frogs, salamanders and caecilians, have smooth skin, most toads have bumpy bodies covered with raised glands, some of which produce toxic secretions. But no amphibians have scales. More on skin later.

Tiny frog eggs can be seen, little white circles surrounded by gelatinous clear globes, as a hand reaches down to lift up a covering and insulating layer of moss.

Tiny jelly-like mantella frog eggs can be seen under a protective and insulative layer of moss.

A caiman lizard pushes free of its egg, its head almost comically large as it pokes out of the white egg shell.

A caiman lizard hatches from a hard-shelled egg, to all appearances a miniature version of an adult.

Reptiles hatch from eggs that have a protective outer layer such as a brittle or leathery shell. Hatchlings are usually miniature versions of an adult, ready to take off in the world. Amphibians lay jellylike eggs that you can see through, and the hatchlings typically look nothing like the adult. Most amphibians undergo metamorphosis, during which they change from an aquatic animal that breathes through gills to an adult that may have gills or lungs, depending on the species.

Amphibians can also breathe through their moist, porous skin, and, in fact, several salamander species and one kind of frog have neither gills nor lungs: They get all of their oxygen through their skin.

An Amazon milk frog tadpole is shown through the side of a clear petri dish filled with water. The tadpole's round main body is trailed by a long, spear-like tail surrounded by a wide translucent, leaf-shaped fin, and two tiny developing legs can be seen growing from where the tadpole's tail meets its body.

Frogs begin their lives as tadpoles. This Amazon milk frog is beginning to metamorphosize into a frog; can you see its little legs forming?

An Amazon milk frog poses on a leaf.

An adult Amazon milk frog.

Because amphibians can breathe through their skin, they are extremely susceptible to toxins in the environment, especially in water. The presence or absence of amphibians in a pond or stream is an excellent indicator of its water quality. Reptiles’ impermeable scales protect them from many pollutants and other toxins in water, on land and even in the air that an amphibian could absorb and be affected by. That’s also why reptiles can live in salt water and amphibians can’t.

Finally, here’s a way to tell the difference by ear: Reptiles hiss, grunt and even roar, but amphibians make spring nights magical with their chorus of mating calls in a pond or wetland.

—Mark Schick, Amphibians special exhibit team