Frogs are not reptiles, although the mistake is understandable due to the commonalities between reptiles and amphibians. Frogs are actually amphibians, a classification which also includes toads, newts, and salamanders, among others. At first glance, frogs may appear to share many characteristics with both reptiles and other amphibians. But frogs have unique physical traits, and other biological characteristics that separate them from reptiles.
Why Frogs are Classified as Amphibians
Frogs are classified as amphibians, not reptiles. Frogs have all the defining biological characteristics of amphibians, including glandular skin for cutaneous respiration, a metamorphic lifestyle with aquatic and terrestrial phases, and strong legs designed for jumping, climbing, and swimming. There are more than 5,000 species of frogs, but all share these distinct traits.
Frogs are spread out across the world and can drastically vary in size, color, and physical characteristics. Some have developed amazing adaptations to survive and thrive in harsh or competitive habitats. But despite this, all frogs share the characteristics noted above that steadfastly connect them to other amphibians.
The Confusion Around Frogs and Reptiles
Frogs are often confused as reptiles because there are many similarities between amphibians and reptiles. Amphibians and reptiles are all cold-blooded (e.g., ectothermic), vertebrae, and egg bearers (though there are a few reptiles that bear live young). They have similar environmental needs and many habitats host both intermingled varieties of reptiles and amphibians. Most amphibians and reptiles need hot or warm environments that can either be dry, moist, or wet.
Amphibians and reptiles also share many physical characteristics, which makes the groups harder to differentiate. For example, many amphibians (frogs included) and reptiles have no outer ear or other external hearing structure. Though they have eardrums and other internal hearing components, there may be minimal or no physical markers of an ear. Instead of an ear like mammals have, many frogs (and other reptiles) have an oval-shaped membrane called a tympanum.
Another frog fun fact is that tympanums actually serve two purposes: they allow amphibians and reptiles to hear while also protecting the inner ear structure. Having some sort of auditory protection keeps out water, mud, dust, and other debris. This means that sound and vibration get through, but everything else is kept out.)
Another biological similarity between amphibians and reptiles is their hearts. Both amphibians like frogs and most reptiles have three-chambered hearts (two atria and one ventricle). However, some reptiles (primarily crocodiles) do have four-chambered hearts (two atria and two ventricles), making them more similar to mammals, birds, and humans. Because of their heart structures, amphibians and reptiles also therefore have closed circulatory systems that use double circulation.
Key Differences Between Frogs and Reptiles
Skin plays a critical role in both amphibians and reptiles, but these groups of animals have very different skin. Frogs and other amphibians have permeable, glandular skin, typically a combination of mucous and poison glands. These poison glands are what make the poison dart frog dangerous. This skin may feel moist or even tacky to the touch. But reptiles have non-glandular skin (as do birds, interestingly), which feels drier and more textured.
For frogs, this skin facilitates respiration (called cutaneous respiration) and water absorption, while also aiding activities like communication and even protection. This skin is beneficial to amphibians because it provides a sort of “backup” system for breathing and drinking. (For the lungless Bornean flat-headed frog, their skin is the only method of breathing.) However, permeable skin also presents a potential downfall, since amphibians like frogs are more easily impacted by their environment. For example, frogs can easily become dehydrated if placed in a hot, arid region.
This difference in skin also translates to how amphibians and reptiles reproduce. A frog’s life journey, like other amphibians, begins when a female lays gelatinous eggs in water. Just like adult frogs, these eggs are dependent on their surroundings and must remain in wet or moist environments to hatch. Additionally, the eggs may need to be directly fertilized once produced (e.g., external fertilization).
Conversely, most reptiles lay amniotic eggs that are self-contained: the amniotic sac provides all the water the embryo needs to develop. So, whereas reptiles must primarily worry about predators, frogs and other amphibians must contend with predators and the environment. And regardless of whether reptiles lay eggs or bear live young, eggs must be fertilized inside a female in order to be viable once laid (e.g., internal fertilization).
Small Differences Between Frogs and Other Amphibians
Amphibians fall under three orders: Anura, Caudata, and Gymnophiona. Frogs and toads comprise the Anura order, characterized by their adaptations for rapid and powerful movements (e.g., their legs). Salamanders and newts fall under Caudata, characterized by their persistent tails and similar adaptations for walking and swimming. And Gymnophiona are the limbless amphibians, also known as caecilians.
At the beginning of this article, we discussed three primarily characteristics shared by amphibians: glandular skin for cutaneous respiration, a metamorphic lifestyle with aquatic and terrestrial phases, and strong legs. Obviously, frogs differ from all amphibians that fall under the Gymnophiona order. But there are other, more subtle differences between frogs and amphibians.
Even within the Anura order, there are differences between frogs and toads. Both have glandular skin, but in toads the glands are physically prominent. Toads are known for their dry, “warty” skin—but these “warts” are actually the glands. Toads also typically have shorter hind legs than frogs, in addition to being more suited to terrestrial habitats instead of aquatic or semi-terrestrial habitats.
Within the Caudata order, the metamorphosis from egg to juvenile to adult is generally less drastic than it is for frogs. For example, frog tadpoles initially have no limbs or gills. But salamander larvae always have limbs (they’re just underdeveloped) and gills. And while a frog’s gills eventually become completely internal, a salamander’s gills may just shrink.
