The frog is an amphibian in the order Anura. There are over 5000 described species of frog in the world, divided into 33 different families, and about 88% of the known amphibian species are frogs. Anura is one of the most diverse orders of vertebrates. The largest frog families are Leptodactylidae (around 1100 species), Hylidae (around 800 species) and Ranidae (around 750 species).

Geographical distribution

Frogs are found all over the world, from the tropics to the sub-arctic regions. They are however not found in true deserts. A majority of the species live in tropical rainforests.

Habits and ecology

Frogs will typically lay their eggs in bodies of water, such as lakes, ponds and puddles. The eggs will develop into larvae (commonly referred to as tadpoles) in the water. Tadpoles are equipped with gills and breathe just like fishes, while adult frogs absorb oxygen from the air like mammals. How aquatic the life of the adult frog will be varies greatly among the various species. Some spend a lot of time in the water and rarely ventures onto land, while others prefer to stay out of water until its time to mate. Adult frogs are carnivores.

Unlike other terrestrial vertebrates, the adult frog has a highly permeable skin through which oxygen can be absorbed. The oxygen is dissolved in an aqueous film on the skin before it gets into the bloodstream. This is why it is so important for a frog never to let its skin dry out. Another problem with having a highly permeable skin is that so many toxins from the environment will pass through it. This makes the frog sensitive to pollutions.  

Taxonomy

Frogs are broadly classified into three different suborders: Archaeobatrachia, Mesobatrachia and Neobatrachia. Archaeobatrachia contains only four families and they are all primitive frogs. There are 28 described species in this suborder and they all have certain physiological characteristics that cannot be seen in other members of the order Anura. Archaeobatrachia tend to be very small and chiefly live in Eurasia and New Zeeland.   

The suborder Mesobatrachia was created in 1993 and currently contains 168 species divided into 6 families and 20 genera. The word meso means middle and Mesobatrachia species are not as old as those found in Archaeobatrachia (archeo means ancient) but not as young as the species in Neobatrachia (neo means new).
Neobatrachia is a huge suborder compared to the other two. It contains 24 families and over 5000 species. Over 96 percent of all described species of frog belong to this suborder. Neobatrachia is commonly divided into four superfamilies – Bufonoidea, Hyloidea, Microhyloidea and Ranoidea – but the classification is controversial and different experts hold different opinions about which species that should be included in each superfamily.  

What’s in a name?

In everyday speech, a distinction is often made between frogs and toads based on their appearance, but this has no taxonomic justification. From a taxonomic perspective, all members of the order Anura are frogs, while only the members of the family Bufonidae are true toads. You can however find species from a lot of other families that have the word toad in their common names, and also quite a few species in the family Bufonidae that are referred to as frogs. 

In everyday language, the distinction between toads and frogs are typically based on appearance. The appearance of the “toads” has changed since they have adapted to dry environments. “Toads” are therefore typically equipped with dry and warty skin, while the frogs have smooth and/or moist skin. There are however exceptions, such as the aquatic Bombina bombina which is called fire-bellied toad in English. This species live in water but does have somewhat warty skin.