Green tree frog. Source
The title "frog" applies to a wide range of creatures from the order Anura ("without tail"). The defining characteristic of these amphibians is that they don't have a tail, as the name of their order would suggest, setting them apart from animals like salamanders or caecilians. Frogs have adapted to live in all sorts of environments. Most people are familiar with them living in or around water, but there are species that also live on land, in trees, and even underground. Literally any environment that an animal is known to live in, there is a frog that lives there too. However, the greatest number of frog species live in tropical forests, frogs like infamous poison dart frog from Central America or the shrub frog (Rhacophoridae) of Asia and Africa.
Triadobatrachus, the first "frog" identified in the fossil record. Source
Before I go any further and define the physical characteristics of frogs, I must first point out the "difference" between toads and frogs. The distinction between the two is that there really isn't a distinction. Toads are frogs, the name "toad" has no taxonomic justification. However frogs of the family Bufonidae are considered "true" toads. Frogs have smooth, permeable skin that allows them to absorb oxygen from the water, but leaves them susceptible to dehydration, whereas "toads" have rougher, thicker skin with "warts" that protects them from water loss. Other than this difference, toads and frogs are almost indistinguishable. That being said, frogs vary drastically from environment to environment. Frogs that spend most of their time in the water have poorly developed lungs (in fact, all frogs have poorly developed lungs) and are able to absorb oxygen through their semi-permeable skin. This allows them to spend a long time underwater, and some species can even survive without their lungs. Given their aquatic life style, frogs have three eyelids, one clear membrane to protect their eyes underwater while giving them vision, and two others for various purposes.
Frogs tend to be carnivorous, but there are omnivorous species and even herbivorous species. Some frogs are toothless, but most have teeth on their upper jaw. These teeth are not meant to cut or chew, instead they are meant to hold in their food (oftentimes living) while they swallow it. Other species that don't have teeth are known to use their hands to shove food into their mouths. Once frogs have gotten a bite of something, they swallow it whole, sometimes retracting their eyes to force food down their throats.
A frog retracting its eyes to swallow. Source
All frogs start their lives as tadpoles, the larval stage between birth and metamorphosis into their adult form. Frogs lay their eggs surrounded individually by a gelatinous bubble that keeps them moist. Sometimes these eggs are laid in water, other times they are kept wet manually by their parents, but always they must be kept wet. Once the eggs hatch, they become fully aquatic tadpoles with internal gills and a tail. These tadpoles spend all their time in water, eating whatever they can with their omnivorous, grating teeth. Once a tadpole reaches the age of maturation, it will go through metamorphosis, where it grows legs and loses its tail and gills. This process can take as little as 24 hours.
Frog eggs. Source
Tadpole. Source
Some frogs have very interesting adaptations. The most commonly known adaptation is the long, sticky tongue. Many frog species have very long, cleft tongues that can be shot out of their mouths at incredible speeds to capture prey. Another famous adaptation are the toxic chemicals secreted out of a frog's glandular skin. Frogs with this adaptation are like the poison dart frog that displays its bright colors to let other animals know that they are deadly. Other poisonous frogs aren't quite as deadly as the poison dart frog, but can make themselves unpalatable to predators. Even other frogs have skin coloration and texture that makes them almost impossible to see.
Goliath frog, the world's biggest frog. Source
Paedophryne amauensis, both the world's smallest frog, and smallest vertebrate. Source
Frogs amaze me with their unique and marvelous adaptions. They are one of the most diverse groups in the animal kingdom, and they look pretty cool too.
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