I've always liked frogs. I remember, when I was probably around 4 years old, being fascinated by the tadpoles that Dad brought home in a big jar from a farm pond. Mum explained about how they'd gradually metamorphose (thought I doubt she used that word!) & we watched their legs slowly grow & their tails disappear as they swam around in an old tub, until the point where they became frogs.
Frogs are amphibians, along with newts & mud-puppies & axolotls and the legless caecilians (which look like a cross between an eel and an earthworm). As a group, frogs are much younger – in geological terms – than the others: most fossil frogs date back only about 50 million years, although the earliest-known frog-like creature, Triadobatrachus, lived about 250 mya in the early Triassic.
Like almost all terrestrial amphibians, adult frogs use not only lungs for gas exchange, but also their skin and the membranous lining of their mouths. (Lungless salamanders are an exception – as the name suggests, they must rely on their skin alone, which is very convenient for those researching amphibian gas exchange.) This reliance on transcutaneous respiration has meant that amphibians are very susceptible to harm due to to chytrid fungus infection, which severely damages the skin and markedly reduces the animals' ability to exchange O2 & CO2 with the atmosphere.
In addition, using your skin as a gas exchange surface means that you have to keep it moist. This means that we'd expect to find frogs only in environments that are humid and damp year-round, & in general that's the case. But there are always exceptions. and the desert rain frog is one of them. Breviceps macrops lives in one of the most inhospitable environments there is, a dry coastal strip of land in Namibia & South Africa. Hardly a place for a frog! It spends most of its time in burrows dug deep enough to reach into moist sand, but comes out at night when the air is cooler & more humid. While there's very little actual rain, moisture-bearing sea fogs roll in from the ocean on at least 100 nights each year, bringing some water to the habitat as the fogs condense onto dunes & vegetation – enough to allow these little amphibians to survive. (There's no actual tadpole stage in their life cycle; little froglets develop directly from eggs in the burrows.)
And like other amphibians, they vocalise to advertise their presence. I hesitate to say the sound is a croak. In fact, it drove my dog to distraction when I played the following clip.
I give you – 'the sonorous war cry of a very angry frog'.