These aren't your run-of-the-mill amphibians; these frogs are downright freaky.

1. Purple Frog (Nasikabatrachus sahyadrensis)

The purple frog, native to India's Western Ghats, is also sometimes called the pignose frog. It spends all but two weeks of the year—monsoon season—more than 26 feet underground, feeding on termites and emerging only to mate. Which apparently isn't easy for these frogs: Two purple frogs were recently caught mating on video for the first time ever, and—thanks to their wide bodies and stubby legs, which are well evolved for life underground, and not so much for getting it on—it involves a lot of scrambling on the male's part as he attempts to get on the much-larger female's back and hang on. You can see the video of the frogs doing their thing here.

2. Black Rain Frog (Breviceps fuscus)

Breviceps fuscus has what humans might call "resting bitch face"—in fact, it's often called the world's grumpiest frog. (In a funny twist, its cousin, Breviceps macrops, has actually been dubbed the world's cutest frog, thanks in part to the little squeaks it makes. Go figure!) This 2-inch-long burrowing amphibian, which is capable of digging tunnels nearly 6 inches deep, is native to the southern coast of Africa. When threatened, they'll puff themselves up so that they look bigger; sometimes they'll do this while they're burrowing so that whatever's after them can't pull them out. These guys know how to make mating go smoothly: The females secrete a sticky substance on their backs so the males don't fall off while the act is taking place; then, he'll hang out in the burrow and guard the eggs.

3. Surinam Toad (Pipa Pipa)

Don't let its name fool you: This animal, found in South American rain forests, is actually considered an aquatic frog. Seven to eight inches long, brown, and flat (in fact, it's one of the world's flattest amphibians), the Surinam toad blends in perfectly with muddy river bottoms. The reproductive cycle of these frogs is definitely a strange affair: The male calls to the female underwater with a clicking sound; she releases 60 to 100 eggs into the water, which the male fertilizes and pushes into her back. Her skin grows up over the eggs until they're entirely enclosed. The young hatch and ride on her back for up to four months as they develop. When they're ready, the toadlets push to loosen the female's skin, pockets on her back open up, and the young pop out. To breed again, she sheds her skin.

4. Hairy Frog (Trichobatrachus robustus)

Gustavocarra / Creative Commons License

This frog is also sometimes called the Horror Frog, and for good reason: When threatened, it breaks its own toes to produce claws, which puncture its toe pads on its hind feet, and uses them as weapons. Breeding males will grow hair-like strands on their sides that are actually skin and arteries, which might allow them to take in additional oxygen from the water while they're guarding their young.

5. Turtle Frog (Myobatrachus gouldii)

The Western Australian Museum calls this odd little Aussie native, the only species in its genus, "a very peculiar frog with a body shape superficially resembling a small turtle with its shell removed." Unlike most frogs, which burrow back into the ground using their hind legs, M. gouldii burrows forward through sandy soils with its muscular arms. The frogs, which can grow to nearly two inches long, don't need water to reproduce; instead, a male will dig a burrow almost 4 feet deep and call to the female from there. After several months of cohabitating, they'll mate and she'll lay as many as 50 eggs in the burrow. The young skip the tadpole stage and instead stay in the eggs, emerging as tiny, fully formed frogs.