The Frog Whose Young Erupt From Under Its Skin

We need to talk, Mom. You’re smothering us. We need space.

Amphibians have it made. Frogs sometimes just up and turn into princes, salamanders can emerge from fire unscathed, and a certain newt almost got to the White House in 2012. And add to these triumphs the fact that egg-laying amphibians don’t have to mess with live birth, an excruciating event that your mother tearfully reminds you of whenever you forget her birthday, which I may or may not have done once in high school.

There is an amphibian, though, that deviates wildly from the reproductive norm. This is the Surinam toad, Pipa pipa, whose females absorb their eggs into their own backs. Here the embedded young develop safely before erupting through their mom’s skin into a world where they’ll need to make damn sure they never forget her birthday, even if it was way back in high school and they had a lot on their plate at the time.

The Surinam toad’s existence begins with some good old sexual somersaults. It’s a highly aquatic frog, and is thus afforded the opportunity of having what herpetologist David Cannatella of the University of Texas calls “acrobatic” mating interactions.

When two Surinam toads love each other very much, the male grasps the female in what’s known as amplexus. “He’s holding on while she is swimming around and often she’s doing somersaults, not just sitting there on the bottom,” said Cannatella. “And that’s really unusual in frogs, because most frogs aren’t that aquatic.” (Toads are a kind of frog, and actually the Surinam toad more resembles a regular frog than a toad — the distinctions are a bit informal and a bit silly.)

A female Surinam toad with her recently hatched young in a nice autumn scene.

“Prior to this the skin on the back of the female, the dorsum, has begun to thicken because of hormonal influence,” he added. As they somersault, the female extrudes eggs one at a time, which the male fertilizes. These land on the female’s back, where the skin “continues to get thicker and grows up around the egg, so essentially the egg is now embedded in this skin.”

Such acrobatics continue for more than 24 hours, and the female can end up with over 100 eggs in her back. Her young will develop in her skin for several months, skipping the tadpole stage entirely.

When they’re ready to leave, one by one they squeeze out of their mother’s back like escape pods jettisoning from a spaceship … that flies underwater. OK fine that’s not a great simile. It’s one of the more unsettling scenes in nature (see National Geographic’s footage below), but this is also an incredible evolutionary adaptation. Instead of laying her eggs elsewhere and exposing them to predation, the Surinam toad mother carries her young in the safety of her body, much like mammals evolved to do with their wombs. And when she’s done, she simply sheds the extra skin and goes about her business.

Yet this is far from the Surinam toad’s only peculiarity.

Yes, that’s how it’s supposed to look.

Its body is so astoundingly flat that Cannatella, a man who has dedicated his life to studying amphibians, has a hard time describing it beyond saying the thing looks like roadkill, which I can confirm as accurate, having once seen a roadkilled frog. The head in particular looks to taper down to an almost razor’s edge.

But this shape is — like every characteristic on the body of any living thing — an adaptation to its environment. The toad spends its days hidden among leaves at the bottom of South American ponds, habitat for which its flat, angular body and brown coloration are perfectly suited. And here it lies in wait for prey before launching a peculiar attack — at least for a frog.

Strangely, the Surinam toad is missing a tongue. “So what’s that related to?” asked Cannatella. “Well, they use suction feeding. When a fish or something swims in front of the frogs they open their mouths really quickly, and they generate suction that pulls the fish in.” No tongue required.

We see this strategy mirrored in a pair of other remarkable amphibians across the Pacific Ocean: two species of giant salamanders in Japan and China. These monsters can grow to 6 feet long, and like the Surinam toad they sport enlarged heads that help boost the vacuum power. They’re nature’s own Dysons.

A close-up of the Surinam toad’s strange fingers, which look suspiciously like those of the Martians in The War of the Worlds.

And also like these giant salamanders, the Surinam toad has puzzlingly tiny eyes. So how exactly does is it detect prey? It actually has the same kind of lateral line as sharks — a system of special sensory structures that detect changes in water pressure — and Cannatella suspects it’s using this to hunt for fish.

Interestingly, tadpoles of other frog species have this as well, but lose it as they mature into adults. The Surinam toad, being highly aquatic, has good reason to retain it. In the murky pond-bottoms that it inhabits, the lateral line would prove far more advantageous over eyesight when detecting prey (and it’s a great conversation-starter at shark parties).

In addition, the Surinam toad feels its way toward prey with its bizarre unwebbed fingers, each of which has four lobes at its tip that each further split into their own lobes, a bit like fractals.

Read more at Wired Science