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Commentary: Plague of frogs

Under CC license from Flickr user Gregory Moine

Science loves a frog. And why not? They're easily disassembled, they produce large numbers of transparent embryos, and they walk  goofy.

But our dissectable friends aren’t doing so well. Scientists have also noted that amphibian extinction rates appear to be anywhere from hundreds to tens of thousands of times higher than usual.

There are many reasons for this, but it certainly doesn't help that frogs are currently in the midst of the deadliest vertebrate pandemic of all time. It's just a lowly skin fungus, albeit one with a fancy name: Batrachochytrium dendrobatidis, or Bd. But when it latches onto amphibians, the mold prevents them from exchanging electrolytes through their delicate skin, leading to cardiac arrest. Over the past few decades, the disease has severely affected more than 200 amphibian species on six continents.

If you go to Lily Pond in Golden Gate Park, though, you'll see that thing is full of frogs. But they’re not your standard Californian variety – those guys are falling to the fungus. These are cheap toy-looking amphibians, with enormous hind claws and fat legs tapering into a simple wedge of a body. Inelegance aside, they are doing very well – no heart attacks for them. That’s partially because they’re resistant to Bd. But it’s also because they carry it.

The frog occupying Golden Gate Park is called Xenopus – the African clawed frog. As you might have guessed from the name, they're not from here, and they're extremely unwelcome. It’s now pretty clear that their spread out of Africa was behind the Bd pandemic. How did they get all the way over here? It’s all about the eggs.

Xenopus is an unusually prolific and hardy egg-layer, which biologists know for a pretty crazy reason: back in the '30s, a zoologist with the stunningly British name of Lancelot Hogben discovered that the frog was a really effective pregnancy test. That’s because it responds to the same hormone that most pregnancy tests look for. Want to know if you’re expecting? Simply inject your urine into one of these frogs, and if it lays a clutch of eggs, you’re pregnant.

This worked so well that, by World War II, British labs were already storing dozens of frogs onsite in large galvanized metal tanks. Even the family planning association clinic in London kept a few on call in the basement.

But the bigger news, at least from the freaky perspective of lab biologists, was that this hormone turned the frog into the egg-laying machine they needed for large-scale developmental studies. By the 1940s, the animals were common clinical and lab equipment all around the world. Xenopus has since played an integral part in our understanding of not just development, but neurobiology, RNA, the cell cycle – it was even the first vertebrate to be cloned.

Unfortunately, the frogs are also notoriously good at escaping from those labs. It didn't take long for wild populations to establish themselves in South America, Wales, and, yes, California. Along with a certain deadly skin fungus, which it’s been spreading for nearly the past seventy years.

In case you're wondering, the frog apocalypse we're now seeing has major consequences for humans. Amphibians are important parts of global food webs. They control pests and they’re promising sources of new medicines. But it’s also worth pointing out that this diversity is important in and of itself.

In order to study anything in a lab, you need to simplify things. That’s what Xenopus is: a simple, generic model. An everyfrog. But whenever you focus in on a model, you end up tuning out the complexity of the real world. Normally, this just means you make some incorrect assumptions. But in this case, something more worrisome is going on – the model is actually making the real world disappear, forever.

There’s an old physics joke about a couple of physicists who are asked to help a dairy farm increase milk production. They come up with a solution, but their equations only works in the case of a spherical cow. Imagine if those equations started bleeding into the real world, cows magically puffing up around the globe. Alternately, just look at Lily Pond, and at the unintended consequences of science’s love for a frog. A pond full of pregnancy tests.
This commentary originally aired on November 13, 2012.