How the Tree Frog Has Redefined Our View of Biology- page 2 | Science | Smithsonian
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A beloved symbol of biodiversity, the red-eyed tree frog, shown here in Panama, has evolved a flexible strategy for survival. (Christian Ziegler)

How the Tree Frog Has Redefined Our View of Biology

The world’s most charismatic amphibian is upending the conventional wisdom about evolution

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(Continued from page 1)

Warkentin’s research “causes us to think more carefully about how organisms respond to challenges even very early in life,” says Eldredge Bermingham, an evolutionary biologist and director of the Smithsonian Tropical Research Institute (STRI, pronounced “str-eye”) in Gamboa, Panama. Warkentin, a biology professor at Boston University, conducts her field studies at STRI. That’s where she showed me how she coaxes the eggs to hatch.

The tadpoles leaping from the wet leaf still have a little yolk on their bellies; they probably won’t need to eat for another day and a half. Warkentin keeps rubbing until only a few remain, stubbornly hiding inside their eggs. “Go on,” she tells them. “I don’t want to leave you here all by yourselves.”

The last of the tadpoles land in the water. Predatory bugs known as backswimmers wait at the surface, but Warkentin says she saved the tadpoles from a worse fate. Their mother had missed the mark, laying them on a leaf that didn’t reach over the pond. “If they were hatching on the ground,” she says, “then they would just be ant food.”

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Warkentin was born in Ontario, and her family moved to Kenya when she was 6. Her father worked with the Canadian International Development Agency to train teachers in the newly independent country. That’s when she got interested in tropical biology, playing with chameleons, and watching giraffes, zebras and gazelles on the drive to school in Nairobi. Her family returned to Canada several years later, but at 20 she went hitchhiking and backpacking across Africa. “That was something that seemed perfectly reasonable in my family,” she says.

Before she started her PhD, she went to Costa Rica to learn more about the tropics and look for a research topic. The red-eyed tree frog’s terrestrial eggs caught her interest. She visited the same pond over and over again, and watched.

“I had the experience—which I’m sure other tropical herpetologists have had before and maybe didn’t think about—if you have a late-stage clutch, if you bump into them, they’ll hatch on you,” Warkentin says. “I bumped into a clutch, and they all were bailing out.”

She had also seen snakes at the pond. “What I thought was, wow, I wonder what would happen if a snake bumped into them,” she says, and laughs. “Like, with its mouth?” Indeed, she found that if a snake appears and starts attacking the clutch, the eggs hatch early. The embryos inside the eggs can even tell the difference between a snake and other vibrations on the leaf. “This is the thing, of going out in the field and watching the animals,” she says. “They’ll tell you things you didn’t expect sometimes.”

Biologists used to think this kind of flexibility got in the way of studying evolution, says Anurag Agrawal, an evolutionary ecologist at Cornell University. No longer. It’s exciting that Warkentin has documented wonderful new things about a charismatic frog, but Agrawal says there’s a great deal more to it. “I think that she gets credit for taking it beyond the ‘gee whiz’ and asking some of the conceptual questions in ecology and evolution.”

What are the advantages of one survival tactic over another? Even a 5-day-old frog has to balance the benefit of avoiding a hungry snake against the cost of hatching early. And, in fact, Warkentin and her colleagues have documented that early-hatching tadpoles were less likely than their late-hatching brethren to survive to adulthood, particularly in the presence of hungry dragonfly nymphs.

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