Frog toxins engage the nose and make them thoroughly unappetizing to humans, and presumably predators as well. If the substances are too strong, a human licker will experience a burning and constricted throat—or far worse.
Some species, such as the tiny golden dart frog, pack enough poison to kill several people.
But highly toxic frogs don't usually catch experienced biologists by surprise: Poisonous species often broadcast colorful warnings with their "don't eat me" colors.
Some nontoxic frogs, however, have developed a mimic appearance to deter predators, which makes them prime candidates for the lick test.
"Some frogs smell so repulsive that you just wouldn't even consider licking them," Clark said.
John Daly, a frog toxin expert and scientist emeritus at the National Institutes of Health (NIH), is Clark's "long-time influence and inspiration," she said.
Daly explained that many scientists use a similar technique—though his version differs from Clark's.
"I press them very hard on the back to get some secretions, and then I taste my finger—cautiously," he said.
(Related: "Poison Frogs Losing Their Toxicity, Study Suggests" [November 7, 2006].)
These days you'll often find Clark in Madagascar, attempting to trace frog skin alkaloids—naturally occurring nitrogen compounds—backward through the food chain to their original sources.
Frogs are generally believed to accumulate the compounds through their diets.
Clark suspects their origin could involve complex symbiosis between ants and alkaloid-producing bacteria.
Testing the theory means identifying the same toxin in a frog and in its prey insects, and eventually in the plants and fungi on which those insects feed.
"What would be really amazing is to see the frogs eating the ants, and the ants and other insects eating the plants, and then we get lucky and the [samples of all three] that we've collected and filmed all have the same toxins," she said.
"That would be the ideal."
But Daly of NIH, whose team has identified and cataloged a definitive list of more than 800 frog alkaloids, suspects that the toxins originate not in bacteria, but solely with the ants and mites that comprise much of the frog diet.
"It's an unanswered question," he said.
"The ants use these [alkaloids] as pheromones, as an alarm, or [as] a trail marker. Can you rely on a symbiotic microorganism making this thing for you? Maybe so, but it may be unlikely."
Clark agreed there are other possible explanations for the frog toxins.
"I believe that many arthropods, including ants, produce their own toxins without [the help of another cooperative organism]," Clark pointed out.
"Many different organisms surely represent the biosynthetic sources of chemicals in frog skin. It is a very open question [that] could take a few lifetimes to sort out," she said.
Clark's love affair with frogs began while exploring parks near her home in suburban Ellicott City, Maryland.
"I got into them at age seven or eight, just like most kids like to catch things and check them out," she said. "I kind of never grew out of that.
"As you can imagine, kids made fun of my love affair with frogs, but who could blame them? I am a bit different."
Valerie's father Bill built a backyard frog pond where his daughter could raise tadpoles. The house soon became packed with a dozen terrariums filled with frogs, snakes, and other creatures, Valerie recalled.
By 1990 Valerie was entering her hand-raised frogs into local jumping races.
When a photo of her blowing on her favorite frog—to encourage jumping—landed in the Baltimore Sun, it was both exciting and worrisome: She had skipped school for the contest.
Luckily, the school principal "thought it was downright hilarious," she said.
"I had no idea that she would latch onto this love of amphibians and other animals like she did," said her mother, Sandy.
Today Valerie, six feet tall (about two meters tall) with long copper hair, "is still kissing frogs in search of the one that will turn into her prince," Sandy said.
Soft Place in Her Heart
As a graduate student, Clark is also exploring unique ecosystems of Guyana's dramatic sandstone mesas, called tepuis. This summer, with Brice Noonan of University of Mississippi, she plans to study frogs in Guyana as well as in French Guiana and Suriname.
She teamed with Florida State University biologist Bruce Means to seek out unknown frog species displaying the hallmarks of toxicity: bright colors, unpleasant odors, and telltale secretions.
The scientists found many frogs seem to have noxious chemicals in their skin.
"We just know we're sitting on a gold mine of incredible stuff," Means said. "These families have been isolated for a long time."
But identifying new species and their toxins means preserving skins for lab study and euthanizing frogs—an experience Clark dislikes.
"Valerie has got a soft place in her heart for frogs, and she hates to do that," Means said. "I'm a little crasser than she is."
But whatever research methods need to be done to save frogs are worth the risk, Clark said.
"They really are important to all of us for many reasons, and the fact that they are threatened is a warning," Clark said.
"[Frogs] are like the canary in the coal mine."
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