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A chemical disguise secreted by this frog’s skin allows the amphibian to live peaceably among fierce, stinging ants.
To protect itself and keep moist, the West African savanna frog spends its days—and much of the dry season—hiding in underground burrows. But it’s usually not alone.
A West African savanna frog hangs out with African stink ants. Photograph by Christian Brede.
The frog often moves into the underground nests of the African stink ant, a highly aggressive species. When disturbed, these ants usually go on the offensive, attacking intruders with their powerful jaws and venomous stingers.
But they don’t seem to mind the West African savanna frog cozying up in their nests, which offer the frog a safe place to hide from other predators and a warm, humid environment to wait out western Africa’s long dry season. (Watch National Geographic’s frog videos.)
It’s all the more surprising given that African stink ants hunt and scavenge for prey that can include frogs. How does this frog trick these ants into leaving it alone?
Skin Deep
German and Swiss researchers working in Benin believe the secret lies in the West African savanna frog’s skin.
To test this theory, they covered mealworms and termites with skin secretions from the frog and offered the insects to the ants. Normally the ants would respond by biting and stinging immediately. But the ants waited 30 seconds to several minutes to attack the secretion-coated insects. (Also see “Trees Trap Ants Into Sweet Servitude.”)
To try to identify the compounds in the frog’s skin responsible for this remarkable effect, they placed a frog in a beaker of pure water, shook it gently for two minutes (to stimulate skin secretion), and collected the results.
The researchers discovered that the secretions aren’t the result of any special food; in fact, they’re produced no matter what the frog eats. That’s not the case with other toxic frogs and toads—like some colorful poison dart frogs—which have poisonous skin as a result of eating small, poisonous insects.
Through chemical analyses, the researchers identified the active ingredients in the frog’s skin: two compounds called peptides, which are like small proteins—and appear to trick the ants into accepting the frog as one of their own.
“Chemical Camouflage”
Ants use their antennae—highly sensitive probes that act as chemical sensors—to discern harmless nestmates from potential intruders. But the peptides in the frog’s skin seem to cause the ants to recognize the frog as a nestmate—or at least not as an intruder.
When approaching the frog, the ants sweep their antennae over its body, picking up its scent. With any other intruder, this would be followed by an aggressive, stinging attack. But the ants tolerate the frog and largely ignore its presence in their nest.
If it looks like an ant …
The paper was published December 11 in the open-access, peer-reviewed journal PLOS ONE.
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