African stink ants (Paltothyreus tarsatus) really don’t like to be disturbed. They live in colonies of several hundred to thousands, and the 2.5-centimeter-long insects vigorously defend their nests, biting intruders with powerful mandibles and stinging them with venom. They mostly eat arthropods but also chow down on frogs and other small vertebrates.
But there’s one frog they don’t eat — the West African savanna frog (Phrynomantis microps). This frog even hides out in ant nests to take shelter from the heat of the day or dehydrating dry season. Their hidey holes also protect the frogs from potential predators because the ants will attack anything that tries to get in.
Some frogs and toads produce toxins in their skin that kill or sicken anything that comes after them, but that’s not what protects the West African savanna frog. Instead, the frog produces two short chains of amino acids — known as peptides — that trick the ants into leaving their anuran intruder alone, say Mark-Oliver Rödel of the Leibniz Institute for Research on Evolution and Biodiversity in Berlin and colleagues. Their study was published December 11 in PLOS ONE.
African stink ants will crawl over a West African savanna frog, but chemicals on the frog’s skin keep the ants from attacking.M.-O. Rödel et al./PLOS ONE 2013
The research team collected 13 frogs from the Pendjari Biosphere Reserve in northern Benin. Back in the lab in Germany, they needed to get samples of the chemicals, unknown at that point, secreted through the frogs’ skin. So the scientists placed a frog in a 250-milliliter beaker containing 10 milliliters of water and shook it for two minutes (though probably not too vigorously, as their paper expressly states that the frogs “experienced no pain, suffering, complaints or harm”).
When the researchers covered termites or mealworms with the frog secretions and presented the potential prey to stink ants, the ants delayed their stinging behavior. Analyses of the secretions came up negative for hydrocarbons and alkaloids — sometimes found in frog skin — as well as for terpenoids and steroids secreted by some toads. They eventually tracked down two small peptides, 9 and 11 amino acids long, and tested them in the field by wetting termites with the chemicals. As when the termites were covered in the frog skin secretions, the ants delayed their aggression.
The peptides “seem to interact with the ants’ antennal chemoreception in a way that the frog is either recognized as a nestmate or at least not as an intruder. When approaching the frog the ants are intensively sweeping the frog’s skin with their antennae, but are not behaving aggressively, as it is always observed with other intruders,” the researchers write. “Most likely the peptides function as an appeasement [chemical] circumventing aggression of ants against the frog.”
The frogs probably don’t absorb the peptides through their food but make them themselves, the researchers say. The chemicals can be found on their skin even after years in captivity eating a lab diet.