Anybody who spends time near a swamp can easily hear that frogs use their voices to chitchat, but it wasn’t until about two decades ago that researchers announced that these animals also converse with water-transported protein pheromones. Now, research shows that frogs banter with airborne chemicals too.
“It’s the first proof that frogs use volatile pheromones” to communicate, says Stefan Schulz, a chemical ecologist at the Technical University of Braunschweig (TU), in Germany. In fact, it’s the first proof that any amphibians communicate using chemicals in the air, he adds (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201106592).
“So few pheromones have been chemically identified in vertebrates, so this is really exciting news,” comments Sarah Woodley, an amphibian biologist at Duquesne University. She points out that biologists had done behavioral studies suggesting frogs used airborne pheromones, but none had been identified until now.
In the new study, Schulz collaborated with TU Braunschweig zoologist Miguel Vences and Katharina Wollenberg, now at Harvard University, who went to Madagascar to study a local family of frogs called Mantellidae. Male Mantellidae frogs have bulbous organs on their inner thighs called femoral glands, and it’s from these sacs that the team isolated two molecules that waft through the air as pheromones, namely 8-methyl-2-nonanol and a macrolide called phoracantholide J.
The team discovered that Mantellidae frogs will hop toward a mixture of these two molecules and that different species have different ratios of them in their femoral glands. Precisely what these frogs are saying with the molecules is up in the air, but Schulz can speculate.
“Frogs occur in high species diversity in these swampy areas—there are about 100 species,” Schulz says. Although the different species croak uniquely, the frog density is so high that “it can be hard to find a mate of the correct species.” Perhaps the odors help with species recognition, he suggests.
The new research also confirms the results of frog genome sequencing, Woodley says. Frog DNA has all sorts of genes for volatile chemical receptors, but nobody knew whether they were functional genes or just an artifact of evolution. “It turns out they may be functional,” she adds.
Schulz’s team isolated a handful of other alcohols and macrolides from the frogs’ femoral glands, including a new natural product called gephyromantolide A. The team also devised a new synthetic route for building the ringed molecules that uses a reaction called Corey–Nicolaou macrolactonization. The route, the shortest such path ever reported, provided enough sample to test which of the additional molecules are pheromones.