Brazilian scientists discovered that the strong odor exhaled by some species of amphibians is produced by bacteria, and would be a way to attract their partners. In a remarkable example of symbiosis, these bacteria help at the time of mating. This discovery about the role of such microorganisms, isolated in the skin of tree frogs, was published in the Proceedings of the National Academy of Sciences (PNAS).
“Tree frogs exhale a strong odor. Occasionally, a specimen of a particular species can even be recognized on the basis of its aroma, but the function of that smell was not yet known. One hypothesis indicated that it could be an aposematic aroma, that is, a chemical warning sign, that would serve to keep predators away, just as striped skunks [Mephitis mephitis] do among mammals, for example,” said Célio Haddad, a professor at the Institute of Biosciences and the Aquaculture Center of the Universidade Estadual Paulista (Unesp), based in Rio Claro, Brazil, and one of the authors of the article.
According to Haddad, this hypothesis was contemplated because many amphibian species, especially poisonous species, exhibit a striking coloration, which functions as a visual warning to scare off predators. “We thought that among the anurans [toads and frogs] that smell might have a similar function,” he said.
This new article is the result of post-doctoral work by Argentine biologist Andrés Eduardo Brunetti, under the supervision of professor Norberto Peporine Lopes. Brunetti conducted his research at the Faculty of Pharmaceutical Sciences of Ribeirão Preto, of the University of São Paulo (USP), with the support of FAPESP.
Tree frogs. (Photo: Gilbert Alvarado)
“The importance and originality of Brunetti’s work is that it points for the first time to a significant difference in the smell exhaled by tree frogs of opposite sexes. No work with anurans has ever suggested this type of behaviour. The results suggest that such a smell serves to allow mutual recognition between males and females of the same species for mating purposes,” Haddad said.
The research was also supported by the FAPESP Program of Research on Characterization, Conservation, Restoration and Sustainable Use of Biodiversity (BIOTA-FAPESP), USP, the National Council for Scientific and Technological Development (CNPq) and the Coordination of Improvement of Higher Level Personnel (Capes), the latter two agencies of the Brazilian federal government.
“In the anurans it is common to see several different species that divide the same lake or swamp. There are also an average of 30 male frogs for each female of the same species. The doubt lies in knowing how the females recognize the males of their species among a multitude of males of different species, all croaking at the same time,” Brunetti said.
“It was known that among anurans the vocalization of males serves the function of attracting females, and that each species has a characteristic song. We verified that the smell would have a similar function, serving as an olfactory signal, which would allow females to recognize the males in their species,” he said.
Biologists were also unaware that there was a difference in the smell of male and female frogs. Brunetti’s primary objective was to understand the chemical composition of the volatile components exhaled from the skin of various species of tree frogs.
His working hypothesis suggested that the aroma would be a chemical warning signal that would serve to deter predators. To verify this hypothesis, Brunetti went to the field in different places in the states of São Paulo and Rio de Janeiro to capture specimens of the tree frog known in Brazil as cará-cará (Boana prasina).
“It is extremely difficult to capture females in the field. At first, we only managed to catch males. When we saw that there could be a sexual difference in the smell of the animals, I went out to the field again with the specific objective of capturing females to make the comparison,” he said.
“During my doctorate at the Museo Argentino de Ciencias Naturales, in Buenos Aires, while investigating the volatile compounds of two other species of toads, I discovered that the secretions were formed by a mixture of between 35 and 42 compounds of nine different chemical types. That’s when we realized that some of those compounds had the specific signature of certain compounds produced by bacteria,” Brunetti said.
The scholar came to Brazil to investigate