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Symbiotic bacteria help frogs find mates (but the real story is all the wrong assumptions we make)

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Adult female (left) and calling male of Boana prasina/Andrés Brunetti

All these stories in recent days about animals finding mates (or not); spring must be coming. And here’s one more: From ScienceDaily:

“Frogs emit a pungent odor. Sometimes a particular species can be recognized by its scent, but until now, the function of this odor was unknown. It was typically assumed to be an aposematic smell, meaning a chemical warning sign that served to repel predators, as in the case of skunks [Mephitis mephitis] among mammals, for example,” said Célio Haddad, a professor at São Paulo State University’s Rio Claro Bioscience Institute (IBRC-UNESP) in Brazil and a coauthor of the article.

According to Haddad, who is also affiliated with the university’s Aquaculture Center (CAUNESP) in Jaboticabal, this hypothesis was considered plausible because many amphibian species, especially when poisonous, are brightly colored, and this serves as a visual alert to frighten predators. “We thought odor might play a similar role among anurans [frogs and toads],” he said. …

“The importance and originality of Brunetti’s research is that for the first time it shows a pronounced difference in the odors emitted by frogs of opposite sexes,” Haddad said. “No other studies of anurans have ever described this type of behavior. The results suggest that the odor serves to permit mutual recognition between males and females of the same species for mating purposes.” …

“In anurans, you often see different species sharing a lake or marsh. In such places, there are 30 male frogs for every female of the same species on average. The question is how the females recognize males of their own species among a multitude of males belonging to several species while they’re all vocalizing at the same time,” Brunetti said.

“It’s well-known that the function of the call of anuran males is to attract females and that every species has a characteristic song. Our findings suggest that odor appears to play a similar role, serving as an olfactory signal that enables females to recognize males of their own species.”

Biologists were also unaware of a difference in the scents of male and female frogs. Brunetti discovered this difference during his research, whose primary goal was to understand the chemical composition of the volatile components emitted by the skin of various frog species.

His working hypothesis suggested that smell was a chemical warning sign that served to repel predators. To verify the hypothesis, Brunetti conducted field surveys at several sites in São Paulo state and Rio de Janeiro state, collecting specimens of the tree frog Boana prasina.

“It’s very hard to collect females in the wild. Initially, we managed to collect only males. When we noticed what appeared to be a sexual difference in their odors, I went into the field again with the specific aim of capturing females for comparison,” he said.

“During my doctoral research at the Argentinian Natural Science Museum in Buenos Aires, while investigating the volatile compounds in two other frog species, I discovered that the secretions were made up of a blend of 35 to 42 compounds in nine different chemical classes. We then realized that some of the compounds had the specific signature of compounds produced by bacteria.” …

Brunetti and colleagues used gas chromatography and mass spectrometry to analyze the diversity of the volatile components secreted by the skin of B. prasina. They found that adult males and females secrete a blend of 60-80 compounds, including alcohols, aldehydes, alkenes, ethers, ketones, methoxypyrazines, terpenes and thioethers.

The compounds were exactly the same in both males and females, but the researchers were surprised to find a pronounced sexual difference in the levels of methoxypyrazines, terpenes, and thioethers.

“These three components were responsible for the difference between males and females. Thioethers and methoxypyrazines are typically produced by microorganisms,” Brunetti said. Paper. (paywall) (access?) – Andrés E. Brunetti, Mariana L. Lyra, Weilan G. P. Melo, Laura E. Andrade, Pablo Palacios-Rodríguez, Bárbara M. Prado, Célio F. B. Haddad, Mônica T. Pupo, Norberto P. Lopes. Symbiotic skin bacteria as a source for sex-specific scents in frogs. Proceedings of the National Academy of Sciences, 2019; 116 (6): 2124 DOI: 10.1073/pnas.1806834116 More.

So, contrary to assumption, 1) smell was important in locating mates and 2) males and females had different smells 3) produced by symbiotic bacteria. One wonders how many other life forms would challenge simple evolution tales if they were closely studied.

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See also: Researchers: When mates are rare, birds help their parents raise more offspring Interestingly, the researchers avoided trying to shoehorn the story into a Darwinian mold. It’s true that the next batch of eggs will share some genes with the babysitter bird but what if neo-Darwinian gene competition isn’t really what is happening at all? The bird can’t find a mate so it starts to engage in nesting behaviour around the nest it grew up in, simply because that’s what it would have done anyway? Then, while we can reasonably guess that the bird who does so helps more chicks who share some genes survive, that’s not the cause of the behavior.

Birds are found to plan like humans for their offsprings’ future Yes, the Darwinbird of pop science can do that! No natural mechanism is remotely suggested, so we must assume that it is sheer mental power, of the sort that we species-ists once thought existed only in humans, that enables the hen bird to plan for her chicks’ future. Shame on us!

A First: Solitary Bees Serve As Stepdads One question comes to mind: For the males, competing for females who are present is cognitively easy. But how do they know about the females who are still larvae? Can they smell them? If not, the behavior seems to require a foresight that is beyond the known cognitive powers of the bee.


Can sex explain evolution?


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