For the first time, scientists took acoustic communication back to its evolutionary roots. Researchers followed acoustic communication's phylogenic path back some 200 million years through evolutionary history.

Scientists began by plotting the relationships among some 1,800 species of tetrapods, including birds, frogs, crocodilians and mammals on a giant evolutionary tree. Next, they surveyed the scientific literature for data on the presence of acoustic communication within each plotted species, drawing connections across the branches of the evolutionary tree.

Using various methods of statistical analysis, scientists attempted to parse the evolutionary tree data for patterns related to the emergence and persistence of acoustic communication.

The research team wanted to know if acoustic communication emerged independently in different groups and whether it was preserved in certain lineages. Scientists also wanted to figure out if it was linked with any other traits, like nocturnal activity.

Their analysis -- published Friday in the journal Nature Communications -- showed the common ancestor for all tetrapods, land-living vertebrates, did not communicate acoustically. Scientists determined the ability to communicate acoustically emerged independently among mammals, birds, frogs and crocodilians between 100 and 200 million years ago.

Their statistical analysis also showed a strong link between vocalization and nocturnal activity. Many nocturnal species evolved communication abilities and later adapted to life during the day, keeping their vocal abilities when they did.

"There appears to be an advantage to evolving acoustic communication when you're active at night, but no disadvantage when you switch to being active during the day," study author John J. Wiens, a professor of ecology and evolutionary biology at the University of Arizona, said in a news release. "We have examples of acoustic communication being retained in groups of frogs and mammals that have become diurnal, even though both frogs and mammals started out being active by night hundreds of millions of years ago."

Authors suspect the fact that birds are their most boisterous song-singing selves at dawn is a remnant from their pasts as nocturnal species.

Researchers were surprised to find acoustic communication to be so stable across lineages, more stable than most other appearance-related communicative traits.

Unexpectedly, scientists found no evidence that vocalization drives diversification. There are 10,000 bird species, all of which vocalize. But crocodiles can make communicative sounds, too, and there are only 25 species. There are just as many lizards and snakes as there are birds, and yet, most are silent. Meanwhile, some 95 percent of the more than 6,000 mammal species communicate acoustically.

"If you look at a smaller scale, such as a few million years, and within certain groups like frogs and birds, the idea that acoustic communication drives speciation works out," Wiens said. "But here we look at 350 million years of evolution, and acoustic communication doesn't appear to explain the patterns of species diversity that we see."

Researchers did find evidence that ecological conditions drive the evolution of signalling like vocalization.

"Here, we show that this idea of ecology shaping signal evolution applies over hundreds of millions of years and to fundamental types of signals, such as being able to communicate acoustically or not," Wiens said.

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