If you are living in the countryside, come outside the house and you'll probably be greeted with a plethora of sounds: chirps, hoots, croaks, and songs. Acoustic communication among vertebrate animals is a familiar experience that's is why it is kind of difficult for us to imagine how is it like if these sounds don't exist. Scientists from the University of Arizona and Henan Normal University in Xinxiang in China conducted a study to find out why the ability of terrestrial vertebrates to create sounds evolve in the first place.
This study by John J. Wiens and Zhuo Chen, published in Nature Communications, is considered the first study to trace the evolution of acoustic communication. The researchers were able to assemble an evolutionary tree for 1,800 species that shows the evolutionary relationships between vertebrates and traced it back to 350 million years ago. Wiens and Chen were able to obtain data from the scientific literature on the presence or absence of acoustic communication within each sampled species and proceeded to map it into the assembled evolutionary tree. In applying these statistical analytical tools, the scientists performed a test to determine three things: find out if acoustic communication had evolved independently in different groups, whether or not it is associated with nocturnal activity, and whether or not it is preserved in a lineage.
According to the results of their study, the tetrapods or the common ancestor of all land-living vertebrates did not have the ability to create sounds to communicate through vocalization meaning these ancient animals do not have the ability to use the respiratory system to generate sound. Wiens and Chen also found out that acoustic communication evolved separately among birds, mammals, frogs, and crocodilians in the last 100-200 million years and are strongly associated with the nocturnal lifestyle.
Since light is no longer available by nighttime, animals will not be able to show off visual cues such as color patterns to intimidate a rival or attract a mate, hence, transmitting sound signals becomes an advantage. Given all the species from their sample, the researchers were able to estimate that more than two-thirds of terrestrial vertebrates have the ability of acoustic communication. Interestingly enough, the lineage that deviated and evolved to be diurnal (those who are active during the day) animals retained the ability to communicate via sound. Wiens explains that there is some advantage in evolving acoustic communication if the creature is nocturnal and there is no disadvantage when these creatures switched to being diurnal. He says that they have examples of acoustic communication being retained in species of frogs and mammals that evolved to be diurnal even though these creatures were originally nocturnal hundreds of millions of years ago.
Birds are among the most active users of acoustic communication and they have continued to do so even after evolving to be diurnal. Interestingly enough, birds also sing at dawn which the researchers speculate are the remnants of its nocturnal ancestry. The results also showed that acoustic communication is a stable evolutionary state -- a remarkable one, at that. According to the researchers, it is possible that once a lineage acquired the ability to communicate through sounds, the tendency to retain such ability might be more stable than other types of signaling such as conspicuous coloration or enlarged showy structures. Lastly, the study revealed that the ability to vocalize is not the main factor of diversification (this happens when a lineage evolves to become a new species).
The authors also pointed out that their findings do not only apply to acoustic communication but also to other evolutionary traits that were driven by ecological conditions.