Elephant seals recognize vocal rhythms to avoid bullies
We recognize friends on the phone by just their voice. Rhythm, inflection and tone clue us in to who is on the other end of the line.
Humans are not the only mammals with this aptitude. Male elephant seals recognize their rivals by rhythmic patterns in their call, according to a study published Thursday in Current Biology. Elephant seals become the first mammal other than humans to show this behavior, and the discovery may help explain why humans enjoy rhythmic things like music.
To uncover this behavior, bioacoustician Nicolas Mathevon headed out before sunrise to the Ano Nuevo State Park in California. More than 4,000 elephant seals gather on the beach from December to March to mate and give birth. At the start of the season, male seals battle to establish a pecking order, sometimes fighting to the death. The strongest control the female harem.
"It is a high stakes environment," Caroline Casey, a University of California, Santa Cruz doctoral candidate in ecology and evolutionary biology and study co-author, told the NewsHour. "We wanted to know what these animals are saying to each other – what information is embedded in these calls that the seals use to avoid a fight, and which components of the calls are important."
Casey had previously shown that elephant seals can identify the calls of their rivals. "Alpha" males sing out to warn low-ranking seals to stay away or fight.
With ringside seats, Mathevon, Casey and their team observed these animals for five years and became accustomed to recognizing individual animals by the rhythms of their voices. They wondered if the seals could do the same.
So the team recorded the vocalizations of a dominant male elephant seal. Back in the lab, they used a computer program to create two variations of the song, which modified only the rhythm, leaving all other natural qualities of the vocalization
Armed with the original and modified recordings, one by one, they sounded the alpha male calls to 10 beta males. Upon hearing the original call, the lowly beta males retreated akin to what they do when avoiding a fight. But, when the beta males heard the call with a considerably modified rhythm, they took no notice, leading the researchers to conclude that the rhythm is key to recognition.
"All social mammals have to recognize members of their network," said Mathevon who works at the University of Lyon, Saint-Etienne in France. "To navigate in the social network, you have to know who is who. What is special about this study is it's the first time that we found a mammal that uses a rhythm to support an individual's unique [vocalization] signature."
A musician divides a beat into long and short tones. A seal also subdivides the pulses of their calls. This study modified the tempo, but Mathevon thinks the seals may even decipher rhythms at a finer level, decoding beats that are subdivided into more complex patterns.
"This research was beautifully designed," said Andrea Ravignani, a researcher at The Seal Rehabilitation and Research Center in Pieterburen, Netherlands, who wasn't involved in the study. His research focuses on the evolutionary basis of rhythm and the origins of music.
Unlike most projects, which use musical recordings to understand how animals make sense of rhythm, this study uses the animal calls, which Ravignani said "is more ecologically relevant."
"Everybody loves music. But we don't understand why," Ravignani said. Because humans and seals don't have a known common ancestor with an affinity for rhythm, Ravignani said this study brings us closer to understanding why humans like language and music, simply by having another species for comparison.
The next step in Mathevon's research will be to look at the seal rhythms on a finer scale, modifying more than just the overall tempo. He plans to also examine female-to-female conversations, as well as communication between adults and their young.