[Above: Two bottlenose dolphins swimming around. Image via Max Pixel and Creative Commons CC0 license.]
Human speaking voices come in a dizzying array of tones. They can be raspy or reedy, lilting or monotone, chirpy or sonorous, nasal or throaty, breathy or booming, and that’s before we even start describing accents.
Most mammals, including humans, use their vocal tracts to make noise, and so slight variations in the anatomies of our voice boxes, throats, mouths, and nasal passages give our voices unique timbres.
Perhaps, not surprisingly, we can use variation in vocal qualities to tell people apart. (Some people have a lot more acuity at this than others, but most of us can recognize our favorite singers, even on songs we haven’t heard before and distinguish our dog’s bark from the neighbors’ dog’s yapping.)
“If you answer the phone, and it’s someone you know very well, you know the voice, and they don’t have to tell you their name,” explains behavioral biologist Laela Sayigh of Hampshire College. “That’s how really every other mammal that has ever been studied identifies each other.”
With one notable exception–dolphins.
Yup, the cleverest animals in the sea seem to be unable to tell each other’s voices apart. Instead, dolphins tell each other apart by listening for “signature whistles,” which function like human names or call-signs.
In a previous study, Sayigh and her colleagues caught wild bottlenose dolphins off the coast of Florida and held them (gently) within a small underwater area while they recorded audio of the dolphins’ whistles, both signature and otherwise.
Later, they caught and held the dolphins again and played the
a mixtape. audio recordings of dolphin whistles back to other dolphins. When the dolphins heard recordings of a relative’s (usually a mom or a sibling) signature whistle, the dolphins would turn their heads toward the sound.
The dolphins even turned their heads toward computer-produced versions of their moms and siblings’ signature calls.
But in a new study, Sayigh and her colleagues tried playing recordings of relatives’ “non-signature” whistles and got no reaction. (Other than the usual “let go of me, human!” behavior.)
[Correction: 10/2/17: In an email, Sayigh pointed out to me that there isn’t really a typical dolphin reaction to being held. “The animals are quite different in how they respond,” she wrote. Also, I was obviously projecting the “let go of me, human!” part. We have no way of knowing what the dolphins were thinking.]
In other words, the dolphins were more likely to react to a computer voice saying “Hello. This is Mom.” than Mom’s voice saying something else.
Sayigh says that anatomical differences combined with dolphins’ underwater lifestyle may explain why dolphins don’t seem to recognize their mothers’ and siblings’ voices. Unlike most mammal calls, dolphins whistle through a system of air sacs located around their blowhole, as opposed to calling through their vocal tract.
When dolphins dive down deep, scientists hypothesize that the increased water pressure could change the shape of those sacs, making the timbre of a dolphin’s whistle an unreliable identifier. No one has done studies on dolphin calls at 100 meters below the surface; doing wild dolphin studies in the shallows is already pretty difficult. But Sayigh thinks that “air sacs might slightly deform, changing the tone of whistles” is a pretty safe conjecture.
“There aren’t many contexts in human society that match that, because we don’t live in a three dimensional environment with very limited visual capabilities,” says Sayigh. “But if you just said, ‘I’m here!’ there could be ten other dolphins that that could apply to, but if you say your own name, then there’s not much question about who’s saying that.”
But naming yourself is easier said than done. Coming up with signature sounds requires the ability to imitate and invent new patterns. The vast majority of animals, including most of our closest relatives, communicate through “hardwired” sounds that they can produce at birth.
“If a human infant doesn’t hear another person talking, they’re not going to learn human speech…The language that you hear is what you learn,” explains Sayigh. “But in the case of most non-human mammals, that production side is pretty much hardwired.”
Dolphins, along with birds and humans, are among the tiny minority of animals that learn their calls by listening, imitating, and inventing “signature” sounds. [Correction 10/2/17: Sayigh asked me to clarify: Humans and birds do not invent signature sounds that are directly analogous to signature whistles. Birds and humans do, however, learn to communicate through imitation.] For researchers interested in the evolution of speech and language, that makes dolphins a very important species. And, like humans, dolphins seem to learn new sounds throughout their lives. Identifying the evolutionary pressures that drove dolphins to evolve their learning style might help us understand where the human gift for gab came from.
Sayigh’s study suggests that dolphins may owe their ability to imitate and riff on other sounds not to incredible smarts (although dolphins are very, very smart) or playfulness but rather their inability to tell their friends’ voices apart.
New research shows that dolphins (probably) can’t tell each other’s voices apart. But that inability may have pushed them to evolve sophisticated vocal learning and to start using signature whistles.
Sayigh et al. “What’s in a voice? Dolphins do not use voice cues for individual recognition.” Animal Cognition, 2017. DOI: https://doi.org/10.1007/s10071-017-1123-5