Two small animals singing in harmony with each other, a burst of cries and chirps incessantly. Close your eyes and listen, you may think it is two small birds in a show of song. But you’d be wrong. In fact, it’s the singing of two Alstom’s singing mice, the tiny rodents found in the cloud forests of Central America that communicate through heartfelt duets with their companions.
Most of these singing mice’s sounds are outside our hearing range, so researchers recorded them at frequencies we can hear as a way to show the beautiful symphony they serve up. But the difficult-to-capture sounds of these mice also disprove a common assumption: in addition to humans, only songbirds can sing. In fact, there are more animals that sing and answer to each other than you might think. So which species can do this, exactly? Do they sing just to find mates and mark territory? Or, like us, is it because they like to sing?
First, we need to understand the difference between song and other sounds. Few researchers have given a definitive answer to this question. Part of Harvard biology professor Brian Farrell’s research takes aim at animal sounds in nature. Farrell explained that at the simplest level, they define a song as a string of tones that may repeat over time to form something like what we call a melody. In short, “all songs are sounds, but not all sounds are songs,” Farrell told a reporter for the Fun Science website. By this definition, a dog barking, a frog or a cicada chirping are not sounds that we necessarily consider songs.
Taking it a step closer, Farrell says, a song may contain a degree of composition, which requires the ability to improvise. Interestingly, singing animals also tend to learn the ability to pronounce words from their parents rather than being born with it; it is believed that this flexible learning enhances the ability to improvise.
This definition is a highly subjective one given by humans. Charles Snowden, a primatologist and retired professor emeritus of psychology at the University of Wisconsin-Madison, says that singing is “a concise way of talking about a specific subset of animal signals that sound a lot like music. When we use this definition, it’s the equivalent of embarking on a journey to uncover the hidden heroines of nature’s operas.
Take, for example, the Mexican free-tailed bat, which uses a shrill tone to attract the attention of females during mating season (so high, in fact, that humans must use special audio equipment to hear it). When a male bat succeeds in attracting the interest of a potential mate, the situation becomes interesting. According to a 2013 study in the British monthly Animal Behavior, male bats will quickly add a series of notes to their simple song, seemingly in order to keep females interested long enough to mate. The bats can quickly reconfigure these pitch combinations to determine exactly what the female likes – a real-life case of improvisation under pressure.
At the same time, gibbons have challenged the status of humans as the most admired singers in the primate world. Not all species of gibbons sing, but the ones that do are capable of singing complex arias – shorter sounds interspersed with long, sobbing voices – which researchers have found to be a common vocal mechanism used by opera singers. Their combinations also depend on the specific context: the researchers found that some gibbons have a distinctive string of predatory warnings that are not heard in regular calls. In addition, some gibbon mates are known for being good at singing duets, which experts believe help strengthen social bonds and mark their territory from other pair-bonded gibbons.
However, these primates aren’t the only animals that enjoy singing in duets. Alstom singing rats also sing duets, and they do so in a courteous manner. Such rats typically produce a series of fast-paced chirps (their songs may contain nearly 100 notes), but studies have shown that one rat’s song never interrupts the other’s. In fact, each rat pauses for a moment after its companion sings, before starting its own song. Neuroscientists have been studying the neural basis of this pause ability to figure out what this tells us about the evolutionary basis of human conversation, which may also be based on this rotation mechanism.
Meanwhile, it’s impossible to discuss singing without also mentioning the beautiful songs of humpback whales, whose songs were first recorded and widely promoted by American biologist Roger Payne in 1970, which sparked the imagination of people. Farrell says the heartfelt songs had such an impact that they are credited with helping to fuel a wave of opposition to whaling throughout the 1970s and eventually led to an almost worldwide moratorium on whaling.
Payne’s recordings also show for the first time that the whales’ whispered songs included distinctive, recurring themes. Farrell said Payne “was really the first to discover that this 20-minute vocalization of the whales was actually a piece of music. Since then, researchers have found that different groups of whales have unique songs that can be used to identify them, and that other whales, including orcas and belugas, also sing.
Why play songs
And what is sung? These are just a few of the species on Earth that sing, and depending on the way we define the original melody of the animal, perhaps many more animals sing as well. But why do some animals sing instead of woofing, baaing or buzzing? Farrell says that in addition to competing for territory, mates and food, animals living in the same sound space have to compete “for bandwidth” in order to be heard by their own kind. Singing has been shown to have the advantage of spreading over long distances and can carry a lot of information in long, repeated chants. Singing is still useful when marking territory, warning other animals of predators, or courting mates with impressive singing, as in the case of free-tailed bats.
But beyond these functional roles, are there animals that sing for the pure enjoyment of singing? There is no single, necessarily correct answer to this question. But Farrell says we do know that animals have fun too, and they have an “emotional life. Both of those things are well established, he says, and there’s a huge literature on them.” There is also growing evidence that animals respond emotionally to music.
For example, researchers have studied the effects of Mozart’s music on rats, which can hear the highest frequency tones in the music, and they found that the music lowered the rats’ blood pressure, which is often associated with feelings of calm. To confirm these findings, Snowden decided to go a step further: 13 years ago, he began working with a cellist named David Thayer to determine whether this connection would still hold true if they composed music specifically for animals. They speculated that these animals would be more likely to respond to music if it contained frequencies that were within their vocal and hearing range and if familiar rhythms based on their heartbeat or vocal patterns appeared.
How to appreciate
Snowden and Thayer conducted two separate studies of cats and a type of monkey called the downy-topped tamarack monkey to measure the animals’ responses to a series of experimental animal songs composed by Snowden and Thayer. First, they composed two unique melodies for the downy-topped tamarisk monkey: one with a sharp, staccato rhythm that reminded these monkeys of an agitating chirp; the other with a high-pitched whine. They also composed a string of slides for cats, with a rhythm in the background that matched the whining sound cats make when they are happy.
In both studies, the specially composed music provoked a response.
Their 2009 study on downy-topped tamarisk monkeys, published in the British journal Biology Letters, showed that they could successfully quiet or excite these monkeys depending on the tune played. In addition, in a 2015 study published in the Dutch journal Applied Animal Behavior Science, their music composed specifically for cats attracted their interest, and the cats were more willing to approach and rub the speaker playing the distinctive song than the usual tune played over the loudspeaker, a sign of their contentment.
Snowden said, “This shows that music has emotions, and if we control these emotional factors, we can change the behavior of animals.” In fact, he says, when another group of researchers tested the aforementioned tunes composed for cats in a real-world setting at a veterinary clinic, “they found that playing music composed for cats during veterinary exams made these animals more calm than playing human music or being quiet.”
Snowden said the ability of music composed for animals to have this effect on them has prompted people to start thinking that the emotional impact of music may have deeper evolutionary roots than we realize, which could reveal the profound impact music has on humans. This is an area that is currently being investigated. In the meantime, can we conclude from this that animals sing purely for enjoyment? Farrell tends to think that there is an emotional element in the tunes of animals, but confirming this is beyond our current research capabilities. He adds that “the most interesting questions are the most difficult to test.”
Considering the happy cries of gibbons, the emotion-filled chirps of singing rats and the heartfelt melodies sung by whales, it’s hard to believe that there isn’t emotion and joy woven into the songs of animals. But that’s another mystery.