Floating along ocean currents in their mother’s shadow, newborn bottlenose dolphins sing to themselves. They create a unique siren of beeps, known as a signature whistle, which scientists have compared to a human name.
Unlike most animals, dolphins cannot use voices as their identifier because it gets distorted at different depths. Instead, they come up with a melody — a pattern of sound frequencies held for a certain amount of time — that they use to identify themselves for the rest of their lives. bottlenose dolphins (Tursiops truncatus) can even imitate the whistles of their friends, call their names when they are lost. Additional information, such as reproductive status, can be conveyed by changing the volume of different parts of the whistle, similar to how people emphasize certain words to add nuance.
But how do dolphins decide what to call themselves?
By eavesdropping on six dolphin populations in the Mediterranean, researchers at the University of Sassari in Italy revealed that differences in signature whistles were usually determined by their habitat and population sizeaccording to a study published in May Scientific Reports. Sound travels differently in different environments, so dolphins create signature whistles that best suit their environment, according to the study authors.
Find the right signature
Dolphins that live among seagrass, the researchers in the Scientific Reports study found gave themselves a short, shrill name compared to the baritone sounds of dolphins living in muddy waters. Meanwhile, small pods showed greater pitch variation than larger groups, which may aid identification when the likelihood of repeat encounters is higher.
But not all scientists consider habitat and group size to be the main drivers of signature whistles. Jason Bruck, a biologist at Stephen F. Austin State University, believes that social factors play a critical role. He points to a study of dolphins living in Sarasota Bay, Florida where dolphins created unique signature whistles with inspiration from community members. Crucially, the dolphins tended to base their whistles on cetaceans they spent less time with. “This avoids the problem of every dolphin being called John Smith,” says Bruck.
Laela Sayigh, a research specialist at Woods Hole Oceanographic Institution in Massachusetts agrees. Based on her work studying cetacean communication over more than three decades, Sayigh estimates that 30 percent of signature whistles are based on their mother’s whistle, while others come up with names that are nothing like their mother’s and closer to that of their siblings. Others still produce a unique whistle that is different from each of their relatives. Marine researchers still don’t know why some bottlenose dolphins base their whistles on relatives and others on lesser acquaintances.
According to Sayigh, factors such as sociability play a role. For example, sociable mothers expose their calves to a wider variety of distinctive whistling tones, giving the calf more sounds to add to their repertoire. However, demonstrating this in wild populations proves difficult.
“It is very difficult to observe which forces affect a calf during the period of development of signature whistles, which require extensive observations,” Sayigh says. “You only look at a very small part of the actual development period. If there is some crucial interaction, you may not capture it.”
Names aren’t the only way dolphins recognize each other; they can also recognize a familiar face by the taste of their urine.
After observing bottlenose dolphins swimming open-mouthed through plumes of urine, marine scientists suspected they could identify other dolphins by the taste of their feces. Unlike many animals, which rely primarily on scent, bottlenose dolphins use their noses as blowholes and do not have an olfactory bulb.
Another study, published earlier this year in scientific progress, confirmed that dolphins can indeed distinguish urine samples from their companions and those of strangers. When the researchers played a signature whistle recording of the same dolphin that provided the urine sample, the dolphins spent longer examining the speaker than when a mismatched whistle was played.
“We can say with some confidence that dolphins have at least two modes of social identification: distinctive whistles and urine signals,” said Bruck, lead author of the study. “Dolphins are excellent whistle-mimickers if they want to, so urine may be more resistant to cetacean identity theft.” However, deception in signature whistles has not been well studied, he adds.
A Bottlenose Bromance
While female dolphins’ signature whistles will barely change throughout their lives, male dolphins can customize their whistle to reflect their best friend’s signature whistle. Linkage bonds between males and males are common in certain populations and can be stronger than the bond between a mother and her calf. “We see this in Sarasota all the time,” Sayigh says. “These male alliances are extremely strong pair bonds where the males are together all the time and they often come together on their signature whistles.”
In addition to an individual signature whistle, groups of dolphins may invent a shared whistle to promote social cohesion. Dolphins often emit the group whistle when they coordinate their behavior with others, such as: looking for food and guarding friends.
Brittany Jones, a scientist with the National Marine Mammal Foundation who specializes in dolphin communication, has studied a group of eight dolphins trained by the United States Navy. Five of the dolphins, who have lived together for 21 years, shared a group whistle which retained enough distinguishing features to identify the speaker.
“These shared whistles, while very similar between dolphins, were slightly more similar within an individual than between dolphins,” Jones says. This suggests that other dolphins may be able to identify who is making the whistle, suggesting it conveys both group and individual identity.
Like the human equivalent, signature whistles are more than just a name. They can reveal family ties, alliances, and possibly a dolphin’s overall environmental landscape. Scientists believe there is still more to discover, such as whether dolphins use their impressionistic skills to cheat and whether they talk about their friends behind their backs. Uncovering the complexity of how these animals use signature whistles can reveal just how imaginative their inner world really is.