Like a dog wagging its tail in anticipation of treats to come, dolphins and belugas squeal with pleasure at the prospect of a fish snack, according to a new study. It’s the first direct demonstration of an excitement call in these animals, says Peter Madsen, a biologist at Aarhus University in Denmark who was not involved in the study.
To hunt and communicate, dolphins and some whale species produce a symphony of clicks, whistles, squeaks, brays, and moans. Sam Ridgway, a longtime marine biologist with the U.S. Navy’s Marine Mammal Program, says he heard distinctive high-pitched squeals for the first time in May 1963 while training newly captured dolphins at the Navy’s facility in Point Mugu, California. “We were throwing fish in, and each time they would catch a fish, they would make this sound,” he says. He describes it as a high-pitched “eeee,” like a child squealing in delight.
Ridgway and his collaborators didn’t think much of the sound until later in the 1960s, when dolphins trained to associate a whistle tone with a task or behavior also began making it. Trainers teach animals a task by rewarding them with a treat and coupling it with a special noise, like a click or a whistle. Eventually only the sound is used, letting the animal know it will get a treat later. The whistle was enough to provoke a victory squeal, Ridgway says. Meanwhile, beluga whales would squeal after diving more than 600 meters to switch off an underwater speaker broadcasting tones. “As soon as the tone went off, they would make this same sound,” Ridgway says, “despite the fact that they’re not going to get a reward for five minutes.” He also heard the squeal at marine parks in response to trainers’ whistles.
Ridgway suspects the squeals are tied to the release of dopamine, a neurotransmitter associated with the reward centers of mammal brains. Since 1956, scientists have identified reward systems involving dopamine neurons in rats, dolphins, monkeys, and humans, among other mammals. Dopamine release can take about 100 to 200 milliseconds, so Ridgway pored over 4 decades of recordings made in open waters to time the animals’ responses. He found that the dolphins take an average of 151 milliseconds to make their squeals and belugas take about 250 milliseconds. Though Ridgway and colleagues didn’t directly measure dopamine in the brain, that’s enough time for dopamine to spark the sound, he and colleagues report online today in The Journal of Experimental Biology.
Ridgway, who is semiretired, says the victory squeals were never a specific research project, but he wanted to publish the results to tie together 52 years of observations.
Marc Lammers, a biologist at the Hawaii Institute of Marine Biology in Kaneohe who studies cetacean communication and behavior, says dopamine release is a novel way to explain these emotive calls. But conditioned responses in captive, trained animals may not necessarily translate to involuntary pleasure sounds in wild animals, he adds. Wild belugas and dolphins emit bursts of these sounds in a variety of settings, from feeding sessions to aggression or courtship aimed at other animals. What’s more, squeals, squawks, or creaks sound differently to us than to cetaceans, which hear at a much higher resolution, Lammers says. “They’re our best attempt at putting a label on a certain type of sound, rather than describing the acoustic quality of the sounds themselves,” he says. “Just because that’s what it looks like or sounds like, or that’s the context—it’s hard for us to know what’s inside the animal’s head.”
Wild dolphins and whales have been shown to produce mews, rasps, buzzes, and creaks to alert other animals when food is available. But this study doesn’t show that victory squeals are an intentional form of communication, either for food sharing or simply emotional sharing, says Paul Manger, a biologist at the University of the Witwatersrand in Johannesburg, South Africa. “To support the idea that somehow there’s a social context to it, they’d have to take the experiments further and show some intentionality; intention to share or deceive,” Manger says. Further work using two dolphins at a time could help clarify whether the calls have some social context, or are just involuntary cries of delight, he says.
(Video courtesy of Sam Ridgway, National Marine Mammal Foundation)