Duke marine-mammal biologists use dolphins to explain stubborn blubber

How much energy does a dolphin burn in a day?

Not only what’s the caloric cost, but can a dolphin afford to forever be dodging boats? That’s so much effort. And how many burned calories keep a mammal warm in cold seawater? How much lost biomass—that’s sea life in this context— is too much before a dolphin can’t get enough to eat?

How do you even measure that, anyway?

You go where you know there are dolphins, and you ask them nicely.

If they don’t want to participate in research activities, “they just swim away, and that’s fine,” says Austin Allen, a Ph.D. candidate in conservation biology at the Duke Marine Lab.

Dolphins are intelligent, meaning researchers and aquatic-facility staff can ask them to play games as part of a study or lift their tail for a blood sample. There’s no forced participation and nothing punitive. The animals always get their full food, and study-related activities hold their attention.

In 2019, Allen spent three weeks at Dolphin Quest in O’ahu, Hawaii, as part of a dolphin-metabolism study designed by Duke evolutionary anthropologists Herman Pontzer and Brian Hare, Hare Lab graduate student Hannah Salomons, and himself. Some of the team worked with the cetaceans of the Dolphin Research Center in the Florida Keys. Studying a dolphin’s daily calorie cost is difficult in the wild—the ocean is a big place, especially if you’re following individual creatures—making facilities like Dolphin Quest and Dolphin Research Center invaluable to marine mammal biologists.

For the study, the results of which were published in the Journal of Experimental Biology in August 2021, dolphins were given doubly labeled water—that is, water containing deuterium, a heavier (but non-radioactive) isotope of hydrogen, and heavy oxygen. Through blood or urine samples, researchers could tell how fast heavy water was leaving the dolphins’ bodies and calculate their metabolic rate.

They found that a dolphin burns less energy per day than expected. Another possible finding—and one that merits further study due to this study’s sample size of ten individuals— is that dolphins forty and older seem to carry a little extra weight, no matter how much they exercise.

That may sound familiar.

“Mammals’ bodies all kind of work in similar ways,” says Chana Kaufman ’20 who participated as an undergrad through the Bass Connections “Are Dolphins Really That Smart and Does It Make Us Like Them More When They Are?” team (which studied energy expenditures as part of its research). Today, she is a veterinary school student at UC Davis. “There are species differences, absolutely, but there are certain patterns across every mammal, humans included. You can kind of extrapolate principles from certain species and apply them to others.”

To clarify the comparison, Allen mentions Pontzer’s paper “Daily Energy Expenditure Through the Human Life Course,” published in August in the journal Science. Its main finding, based on a sample size of 6,400, was that there’s no midlife metabolism dip in humans— rather, it drops off after sixty. The dolphin lifespan isn’t all that well-known, Allen adds, but it’s rare for a dolphin to live into its forties, fifties, or even sixties.

“It might just be that that drop-off instead of sixty in humans is closer to forty in dolphins,” he ventures. “It’s hard to compare with the different lifespans.”

This didn’t express itself in the dolphins’ energy levels, though. Some of the older animals were quite active, Allen says. These are individuals, after all, with distinct habits and personalities.

Kaufman, who was active in the Duke Canine Cognition Center and its Puppy Kindergarten as an undergraduate, called them “sea puppies” because of their similarities with dogs. She recalls throwing a football to her favorite, a big twenty-four-year-old male named Pax.

“If you walked along the walkways or the decks, they would swim up and follow you and look at you,” she says. “If you walked past without saying hi, they would chirp at you.”

Indeed, dolphins are excited to see you every morning, Allen says. He loves working with the animals, and his research— which, yes, involves asking dolphins to swim around and play and be ridiculous— ties directly into whether wild dolphins have enough available prey to swim around and hunt and be prolific.

From a conservation standpoint, finding out how much it costs to be an animal is critical to measuring non-lethal, detrimental, cumulative effects at the population level. Energy, he continues, is one of the more direct ways to influence survival and reproduction. Dolphins won’t be doing enough of either without an adequate food supply.

“It’s not as obvious as an animal getting caught in a net, but I think it’s really critical,” Allen says.