Counting the breaths of wild porpoises reveals their revved-up metabolism

By counting harbor porpoise breaths, researchers have come up with a new way to judge the animals’ hard-to-measure metabolism. The trick shows that the animals can burn energy more than twice as fast as humans.

Researchers analyzed the several thousand puff-huff respiratory sounds recorded per day from each of 13 harbor porpoises swimming freely in Danish waters. Including just everyday staying-alive body processes plus hunting and other activities, the animals’ average total energy use ranged from 7.8 to 31 megajoules per day, researchers report December 6 in the Journal of Experimental Biology.

The five adult porpoises (Phocoena phocoena) studied averaged 21.7 megajoules per day. A typical human weighing about as much as a full-grown porpoise, however, needs only about seven to nine megajoules of energy daily, says study coauthor Peter Teglberg Madsen, an eco-physiologist at Aarhus University in Denmark.

Madsen says the animals’ high energy needs in the chilly waters close to Danish shores leave him “somewhat worried.” Harbor porpoises there depend on small fish, even down to pinkie finger–sized ones. But to survive with such a high metabolic rate on small prey demands steady hunting. And Madsen fears that increasing human disruptions in the ocean are making that difficult for the ocean mammals (SN: 2/13/18).

Metabolic rates of marine mammals “are certainly more than an abstract scientific question,” says marine biologist Andrew Read, who directs the Duke University Marine Lab in Beaufort, N.C., and was not involved in the project. Advocates of culling marine mammals believed to compete with human fisheries often cite the animals’ revved-up metabolisms and voracious hunting habits to support their point, he says.

But figuring out how much food a free-swimming mammal really needs to sustain its metabolism is hard. That’s where the breaths come in. As an animal fuels its basic body processes and daily activities, it consumes oxygen and produces carbon dioxide. For small land animals, researchers have devised lab setups to measure gas fluctuations in breath and blood, but that’s tricky for big sea animals.

In the past, researchers have used data on land mammals to speculate about their counterparts in the sea. But the few actual measurements of these gas fluctuations in young porpoises in captivity have suggested their metabolic rates are higher than similarly sized land mammals. Captivity, however, might be stressing the animals and driving up their rates.

So Madsen and his colleagues wondered if porpoise breathing rate, something people can reliably count in free-swimming mammals, might give clues to the animals’ total daily energy needs, called the field metabolic rate. To connect breaths with energy use, study coauthor Laia Rojano-Doñate, also at Aarhus, turned to three captive porpoises kept in a net pen in a fjord, where the animals experienced natural ocean temperatures, salinity and water movements.

Records showed how much food the porpoises had eaten, so researchers could get a rough estimate of how energy needs fit with breathing rates. To get a better sense, however, researchers gave one porpoise “doubly labeled water,” with both the oxygen and hydrogen in unusual forms that are easy to monitor. That let researchers track oxygen use in the porpoise body during daily activities. During these tests, students also took shifts counting blow-hole openings to indicate breaths. At the end, researchers could predict how breath number and animal size signal metabolic state.

To get breath counts on wild porpoises, Rojano-Doñate went out with rescuers trying to save porpoises accidentally tangled in fishing nets. From 2012 to 2016, the team attached small recorders to 13 porpoises that piggybacked on an animal for about six to 38 hours. After the heroic tedium of tallying breath sounds in the recordings (more than 6,000 per day in one animal), the researchers then calculated the metabolic rates that are reported in the study.

The project has convinced ecological modeler Cara Gallagher of the porpoises’ high energy needs. She’s now at Aarhus but didn’t worked on the breath-count project and had even used old, lower estimates of porpoise metabolism in her master’s project, published in 2018, at San Francisco State University. “There was so much controversy,” she says. Now “we actually have data on it.”

A doubt lingering in Read’s mind is whether harbor porpoises routinely feed in binges on extreme tiny fish. Madsen and colleagues have described such episodes, such as a porpoise downing some 550 young fish in a frenzy, and argue that’s typical for local porpoises. Read has called for more widespread monitoring of routine feeding, though that’s not an easy thing to do. Time for the next porpoise-watcher innovation.