Human earwax, whether removed by a curious finger or an ill-advised cotton swab, is usually tossed in the nearest garbage shortly after its removal. But this sticky substance can hang on to clues about health that build up in the ear canal over time—including in the giant ears of whales.

Luckily, museum curators around the world have had the good sense to hold onto massive plugs of earwax pulled from dead whales over the centuries.

Thanks to those plugs, scientists have now discovered a record, hidden in earwax, of how human activities have stressed out whales over the past century and a half. Stephen Trumble, a comparative physiologist at Baylor University, and his colleagues published the findings this month in Nature Communications.

It turns out we’re incredibly stress-inducing—from whaling to war to climate change, our actions have been affecting whales, even if we don’t interact with them directly.

Waxy Records

Each earwax plug, which can be more than a foot and a half long (50 centimeters) and weigh two pounds (about a kilogram), contains a wealth of information about the environmental conditions the animal lived in, as well as the health of the whale itself over its lifespan.

And because the wax is added in layers—similar to tree rings—researchers can get a time series of data on everything from pesticide contamination to reproductive cycles.

But it was the animals’ response to human activities that Trumble and his colleagues were especially keen to examine. And one of the best ways to do that is to measure the levels of hormones such as cortisol that are released when an animal is stressed.

Obtaining long-term data on whale hormone levels is incredibly difficult. It’s basically impossible to track and sample from individual animals over their entire lifetime. A whale’s baleen, which it uses to filter food, contains roughly 10 years of information, but the animals can live for 50 to 100 years, so that’s just a glimpse at their life.

Their earwax plugs, on the other hand, provide decades of data.

Extracting such information, however, is no small feat, says Trumble. Separating layers of wax for analysis—each of which contains information from approximately six months of the whale’s life—takes days of careful work.

But the result is worth it. “Being able to put together a picture of what stressors are involved and then the response of the whale—especially over lifetimes—is unprecedented,” says Trumble.

Nick Kellar, a cetacean biologist with the NOAA’s Southwest Fisheries Science Center in La Jolla, California, agrees. “This represents the best available science on the non-lethal effects of whaling and is a major advancement in this field,” he says.

View Images Fin whale populations were hit hard by whaling in the 20th century, and the animals are still targeted by whalers despite being endangered. Photograph by Tui De Roy, Minden Pictures/Nat Geo Image Collection

War and Global Warming

In the new study, hormone profiles from 20 fin, humpback, and blue whales revealed a tight connection between whaling activities and stress from the late 19th century to the 1970s, when legislation dramatically reduced the hunting of whales.

“The result that surprised us was the correlation itself,” says Trumble. While the researchers expected whaling to increase stress, they didn’t expect hormone levels to drop in lock-step with reductions to hunts. “These whales truly mirror their environment and can be used in a way similar to the canary in the coal mine,” he adds.

Hunting wasn’t the only source of stress that the researchers saw, either. From 1939 to 1945, elevated cortisol levels indicated that the whales’ stress levels were high, even though fewer whales were being harpooned. But there was another stressor at the time: global war. “We suspect this increase in cortisol during World War II is probably a result of noise from planes, bombs, ships, et cetera,” says Trumble.

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After about 1970—and especially after 1990— the researchers saw a worrying trend: Cortisol levels also increased rapidly alongside rising water temperatures. This suggests that climate change, too, is stressing the whales.

There are lots of ways that increased temperatures could affect the animals—from altering the location and abundance of prey to direct effects on their physiology from hotter waters (after all, it’s not like whales can flip on the air conditioning when their homes get uncomfortably warm). Trumble says he and his colleagues are still trying to narrow down exactly what about climate change is causing the increase in stress.

That will require more research, since the connection to temperature is based on plugs from just six whales. In addition, Kellar says that he’d like to see other variables such as the animal’s cause of death taken into account, as the natural aging process can affect hormone levels.

That doesn’t mean there’s no climate connection, though. “The weaknesses in the study are merely a reflection of inherent difficulties of trying to address an historic question at this scale,” Kellar says.