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Whales are big, whales are long-lived, and whales have paddle-shaped flippers instead of dexterous hands. These three traits inexorably lead to a fourth: Over time, whales accumulate a lot of earwax.

Whale earwax forms like yours does: A gland secretes oily gunk into the ear canal, which hardens and accumulates into a solid, tapering plug. In the largest whales, like blues, a plug can grow up to 10 inches long, and looks like a cross between a goat’s horn and the world’s nastiest candle. Fin whale wax is firmer than blue whale wax, bowhead whale wax is softer and almost liquid, and sei whale wax is dark and brittle. But regardless of size or texture, these plugs are all surprisingly informative.

As whales go through their annual cycles of summer binge-eating and winter migrations, the wax in their ears changes from light to dark. These changes manifest as alternating bands, which you can see if you slice through the plugs. Much as with tree rings, you can count the bands to estimate a whale’s age. And you can also analyze them to measure the substances that were coursing through the whale’s body when each band was formed. A whale’s earwax, then, is a chronological chemical biography.

Stephen Trumble and Sascha Usenko from Baylor University have worked out how to read those biographies. And they’ve shown that whale earwax not only reveals the lives of their owners, but the history of the oceans. Hunting, abnormal temperatures, pollutants—it’s all there. If all of humanity’s archives were to disappear, Trumble and Usenko could still reconstruct a pretty decent record of whaling intensity by measuring the stress hormones in the earwax of a few dozen whales.

Fin whale earwax (Stephen Trumble)

The duo first tested their idea of studying earwax by analyzing the plug from a single blue whale—a 12-year-old male that was fatally struck by a ship off the coast of Santa Barbara in 2007. They could tell that the whale became sexually mature when it was 9 years old, as that’s when testosterone levels in the plug shot up by 200 times. They showed that the stress hormone cortisol peaked a year before that, perhaps a sign of the creature’s changing body and mind. They found traces of pesticides and flame retardants that were especially concentrated in the whale’s first six months of life, and had likely been passed down in its mother’s milk. “I was surprised at how well [the technique] worked, not only for persistent chemicals but for hormones that typically rapidly degrade,” Usenko told me at the time.

That was just one earwax plug, but it was surprisingly easy to get more. They just had to call curators at the right natural-history museums. “Museums are notorious for collecting everything, and waiting for the science to catch up,” Trumble says. “We called Charles Potter at the Smithsonian Institution, and he said, ‘It’s interesting you called because we have pallets and pallets of these ear plugs sitting around, and we’re thinking of throwing them away.’ Instead of being thrown away, those ear plugs are now objects of wonder.”

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Trumble, Usenko, and their colleagues ended up measuring cortisol levels in the plugs from 20 blue, fin, and humpback whales, the oldest of which had been born in 1871. The team measured how this stress hormone varied over the lifetime of each animal, relative to the lowest levels found in each plug. They then combined these readings into a 146-year chronicle of whale stress, which they compared to a record of all whaling data from the 20th century. “We plotted the two together, and were like: ‘You’ve got to be kidding me,’” says Trumble.

A graph comparing whaling intensity with earwax cortisol. (Trumble et al, 2018, Nature communications)

The two data sets matched beautifully. When whaling increased, cortisol levels rose, hitting their peak during the heyday of whaling in the early 1960s. After moratoriums were adopted in the 1970s, whaling harvests fell by 7.5 percent a year and cortisol levels in earwax fell by 6.4 percent a year.

To an extent, that’s not surprising: Of course, whales would be more stressed if their pod-mates are being harvested. Still, it’s astonishing just how well the two data sets match. Trumble and Usenko could get a pretty good picture of global whaling efforts through the lived experiences of 20 whales.

There are a few discrepancies, and they’re telling. For example, whaling fell away during World War II while cortisol levels rose by 10 percent. The oceans may have been relatively free of harpoons, but they were instead filled with battleships, submarines, depth charges, and the sounds of warfare. Those indirect disturbances, it seems, were just as stressful to the whales as their hunters had been—and they continue today.

Since the 1970s, whaling has dwindled to negligible levels in the Northern Hemisphere, but if anything, cortisol levels have risen—slowly at first, and then more dramatically in recent decades. Trumble and Usenko showed that this rise correlates with the number of days when ocean temperatures were unusually high.

The team’s 146-year chronicle also has a gigantic spike in the early 2000s when cortisol levels seem to shoot through the roof. That’s because of the very first blue whale they studied. It was the only individual whose life spanned those particular years, and for whatever reason, it spent those years in an extreme state of stress. Was it reacting to the noisy shipping lanes that crisscross California’s waters? Was it suffering from the mercury, pesticides, and other pollutants in its body? No one knows, but its cortisol was hitting highs that haven’t been seen since the days when people killed whales in the hundreds of thousands. “When I look at that, I think: Here’s an individual that’s under stress levels as if it’s being whaled,” says Usenko.

“I think this is going to revolutionize our studies of whale biology,” says Kathleen Hunt from Northern Arizona University, who was not involved in the work. “Whale biologists are used to gleaning tiny bits of information from samples like a single blubber biopsy, one or two fecal samples, or a few photographs scattered over years. An earwax plug is more like 200 samples in a row, taken from the same animal, every 6 months, for its whole life.” They’re like the ice cores that climate scientists use to peer back into the Earth’s distant past.

The plugs are especially informative because whales are so long-lived. They can take a decade to mature, go for years between pregnancies, and spend much longer recovering from episodes of trauma. “We’ve never really had a way to track individual whale stress responses over those sorts of timescale before, and it’s very exciting,” says Hunt.

The team is now examining the wax for pregnancy hormones, chemical isotopes that reflect the whales’ diet, and other telltale molecules. “We’re getting tons and tons of data from these earplugs that we’ve only ever assumed,” Trumble says. And he’s not running out of material to work with. “The Canadian Museum of Nature in Ottawa has 4,000 ear plugs, and we had 100 shipped to us. We’re getting quite deep into this.”