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.”

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