In a whale’s earwax lie clues to its entire life. Some species of whale build up large “earplugs” of fatty, waxy material that can trap hints about the hormones that coursed through the beast and the pollutants it swam through.

In a paper published this month in Nature Communications, researchers used earplugs recovered from 20 whales to explore how their stress levels have responded to changes over the last 200 years. They found that the whales’ stress levels moved in concert with being hunted, rising as whaling levels reached fever pitch, and plummeting as whaling levels reduced. But since the 1970s, stress levels have been steadily climbing again, keeping step with warming ocean waters.

The ear is the window to a whale’s soul

Tracking even the most obvious behaviors in wild animals can be a tricky business—for instance, nobody knows for sure where great white sharks go to breed. Even knowing how many animals there are in a population can be difficult. Figuring out how stressed whales have been is a near-impossible task, but a crucial one: stress affects the health of individual whales, which in turn affects the health of the population. So tracking stress levels could be useful for developing a comprehensive whale conservation strategy.

That’s why earplugs are such a boon. Over the course of a whale’s life, the waxy material is deposited in its ear canal, leaving a roughly foot-long structure that can be recovered after the animal’s death. Much like a tree’s rings, the layers in the wax can tell a story about the whale’s life. With a layer being deposited every six months, it’s possible to work out how old the whale is and get some clues about the experiences it faced throughout its history.

Biologist Stephen Trumble is no stranger to whale earwax. He’s previously used a blue whale’s earplug to track changing levels of pollution that the whale was exposed to. But his more recent research has ramped things up dramatically, using earplugs from 20 fin, humpback, and blue whales that lived through the 19th and 20th centuries. This allowed him to get a much more powerful sense of what the whales have been through.

To do this, Trumble and his colleagues looked at the whales’ cortisol levels, since the hormone can be used as an indicator of how stressed a mammal is. The researchers carefully shaved the earplugs into their six-monthly layers and then took cortisol readings from each. Because some whales might be more laid-back while others are more anxious, what counts as “high” cortisol levels might differ from one individual to another. To account for that, Trumble and colleagues set a “baseline” level for each individual, comparing how much cortisol raised and dropped compared to the norm for that whale.

Harpoons and bombs

This method gave them 942 earwax layers that they could use to compare to whaling data from across the 20th century, around the time when humans were figuring out how to get really efficient at hunting huge numbers of whales. During the 1930s, the authors write, approximately 50,000 fin, humpback and blue whales were hunted.

Whaling took a short break between 1939 and 1945 while humans focused on killing each other instead. But it picked up afterwards, ramping up to the killing of nearly 150,000 animals from those three whale species in the 1960s. Finally, the whales caught a break with international protective legislation in the 1970s.

The cortisol levels in the earplugs tracked these changes astonishingly closely, with one surprising anomaly: a small spike in stress during the war years, despite the drop in whaling activity. This can probably be explained by naval battles and increased numbers of ships creating new and distinct stresses for whales in those years, the authors suggest.

Swimming in hot water

Since the 1970s, however, things have not remained peachy. With whaling counts “reportedly zero in the Northern Hemisphere,” write Trundle and his colleagues, cortisol levels have nonetheless been climbing again, “with recent peaks reaching near the maximum levels observed before the whaling moratorium.”

There’s a long list of possible reasons for this, including increasing levels of tourism, recreational fishing, shipping, and pollution. But the best data available is for sea-surface temperature, and so that’s what Trundle and his team looked at. They found that cortisol levels in the whales correlated reasonably strongly with warming waters—not as strongly as they had correlated with whaling, but this isn’t a surprise given that warming is only one of many stressors that’s been affecting whales over the last 40 years. What is a surprise is the possibility that warming, combined with other human-created pressures, could be nearly as stressful for whales as hunting them in huge numbers.

Although the researchers had access to a healthy number of wax layers, that data all came from just 20 whales—and at any given time throughout the 20th century, only a few of those whales were alive. So, the data is still fairly limited, and these results give only a very small window into the world of whale stress. A bigger sample size is surely on Trundle’s wishlist, but earplugs aren’t recovered every day.

As more whale earplugs are recovered, more ambitious projects can be attempted. Looking at other stress factors, like overfishing and sea-ice decline, will also be useful. But this work is a huge leap, helping us gain insight into how human activities are continuing to hurt whales long after we’ve stopped hunting them.

Nature Communications, 2018. DOI: 10.1038/s41467-018-07044-w  (About DOIs).