Thu. Dec 1st, 2022
A chart next to what looks like a tree cut in half, but is actually whale earwax.
enlarge / A whale earplug, which shows the layers that have accumulated over the course of a lifetime, and the corresponding levels of cortisol.

A whale’s earwax contains clues for its entire life. Some whale species build up large “earplugs” of greasy, waxy material that can catch hints about the hormones flowing through the beast and the pollutants it swam through.

In an article published this month in nature communication, researchers used earplugs recovered from 20 whales to examine how their stress levels have responded to changes over the past 200 years. They found that the whales’ stress levels moved with the hunt, rising when whaling levels reached fever and plummeting when whale levels declined. But since the 1970s, stress levels have steadily risen again, keeping pace with warming ocean waters.

The ear is the window to a whale’s soul

Tracking even the most obvious wildlife behaviors can be a tricky business — no one knows for sure where great white sharks go to breed, for example. Even knowing how many animals there are in a population can be difficult. Figuring out how stressed whales are is an almost impossible task, but a crucial one: Stress affects the health of individual whales, which in turn affects the health of the population. Thus, monitoring stress levels can be helpful in developing a comprehensive whale conservation strategy.

That’s why earplugs are such a blessing. Over the course of a whale’s life, the waxy material is deposited in its ear canal, leaving an approximately 12-foot-long structure that can be repaired after the animal’s death. Like the rings of a tree, the layers in the wax can tell a story about the whale’s life. Because a layer is deposited every six months, it is possible to find out how old the whale is and get some clues about the experiences it has had throughout its history.

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

To do this, Trumble and his colleagues looked at the whales’ cortisol levels, because the hormone can be used as an indicator of how stressed a mammal is. The researchers carefully shaved the earplugs into their semiannual layers and then took the cortisol measurements from each. Because some whales may be more relaxed while others are more anxious, what counts as “high” cortisol levels can vary from person to person. To explain that, Trumble and colleagues set a “baseline” level for each individual, comparing how much cortisol increased and decreased compared to the norm for that whale.

Harpoons and Bombs

This method gave them 942 layers of earwax that they could use to compare with whale data from the 20th century, around the time when people were figuring out how to become really efficient at hunting massive numbers of whales. In the 1930s, the authors write, about 50,000 fin, humpback and blue whales were hunted.

Whaling took a brief hiatus between 1939 and 1945, as people concentrated instead on killing each other. But it then increased, increasing to the killing of nearly 150,000 of those three whale species in the 1960s. Finally, in the 1970s, the whales broke international protective law.

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 decline in whaling. This could likely be explained by naval battles and increased numbers of ships that created new and apparent tensions for whales during those years, the authors suggest.

Whale cortisol levels (black) closely track whaling (blue) throughout the 20th century.  Between 1939 and 1945, despite whaling declining, a spike in cortisol could be explained by other war-related stresses for whales.
enlarge / Whale cortisol levels (black) closely track whaling (blue) throughout the 20th century. Between 1939 and 1945, despite whaling declining, a spike in cortisol could be explained by other war-related stresses for whales.

nature communication

Swimming in warm water

However, things have not been rosy since the 1970s. With whaling “reportedly zero in the Northern Hemisphere,” write Trumble and his colleagues, cortisol levels are nevertheless rising again, “with recent spikes approaching maximum levels seen before the whale moratorium.”

There is a long list of possible reasons for this, including increasing tourism, recreational fishing, shipping and pollution. But the best data available is for sea surface temperature, which is what Trumble and his team looked at. They found that the cortisol levels in the whales correlated fairly strongly with warming waters — not as strongly as they had correlated with whaling, but this is no surprise given that warming is just one of many stressors that have hit whales over the past 40 years. year. What’s surprising is the possibility that warming, coupled with other human-induced pressures, could be almost as stressful for whales as hunting in large numbers.

While the researchers had access to a healthy number of wax layers, that data was all from just 20 whales — and only a few of those whales were alive at any point in the 20th century. So the data is still quite limited and these results give only a very small glimpse into the world of whale stress. A larger sample size is certainly on Trumble’s wish list, but earplugs aren’t rediscovered every day.

As more whale plugs are recovered, more ambitious projects can be attempted. Looking at other stressors, such as overfishing and sea ice decline, will also be helpful. But this work is a huge step forward and helps us understand how human activities continue to harm whales long after we’ve stopped hunting.

nature communication2018. DOI: 10.1038/s41467-018-07044-w (About DOIs).

By akfire1

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