Unlike humans, who have ways to remove their earwax (that doesn’t mean resorting to using a Q-tip, which is really, really discouraged), the wax in whales’ ears simply accumulates, creating a plug of hardened excretions that stays with them their entire life. That turns out to be great for scientists, who have now shown that the gunk in a whale's ear can tell us how much we’ve been stressing these giants of the sea for the last 150 years.
The scientific value of whale earwax has been known for decades, since it allows scientists to determine the age of a whale. Six years ago, biologist Stephen Trumble and his colleagues at Baylor University demonstrated that a whale’s earwax can tell us how many pollutants they are exposed to over their lifetime. Much like tree rings, whale earwax accumulates in layers or laminae, each of which represents about six months of life. By slicing up the wax and testing each six-month period, Trumble found that the wax revealed pollutants the whale encountered and a record of hormones in its body.
Now, Trumble and his team looked at 20 earwax plugs from three baleen whale species—humpbacks, fin and blue whales—from both the Pacific and Atlantic Oceans to determine their cortisol levels, a hormone that is released during times of stress. Overall, the wax plugs cover 150 years, from 1870 to 2016. It turns out that the whales’ stress levels rose and fell with human-induced stress. The results appear in the journal Nature Communications.
“This is the first-ever study to quantify temporal stress patterns in baleen whales,” Trumble says in a statement. “While the generated stress profile spans nearly 150 years, we show that these whales experienced survivor stress, meaning the exposure to the indirect effects of whaling, including ship noise, ship proximity and constant harassment, results in elevated stress hormones in whales spanning vast distances.”
According to London’s Natural History Museum, which contributed eight earwax plugs, including the oldest sample to the project, cortisol increased in the 1920s and 1930s when whaling in the Northern hemisphere ramped up to industrial levels, with 50,000 baleen whales taken in the 1930s.
Whaling tapered off during World War II, but surprisingly whale stress levels did not. “The stressors associated with activities specific to WWII may supplant the stressors associated with industrial whaling for baleen whales,” co-author Sasha Usenko says. “We surmised that wartime activities such as underwater detonation, naval battles including ships, planes and submarines, as well as increased vessel numbers, contributed to increase cortisol concentrations during this period of reduced whaling.”
The cortisol peaked with the height of industrial whaling in the 1960s when 150,000 of the whales were harvested. But when whaling moratoriums went into effect in the early 1970s, stress levels dropped dramatically. However, stress levels among the cetaceans has slowly continued to increase since then, likely caused by less targeted but equally concerning human-driven, non-lethal stressors, including noise from ship traffic, pollution and rising sea temperatures caused by climate change.
“The result that surprised us was the correlation itself,” Trumble tells Christie Wilcox at National Geographic. “These whales truly mirror their environment and can be used in a way similar to the canary in the coal mine.”