Researchers have used a rather intriguing method to find out what makes whales stressed – and, surprise surprise, it looks like we’re to blame.

In a study published in the journal Nature Communications, a team led by Baylor University in Texas studied the earwax of fin, humpback, and blue whales living in the Pacific and Atlantic Oceans between 1870 and 2016. This is the first study to ever look at stress over time in baleen whales.

This earwax came from earplug laminae, collected by museums, which is a growth layer found in whale earwax. Studying this layer, the researchers could examine the cortisol levels in the whales, which is a stress-response hormone, and matched it up to key moments in history.

And amazingly, they were able to show that cortisol levels increased in the 1960s, when whaling was at its peak – up to 150,000 of the animals were “harvested”. This represented the highest cortisol levels found in the 20th century. They were also highly stressed during increased levels of whaling in the 1920s and 1930s.

But cortisol levels were also found to increase during World War II. Despite whaling activities actually declining, the researchers think that the impact of war could have caused the whales to become stressed.

"The stressors associated with activities specific to WWII may supplant the stressors associated with industrial whaling for baleen whales," Dr Sascha Usenko, one of the study’s co-authors, said in a statement.

"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.”

Whale cortisol levels reached their lowest point in the mid-1970s, when whaling decreased to reportedly zero in the Northern Hemisphere. However, cortisol levels have still increased steadily to the modern day, suggesting that other human-caused stresses and the warming of sea surface temperatures may be playing a part.

The research is important because whales are thought to be a good indicator of the effect humans are having on the marine habitat. It also highlights just how much of an impact humans can have on these large whales, showing that key moments in our history have affected not just us.

"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," Dr Stephen Trumble, the study’s lead author, said in the statement.