Whaling left traces in whales’ earwax

Whaling left traces in whales' earwax

Researchers say their stress levels can be read in the earwax of certain coarse whales.

A time series spanning some 150 years even shows when a particularly large number of these animals were killed and other whales therefore produced particularly high levels of stress hormones, writes a team led by biologist stephen trumble and environmental researcher sascha usenko from baylor university in texas in the specialist journal "nature communications. Whales’ stress levels also rose during the second world war.

The researchers studied three species of baleen whales, which filter their food from the sea by means of adjacent comb-like plates called baleen whales. Over the lifetime of a baleen whale, earwax gradually forms a cone that can grow several decimeters long and, like a tree, has a kind of annual rings. The team analyzed such eartips from 20 fin, humpback and blue whales from the atlantic and pacific, spanning a period from the 1870s to 2016. According to the authors, industrial whaling is clearly reflected by the stress hormone cortisol contained in the animals’ earwax: the more whaling, the higher their stress. The whales had an estimated age of 2 to 63 years old.

Both stress levels and industrial whaling peaked in the 1960s. After that, whaling and stress dropped significantly. In addition to industrial whaling, the study found that the second world war also had an impact on the location of the sea suckers. Although whale catches decreased during this time, stress levels increased slightly. Researchers attributed this to sea battles, increased shipping, underwater detonations and submarines.

And changes in the temperature of the water surface could also be decisive, according to the study: whales could move to other habitats, for example, if their prey were in other places. According to researchers, the anomalies in surface temperature, which have become more pronounced since the 1970s, were also reflected in the cortisol content of the ear cones. These anomalies had replaced whaling as a source of stress, researchers suggest. Other possible stressors such as the catch of huge amounts of krill by humans or the receding sea ice due to climate change had to be examined.

The research team had already determined in 2013 that whale earwax can provide information about sexual maturity or the animals’ exposure to pollutants.