Baleen Whale Cortisol Levels Reveal a Physiological Response to 20th Century Whaling

Today I learned: whaling in the 20th century indirectly stressed out many different species of whales. A group of scientists looked at the earplugs of three different types of baleen whales: fin, humpback, and blue. A whale earplug is a collection of ear wax that has alternating lines that tell the age of a whale (like tree rings do) and store both endogenous (i.e. hormones) or exogenous (i.e. pollutants) chemicals. Because of this, they could measure the amount of cortisol, a stress hormone, in each whale during many different years. They found that when whaling numbers peaked, there was also a peak in the average cortisol levels. Interestingly, when whaling decreased during World War II, they still saw slightly higher cortisol levels, which suggests that wartime noises (i.e. submarines, bombing/explosions, planes, etc.) also increased the stress in the whales. The sharp drop in cortisol levels in the 1970s was connected to the start of the Marine Mammal Protection Act that put a moratorium on whaling. However, cortisol levels have been slowing increasing since then, indicating that other stressors are present (i.e. increased sea temperatures, over-fishing, and noise).

Graph showing that when whaling numbers peak, there is a peak in cortisol levels

The graph shows that from 1900-2000, when whaling numbers (blue) increased, there was also an increase in cortisol levels (black). Highlighted in grey is a period corresponding to WWII where the whaling numbers were low, but cortisol was still high. Also shown is a red line to indicate when the Marine Mammal Protection Act went into effect in 1972 to emphasize the sharp drop in cortisol levels that followed.

Reference: Trumble, Stephen J., et al. “Baleen Whale Cortisol Levels Reveal a Physiological Response to 20th Century Whaling.” Nature Communications, vol. 9, no. 1, 2018, doi:10.1038/s41467-018-07044-w.