Ear wax in whales reveals information about their levels of human-induced stress

In November 2018, Nature Communications published several articles about the new work of Steven Trumble and Sascha Usenko. The two scientists from the Baylor University discovered that ear wax plugs in whales store memories about events in world history. Additionally, the animals react to all kinds of stress, starting with pregnancy and ending with whaling and sea battles.

Just like in humans and other mammals, whales’ ears produce ear wax. According to Usenko, it is a defence mechanism. This way, the organism discharges any unneeded waste through the ear. As whales were evolving from land creatures to marine, their outer ear canal was isolated from the outside environment in order to prevent cold water from getting too deep inside. As a result, the wax was ‘trapped’ and started to accumulate and form a plug. A wax plug of a baleen whale can be as long as 25 cm and can look like a piece of wood or the horn of a goat. It is accumulated in a whale’s ear throughout its life, which can be as long as 100 years.

The ear wax is secreted from the body together with various substances, such as the hormone cortisol. Cortisol can illuminate the levels of stress in the whale. The plug is accumulated in layers. A new ‘portion’ of ear wax with a new mix of various hormones is added to the previous layer of wax every day. This way, the plug can reveal how the levels of stress changed in the whale throughout its life and as a result, it functions as a register of the whale’s chemical biography.

The research revealed that the whales are greatly impacted not only by what happens among them but by human activity as well. For example, the more active whaling is, the faster the whales’ population decreases. The noisier the waters get, the higher the levels of cortisol in whales are. When whaling reached its peak in the 1960s, the overall level of cortisol in the studied whales rocketed. Consequently, after whaling was prohibited in the 1970s, the levels of cortisol started dropping gradually by 6.4 per cent per year. During World War Two the whales were not killed as much, but there were a lot of sea battles and a lot of bombs, which created noise. Because whales can hear across thousands of kilometres, they could feel the stress from bombings even if they were far away from the bombs themselves. During that war, the overall level of cortisol in studied whales increased by 10 per cent.

The research is unique and important in several ways. Before Usenko and Trumble, the ear wax plugs were never studied. The research illustrates how read-outs such as these can be useful in evaluating human impact. This, in turn, allows to predict and evaluate to what extent whales can adapt to a fast-changing environment.

Image credit: Navin75 via Flickr

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