Whales had more cortisol in their bodies while the global whaling industry was at its peak.

A new study, published in Nature Communications, showed that fin whales, blue whales and humpback whales experienced a physical stress response when they were hunted on an industrial scale. Earwax plugs form continually over the lifespan of baleen whales and are made from waxy material deposited in the ear canal. They contain fats and proteins that can reveal the type and quantities of hormones inside a whale's body at various points in its life. What can we learn from whale earwax? In this study, led by researchers at Baylor University in Texas, the team were particularly interested in cortisol, a hormone that mammals produce at times of stress. Researchers examined whale earplugs from the Museum's reference collection, the collection of the Smithsonian National Museum of Natural History in the USA and samples taken of recent strandings.

The scientists created lifetime 'stress profiles' of 20 whales from three species: humpback, fin and blue whales. These were some of the most hunted species in the twentieth century, when the global whaling industry boomed. The profiles resulted in a 146-year dataset, measuring the stress responses of animals from 1870 to 2016. The whales were particularly stressed during the 1920s and 1930s, at a time of extremely high whale harvests in the northern hemisphere. About 50,000 fin, humpback and blue whales were killed in the 1930s. Extracting earplugs Whale earwax can reveal a whale's age, the environmental pollutants it has been exposed to and the hormonal responses throughout its life. Richard Sabin, curator of marine mammals at the Museum, helped the researchers to access the historic whale collections held in London, which include the precious earplugs. He says, 'In the middle of the twentieth century, Natural History Museum scientists showed that baleen whale earplugs could be used to determine how old an animal was when it died. 'This was an important step forward, as it allowed more accurate estimates to be made of the ages of animals taken by commercial whaling activities. Earplugs had been of interest to scientists since the start of the century. 'Fortunately, very early examples exist in the collections at the Museum, and it was one of these - from a North Atlantic fin whale estimated to have lived from about 1870 to 1909 - which became the oldest earplug examined as part of this latest research. This allowed the project to extend its study back into the nineteenth century.' The Museum contributed eight earplugs from its research collection for this study, covering the period from 1870 to 1956. Richard adds, 'This research really shows the power and significance of archived natural history collections. Each year, new analytical techniques allow more information to be extracted from the often centuries-old preserved remains of museum specimens.'

The underwater war It isn't just whaling that has had an impact on cetaceans. Baleen whales have also been subjected to other anthropogenic disturbances over the last 100 years, including shipping traffic and increased fishing. The study also showed that the Second World War (1939-1945) provoked a stress response from the animals. Commercial hunting had slowed dramatically at the time, but researchers found that the cortisol levels in whales kept increasing. The data suggests that naval battles and underwater bombs, for example, are just as difficult for baleen whales to deal with. Wartime activities may have simply replaced the whaling industry as a source of stress. Baleen whales may be particularly affected during a period of global conflict because they migrate so far around the world during their lifetimes, increasing the chances of them coming into contact with battles at sea.