A whale's earwax works like tree rings, recording its whole life story
Unlike most mammals, baleen whales never shed their earwax — it builds up in layers over the whale's entire life, sometimes 50 to 100 years. Slicing a plug open lets scientists read hormone levels layer by layer, and a 2018 study used plugs spanning 1870 to 2016 to show whale stress hormones spiking during peak 20th-century whaling and staying high through World War II.