Learning the story of a blue whale's life is easy, if a little disgusting: It's all in the earwax. It forms a tube in the animal's long ear canal, "kind of like a candle that's been roughed up a bit," a researcher tells NPR. When extracted, the substance can tell you approximately how old the whale is, when it was feeding and migrating, and more—including when it was exposed to certain pollutants, the Smithsonian reports.
Scientists examined the 10-inch "earplug" of a blue whale that washed up on California's coast in 2007. The animal had been exposed to mercury, DDT, and other pollutants, both from its mother and likely through the krill it ate. "DDT was banned 30 years before this animal was born, but it was still exposed to DDT over its entire lifetime," says the researcher. In the future, experts may learn more from decades-old earwax samples kept in museums. "For a majority of the species on the planet, lifetime profiles such as these are simply unattainable," the scientists note. But they believe similar research is possible in several species of baleen whale.