We all get earwax, even whales. But whales don’t have fingers or doctors with syringes and so their earwax stays there, building up, year after year. Yuk, you might think. But we can learn a lot from whales’ earwax.

The wax accumulates into a long plug which is trapped inside the whale’s head. It builds up in layers in the ear canal, adding one layer every six months or so. Considering a whale can live 200 years, that can be a lot earwax. The plugs can be many centimetres long. (But the whole whale can be nearly 30 metres long, so there’s plenty of space inside.)

This scientists can assess the age of a whale by the number of layers in its earplugs—just like counting rings in a tree trunk to judge the age of a tree. But they can tell more than just how old the whale was. The earplug contains evidence of chemicals the whale was exposed to from pollutants to hormones produced by its own body. The earplug of a female whale will have hormonal signatures for all her pregnancies, for instance. Cortisol, a hormone produced when an animal is stressed, is also preserved in earplugs, so we can learn about stressful times in the whale’s life (but not what caused the stress). Pollutants might indicate where it had been feeding, The whale’s life story can be pieced together from its ear wax.

Sadly, the earplug can only be removed until after the whale has died. It’s not just a matter of ferretting around in its ear—scientists have to cut the skull open to get to the ear plug. There are earplugs collected from dead whales in the historic collections of museums around the world, most so far untouched and ripe for analysis.

You can read more about whale earplugs at National Geographic and Live Science. You can see whale earplugs at the Whales exhibition at the Natural History Museum in London until 28 February 2018.