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Aah, the things we humans do for science. Like studying a giant (25.4cm!) column of earwax extracted from the skull of the biggest animal on earth – the blue whale. But why, you ask. Well, since whales are not particularly diligent about cleaning their ears (by which we mean they never do), the gunk that accumulates in there over a lifetime becomes a waxy transcript of information scientists can use to learn more about the state of these endangered animals and their habitats – including clues about age, hormone levels and exposure to harmful contaminants in the oceans. 

To unlock these secrets sealed in wax, scientists from Baylor University in Texas dissected an 'earplug' retrieved back in 2007 from a 12-year-old blue whale that had died in a collision with a ship and washed up on shore. But what did the wax tell them that other parts of the whale, say blubber, couldn't? The key difference is that the earplug is made up of layers that accumulate one on top of the other (sort of like rings on a tree) throughout the whale's lifetime. Each layer corresponds to a period of the animal's life – light layers form during periods of feeding and alternate roughly every six months with darker ones formed during periods of migration. This chronological archive of wax helped the researchers to not only accurately estimate the animal's age at the time of its death, but also revealed a lifetime's fluctuations in the whale's hormone levels as well as patterns of exposure to harmful pollutants that left their mark in the wax. "Currently, obtaining lifetime chemical profiles from birth to death is extremely rare and difficult for most of Earth’s animals," the researchers note. 

2013 09 17 Whales Life Story In Earwax 01
Image credit: Trumble et al, 2013. PNAS

As part of the study, which was published recently in the Proceedings of the National Academy of Sciences, researchers tested the wax for hormones like cortisol (stress hormone) and testosterone (developmental hormone), as well as various pollutants that are known to persist in the environment. They found traces of mercury, flame retardants, several pesticides and even DDT (which was banned in 1972), and – crucially – were able to estimate roughly when in its lifecycle the whale was exposed to these by measuring their concentrations in the layers of wax. Their findings show that some of the pollutants accumulated in the wax throughout the whale's lifetime, while others, like the now-banned DDT, were likely to have been transferred to the whale from its mother. In the image above, the position of the ear plug in the whale's skull is shown in diagram A, while the actual plug is shown at B (with plug cross-sections at C and D).

Tracking lifetime exposure to potentially harmful pollutants is important not only because blue whales are in serious decline (global numbers stand at just 3-11% of populations 100 years ago, according to the IUCN), but also because the animals are a good indicator species – which means studying their wellbeing in a given environment helps researchers to gauge the health of the ecosystem as a whole. "We anticipate this technique will fundamentally transform our ability to assess human impact on these environmental sentinels and their ecosystems," say the researchers.