By Roberta Attanasio, IEAM Blog Editor
Earthworms, swallows, bats, river otters, dolphins, and our pets – these are some of the animal sentinels alerting us to the presence of contaminants in our environment. Now, there is one more species that can be called sentinel – the blue whale.
This sentinel is sort of special, and it has made the news just a few months ago – all because of the earplug, a tell-tale build-up of wax that accumulates in the ear canal and can be over a foot long. By examining the earplug, a lifetime story of exposure to stress and pollutants can be unveiled along with the whale’s age – a finding recently published in Proceedings of the National Academy of Sciences. The article, “Blue whale earplug reveals lifetime contaminant exposure and hormone profiles,” is the result of a collaborative study that included researchers from Baylor University, the Santa Barbara Museum of Natural History and the Smithsonian Institution.
The earplug is sealed off from the external environment, and remains in place for the entire whale’s life. It is similar to a tree ring – the earwax is continuously produced, forming alternating dark and light layers that represent seasons of feeding or migration, with each layer corresponding to six months of the whale’s life. A few years ago, the researchers collected the earplug from a dead male blue whale that washed ashore on a California beach after being struck and killed by a ship.
By analyzing the earplug layer by layer using novel analytical methods, the researchers found presence of testosterone, the stress hormone cortisol, mercury, and organic contaminants such as pesticides and flame retardants.
They found that, at age 10, the whale became sexually mature, as indicated by increased testosterone levels that were accompanied by a spike in cortisol. The researchers believe that the spike might have been caused by breeding competition or by the social bonds the whale formed at that time. In addition, they discovered that levels of stress hormones doubled over the whale’s life.
Significant exposure to anthropogenic chemicals such as dichlorodiphenyltrichloroethanes (DDT), chlordanes, polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers occurred in the early, most vulnerable stages of the whale’s life, most likely when the whale was still nursing. The earplug analysis also showed evidence that the whale had been exposed to mercury two times, first at around age 5 and then again at around age 10.
Most pollutants could also be identified in the blubber, confirming that both blubber and earwax can be used to reliably identify different types of exposures – however, only the earplug can be used to track the timing of exposure.
These studies will be extended by analyzing earplugs collected from dead beached whales all around the world – indeed, many other species of baleen whales continuously accumulate layers of wax similarly to blue whales. In addition, the researchers plan to go back in time and analyze archived museum earplug samples that were harvested in the 1950s.