Giant gobs of earwax tell a whale of a tale

A life history of hormones and pollutants preserved in a blue whale’s ear wax.

The largest animal on Earth, even a young blue whale's ear wax plug was already 25cm across.

Tree rings can paint a telling picture of an ancient redwood or an old maple: how old it is, when environmental conditions were favorable, and when the tree faced a tougher time. Scientists have now found an even richer record of an individual’s history, and it comes from an unlikely place: a whale’s ear. In this week’s issue of PNAS, a group of researchers reports the wealth of information that can be found buried in the earwax of a blue whale.

Throughout their lives, baleen whales develop wax in their ears, just like humans do. Every six months, a new layer, or “lamina,” is deposited. The laminae have regular color differences due to annual feeding and migration patterns, allowing researchers to distinguish each and every six-month period in the whales’ lives. Since whales don’t clean their ears like we do, their entire history is recorded in the wax plug by the time they die. These earplugs have long been used by scientists to figure out the age of whales, but it turns out the wax can tell us much more about whales’ lives than just how old they are.

A whale’s blubber tends to accumulate various compounds that the whale is exposed to, both ones that the animal itself produces (such as hormones), and ones they come into contact with in the environment (such as pollutants). The researchers hypothesized that earwax, which is also a fatty deposit, might also harbor these same compounds. So when a nearly 70-foot male blue whale was struck by a boat and ended up dead on a beach in Santa Barbara, they seized the opportunity and grabbed its earplug.

The 10-inch earplug had 24 laminae, indicating that the whale was 12 years old. The researchers separated and analyzed each layer individually to measure levels of cortisol, testosterone, and various environmental pollutants encountered by the whale during its life.

Based on the peak testosterone level, the researchers determined that the whale likely reached sexual maturity between 113 and 126 months. This estimate agrees with—and even improves on the accuracy of—other ways to assess sexual maturity in whales, such as the ratio between their age and length.

Just after the whale reached sexual maturity, the earplug shows that the animal experienced peak levels of cortisol, a stress hormone. It’s likely that competition with other males triggered high levels of stress in the whale during its first breeding season. The whale’s overall cortisol levels doubled over its lifetime, although the researchers aren’t sure what to ascribe this to; it could be due to behavioral factors such as weaning, ecological factors like decreasing food availability, or environmental factors such as intensifying boat noise.

Some of the most telling results came from analyses of persistent organic pollutants, such as DDT, that ended up in the whale’s earplug. The researchers found measurable levels of 20 different pesticides used in the last several decades. Somewhat surprisingly, the highest concentrations of these pesticides were encountered during the first six months of the whale’s life; these peaks may be due to what the researchers called “maternal transfer.” In other words, pesticide residue could have been transferred to the young whale from its mother via her milk, which suggests that the mother must have had significant levels of these chemicals.

Mercury, too, was present in the earplug. There were two distinct peaks, one between 60 and 72 months, and another between 120 and 126 months. Although the researchers can only speculate about these spikes, it’s possible that the whale encountered high levels of mercury along regular migration routes.

Obviously, there’s a major drawback of this research: it has a sample size of just one whale. But it’s a proof-of-concept study more than anything else. It shows that it is possible to measure much more than just a whale’s age with its earwax. In fact, examining earwax may be more accurate than other techniques used to identify certain compounds in marine mammals. Although blubber does accumulate various hormones and pollutants, it’s impossible to use blubber to pinpoint when a whale encountered these compounds; the distinct laminae of the earplug tell a much more detailed story of the whale’s life.

While it’s an exciting avenue for further research, many of the results here are well-educated guesses and well-reasoned speculation. Researchers will need the earplugs of many more whales to flesh out the possibilities raised by this study. But when we do get this information, it could produce an intimate and telling portrait of some of the most enigmatic and hard-to-study animals on the planet.

PNAS, 2013. DOI: 10.1073/pnas.1311418110 (About DOIs).