This is the secret to discovering a whale's life history

Whales can live longer than humans do, but often there is only one way for researchers to find out the age of a particular whale. They have to go deep inside the whales' ears.

Like other mammals, whales produce earwax, which stays in their ears for their whole lives. Research now shows these waxy buildups hold clues to a whale's entire chemical and hormonal history, and can help reveal how the lives of these giants have changed over the last century.

Starting from birth, many baleen whale species accumulate loads of earwax that develop into enormous wads of lipids (fatty acids), waxes and keratin known as "earplugs." Unlike the human analog, however, these earplugs don't block out sound. "If anything, the leading science suggests that earplugs might help the whales funnel external sound to the eardrum," Baylor University biologist Stephen Trumble told io9.

The earplugs form in alternating light- and dark-colored layers called laminae. Importantly, the laminae grow in regular intervals, which may be different for each whale species — humpback whales, for example, seem to produce a new lamina each year.

The laminae can be used as a kind of tree ring for the whale, revealing both how long it lived and what its environment was like. "As you go toward the center of the earplug, you go back in time," said Sascha Usenko, a Baylor environmental scientist. In fact, scientists have used the earplugs to estimate whale ages — postmortem, as they cannot be extracted while the animal is alive — for decades, Trumble added.

Earplugs and the Environment

Over the last century, the environment has changed drastically for whales (and other marine mammals), as they must now deal with increased anthropogenic influences, including environmental noise, ship strikes and pollution. A long-standing question is how the animals are dealing with these changes. Are the whales now exposed to more toxic chemicals throughout their lives? And are they more stressed than they were in the past?

To answer these questions, scientists often turn to biopsies. "Anthropogenic chemicals love lipid-rich matrices," Usenko explained. "You see them in oils, in fat and in blubber." The same goes for hormones. However, to see how whales' chemical and hormone profiles change over time, you'd have to repeatedly take samples from them. And if you come across an unknown whale that is already dead, biopsies won't tell you much about its history.

Since whales' earplugs also have waxy, lipid matrices, Usenko, Trumble and their colleagues had a good hunch that the earplugs would also store contaminants and hormones. "They should be preserved in there and trapped as the next layer builds," Usenko said. "This would allow us to reconstruct profiles from birth to death."

To test this, the team extracted a 10-inch (25.4 cm) earplug from a 70-foot (21.2 m) male blue whale that died off the coast of Santa Barbara, California from a ship strike in 2007. From the laminae, they determined that the whale was about 12.5 years old, which matched with age estimates based on length.

This is the secret to discovering a whale's life history

Courtesy of PNAS: "Illustration of a blue whale earplug. (A) Schematic diagram showing the location of the earplug within the ear canal: (a) whale skull, (b) tympanic bulla, (c) pars flaccida/tympanic membrane (“glove finger”), (d) cerumen (earplug), (e) external auditory meatus, (f) auditory canal, (g) muscle tissue, (h) blubber tissue, and (i) epidermis. (B) Extracted blue whale earplug; total length 25.4 cm. (C) Earplug longitudinal cross-section. (D) View (20×) of earplug cross- section showing discrete laminae."

The researchers then analyzed each layer for mercury and 42 "persistent organic pollutants," which included pesticides, polychlorinated biphenyls (PCBs, often used as coolant fluids) and polybrominated diphenyl ethers (PBDEs, often used as flame retardants). They also measured the laminae's concentrations of different hormones, including testosterone and the stress hormone cortisol.

Evidence of Stress and Puberty

Based on a major testosterone spike, the researchers could tell that the animal reached puberty between 114 and 126 months old. Previous estimates based on body length and other methods suggested blue whales reach sexual maturity between 60 and 180 months old. "So we were right there in the middle," Trumble said.

Measurements also showed that the animal's cortisol levels doubled over its lifetime, and that the whale had a few peaks in stress, including one during puberty. The increase in cortisol could be associated with a number of factors, including food availability, migration and changes in social status, the researchers said.

Looking at the chemical profiles, the scientists discovered that 96 percent of the whale's "organic burden" was made up of four historic-use pesticides (including DDT) and their metabolites, and one PCB. They also found that the whale received a large amount of contaminants — about a fifth of its total lifetime burden — from its mother during gestation and nursing.

Once on its own, the whale continued to take in pollutants from its food, and the mean concentrations of the contaminants increased over time. The overall increase in contaminant burden and cortisol concentrations may be correlated, but the researchers say it's impossible to tell with only a single animal to go off of.

The whale's mercury levels spiked twice in its life — the team believes these peaks were likely due to regional (California) environmental or anthropogenic increases in mercury.

All in all, the study is a proof of concept. The research really highlights how scientists can use whale earplugs to look at the human impact on these "environmental sentinels," Usenko said. "It's a new field of research — the sky is the limit," he said.

Trumble says researchers could go back and analyze archived samples of earplugs, which will then help answer questions about excess noise and pollution. "If one was collected in the 60s, and the whale was 40 years old, then we have a record from the 1920s to the 1960s," he said. "We could see how animals responded to the environment then and compare that with animals today."

If it turns out that whales and other marine mammals are in greater danger now than in the past due to human influence, then people can try to take steps to mitigate the problem. Of course, it's ultimately up to policy makers to decide what to do. "We can only provide the science," Usenko said.

Check out the research in the journal PNAS.

Top image via John Calambokidis Ph.D., Cascadia Research.