Two biologists at Baylor University in Texas, Sascha Usenko and Stephen Trumble, and colleagues have dissected a giant gob of earwax from a blue whale to measure pollutants to which the whale was exposed.
The earwax (known as an earplug) can also be used to measure levels of stress and sexual maturity.
It sounds odd to go spelunking for information on chemical contaminants in the inner ear of a dead whale, but Usenko and Trumble say that the method yields more data than previous methods, such as taking samples of whale blubber.
“We could get measurements of what chemicals they were exposed to and the hormones in their body at six-month increments,” Usenko said. (Take a blue whale quiz.)
Adds Trumble: “We can not only get measurements of different chemical exposures in different time periods, but also from different places around the world.”
Enter the Earwax
Blue whales are the largest living animals today. They weigh up to 190 tons (380,000 pounds) and measure up to 89 feet (27 meters) long. Despite their size, whales, which roam the ocean, are difficult to study. Commercial whaling in the early 19th century caused their numbers to plummet, but they haven’t made much of a comeback. (Watch a blue whale video.)
One reason may be organic pollutants that accumulate in the whale’s fatty blubber, though the specifics are not understood as yet.
Researchers can take a sample of blubber, but that method only reveals the mammal’s exposure to contaminants at one point in time. Usenko and Trumble wanted to know how those levels changed over the span of the whale’s lifetime. The problem was identifying something fatty like blubber that accumulated over time and allowed scientists to get a historical picture.
Scientists and museums had collected the waxy, fatty deposits of earwax from the ear canals of dead blue whales and other species for hundreds of years. The earplugs were displayed or (more likely) placed into storage for later study. Usenko and Trumble thought this might provide some answers.
“Historically, people had used earplugs from certain baleen whale species to help estimate the age of the whale. You can count the layers [of earwax] like tree rings and basically go back in time. Once we knew this and combined it with the knowledge that contaminants like to accumulate in lipid-rich matrices like the earplug, we could start asking whether earplugs could be used to make these measurements,” said Usenko, whose study appears today in the journal Proceedings of the National Academy of Sciences.
When a blue whale was struck and killed by a boat in the waters off Santa Monica, California, researchers harvested a 10-inch (25-centimeter) long piece of earwax. But even the world’s largest Q-tip is of no use in removing the earplug.
“You can only get to the earplug after the animal has died,” Trumble said, by cutting into the skull with a large, sharp knife.
Analysis of the earplug revealed that the whale deposited one layer roughly every six months. The dead whale had 24 layers of earwax and was estimated to be around 12 years old, which corresponded to other estimates of its age. (Explore a National Geographic blue whale interactive.)
In dissecting the layers of earwax, the scientists discovered that during its first year of life the blue whale had a spike in the concentrations of organic pollutants (mainly pesticides like chlordane and PCBs), which they believe occurred during nursing. The high fat content of blue whale milk (scientists estimate that the milk contains between 30 to 50 percent fat) further concentrates these compounds.
Usenko and Trumble also noticed a spike in mercury in the earplug shortly after the blue whale’s fifth birthday, although they don’t yet know the significance of the spike.
Lastly, the researchers could track stress levels and reproductive maturity over the course of the whale’s life by measuring concentrations of the stress hormone cortisol and testosterone. The whale showed a massive spike in testosterone at ten years of age, corresponding with the animal’s sexual maturity. Cortisol levels spiked in the next layer, probably due to breeding and social competition.
The study shows that earplugs can potentially allow researchers to track a whale’s exposure to different chemicals over time, as well as some of the physiological changes exposure may have caused, the researchers say.
“We can go back in time and determine when the animal was exposed. If it’s undergoing development or sexual maturity or something like that, exposure can be really important,” Usenko said.
The hundreds of well-preserved samples in museum collections will enable scientists to gather historical data, as well.
Hopefully, knowledge gained from measuring the effects of contaminants on whale health will lead, in time, to a rebound in population levels.