Pesticides. Flame retardants. Mercury. These are just a few of the chemicals that whales are exposed to as they swim about in the ocean. How do we know this? Their ears told us.

Specifically, their ear wax. It turns out that blue whales are constantly generating new layers of wax in their ear canals. This builds up into an "earplug" that last the whale's entire lifetime. And similar to the rings inside a tree's trunk, scientists have long used the waxy layers of these plugs to determine a whale's age.

But two scientists from Baylor University wondered what clues besides age the earplugs would provide. They theorized that the wax would also provide a catalog of two types of chemicals: the hormones the whales naturally secreted and the chemicals they were exposed to in the ocean.

They got a chance to test their theory after a 21-meter blue whale was struck and killed by a ship in 2007. The whale's carcass revealed an earplug with 24 discrete layers. With each later (called a laminae) taking about six months to form, that put the male whale at about 12 years old (probably just around the age of sexual maturity). Here's what it looked like:

The earplug, originally collected by the Santa Barbara Museum of Natural History, provided the Baylor scientists — professors Stephen Trumble and Sascha Usenko — the opportunity to test for what it contained. They found a lot.

Some of the substances in the wax belonged there. The scientists found varying levels of cortisol, for example. The whale produced cortisol at a fairly regular level for its first 126 months, but levels shot up 800 percent after that. This was shortly after testosterone levels also increased, indicating that the young whale had reached sexual maturity at roughly between 114 and 126 months. "Our research was able to improve upon estimates of sexual maturity for blue whales," Trumble said in a press release. "Previous estimates provided a 10-year range of maturity and we have been able to pinpoint exactly when the whale in the study hit sexual maturity. Our research was able to shed new light on the life cycle of whales."

More troubling was the presence of 42 chemicals that did not belong in the whale's body: pesticides (including DDT, which was banned in the U.S. back in 1972), 15 different types of PCBs, mercury and other substances. Many of these chemicals were present in the very first layer of wax, indicating they may have been transmitted to the young whale through its mothers' milk.

This new information, which Usenko calls "a new field of research," provides clues into how humans affect marine life. "You have this 100-year-old question: How are we impacting these animals? There is ship traffic, environmental noise, climate change and contaminants. Now, we are able to provide definitive answers by analyzing whale earwax plugs," he said.

The scientists say researchers could now go back to examine other earplugs in museum collections to see what evidence they yield so we can see how environmental contaminants have affected whales over the past six or seven decades.

Related on MNN: