Last week we discussed the general topic of Whales and their hearing, but there is another chapter to this story that combines whales, their hearing and Audiology. This week Hearing International looks at the Blue Whale, the largest living animal in the world today. These mammals weigh up to 190 tons (380,000 pounds) and measure up to 89 feet (27 meters) long. Despite their size, whales are difficult to study, both because of their ocean-roaming ways and their small number, resulting from commercial whaling.
In the 19th century whaling was one of the most prominent businesses in America. Hundreds of ships set out from ports, mostly in New England, and roamed the globe, bringing back whale oil and other products made from whales. While it was American ships that created a highly organized industry, the hunting of whales dates back thousands of years. And throughout recorded history, the enormous mammals have been highly prized for the products that can be harvested from them.
One prized product was oil made from whale’s blubber that was used both for lighting and for lubrication. Whale bones were used to make a variety of useful products. A typical American household of the 19th century might contain several items manufactured from whale products, such as candles and corsets made with whalebone stays.
It is estimated that the population of blue whales in the 19th century was about 275,000; by the 1990s their number had plummeted to about 1,000. The population has been slow to recover since each whale is carried for 7 months to gestation and females give birth only about every 3 years. Hunting blue whales is outlawed in most of the world and the population of the species is recovering to some degree, Now, their estimated population is about 15,000. The International Whaling Commission estimates the rate of Blue Whale population increase at 8.2% per year from 1978 to 2004. (Click on the Blue Whale picture for a video)
The Ears of Whales
The tiny human cochlea is located in a bony structure in the skull called “the bulla of the temporal bone.” The temporal bone is considered to be the hardest bone in the body. Almost all other mammals share this structure (the bulla), as a protective shield around the inner ear. The human bulla is quite small, about the size of a large marble, but that of a whale’s (and especially, a blue whale) is enormous, reaching the size of a Nerf football. The real story here is not the anatomy or physiology of the whale ear, nor does it have anything to do with comparative anatomy with humans. The real story is Whale cerumen.
Almost everything about blue whales is big, except their pinhole ear canals. As in humans whale cerumen builds up over time. But for whales with only a pinhole of an ear canal it is difficult to get rid of cerumen, or earwax. Thus it builds up over an animal’s entire lifetime. Over time it forms a stick like a crayon or a candle, but waxy, roughed up, fibrous, with a familiar yellow-brown coloration. These “plugs” from whale ears offer detailed information about the life of the whale, where they have been, and the specifics of their ocean environment. Researchers now know that examining these plugs tells them about a whale’s lifetime exposure to pollution. Alternating layers of dark and light in the plugs correlate to seasons of feeding or migration. So the plugs have also been used to determine a whale’s age.
In the latest study, scientists analyzed an earplug from an endangered blue whale killed by a ship near California. They found that levels of stress hormones doubled over the whale’s life.
This study by Trumble and Usenko (2013) in the Proceedings of the National Academy of Sciences also found evidence that the whale had been exposed to pesticides such as DDT, with the highest levels during the whale’s first six months of life. The whale was likely exposed to the pesticides in its mother’s milk. They also found a couple of peaks of exposure to mercury. Historically, scientists have used whale blubber to determine hormone and chemical exposure, but that method only provides information over short, finite periods of time and can be difficult to obtain and cost-prohibitive. With these new data scientists are now able to assess the human impact on individual whales and multiple generations, as well as on marine ecosystems. Trumble and Usenko ask a 100-year-old question: How are we impacting these animals through our ship traffic, environmental noise, climate change, and contaminants? Now, they are able to provide definitive answers by analyzing whale earwax plugs.
In addition to using plugs of whale cerumen to determine whales’ lifetime exposure to chemicals and environmental pollutants, the plugs also provide time-specific biological information about whales. Trumble and Usenko’s research was able to improve upon estimates of sexual maturity for blue whales. Previous estimates provided a 10-year range of maturity, but, studying the plugs, they were able to pinpoint exactly when the whale in the study hit sexual maturity, shedding new insight into the life cycle of whales.
Additionally, their new methodology will enable them to gain a deeper understanding of whales today and those that lived decades ago by analyzing archived museum earplug samples that were harvested in the 1950s. Critical issues such as the effects of pollution, use of sonar in the oceans and the introduction of specific chemicals and pesticides in the environment over long periods of time. (Click on the Baylor picture for an interesting video) There is so much additional information that can be mined from studying earplugs. So, much of our information about our world is derived…….. From the Ears of Whales!