Surviving in the Dark Using Biosonar by Carol Turner

Dolphin

Most of the ocean is a dark and noisy place. The top 200 meters (656 feet) is known as the sunlight or photic zone, and it’s full of marine life. Despite the name, it’s often difficult to see more than a few feet away in this zone, even during daylight hours.

Below the sunlight zone lies the vast majority of the ocean — a murky, mysterious world about which we know little. The ocean layer from 200 meters down to 1000 meters (3280 feet) is aptly named the twilight zone, and below that is the midnight zone, a very dark place indeed. Further down in some spots are trench zones, created by the separation of tectonic plates. These extremely deep environments host a diverse array of creatures who feed either on each other or on organic matter that trickles down from the upper layers.

Water is an efficient conductor of sound, and all zones are filled with the noise of vessel traffic, booms and explosions from oil and gas exploration, sonar waves from naval training exercises, and vocalizations from marine life. In the absence of light, many ocean creatures use sound to navigate, communicate, find food, and avoid being eaten. A sub-group of these processes is called echolocation, or biosonar.

What is biosonar?

Biosonar is the innate ability to issue a sound and identify objects by processing the sound’s echo. The sound itself is usually a click or a pulse of some sort. Using a layer of fat in their forehead area, called a melon, these marine mammals can control and focus the sound. A similar mechanism, for example, might be the way you mess around with the fancy showerhead installed in your bathroom, switching instantly from a wide gentle spray to a narrow, more powerful spray.

The echo is then processed by the animal using fat and tissue around the jaw and inner ear. The response time of the echo can indicate how far away an object is, and the animal can glean the size and shape of the object, its speed and direction, and even its composition.

These extremely sophisticated biosonar systems have been identified and studied in toothed whales or odontocetes. Odontocetes include orcas, sperm whales, belugas, narwhals, beaked whales, several species of porpoise, and many other species of whales and dolphins (72 in total).

Darlene Ketten, Ph.D., a marine research scientist at Woods Hole Oceanographic Institution in Massachusetts, has used a CT scanner to create three-dimensional models of the inner ears of 16 species of toothed whales. The models revealed that each species “had evolved its own specialized auditory structures to hear specified frequencies and wavelengths, depending on the demands of its respective hunting environment.” (Source: War of the Whales, by Joshua Horwitz).

Early studies of biosonar involved experiments with bats, which navigate using high frequency sound that can’t be heard by the human ear. It wasn’t until technology provided the tools to pick up high frequency sounds that scientists were able to figure out how bats could avoid obstacles in the pitch black. In the 1940s, a biology student named Donald Griffin came up with the term “echolocation,” and his bat experiments caught the attention of radar and sonar specialists at the U.S. Navy. The Office of Naval Research (ONR) began funding research into cetacean biosonar, with the aim of supporting its antisubmarine warfare efforts.

Most studies of biosonar in marine mammals have involved dolphins. Dolphins have demonstrated amazing detection abilities, such as locating an object the size of a tangerine from 300 feet. They can differentiate between metal types and can find buried objects. They are so adept at locating and identifying objects that several navies have used them in hunts for underwater mines and other missions.

Most research funded by the Navy

In the United States, most biosonar research is still being funded, at least in part, by the U.S. Navy or other agencies of the federal government. Ironically, the Navy’s use of active sonar in war games can distress and harm odontocetes. Since the early 1990s, more than a dozen mass strandings have been recorded shortly after naval training exercises using active sonar technologies. The animals showed symptoms such as bleeding in the eyes and ears, and bubbles in their organs. Some survive and can be guided back out to sea but others are found dead or die soon after. The Navy has been quick to send investigative teams to those scenes and now employs observers during exercises. If they spot animals in the vicinity, they abort sonar activities. Obviously, however, their primary aim is to continue their training exercises.

Locally, a group of advocates has developed a growing network of hydrophones (passive sonar) that monitors the vocalizations of orcas and other marine mammals in the Salish Sea. Check out www.orcasound.net.

Next month: The story of mass whale strandings after naval exercises and how ocean conservationists and the U.S. Supreme Court got involved, as described in Joshua Horwitz’s excellent book, War of the Whales.

Leave a Comment

Your email address will not be published. Required fields are marked *

*