June 12, 2013
High-Pitched Echolocation Helps Harbor Porpoises Avoid Killer Whales
Lee Rannals for redOrbit.com — Your Universe Online
Researchers from the University of Southern Denmark (SDU) have determined why harbour porpoises are doing so well in coastal and busy waters.The team wrote in the journal Frontiers in Physiology that these animals are able to thrive through the Northern Hemisphere due to their sophisticated echolocation abilities. Coastal waters like the ones harbor porpoises live in can be challenging for whales due to the risk of beaching and being caught in a fisherman's net. However, these waters also offer plenty of fish, making it very attractive for porpoises.
This whale species is extremely skilled at navigating, locating prey and avoiding hazards near the coast due to its echolocation. However, what sets them apart from other whales that have this ability is they emit a constant stream of sonar clicks that are higher in frequency than those of any other toothed whales.
Porpoise clicks only last for a hundred-millionth of a second and are about 130 kHz, which is about 110 kHz higher than a human can hear and 70 kHz higher than a dog can hear. The researchers believe that these whales make their clicks so high due to the killer whales.
"Over millions of years the porpoise has evolved its ability to emit very high frequency click sounds that killer whales have difficulty hearing since they cannot hear sounds that are much higher than about 100 kHz. Killer whale hearing is best at around 20 kHz, so it is hard for them to detect a porpoise," explains Lee Miller, from the Institute of Biology at SDU.
The ancestor of whales emerged about 50 million years ago, while the first toothed whales began using echolocation 30 million years ago. However, the researchers say that 5 to 10 million years ago, when the killer whale emerged, evolution began to favor the toothed whales that could avoid being captured by killer whales.
"One way to avoid being eaten was to emit echolocation sounds that were difficult for killer whales to detect — thus an ability favored by evolution," notes Lee Miller and colleague Magnus Wahlberg in their research article.
Lee Miller said the porpoises' 130-kHz frequency also works effectively because it is at these frequencies that natural noises in the ocean is the lowest.
"Thus porpoises can better hear the echoes from objects and prey when using these clicking sounds," the researcher said.
Despite the reputation of a species like the killer whale, not all whales are bad. In January two behavioral ecologists from the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Germany discovered a group of sperm whales that adopted a dolphin with a spinal deformation. The relationship has been a bit of a puzzle to scientists, particularly because the two species do not have a good history with one another.