Chuck Bednar for redOrbit.com – @BednarChuck
When hunting, porpoises have the ability to switch their echolocation beam from a wide field to a narrow one and vice versa, researchers from Aarhus University in Denmark reported in a study published earlier this month in the journal eLife.
According to BBC News, the research reveals how the aquatic mammals (as well as their cousins the whale and the dolphin) send out a series of clicking and buzzing noises, then honing in on the echoes given off by their would-be prey. Essentially, they can fine-tune their sound beams like a flashlight to trap a fish and prevent it from escaping, the UK media outlet added.
“Toothed whales use sonar to detect, locate, and track prey,” the study authors wrote. “They adjust emitted sound intensity, auditory sensitivity and click rate to target range, and terminate prey pursuits with high-repetition-rate, low-intensity buzzes.”
“However, their narrow acoustic field of view (FOV) is considered stable throughout target approach, which could facilitate prey escape at close-range,” they added. “Here we show that, like some bats, harbor porpoises can broaden their biosonar beam during the terminal phase of attack but, unlike bats, maintain the ability to change beamwidth within this phase.”
They studied video, MRI, and acoustic-tag recordings and found that the flexibility of this sonar beam is modulated to accommodate changing spatial relationships with their prey, as well as the acoustic complexity of their surroundings. While whales and bats generate and transmit sounds using different techniques, researchers explained that both creatures can adaptively change their FOV, indicating that this trait played an important role in the evolution of echolocation.
Danuta Wisniewska of Aarhus University told BBC News that she and her colleagues hope that uncovering these acoustic secrets could help them develop a way of keeping porpoises and other types of toothed whales from becoming trapped in fishing nets.
The research was conducted using harbor porpoises in a semi-natural enclosure at a conservation research facility in Denmark. At the center, the creatures still had access to the seafloor, and the only thing separating them from a nearby harbor is a net. Fish were able to enter the area so that the porpoises could continue to hunt while the scientists monitored their activity.
The creatures were outfitted with sound-detecting tags and an array of microphones was place to detect sound elsewhere in the enclosure, the website explained. Wisniewska and her co-authors conducted a series of experiments to determine where the sound energy was being directed. One of the experiments involved adding fish into the water to entice the porpoises to hunt.
They found that the porpoises switched from a exploratory clicking type of echolocation to a more intense, focused and high-frequency buzz to obtain a continuous echo from their would-be prey. The beam, which is produced by a fatty structure in the head known as the melon, could be broadened or narrowed by as much as 50 percent, the researchers reported.
“If you were trying to find your car in a car park, you could use a narrow beam over a long distance and still see a lot. But when you’re trying to get your keys into the car, you would switch to a wider beam,” Wisniewska told the BBC. “This is similar to what we see in porpoises.”
“My research suggests that they really attend to their target, so we could be seeing a sort of attention blindness,” she added, explaining that the creatures could be focusing so intently on their prey that they ignore their surroundings, thus wandering into a fishing net.
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