The physics of porpoises
When it comes to catching prey, porpoises really use their heads. Andrew P Street reports.
There are many enduring mysteries surrounding dolphins and porpoises. Are they really as happy as they seem? Are they laughing with us, or at us? But one of the more serious – and better researched – questions is, how does their natural sonar actually work?
We know in general terms how cetaceans echolocate – they generate high frequency noises which echo off an object, thereby building up a sound-picture in the brain which allows them to work out their surroundings, any possible threats and the presence of prey. Other animals do it too, most obviously bats, and humans have built machines that do the same thing.
The question, however, is this: how do they do it so well? The accuracy and directionality of the biological sonar of porpoises wildly outperforms anything humans can design, and there's no obvious reason for it.
That's not least because we know how they do it: the clicks are made in the nasal passage and beamed out through the animal's forehead as an ultrasound beam. The returning echoes are picked up by the jaw where a passage of fat transmits it to the inner ear where nerve impulses are processed by the brain.
The mystery is that the wavelengths generated by the clicks are far too large to capture the detail of the often tiny objects which the animals are evidently able to sense. The physics would appear to be impossible, yet the results are observable. So: how the hell do they do it?
A likely-sounding answer has come with a research paper with the catchy title Directional acoustic wave manipulation by a porpoise via multiphase forehead structure, published in the journal Physical Review Applied, and written by a team headed by Yu Zhang, of Xiamen University in China.
Zhang and colleagues suggest that the answer lies in the unique qualities of the porpoise forehead. The mix of bone, air sacs and muscle within it allow the animal to fine-tune their acoustic field. To put it another way, they can use their forehead as a natural metamaterial, altering its physical structure to exhibit properties that would not normally occur.
This research expands on the findings of a 2016 paper, also led by Zhang, published in the Journal of the Acoustical Society of America, which suggests that the finless porpoise (Neophocaena phocaenoides) is able to manipulate its skull, forehead, muscle and melon (a unique tissue mass between the blowhole and snout), to tune the acoustic beam from its forehead.
By changing the shape and density of its forehead tissues, the researchers found, the porpoise is able to widen or narrow the echolocation sound beam. The forehead, they concluded, was effect a tuneable instrument.
In other words, dolphins and porpoises would appear to have developed a natural metamaterial that gives them super-senses. We can probably conclude they're laughing at us, then.