New images reveal for the first time how dolphins 'see' humans
Extraordinary images conjured from a dolphin's sonar have revealed how the creature "sees" a human swimmer using echolocation.
Scientists produced the blurred outlines of a submerged man after recording the sonar signals with an underwater microphone and converting them into pictures.
Sophisticated processing was used to transcribe the way sound waves imprint themselves on water and create the images.
A similar technique was used three years ago to capture a dolphin's echolocation view of inanimate submerged objects, indicating that cetaceans really can "see" with sonar.
Team leader Jack Kassewitz, from the US-based SpeakDolphin project promoting communication between humans and dolphins, said: "This is the first time we have captured a what-the-dolphin-saw image of a submerged man.
"We employed a similar technique in 2012 to capture a dolphin's echolocation picture of a flowerpot and several other submerged plastic objects but the present research has confirmed that result and so much more."
The research, conducted with the help of British experts, took place at the Dolphin Discovery Centre in Puerto Aventuras, Mexico.
Underwater swimmer Jim McDonough wore a weight belt and exhaled most of the air in his lungs to overcome his natural buoyancy.
A decision was taken not to use breathing apparatus that would generate bubbles which could affect the results of the experiment.
The echo signal was sent via email to a UK scientist who has pioneered new technology called CymaScope that makes sound visible.
Acoustics physicist Dr John Stuart Reid, from Keswick, who heads the CymaScope team, said: "When I received the recording Jack had told me only that it might contain an echolocation reflection from someone's face. I noticed the file name 'Jim' so I assumed that the image, if it existed within the file, would be that of a man's face.
"I was somewhat dubious whether this could be achieved because the imaging we had carried out in 2012 was of simple plastic objects that had no inherent detail, whereas a face is a highly detailed form."
He added: "I listened to the file and heard an interesting structure of clicks. The basic principle of the CymaScope instrument is that it transcribes sonic periodicities to water wavelet periodicities - in other words, the sound sample is imprinted onto a water membrane.
"The ability of the CymaScope to capture what-the-dolphin-saw images relates to the quasi-holographic properties of sound and its relationship with water ... When I injected the click train into the CymaScope, while running the camera in video mode, I saw a fleeting shape on the water's surface that did not resemble a face. I replayed the video, frame by frame and saw something entirely unexpected, the faint outline of a man."
Computer processing enhanced and cleaned up the image producing more detail, including the weight belt worn by Mr McDonough.
The results suggest that dolphins can sense more than the shadow of an object with their echolocation, and are able to resolve surface features, said Mr Kassewitz.
He added: " The dolphin has had around 50 million years to evolve its echolocation sense whereas marine biologists have studied the physiology of cetaceans for only around five decades and I have worked with John Stuart Reid for barely five years. Even so, our recent success has left us all speechless.
"We now think it is safe to speculate that dolphins may employ a 'sono-pictorial' form of language, a language of pictures that they share with each other. If that proves to be true an exciting future lies ahead for inter-species communications."