SpeakDolphin.com, CymaScope laboritory
Original CymaScope image of how a dolphin saw a submerged man (left) and the computer enhanced version of the same image (right).
Originally designed to live on land, marine mammals are a diverse, charismatic group of animals that include more than 120 species. The animals share key characteristics of land mammals. They have hair, breathe air, give birth to live young, which feed off mother's milk when young. They have warm bodies and usually thick blubber to keep their body temperatures high. The bottlenose dolphin is probably the most widely recognized marine mammal, easily spotted just offshore from beaches around the world. Small groups of 20 or less can live in close proximity to shorelines, but groups living more offshore can reach several hundred. Bottlenose dolphin calves stay with their mothers for up to six years, learning how to hunt and become good dolphin citizens. Full-grown dolphins reach eight to 12 feet in length and can weigh up to 1,430 pounds. The bottlenose dolphin is protected in U.S. waters.
What makes them "marine" depends on the animal. They either live mostly in the sea or, like polar bears, depend on the ocean for food. The largest in the group are whales -- including humpback whales. These massive animals reach up to 50 feet in length and weigh up to 79,000 pounds. To maintain their weight, the animals feed on tons of krill and fish. They neared extinction due to whaling, but have recovered somewhat since a 1966 moratorium on whaling was introduced.
While polar bears live mostly on land or ice, they are excellent swimmers and have been known to swim up to 45 miles a day. The massive animals, weighing up to 1,500 pounds, hunt mostly seals. In recent years, biologists have observed that the bears are swimming now more than ever as melting stretches the distances between Arctic ice flows. Because they depend on sea ice to hunt seals, the polar bear is considered threatened as global warming melts and thins ice in this region.
This member of the weasel family is also the smallest marine mammal, with females weighing about 60 pounds and males weighing up to 90 pounds. They may be small, but they're also clever. They're the only marine mammals known to use tools. They use stones to break open clams and store food they gather in the folds of their armpits! Another feature that sets them apart is their lack of blubber. These marine mammals depend mostly on their fur to stay warm. That feature makes them particularly vulnerable to oil spills, which can compromise their fur's insulating effect.
Immediately recognizable by its long tusks and whiskers, the sea walrus is a hefty, flippered member of the Odobenidae family and is, in fact, the last living member of this group. Since both the males and females have big tusks and not much for teeth, the animals feed by sucking up shellfish from the ocean floor. So, just what are those tusks for? The longer they are (they grow to be up to four feet long in males), the higher an animal is ranked in the group. Males attack each other with their tusks to establish dominance. The ivory appendages are also handy for poking holes in the winter ice and for helping the animals pull themselves out of the water.
Manatees, also known as sea cows, are gentle herbivores that live in marshy areas in tropical and subtropical waters. The average adult manatee can weigh up 1,200 pounds and is around 10 feet long. Because of their slow metabolism, these animals can only survive in warm waters. Due to the unusually long, cold winter this year in part of the southeastern United States, populations of manatees throughout Florida were devastated. During the day, manatees usually like to stay close to the surface. At night, manatees will often sleep about three to 10 feet below sea level. This is why these gentle animals are so often accidentally injured, maimed or killed by passing boats.
Found up and down the North American coastlines, these marine mammals spend half of their lives swimming. Although they can reach up to six feet in length and weigh around 180 pounds, when on land and in plain sight harbor seals may not be easy to spot. Their spotted brown or tan fur allows harbor seals to blend in with sand and rocks. Unlike their very vocal relatives -- sea lions and elephant seals -- harbor seals are quiet creatures that make little noise. They like to hang out on beaches, sand bars and rocks during low tide to bask in the sun and sleep, but they never go far from the water. At the slightest sign of danger, they will quickly slip back under the waves. These expert swimmers have been known to plunge to depths of more than 1,600 feet and stay underwater up to 28 minutes.
In a scientific first, researchers have just reproduced what a dolphin saw as it encountered a male diver.
This “what the dolphin saw” image of the submerged man reveals that dolphin echolocation results in fairly detailed images. What’s more, it’s now thought that dolphins may share such images with each other as part of a previously unknown marine mammal language.
Research team leader Jack Kassewitz of SpeakDolphin.com said in a press release that “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.”
For the research, which took place at the Dolphin Discovery Center in Puerto Aventuras, Mexico, Kassewitz had colleague Jim McDonough submerge himself in front of the female dolphin “Amaya” in a research pool at the center. To avoid bubbles from a breathing apparatus (which might have hurt the later recreation of the image), McDonough wore a weight belt and exhaled most of the air in his lungs to overcome his natural buoyancy before positioning himself against a shelf in the pool.
As Amaya directed her echolocation beam to McDonough, high specification audio equipment was used to record the signal. Team members Alex Green and Toni Saul handled that part of the project.
Green and Saul then sent the recording to the CymaScope laboratory in the U.K., where yet another colleague, acoustic physics research John Stuart Reid, imprinted the signal onto a water membrane and then computer enhanced the resulting image.
“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, which will be described in a forthcoming science paper on this subject,” Reid explained.
His fellow teammates thought they had captured an echolocation image of McDonough’s face, so that was what Reid was expecting to see. Instead, as he told Kassewitz in a note at the time, the signal translated to “what appears to be the fuzzy silhouette of almost a full man. No face.”
As it turns out, Amaya had been echolocating on McDonough from several feet away before she came in closer, so the researchers captured one of those farther away signals.
Kassewitz said, “Having demonstrated that the CymaScope can capture what-the-dolphin-saw images, our research infers that dolphins can at least see the full silhouette of an object with their echolocation sound sense, but the fact that we can just make out the weight belt worn by Jim in our what-the-dolphin-saw image suggests that dolphins can see surface features too.”
It could be that dolphin echolocation signals result in much clearer, more detailed mental images, and that it’s our technology that isn’t yet fully attuned to what the marine mammals are precisely seeing.
As Kassewitz said, “The dolphin has had around fifty 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.”