Fleeing fish beware: The Guiana dolphin has a super Spidey sense. But instead of danger, the dolphin detects faint electrical fields generated by such things as contracting muscles, a beating heart and pumping gills — telltale signs of potential prey.
The dolphin is the first true mammal with these super sensory powers, scientists report. It detects electrical fields using organs on its snout that were once considered simple remnants of long-lost whiskers. Electroreception — the ability to sense these bioelectric fields — has already been described in sharks, amphibians, fish and some egg-laying mammals.
“We were really surprised to find this in the dolphin. Nobody had expected it,” says sensory biologist Wolf Hanke of the University of Rostock in Germany.
Hanke and his team first suspected the Guiana dolphin (Sotalia guianensis) had electropowers based on the size of organs called vibrissal crypts on its snout. Earlier work suggested the crypts, shaped like pits, have a rich blood supply. “We thought they must have some function — they were pretty big — and otherwise would have disappeared during evolution,” Hanke says of the crypts. When the team considered the dolphins’ lifestyle, the idea became even more plausible. Scientists think the dolphins, which live off the eastern edge of Central and South America, are benthic feeders, gulping fish from the seafloor. The resulting plumes of sediment can limit visibility and echolocation, meaning a different way of detecting prey would be especially helpful.
The team reports the dolphin’s ability based on behavioral tests and an examination of the snout organs online July 27 in the Proceedings of the Royal Society B.
“This is a major breakthrough,” says sensory physiologist Peter Madsen of Aarhus University in Denmark, who would like to see additional dolphins tested. “I think they’ve demonstrated in a convincing way that this dolphin species can use electroreception, and in a way that’s sensitive enough to potentially detect prey.”
First, Hanke and his team examined a cross section of crypts taken from a dolphin that had died of natural causes. Under a microscope, the structures resembled electroreceptors in the egg-laying platypus and echidna — and looked a lot like a whisker follicle. But 300 nerve fibers were plugged into it. “That’s a lot,” Hanke says. “It’s not quite as much as pinnipeds [such as seals], but it’s more than a rat’s whisker.”
The scientists then tested whether a Guiana dolphin could perceive electrical fields. The researchers trained Paco, a 28-year-old male in captivity, to place his snout 10 centimeters from two electrodes, which would randomly emit either a current or no stimulus. When Paco detected a current, he would swim away. When he didn’t, he stayed put. Paco’s perceptive capabilities were then put to the test over hundreds of trials, using signals similar in strength to those generated by the dolphins’ natural prey. Paco could perceive a current as weak as 4.6 microvolts per centimeter, much too faint for humans to perceive. “That’s a factor of about 10,000 or so below what a human can feel when he touches a 12-volt battery with his tongue,” Hanke says.
When researchers covered Paco’s snout with a plastic shield, he didn’t react to signals of any strength.
The scientists suggest that the crypts — originally responsive to mechanical, whisker-generated stimuli — evolved to respond to electrical stimuli instead. The necessary machinery is already there, Hanke says. “You just need to grow the nerve a little bit further and you have a basic electroreceptor.”
Marine biologist Paul Nachtigall of the University of Hawaii at Manoa is curious about whether related species have the same ability to detect natural electrical fields, which animals generate in many ways. “Everything that I’m doing when I’m talking, when my brain is working, is making an electrical field. And water carries electricity,” he says.
The finding, adds Madsen, is “a beautiful example of what’s called convergent evolution, where animals find the same solution to the same problem, but from different starting points.”
