Dolphins are famous for their ability to hunt prey via echolocation. Now, scientists have discovered that at least one dolphin species, the Guiana dolphin, can also detect fish by tuning into their electrical fields. It is the first time this sense has been reported in a marine mammal—or in any placental mammal. The researchers expect that electroreception, as this sense is called, will be found in other cetacean species. Until this discovery, it was known only in fish, amphibians, and two egg-laying mammals, or monotremes, the platypus and echidna.
All animals generate weak electric fields from the activity of their muscles and nerves. Species with electroreceptors can sense this bioelectric field and use it to spot prey that they can't see. And visibility is a real problem for Guiana dolphins, which live off the western Atlantic coast of Central and South America and hunt fish in turbid water and muddy sediments.
Working with two elderly captive Guiana dolphins at the Dolphinarium of Allwetterzoo Münster in Germany, researchers began to suspect that the animals might have electroreceptors "because you can see dark pits on their snouts," says Wolf Hanke, a sensory biologist at the University of Rostock in Germany and one of the study's authors. At some point in the dolphins' evolutionary past, an ancestor had whiskers sprouting from these pits, and most researchers thought they no longer served a purpose. But infrared photographs of the pits, called "vibrissal crypts," showed them lighting up with activity, Hanke says, "so we knew they must have some function."
When one of the two male dolphins died at age 29, Hanke and his team took tissue samples from his rostrum, or beak, and studied the cellular structure of the crypts. The pits showed no sign of hair shafts or other features associated with whiskers; instead, they most closely resembled the electroreceptors found in many species of fish and in platypuses. But dolphins, fish, and platypuses each evolved the receptor cells independently, from different organs unique to each lineage. "This is a case of convergent evolution," Hanke says. The dolphins' cells also contained a gel-like substance, similar to a gel found in the receptors of fish and a mucus in those of the platypus. Hanke suspects the gel helps conduct the electrical signals.
After discovering the electroreceptors in the deceased Guiana dolphin, the researchers then tested the other dolphin, named Paco, to see if he would react to a weak electrical signal like that generated by small- to medium-sized fish, his natural prey. They trained Paco to position himself in a dolphin-holding station and then delivered electrical signals via two electrodes 10 centimeters from Paco's rostrum. The scientists trained Paco to leave the station when he sensed a signal; he was then rewarded with a treat. When he did not detect a signal, he remained in the station.
The scientists conducted 186 trials, presenting the dolphin with a range of electrical signals from low to high. The dolphin responded well even to low signals, displaying an electrosensitivity similar to that of platypuses, the researchers report today in the Proceedings of the Royal Society B. When they covered his beak with a plastic shell that blocked the crypts, Paco could no longer sense any electrical stimulus.
Until these experiments, Paco probably had rarely had a chance to use his electroreceptors, because he had spent much of his life in captivity. That would be rather like keeping us from using our sense of smell for many years, Hanke says. Having the opportunity to use his electrosense seemed to make Paco "more alert and attentive," Hanke says, although the researchers had no hard data for this. In the wild, the Guiana dolphins probably use their electroreceptors to detect prey at close range while targeting more distant fish with echolocation. Hanke and others now say scientists should investigate whether other bottom-feeding cetaceans, such as river dolphins and pygmy sperm whales, share the ability. He thinks it's possible that even bottlenose dolphins, the most well-known of dolphin species, have electroreception because of the pits on their beaks and propensity for bottom feeding—sometimes even with sponges on their noses.
The study opens a new door to cetacean researchers, who now have a "new sensory system" to explore, says Paul Nachtigall, a sensory biologist at the University of Hawaii, Manoa. "We have been so impressed by hearing and echolocation that we've ignored other, possible sensory systems in cetaceans," he says. Indeed, he notes, the findings of Hanke's team are so "unusual that when they first reported their findings at a scientific conference, they were met with skepticism."
That skepticism has given way to full-blown praise, says Peter Teglberg Madsen, a sensory biologist at Aarhus University in Denmark. "This is a major breakthrough and a beautiful example of convergent evolution where vertebrates at least five times have independently evolved" this ability "by modifying different cell types to serve the same function." Moreover, Madsen says, there's a lesson in Hanke's findings: "It emphasizes that we need to be open to the fact that animals gather information in ways that are very different from those of humans."