Guiana dolphin with its vibrissal crypts covered by a plastic shell.
Credit: A. Liebschner
Vibrissal crypts of a Guiana dolphin.
Credit: A. Liebschner
SYDNEY: Dolphins can sense the electric fields produced by their prey, and are one of the only known mammals with this ability.
While many predatory fishes and amphibians can perceive electrical fields, the only mammals (but technically monotremes) thought to possess this ability were the Australian platypus and echidna.
Researchers have now shown that the Guiana dolphin, a species that lives in the coastal waters of South America, can perceive these electrical fields using specialised structures on its snout. The murky lifestyle of the dolphin indicated to the researchers that these structures, known as vibrissal crypts, could function as electroreceptors.
"We noticed the relatively large vibrissal crypts and thought they must be good for something," said Wolf Hanke from Rostock University in Germany, lead author of the paper published in Proceedings of the Royal Society B.
Sensing fish
The researchers examined the vibrissal crypts from a deceased 29-year-old male Guiana dolphin. "When one of the animals in the dolphinarium died from old age, we had the opportunity to look at the structure closer and found that it has some basic, remote similarity with electroreceptors in fish and monotremes," Hanke said.
A 28-year-old Guiana dolphin was then trained to respond to electrical stimuli of the same magnitude as those that would be generated by small- to medium-sized fish, its natural prey. If it sensed an electrical stimulus during the trial, the dolphin would leave its station.
Using a battery-powered circuit that passed electric currents between two copper wire electrodes, the researchers generated the stimuli 10cm from the dolphin's snout. The trails showed that dolphin responded well to stimuli below 10 mV cm21.
The researchers also carried out a series of control tests with a plastic shell covering the vibrissal crypts. The dolphin did not respond to high-intensity electric field stimuli while wearing the shell, suggesting that the responses were due to electroreception.
"It is nice to see evidence from natural behaviour that the receptor is being used in the context of receiving and sensing electrical currents," said Carl Hopkins from Cornell University in the U.S., who was not involved in the study. He said it is tricky to name something as an electrical receptor, as an electric current could possibly activate other receptors, such as pain or temperature, by stimulating the neurons.
Advantage for bottom feeders
The Guiana dolphin feeds on bottom dwelling species, sometimes digging into the mud to catch their prey. "In the calm, shallow, coastal waters that these dolphins inhabit, often close to river mouths, a lot of silt and mud can be expected on the ground that makes the water turbid as soon as the dolphin starts digging," Hanke said.
The receptors increase the Guiana dolphins' ability to locate their prey in murky water or while digging in the substrate. The vibrissal structures are also present on other cetacean species, including the bottlenose dolphin, which has been observed burying itself in sand to catch its prey in the Bahamas.
Paul Nachtigall from the University of Hawaii said it would be unusual if the Guiana dolphin was the also species to have developed electroreception and more research needs to be done in this field. "It would help to explain how some odd species like sperm whales get wriggling food into a very small mouth around a very large nose," he said.
"Touch is helpful [up] close, but electroreception would take over in the area between what an animal echolocates and what it touches. In the depths where many cetaceans (and order that includes whales, dolphins, and porpoises) echolocate after single, fast creatures, this is a critical area."
