Bottlenose Dolphins Sense Electricity

Bottlenose dolphins.

Source: Schnapps2012, via iStock.

Even buried in the ocean floor, fish may not stand a chance against a hungry bottlenose dolphin. Dolphins have keen vision, an exceptional sense of hearing—and they can sense electric fields emitted by their prey, according to new research.

Researchers at Nuremberg Zoo and the University of Rostock (Germany) teamed up to find out just how sensitive these marine mammals are to electric fields. The results may explain how dolphins detect hidden fish at close range and even provide clues as to why the animals sometimes strand themselves on beaches.

“Dolphins are fascinating animals that have been of public and scientific interest for so many decades now,” says Tim Hüttner, lead author of the study. “And yet, in the 21st century, we are still able to discover something completely new.”

Electrosensitive Snouts

Passive electroreception, the ability to detect and analyze electric fields in the environment, is found in many aquatic vertebrates, primarily fishes. Among mammals, it has been experimentally demonstrated in the platypus, the short-beaked echidna, and the Guiana dolphin.

These findings inspired Hüttner, with Guido Denhardt and colleagues, to investigate another marine mammal, the bottlenose dolphin. In a previous experiment, they found that, like Guiana dolphins, bottlenose dolphins possess rows of dimples along their snouts, known as vibrissal pits. The team showed that these pits are very similar in structure and innervation to those of the electroreceptive Guiana dolphin and also resemble the structures that sharks use to detect electric fields, called the ampullae of Lorenzini. And in behavioral tests, captive bottlenose dolphins responded spontaneously to weak electric fields.

A dolphin (Dolly) resting her jaw on a bar ready to test her sensitivity to an electric field.

Source: Tim Hüttner, used with permission.

For the new study, the researchers set out to determine bottlenose dolphins’ detection thresholds for weak electrical direct current (DC) and alternating current (AC) fields. The subjects were Dolly and Donna, two bottlenose dolphins living at Nuremberg Zoo. Each dolphin was trained to rest its jaw on a submerged metal bar with electrodes immediately above its snout. The animals were taught to swim away within 5 seconds of perceiving a stimulus (such as a sound) or remain stationary in the absence of a stimulus.

Hüttner says that once the dolphins understood the rules of the task, the researchers interspersed the first electric DC stimuli—and the dolphins responded immediately.

“Both of them responded correctly from the very first trial, and Dolly’s performance was nearly perfect over the first 100 trials,” he says. “Then, step by step, we decreased the intensity of the electric field and were able to determine a sensory threshold.”

In the end, Donna proved to be slightly more sensitive, sensing DC electric fields that were 2.4μV/cm, while Dolly detected fields as low as 5.5μV/cm. These detection thresholds are comparable to those demonstrated by the Guiana dolphin.

The researchers also tested the dolphins’ sensitivity to AC fields, since living animals produce both static and pulsing electric fields through breathing and other movements. Detection thresholds for AC fields (presented at 1, 5, and 25 Hz) were generally higher than those for DC fields, and the dolphins’ sensitivity for AC fields decreased with increasing frequency.

Super Senses

What are bottlenose dolphins using their electric sense for? Their detection range for weak electric fields is much lower than that of sharks, which are noted electrical specialists, says study co-author Guido Denhardt.

Bottlenose dolphins crater feeding, burrowing in the sand with their rostrums, on a sand bank in The Bahamas.

Source: Shane Gross, used with permission.

“Dolphins may use this sense for close-range detection of bottom-dwelling fish, when other senses are restricted,” he says. “Dolphins will dig into the sediment up to half a meter when hunting, limiting the usefulness of vision and echolocation. This electric sense might help them detect and snap up buried fish.”

Denhardt and Hüttner also suspect that dolphins and other toothed whales may use this sensory ability for large-scale orientation. They say that any electrosensitive animal has the potential to detect the planet’s magnetic field. A dolphin swimming in saltwater through the Earth’s magnetic field at a speed of 10 m/s could generate a detectable electric field of 2.5μV/cm across its body. Swimming faster, it would be even more likely to sense the magnetic field, potentially allowing it to navigate the globe by magnetic map.

The researchers think that their findings may even help explain the phenomenon of mass strandings, in which large numbers of apparently healthy toothed whales beach themselves.

“This is the first hint that electroreception might be a sensory explanation for mass strandings,” says Hüttner. “Hopefully, this will lead to more research that will help us understand why dolphin strandings are often associated with geomagnetic anomalies or with alterations of geomagnetic fields caused by solar storms.”

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For Denhardt, the findings also underscore how important it is to not reduce an animal to one sensory modality. He says that dolphin sensory research tends to be dominated by studies of echolocation, since it is such a special modality. But dolphins possess a sensory arsenal.

“They have echolocation, of course, as well as passive hearing. They have very good vision. They have the senses of touch and smell. And now we know that they also have electroreception,” says Denhardt.

“I think we have to consider the animal as a whole. The perceptual world of these animals is complex, and electroreception is one part of that.”