Coming after chimpanzees and followed by octopuses, dogs, orangutans, crows and ravens, African grey parrots, and bumblebees, dolphins are among the most intelligent animals in the world.
Born tail first, bottlenose dolphin calves come out of their mother’s body equipped with two slender rows of whiskers along their beak-like snouts – much like the touch-sensitive whiskers of seals – but the whiskers fall out soon after birth, leaving the youngster with a series of dimples known as vibrissal pits.
Recently Dr. Tim Hüttner and Dr. Guido Dehnhardt from the University of Rostock and the Marine Science Center in Germany began to suspect that the dimples may be more than just a relic and could allow the adult mammals to sense weak electric fields.
Taking an initial close look, they realized that the remnant pits resemble the structures that allow sharks to detect electric fields, and when they checked whether captive bottlenose dolphins could sense an electric field in water, all of the animals felt the field.
“It was very impressive to see,” said Dehnhardt, who published the extraordinary discovery and how the animals could use their electric sense in the Journal of Experimental Biology under the title “Passive electroreception in bottlenose dolphins (Tursiops truncatus): implication for micro and large-scale orientation.”
Hüttner and Dehnhardt suspect that the dolphin’s ability to feel electricity could help them on a larger scale. “This sensory ability can also be used to explain the orientation of toothed whales to the earth’s magnetic field,” Dehnhardt explained.
He added that dolphins swimming through weak areas of the earth’s magnetic field at a normal speed of 10m/s could generate a detectable electric field of 2.5μV/cm across their body. And, if the animals swim faster, they are even more likely to sense the planet’s magnetic field, allowing them to use their electric sense to navigate the globe by magnetic map.
To find out how sensitive bottlenose dolphins are to the electric fields produced by lifeforms in water, they teamed up with Lorenzo von Fersen at Nuremberg Zoo and Lars Miersch at the University of Rostock. First, they tested the sensitivity of two female bottlenose dolphins named Donna and Dolly to different electric fields to find out whether they could detect a fish buried in the sandy sea floor.
After training each animal to rest its jaw on a submerged metal bar, a large team taught the dolphins to swim away within five seconds of feeling an electric field produced by electrodes immediately above each dolphin’s snout.
Gradually decreasing the electric field from 500 to 2μV/cm, the team kept track of how many times the dolphins departed on cue and were impressed – Donna and Dolly were equally sensitive to the strongest fields, exiting correctly almost every time. It was only when the electric fields became weaker that it became clear that Donna was slightly more sensitive, sensing fields that were 2.4μV/cm, while Dolly became aware of fields of 5.5μV/cm.
Electric fields produced by living animals aren’t just static
The pulsing movements of gills on fish cause their electric fields to fluctuate, suggesting that so could Donna and Dolly sense pulsing fields as well This time the team pulsed the electric fields 1, 5, and 25 times per second while reducing the field strength.
Sure enough, the dolphins could sense the fields, but neither of the dolphins was as sensitive to the alternating fields as they were to the unvarying electric fields. Dolly could only pick up the slowest field at 28.9μV/cm, while Donna picked up all three of the oscillating fields, sensing the slowest at 11.7μV/cm.
What does this new super sense mean for dolphins in practice? Dehnhardt said that “the sensitivity to weak electric fields helps a dolphin search for fish hidden in sediment over the last few centimeters before snapping them up, in contrast to sharks – the electrosensitive “superstars” that are capable of sensing the electric fields of fish within 30 to 70 centimeters.
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