Bottlenose dolphins have a seventh sense: they feel electricity

Dolphins are one of the few mammals that have six senses: taste, smell, hearing, sight and touch of other species, they add echolocation. Thanks to the bounce of their vocalizations, they are able to detect a small fish almost 100 meters away. Now, a series of experiments have confirmed that the bottlenose dolphin (Tursiops truncatus), the most common of the aquariums, has a seventh sense: they are able to detect electric fields. This ability would help them hunt the fish that hide at the bottom of the sea. The discoverers also believe that this electroreception helps them orient themselves by following the Earth's magnetic field.

Although there are many fish, especially elasmobranchs (rays and sharks), and some amphibians that detect low-intensity electric fields, it is extremely rare among mammals. So rare that only two of the rarest animals on the planet have this ability: the platypuses and the Australian echidna, both monotremes that lay eggs and have a single orifice, the cloaca, where the digestive, urinal, and reproductive tract converge. In 2011, a group of German scientists discovered that a species of odontocete cetacean, the coastal dolphin, perceived electrical signals. This dolphin, native to the South Atlantic of the Americas, from the Caribbean to the coasts of Brazil, hunts the fish that hide on or under the sand at the bottom of the sea. Now, part of the team that made that discovery has proven that bottlenose dolphins also have this ability.

The electroreception in the coastal dolphin led the director of the Center for Marine Sciences at the University of Rostock (Germany), Guido Dehnhardt, to think that it would not be the only dolphin with this seventh sense. Dehnhardt, one of the authors of the 2011 discovery, was convinced that bottlenose dolphins must also have this ability. "Both species follow a benthic feeding strategy," he says in an email. He means that both species eat fish that live on the bottom, and if the coastal dolphin is able to detect the electricity generated by the fish, why shouldn't the bottlenose do it?

In the image, Dolly, one of the protagonists of the experiment. In the center of the image you can see, on the snout, the hairy cavities that allow it to perceive electric fields. Tim Hüttner

All living organisms generate electric fields around their body when they are in the water and that is the signal that dolphins would detect. Tim Hüttners, Dehnhardt's pupil at the German university, explains: "These electric fields are generated due to neuronal activity or muscle movement." Fish also generate a field around them when the mucous membranes of the mouth and gills "come into direct contact with the ocean and release ions into the surrounding water," he said. Water, thanks to the salt it contains, helps to propagate these fields that can be detected by animals that have developed systems to perceive them. This is why sharks are successful at short distances (smell at long distances).

To test the existence of this sense in bottlenose dolphins, Hüttners and Dehnhardt recruited Donna and Dolly, two females of this species who live in the aquarium of Nuremberg (Germany). They created a system in which they had to touch a ball when they detected an electric field and if they got it right, they received a prize herring. The experiments, carried out over the past three years and whose results have just been published in the Journal of Experimental Biology, showed that both animals had a high sensitivity to electric fields. Although there are some differences between the two, they felt fields generated by both alternating and direct current. To measure how long, they started with a field with an electrical potential of 500 microvolts per centimeter (μV/cm) and worked their way down.

Donna and Dolly were equally sensitive to stronger fields. With the intermediates, the percentage of correct answers was always above 80%. Only with the weakest electric fields did the former prove to be slightly more sensitive, detecting fields of 2.4 μV/cm, while Dolly perceived fields of 5.5 μV/cm. A microvolt is equal to one millionth of a volt. By comparison, platypuses, which also feed on animals hidden at the bottom, in their case in rivers, capture crabs, shrimp or insects that give themselves away with electric fields of between 25 and 50 microvolts.

"At birth, dolphins still have hair follicles [vibrissae like those in the human nose] with a hair that function as mechanoreceptors, but they lose their hair soon after birth."

The seventh sense of these dolphins seems to be found in sensors reminiscent of the whiskers of cats or seals. "At birth, they still have hair follicles [vibrissae like those in the human nose] with a hair that function as mechanoreceptors (tactile information), but they lose their hair soon after birth and only the empty cells remain," explains Hüttners. For a long time, it was thought that these hollows above the muzzle were reminiscences of the past that had lost their function. But nothing could be further from the truth: "According to our tests and a previous study with a Guiana dolphin (the fishing dolphin, Sotalia guianensis) the vibrisal cells transform from a mechanoreceptor to an electroreceptor," he adds.

Just by contracting their muscles or exchanging ions with water, aquatic animals generate fields of between 50 and 500 μV/cm. Although the authors of the experiments did not use live fish to perform them, they believe that electroreception is key for the dolphins to be able to feed. These animals already have echolocation. But when they're inches away from a prey hidden at the bottom, the sand interferes with the echo signal, returning wrong locations. Although the electric field attenuates with distance, in proximity, it betrays the presence of loot.

German biologists point to a second function of this seventh sense. The nerve endings in those holes above the muzzle would have become a kind of magnetometer. "Electric and magnetic fields are always connected," Hüttners recalls. When a conductive body moves through a magnetic field, it generates an electric field. "This is called electromagnetic induction and it occurs in sharks and possibly in dolphins," says the researcher. As they swim through the Earth's magnetic field, they generate an electric field around their body. "This electric field could be strong enough to be detected by the animal itself, providing map-like information that it can use to orient itself in the ocean," Hüttners concludes. This would help explain the connection between many of the cetacean strandings on beaches following a solar storm or magnetic anomaly.

The main goal of these experiments with bottlenose dolphins was to show that "electroreception doesn't just occur in one species, but is probably an ability of perhaps most toothed whales," says Dehnhardt, senior author of this research. The problem is going to be checking it out, although there are hints that this is the case. This is the case with sperm whales. They are also odontocetous cetaceans, as well as the heaviest animal on the planet. Dehnhardt remembers how these sea giants died by the dozens snagged in submarine cables. Like dolphins, they also feed on benthic fish and, in their search, they would encounter the wires, breaking more than one. But in more recent decades, the death of these whales due to the power lines has ceased to be reported. The explanation could be, says the German scientist, "a first indication of the ability of these odontocetes to perceive electric fields." Early telegraph systems and later telephone systems used wires with metal cores that generated powerful electromagnetic fields that could have attracted electroresponsive whales. Neither coaxial cables nor fiber optics generate those fields in their environment. That's why sperm whales don't tangle with them anymore.

You can follow MATERIA on Facebook, X and Instagram, or sign up here to receive our weekly newsletter.