Dolphins are one of the few mammals that have six senses: Echolocation is added to the taste, smell, hearing, sight and touch of other species. Thanks to the resonance of their vocalizations, they are able to detect a small fish at a distance of almost 100 meters. Now a series of experiments have confirmed that the bottlenose dolphin (Tursiops truncatus), which is most commonly found in aquariums, has a seventh sense: it is able to detect electric fields. This ability would help them hunt fish that hide on the ocean floor. The explorers also believe that this electroreception helps them orient themselves to the Earth’s magnetic field.
Although there are many fish, particularly elasmobranchs (rays and sharks), and some amphibians that sense low-intensity electric fields, this is extremely rare among mammals. So rare that only two of the world’s rarest animals have this ability: the platypus and the Australian echidna, both monotremes that lay eggs and have a single opening, the cloaca, where the digestive, urinary and reproductive tracts come together. In 2011, a group of German scientists discovered that a type of odontocete whale, the coastal dolphin, detects electrical signals. Native to the American South Atlantic, from the Caribbean to the coasts of Brazil, this dolphin hunts the fish that hide on or under the sand at the ocean floor. Now part of the team that made this discovery has confirmed that bottlenose dolphins also have this ability.
The electroreception in the coastal dolphin led the director of the Marine Science Center at the University of Rostock (Germany), Guido Dehnhardt, to suspect 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 said in an email. It refers to the fact that both species eat bottom-dwelling fish. If the coastal dolphin is able to detect the electricity produced by fish, why shouldn’t the bottlenose dolphin do the same?
In the picture Dolly, one of the protagonists of the experiment. In the middle of the picture you can see the hair cavities on the snout, which enable it to perceive electric fields.Tim Hüttner
All living organisms produce electric fields around their bodies when they are in water, and that is the signal that dolphins would detect. Tim Hüttners, a Dehnhardt student at the German university, explains it: “These electric fields are created by neural activity or muscle movement.” Fish also create a field around themselves when the mucous membranes of the mouth and gills “come into direct contact with the “They come from the sea and release ions into the surrounding water,” he explains. Water, thanks to the salt it contains, contributes to the spread of these fields, which can be perceived by animals that have developed systems to perceive them. This is what sharks owe their success at short distances (at long distances it is thanks to smell).
To verify the existence of this sense in bottlenose dolphins, Hüttners and Dehnhardt recruited Donna and Dolly, two females of this species living in the Nuremberg Aquarium (Germany). They devised a system where they had to touch a ball when they detected an electric field, and if they got it right they received a prize. The experiments conducted over the past three years, the results of which have just been published in the Journal of Experimental Biology, showed that both animals had great sensitivity to electric fields. Although there were some differences between the two, they sensed fields created by both alternating and direct current. To measure how long it took, they started with a field with an electrical voltage of 500 microvolts per centimeter (μV/cm) and went downwards.
Donna and Dolly were equally sensitive to the stronger fields. At the intermediate levels, the proportion of correct answers was always over 80%. The former proved to be slightly more sensitive to only the weakest electric fields, detecting fields of 2.4 μV/cm, while Dolly detected fields of 5.5 μV/cm. A microvolt is equal to a millionth of a volt. For comparison: Platypuses, which also feed on animals hidden on the ground, in their case in rivers, catch crabs, shrimps or insects that reveal themselves with electric fields between 25 and 50 microvolts.
“Dolphins still have hair follicles when they are born [vibrisas como las de la nariz humana] with hair that acts as mechanoreceptors but loses its hair shortly after birth.”
These dolphins’ seventh sense appears to lie in sensors reminiscent of the whiskers of cats or seals. “At birth they still have hair follicles [vibrisas como las de la nariz humana] However, “children with hair that act as mechanoreceptors (tactile information) lose their hair shortly after birth and only empty cells remain,” explains Hüttners. For a long time it was believed that these holes above the snout 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 vibrissa cells convert from a mechanoreceptor to an electroreceptor,” he concludes.
Simply by contracting their muscles or exchanging ions with water, aquatic animals generate fields between 50 and 500 μV/cm. Although the authors of the experiments did not use live fish to conduct them, they believe that electroreception is crucial to the dolphins’ feeding. These animals already have echolocation. But when they are inches away from prey hidden on the ground, the sand interferes with the echo signal and returns false locations. Although the electric field becomes weaker with increasing distance, it reveals the presence of prey in close proximity.
German biologists point out a second function of this seventh sense. The nerve endings in these holes on the snout would have become a kind of magnetometer. “Electric and magnetic fields are always connected,” remembers Hüttners. When a conducting body moves through a magnetic field, it creates an electric field. “This is called electromagnetic induction and occurs in sharks and possibly also in dolphins,” explains the researcher. As they swim through the Earth’s magnetic field, they create an electric field around their bodies. “This electric field could be strong enough to be perceived by the animal itself and provide information similar to a map that it can use to orientate itself in the ocean,” concludes Hüttners. This would help explain the connection between many whale strandings on beaches following a solar storm or magnetic anomaly.
The main goal of these experiments with bottlenose dolphins was to show that “electroreception is not unique to one species, but is likely an ability of most toothed whales,” says Dehnhardt, senior author of this research. The problem will be verifying this, although there is evidence that this is the case. This is the case with sperm whales. They are also odontocete whales and the heaviest animal on the planet. Dehnhardt remembers how dozens of these sea giants were caught in underwater cables. Like dolphins, they also feed on bottom fish and in their search they came across cables, more than one of which broke. But in recent decades, the deaths of these whales due to the power lines have not been reported. The explanation, says the German scientist, could be “a first indication of the ability of these odontocetes to sense electric fields.” Early telegraph and later telephone systems used metal-core cables that generated strong electromagnetic fields that could have attracted electroreceptive whales. Neither coaxial cables nor fiber optics generate these fields in their surroundings. That’s why sperm whales no longer tangle with them.
You can follow THEME on Facebook, X and Instagram, or sign up here to receive our weekly newsletter.