Bottlenose dolphins become one of the few known mammals with a ‘seventh sense’

The Initial research into the sensitivity of bottlenose dolphins to electric fields has shown that some can detect direct current (DC) electric fields as weak as 2.4 microvolts per centimeter, even better than the measured capabilities of platypuses. Although they are still less capable than sharks and rays in this regard, the finding suggests that electroreceptivity may play a more important role in dolphin survival than previously suspected.

Dolphins have small pits on their faces that are rich in nerve endings, known as vibrissal crypts. A 2022 study confirmed this This allows them to detect weak electric fields, but they do not provide an indication of how weak they may be. For species that live in murky rivers or estuaries, it makes sense to develop alternatives to underwater vision, but for dolphins that live in clearer waters, such capabilities may prove unnecessary.

However, it appears that even in their often crystal clear waters, bottlenose dolphins find electrosensitivity useful enough that they have retained it to a significant degree.

Dolphins are not the easiest test subjects, but a team led by Dr Tim Hüttner from the University of Rostock tested two female dolphins, Dolly and Donna, from the Nuremberg Zoo. Their enclosure consists of nine tanks, providing ample opportunity to separate the two from each other and the rest of the pod.

Once a day, each dolphin placed its nose in a headpiece with two electrodes that can produce weak electric fields in the water around them. Dolly and Donna were trained with fish rewards to leave the station if they detected an electric field, and to stay if they did not.

The field strength started at 500 µV cm−1 and was gradually reduced. By comparison, platypuses, the first mammals found to be electrosensitive, can detect fields of 25-50 µV cm2.−1. It turns out the Dolphins can do better than that. After achieving a 96 percent success rate at starting field strength, the two fared less well, but still much better than chance, with lower fields. Dolly’s performance reached a random level at 5.5 µV cm, and she lost the motivation to keep playing underneath. Donna turned out to be more sensitive and detected fields up to 2.4 µV cm−1and performs well, not far above that.

Both dolphins appeared to be less adept at detecting alternating current fields, requiring field strengths up to ten times higher at 1 Hz, and having even more difficulty at higher frequencies.

“Weak bioelectric fields are a reliable short-range source of information for passive electroreceptive animals, as all organisms produce direct current (DC) electric fields in water,” the authors write. These fields are created by the ion current of fish or crustaceans, and are modulated by low-frequency AC potential from muscle activity.

Predators can hunt through these fields, especially if their other senses are blocked. For some fish, the ability to detect electric fields is so essential that they produce their own weak electrical discharges, allowing them to sense a disturbance in the force caused by moving prey.

More often, however, electroreception is purely passive, detecting the fields created by others. It is suspected that this may also extend to the ability to orientate itself relative to the Earth’s magnetic field, not directly as migratory birds do, but through electromagnetic induction in seawater.

Electroreception is so useful that it has evolved many times in different branches of the animal family tree, but it is only known in mammals such as platypuses, echidnas and some dolphins. The latter is particularly curious, as their ability to echolocate may seem to make this unnecessary.

Guyana dolphins were the first dolphin species to have electroreceptivity demonstrated. They live in estuaries around the South American coast and often swim far upstream. They face a particularly muddy environment, and much of their diet comes from fish hiding in the sediments on the sea floor. The ability to detect electric fields that these fish produce offers clear advantages.

Bottlenose dolphins have a much more diverse diet. Just as they have developed remarkably innovative methods of safely accessing fish in traps and protecting themselves from sharp objects, it appears that they have also honed their senses over many generations. Being able to see, hear, taste, smell and touch the world, as well as detect it through echolocation and sense its electrical fields, may make some creatures overwhelmed by the overload of information, but it seems dolphins integrate it all. The authors suggest they use echolocation to detect prey at a distance, and electric fields for close-range work.

The research has been published open access in the Journal of Experimental Biology

An earlier version of this article appeared in December 2023.

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