Scientists have long known that dolphins can detect electric fields emitted by fish and use this ability to hunt their prey. However, how they’ve been able to do so remained a mystery — until now.
A new study focusing on bottlenose dolphins reveals that dolphins are able to sense electric fields because of the dimples (also called vibrissal pits) on their beak-shaped mouths.
When a baby dolphin is born, the dimples on its mouth harbor whiskers that soon shed as the dolphin matures. Until now, scientists believed these whisker pits or dimples only served as remnants of a dolphin’s childhood whiskers and had no other use.
However, a team of researchers from the University of Rostock and Nürnberg Zoo in Germany conducted some experiments, revealing the real purpose of the dolphin dimples.
Discovering the secret behind dolphin dimples
According to the researchers, all living organisms generate electric fields around their bodies when in water. This is because neuronal activity, such as muscle contractions and nerve fiber activity, generates some electric fields.
Electric fields also result from charged ions generated as part of normal biological processes. For example, fish emit bioelectric fields around their mouths and gills because mucous membranes in these areas come into direct contact with the ocean and release ions into the surrounding water.
The salty seawater itself is loaded with charged ions that help to propagate these fields from the bodies of the fish. And these electric fields are then used by sharks, rays, and dolphins to catch their prey.
When we asked the researchers, how they first realized that dolphins used their dimples to detect electric fields, Tim Hüttner, one of the study authors and a researcher at the University of Rostock, told ZME Science:
“The whisker pits were clearly visible as hot dots on the upper jaw of the dolphins indicating that they are still functional. An accompanying study then revealed that the histology of these crypts of the Guiana dolphin and the bottlenose dolphin resembles the basic structure of known electroreceptors as found in sharks and rays, the so-called ampullae of Lorenzini.”
Furthermore, “A behavioral test with a male Sotalia guianensis then confirmed this as a Guiana dolphin responded to weak electric fields in a psychophysical experiment. Following this, we then started the study at Nuremberg Zoo with the bottlenose dolphins,” Hüttner added.
They conducted an interesting experiment involving two bottlenose dolphins, Donna and Dolly, to test their sensitivity toward static electric fields.
How sensitive are dolphin dimples?
They performed two separate tests. First, they buried fish in deep sandy soil and detected the electric field levels to which Donna and Dolly were sensitive. Next, they detected artificial electric fields produced by electrodes attached to their mouths.
For the second test, the researchers first trained the dolphins to rest their jaws on submerged metal bars and swim away every time they felt an electric field from the electrodes.
They also adjusted the electric field multiple times to find the levels that the dolphin whisker pits were most sensitive to.
“Gradually decreasing the electric field from 500 to 2 μV/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 evident that Donna was slightly more sensitive, sensing fields that were 2.4 μV/cm, while Dolly became aware of fields of 5.5 μV/cm,” the study authors note.
“Therefore, we successfully show that the two bottlenose dolphins are able to detect DC electric fields as low as 2.4 and 5.5 µVcm−1, respectively, a detection threshold in the same order of magnitude as those in the platypus and the Guiana dolphin,” they added.
However, marine animals such as fish don’t always produce static electric fields. The internal movement of their respiratory organs can lead to the generation of fields of varying strength.
Testing the dimples against pulsing electric fields
The researchers produced low-strength electric fields that pulsed one to 25 times per second and noticed that the dimples were able to detect the signals. However, their sensitivity to these AC electric fields decreased with increasing AC frequency.
A similar phenomenon is also observed in sharks and rays. Moreover, both dolphins were more sensitive to DC (static) electric fields than AC signals (varying).
“Dolly could only pick up the slowest field at 28.9 μV/cm, while Donna picked up all three of the oscillating fields, sensing the slowest at 11.7 μV/cm,” the researchers note.
Why are these findings important?
It took the researchers about three and a half years to train the dolphins, measure the sensitivity of their dimples, and come across the abovementioned DC and AC electric field levels.
The current study suggests that dolphins use their dimples to sense electric fields and find food but the utility of these biological sensors possibly goes beyond foraging.
For instance, by establishing electroreception in dolphins it is possible that the animals are able to detect the Earth’s geomagnetic field through electromagnetic induction.
Moreover, being aware of the electrically sensitive dimples might also help scientists to come up with better dolphin conservation strategies.
“We show, that even in the well-known bottlenose dolphin, one of the best-studied mammals in the world, we were able to find a “new” sensory modality, showing that we do not know everything yet. And knowing more about an animal’s behavior, physiology, and in our case, dolphin sensory ecology always helps to better understand the species and yes, this can lead to better-informed conservation plans,” Hüttner told ZME Science.
The study is published in the Journal of Experimental Biology.