If you look carefully at the snout of a dolphin, you’ll see two rows of tiny pits, known as vibrissal crypts, When dolphins are born, these pits house whiskers that soon waste away to leave empty craters. It’s tempting to think that the crypts as useless evolutionary throwbacks to a time when the ancestors of dolphins used whiskers to feel their way about. But these structures are far from useless. In at least one species of dolphin, they can sense electricity.
Nicole Czech-Damal from the University of Hamburg discovered this amazing ability by studying the Guiana dolphin, also known as the costero. It looks a lot like the familiar bottlenose dolphin, but its vibrissal crypts are far larger. Back in 2000, these prominent pits intrigued Guido Denhardt, who decided to look at them using a heat-sensitive camera. He found that the dolphin’s crypts produce spots of intense heat, burning as brightly on the camera as the whiskers of harbour seals.
The heat spots implied that, contrary to what people had thought, the vibrissal crypts are fuelled by a strong supply of blood. They are not useless vestigial organs – these crypts do something.
Czech-Damal confirmed that by dissecting the crypts of a Guiana dolphin that had died of natural causes at Munster Zoo. She saw that each pit looks like a long pitcher, surrounded by blood vessels. They are also suffused with branches of the trigeminal nerve, which carries information from the senses to the brain.
They were clearly sense organs, and they looked rather similar to structures that allow sharks and platypuses to sense electric fields. Czech-Damal began to suspect that the dolphins were using their crypts for the same purpose.
To test that idea, she worked with Paco, another of Munster Zoo’s resident Guiana dolphins. Paco’s keepers trained him to put his head in a hoop, at a fixed point in front of two electrodes. The electrodes produced small electric fields, of similar strength to those given off by a small fish. Eventually, Paco learned to stay in place if he didn’t detect an electric field, and to swim out of the hoop if he did. He could detect any field above 4.6 microvolts per centimetre – more than enough to detect a fish.
When the keepers covered Paco’s vibrissal crypts with a plastic shell, he no longer responded to even intense electric fields. If the shell has small holes that allowed sea water to reach the crypts, Paco’s electric sense returned.
Dolphins, of course, are not short of super-senses. They navigate and hunt with sonar, producing clicks and timing their rebounding echoes. The evolution of this ability could explain why they lost the close-range whiskers of other mammals in the first place. But it seems that the structures that once housed whiskers have evolved a new ability, and one that supplements the invaluable sonar.
The Guiana dolphin often roots around the seafloor in search of hidden prey, and divers have seen them kicking up mud-plumes while they feed. They also live in shallow waters of coasts and estuaries, where silt and mud can turn the water into a turbid mess. Their electric sense could help them to cut through the murk. Even buried fish cannot hide from a hunter that can sense the weak electric fields that they inevitably produce.
This ability, known as electroreception, is fairly common among fish. Predators like sharks and rays use these fields to track their prey, while some electric fish can produce their own fields, using them to find their way around, talk to each other, and even stun their prey. Only a small number of mammals share the same ability. So far, the exclusive list includes the four species of echidna, and the duck-billed platypus. Now, the Guiana dolphin can join them. It is ten times more sensitive than the platypus is, but probably a million times less sensitive than a shark.
It’s probably not the only dolphin with this ability. Wolf Hanke, who led the study, says, “Other dolphins have these structures too, like the well-known bottlenose dolphin. Its vibrissal crypts are, however, smaller. For now, no one knows if the bottlenose or other dolphins use their crypts in the same way that the Guiana dolphin clearly can. But Hanke adds, “It is striking that bottlenose dolphins have been observed digging for food in the ground, where electroreception would be most useful.”
Hanke’s team is starting to look at the crypts of other species (his paper tantalisingly references “Czech-Damal et al. In preparation”). They are also trying to find out how the vibrissal crypts actually work. “We have reason to believe that the dolphin allows a tiny current to flow through the wall of the vibrissal crypt, where it has to pass nerve endings that will be excited by that current,” says Hanke. To aid with this, the crypts contain a gel-like substance that might conduct electricity. Other electric-sensing animals use a similar trick – sharks and other fish have jelly-filled pits, and platypuses top theirs up with mucus.
Reference: Czech-Damal, Liebschner, Miersch, Klauer, Hanke, Marshall, Dehnhardt & Hanke. 2011. Electroreception in the Guiana dolphin (Sotalia guianensis). Proc Roy Soc B http://dx.doi:10.1098/rspb.2011.1127