The so-called vibrissae are used by seals as sophisticated antennae for underwater hunting, as a study shows: Researchers looked at the snout of elephant seals during their forays in the dark deep water and thus showed the central role of the whiskers in the hunt. When prey is nearby, they tensely stretch the long structures forward, moving them back and forth rhythmically. This allows them to perceive subtle water movements caused by their prey, the researchers explain. It is becoming clear, they say, that this system is another sophisticated adaptation of mammals to hunting in the dark seas, alongside the whales’ biosonar.
Cats, mice and even seals are equipped with them: Many mammals have whiskers on their snouts. Some studies have already dealt with the meaning of these structures, called vibrissae in the technical language. One thing is clear: these are the antennae of a perception system. At the base of the vibrissae are nerves that record fine movements of the hair and transmit them to the brain. In addition, they can be moved via muscles and thus specifically aligned as required. This allows the “bearded” animal species to gain additional information about their surroundings, especially in the dark. As studies have shown, some species not only register contact with objects: rats can even determine the wind direction with their whiskers and thus localize sources of smells, for example.
Hairy Sensory System in Water
But what role do the sensory structures play for mammals hunting in the water? Basically, earlier studies have already shown an important role of the whiskers for orientation in the liquid medium. For example, seals kept in captivity have been shown to be able to follow moving objects in the water even when blindfolded. This has been attributed to their ability to use their whiskers to detect hydrodynamic traces in the water. However, these earlier studies were conducted in experimental settings and under artificial circumstances. The researchers led by Taiki Adachi from the University of California in Santa Cruz have now for the first time investigated under natural conditions how seals use their whiskers when hunting in dark water depths.
They chose the largest representatives of this group of animals as test animals: elephant seals. Interestingly, studies have shown that these animals have the highest number of nerve fibers per whisker of any mammal. Elephant seals dive enormously deep when hunting: they prey on sea creatures in dark depths of up to over 600 meters. Other mammals also hunt there – but the toothed whales have biosonar, which they use to locate their prey. The seals lack this system, but unlike whales, they have whiskers. For their study, the researchers attached small cameras to the heads of female elephant seals so that their snouts and whiskers were in their sights. The device was equipped with a red/infrared LED light that provided illumination that the animal could not see. This made it possible to record the hunting behavior largely without interference.
Sophisticated biosensor technology
As the team reports, the evaluations showed: On the way to and from the hunting grounds at depth, the whiskers of the elephant seals took a laid-back position. In the area where prey was expected, the animals raised their whiskers, then hung forward. They then also performed rhythmic movements – they slightly pulled their whiskers back and forth. According to the researchers, this behavior has also been observed in land mammals when they try to detect subtle signals from their environment. In the case of the elephant seals, it has now been shown that they apparently use the concept to detect fine movement patterns in the water, which can provide clues to nearby prey. The recordings also show that the tensing of the antenna system is a strain on the animals. That’s why they keep their whiskers loose when they’re not needed, the researchers explain.
Video © Taiki Adachi et al. (2022)
As part of their study, they also investigated the role played by the whisker sense compared to the use of the eyes when elephant seals catch prey. Because it can be assumed that they use them to perceive the light effects that some marine animals produce through bioluminescence. However, the results showed that visual perception plays a comparatively minor role in hunting. The camera’s light sensors showed that the elephant seals only saw the bioluminescence of the prey in about 20 percent of successful prey attempts.
The current study thus complements previous whisker studies and expands the knowledge in the field of animal sensory performance, the researchers conclude: “Our results shed light on the question of how deep-diving seals localize their prey without the biosonar used by whales, and illustrate another adaptation of mammals to total darkness,” says Adachi. He and his colleagues now want to continue working on the topic: “We are planning comparative studies on other mammals in order to better understand how the whisker sense shapes the natural behavior of the individual mammal species in different environments,” says the scientist.
Source: Research Organization of Information and Systems, professional article: PNAS, doi: 10.1073/pnas.2119502119