Marine mammals catch their prey at depths of up to a kilometer. Elephant seals do this by rhythmically moving their whiskers back and forth. In this way they detect small water movements that fish cause.
That writes an international group of researchers this week in the PNAS after investigation at the northern elephant seal †Mirounga angustirostris), which occurs along the west coast of North America.
Elephant seals are sizable seals that hunt at depths between 400 and 600 meters for small fish species such as lanternfish. Adult males can reach six meters in length and 3,000 kilograms in weight. With a length of three meters and a weight of 650 kilos, the females are much smaller. Other deep divers, such as toothed whales, use echolocation to locate prey such as squid. Elephant seals cannot do this.
By studying blindfolded harbor seals in captivity, scientists previously discovered that whiskers play an important role in hunting. Seals use this to follow the water movements that arise behind swimming fish and can therefore observe prey at a distance of forty meters. Blind seals living in the wild were also able to gather enough food, even though they could not see their prey.
Rhythmic back and forth
Elephant seals have the highest number of nerves per whisker of all mammals, including rodents. Researchers therefore suspected that these animals also use their whiskers to hunt. Such experiments had just never been conducted in the wild.
The scientists tied an infrared video camera and measuring equipment to the cheeks of five females. This allowed them to capture whisker movement and depth. The camera was able to film prey, without the elephant seals being able to see the light from the camera. The rods in the retina only perceive light with a short wavelength and the chosen LED lighting had a long wavelength.
The authors found that the elephant seals kept their whiskers retracted at the start of their dives until they reached the depths where their prey lives. While hunting, the animals rhythmically waved their whiskers back and forth. As they rose to the surface again, the elephant seals pulled in their hair. According to the researchers, the animals do not stretch their whiskers continuously because this consumes too much energy.
A video from the researchers showing an elephant seal moving its whiskers back and forth.
Some elephant seal prey glows in the dark, also known as bioluminescence. The researchers therefore looked at whether the hunting animals used their whiskers differently when luminous prey was nearby. This appeared to not be the case. In addition, the light sensors showed that a fifth of the prey used bioluminescence. The rest of the prey was invisible. According to the researchers, this confirms the hypothesis that the animals can catch their meal by touch.