Northern elephant seals occur in the eastern Pacific and commonly breed on islands off the coast of California. They have been recorded diving to great depths in order to forage for their favorite fish and squid, and scientists have long wondered how they locate their prey in the blackness of the deep ocean.
Sunlight does not penetrate very deeply into ocean water so elephant seals that dive deeply will not be able to see their prey. Some marine organisms do emit bioluminescence, and it has been postulated that the large eyes of the seals are adapted for detecting this source of light. However, not all potential prey species are bioluminescent, and this form of light is very feint in the great dark abyss anyway.
Toothed whales make use of echolocation or biosonar to detect prey in the darkness, but elephant seals do not have these adaptations. They do have spectacular whiskers, however, that have been studied in the past in order to understand their function. Known as “vibrissae”, these thick, mobile facial whiskers are powerful sensory organs used to detect vibrations. The word “vibrissae” comes from the Latin word “vibrio” which means “to vibrate.”
Previous studies have investigated the functioning of isolated whiskers, proposed artificial models, and observed whisker use on captive animals. But until now, researchers have not understood the movement and function of the mammals’ facial whiskers in their natural environment because of the challenges of observing the seals when they forage in the dark depths. In a new study, published today in the journal Proceedings of the National Academy of Sciences, researchers have now used tiny video loggers to record exactly what goes on with a seal’s whiskers as it searches for prey in the depths.
The researchers placed small video loggers on free-ranging female northern elephant seals, choosing elephant seals in particular because of their highly sensitive whiskers. These seals have the highest number of nerve fibers per whisker of any mammal. The researchers mounted the video loggers on each seal’s cheek to observe how the seal moves and uses the whiskers in front of its mouth as it searches for food in the dark ocean. Northern elephant seals feed mostly on pelagic fish and squid, but will forage on the ocean floor as well.
Each data logger had an LED red/infrared-light flash that enabled the researchers to see in the dark depths, and to observe what the seal did with its whiskers as it searched for and approached its prey. The light was not visible to the seal and so the procedure did not disturb the animal in its quest to find food.
The cameras showed that the seals moved their whiskers forwards and backwards a lot as they approached prey. With their whiskers extended forward, ahead of the mouth, they were able to detect very slight movements of the water and locate prey in this way. They performed rhythmic whisker movement, extending and retracting their whiskers, to search for hydrodynamic cues in the water just ahead of their mouths. Terrestrial mammals do a similar thing when they are searching their environments for odors and information.
The team took into consideration the possibility that light provided by the bioluminescence in some prey might help the seals in their hunt for food. But their findings reveal that, while bioluminescence is important, the seals’ sensitive whiskers are the primary method the mammal uses to find its prey.
The seals’ whiskers allow them to search for, pursue, and capture prey. “Our findings solve a decades-long mystery about how deep-diving seals locate their prey without the biosonar used by whales, revealing another mammalian adaptation to complete darkness,” said Taiki Adachi, project researcher at the National Institute of Polar Research / Assistant Project Scientist of University of California, Santa Cruz.
This research complements earlier whisker studies conducted on mammals in captive conditions, and it propels forward the field of sensory ecology of foraging. “The next step is conducting comparative field studies on other mammals to better understand how whisker-sensing shapes natural behavior in each mammalian species under different environments,” said Adachi.