Whisker-cams unravel the mystery of how seals feed in the dark
Using their sonar system, toothed whales are able to zero in on prey in the pitch black conditions of the deep sea. Seals lack such a system, but still catch prey in the same sunless conditions. New research now shows how their whiskers allow them to do so.
The study was conducted by scientists from the University of California - Santa Cruz, the University of Tokyo, and Japan's National Institute of Polar Research. It built upon previous studies which were based on observations of individual whiskers, artificial whiskers, or seals that were held in captivity.
For this project, the team adhered tiny video loggers to the cheeks of nine female northern elephant seals at California's Año Nuevo State Park. Each device was equipped with an infrared LED, which produced illumination that was visible to the camera, but invisible to the seal. The seals were captured and sedated in order to apply the loggers, then recaptured a few days later in order to retrieve the devices.
Based on analysis of a total of 9.4 hours of footage obtained from five of the seals, it was found that once the animals reached a depth of about 200 m (656 ft), they began rhythmically extending and retracting their whiskers. When the whiskers were extended, they appeared to be detecting the waterborne vibrations made by small fish as they swam nearby – the longer the fish appeared on camera, the longer the whiskers remained extended.
This sensing technique not only alerted the seals to the presence of the fish, but allowed the seals to track the location of the fish relative to themselves. In some cases, the seals were able to visually locate squid that gave off bioluminescent signals. Such incidents only accounted for about 20 percent of the prey captured in deep waters, however – the whiskers were responsible for all the rest.
"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 project researcher Taiki Adachi, who is affiliated with both UC Santa Cruz and the National Institute of Polar Research. "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."
A paper on the research was recently published in the journal Proceedings of the National Academy of Sciences of United States of America.