Unlike humans, most mammals have mobile facial whiskers, yet their natural movement and function are unknown due to observational difficulties, even in well-studied terrestrial whisker specialists (rodents). We report a remarkable case of whiskers contributing to mammal foraging in an extreme underwater environment: the deep, dark ocean. Our animal-borne video cameras revealed that elephant seals captured moving prey by sensing water movement. Their whiskers extended forward ahead of the mouth. Seals performed rhythmic whisker movement to search for hydrodynamic cues, a whisker movement homologous to terrestrial mammals exploring their environment. Based on direct observations, we show how deep-diving seals locate their prey without the biosonar used by whales, revealing another mammalian adaptation to complete darkness.
The darkness of the deep ocean limits the vision of diving predators, except when prey emit bioluminescence. It is hypothesized that deep-diving seals rely on highly developed whiskers to locate their prey. However, if and how seals use their whiskers while foraging in natural conditions remains unknown. We used animal-borne tags to show that free-ranging elephant seals use their whiskers for hydrodynamic prey sensing. Small, cheek-mounted video loggers documented seals actively protracting their whiskers in front of their mouths with rhythmic whisker movement, like terrestrial mammals exploring their environment. Seals focused their sensing effort at deep foraging depths, performing prolonged whisker protraction to detect, pursue, and capture prey. Feeding-event recorders with light sensors demonstrated that bioluminescence contributed to only about 20% of overall foraging success, confirming that whiskers play the primary role in sensing prey. Accordingly, visual prey detection complemented and enhanced prey capture. The whiskers’ role highlights an evolutionary alternative to echolocation for adapting to the extreme dark of the deep ocean environment, revealing how sensory abilities shape foraging niche segregation in deep-diving mammals. Mammals typically have mobile facial whiskers, and our study reveals the significant function of whiskers in the natural foraging behavior of a marine predator. We demonstrate the importance of field-based sensory studies incorporating multimodality to better understand how multiple sensory systems are complementary in shaping the foraging success of predators.