With the casual efficiency of an animal that has done this ten thousand times, a harbor seal on a California research pier surfaces with a fish in its mouth and hauls itself onto a floating platform. What transpired during the dive is less observable from any reasonable distance. The water in which the fish had been swimming just prior to the seal’s arrival was still in motion, with tiny vortices and pressure waves spinning off the fish’s tail in patterns that continued in the water long after the fish had changed course. The fish escaped the seal’s notice. The water where the fish had been was read by it.
In situations where vision is just not possible, deep-diving seals hunt in this manner. not additional sensing. not a contingency plan. the main mechanism. Seal vibrissae, the technical term for whiskers, are doing the navigational and predatory tasks that most people would suppose require either sight or echolocation in the deep ocean, below the photic zone, where even bioluminescent light contributes just slightly to what an animal can sense. Seals are not echolocators. They have followed a separate evolutionary route, which seems unlikely until you consider the structure of a seal whisker.
The base of each whisker is connected to a blood-filled follicular sinus that contains thousands of nerve endings, making it one of the mammalian sensory organs with the highest mechanoreceptor density. The follicle that surrounds each whisker is essentially a chamber of fluid and nerve tissue tailored to transform incredibly slight physical pressure changes into neural impulses; the analogy to a fingertip is imperfect but directionally helpful. When a fish swims thru water, it moves the water in a certain way. When a fish changes course, it leaves behind a vortex ring signature in the water, which is a spinning column of disturbed fluid that lasts for a few seconds and provides details on the fish’s size, speed, and trajectory. All of that is received as a signal by a seal whisker in that water column.
Once you know to look for it, the structural architecture of seal whiskers is one of those features that really matters. Seal vibrissae have an undulating, wavy cross-section with alternating ridges and valleys down the length of the hair, in contrast to the smooth, cylindrical whiskers of most mammals. At first glance, such shape appears to be a small anatomical anomaly. Its role is far from insignificant. The self-induced vibrations produced by a smooth whisker traveling thru water at the speed of a swimming seal would easily drown out the far more subdued signals from external hydrodynamic sources. This self-generated vibration is prevented by the wavy geometry’s disruption of the fluid dynamics surrounding the whisker, keeping the device quiet enough to detect external signals at astounding sensitivities and distances.
Instead of using their whiskers passively, seals use them aggressively. A rhythmic whisking motion, a back-and-forth sweep of the facial vibrissae, has been seen by researchers studying hunting behavior. This action is similar to how terrestrial rats use their whiskers to investigate surfaces in the dark. This motion creates a spatial image of what has just passed thru the area and in which direction by searching the surrounding water for hydrodynamic trails in the deep ocean. The research record shows that seals can recognize and react to the particular vortex ring patterns produced by fish performing quick escape maneuvers, the kind of abrupt direction change fish use to confuse visual predators. This image appears to contain enough detail to track evasive prey.
The 20 percent bioluminescence contribution to deep-sea hunting is one of those figures that, once you see it, kind of changes your perspective on the deep ocean. Unconsciously, most people believe that light, no matter how dim, serves as the main conduit between predators and prey. That idea just isn’t true in the settings where elephant seals regularly dive—below 1,500 meters. It’s an incidental light down there. Information passes thru the water itself, and the whisker is the device that reads it.
