Seals, marine mammals known as pinnipeds, are highly effective predators adapted to hunting in the ocean’s complex environment. These animals, including true seals and fur seals, range widely in size and habitat, from coastal waters to the vast depths of the open sea. Their success stems from specialized sensory tools and remarkable physiological capabilities that enable them to locate, pursue, and capture prey where other mammals cannot. Understanding how seals hunt involves examining the sophisticated mechanisms they use to track targets underwater and the distinct behavioral strategies they employ to secure a meal.
Specialized Sensory Tools for Underwater Tracking
Seals use their highly sensitive vibrissae, or whiskers, as a primary tool for locating prey, especially in murky, low-light conditions where vision is limited. These whiskers are not merely for touch; they are specialized hydrodynamic sensors capable of detecting minute water movements. As a fish swims, it leaves a trail of swirling water disturbances, known as vortices or eddies, and the seal’s whiskers can detect this trail long after the prey has passed.
The whiskers on a seal’s muzzle have a unique, wavy or undulating cross-section, which is a structural adaptation that helps suppress self-induced vibrations. This design prevents the seal’s own swimming motion from generating noise, enhancing the signal-to-noise ratio for detecting faint hydrodynamic trails. Studies have shown that a blindfolded seal can accurately follow a simulated hydrodynamic trail for tens of seconds, demonstrating the precision of this sensory system. This allows them to track a fish’s movements, determine its direction, and even estimate its size and speed from a distance.
While the vibrissae are primary, seals also possess acute low-light vision and excellent underwater hearing, which complement their tactile tracking. Their large, spherical eyes are adapted to gather light efficiently in the dim depths, though they are generally considered color-blind. Pinnipeds can hear sound frequencies up to 70,000 Hz, giving them an auditory advantage for tracking vocalizing prey or navigating in the dark. These combined senses ensure that seals are rarely at a disadvantage.
Primary Hunting Strategies and Prey Capture
Seals employ varied hunting strategies tailored to the type and behavior of their prey, which can range from small schooling fish to large penguins or squid. The most common method is active pursuit hunting, where seals use bursts of speed and agility to chase down fast-moving fish like herring or salmon. Harbor seals, for instance, are known to adjust their tactics, using stealth and suction feeding for cryptic bottom-dwelling prey, but switching to swift raptorial biting for conspicuous fish swimming in the open water.
Another effective technique is ambush hunting, which is especially common for species living near ice floes. Some seals use stalking techniques, such as waiting near known concentrations of prey, or using the cover of darkness to approach fish hiding near rocks before pinning them down.
While many seals are solitary hunters, some species will occasionally engage in loose cooperative hunting to increase their success rate against dense schools of fish. California sea lions have been observed herding schools of fish into dense, manageable groups, making individual capture easier. Small fish are typically swallowed whole underwater, often using powerful suction feeding to draw the meal directly into the mouth. Larger prey, such as sea birds or bigger fish, must be brought to the surface and processed with a behavior known as “shake feeding,” where the seal violently shakes the prey to tear it into manageable, swallowable pieces.
The Physiology Enabling Deep Pursuit
The demanding nature of deep or prolonged underwater hunting is made possible by a suite of physiological adaptations known as the mammalian dive reflex. This reflex is automatically triggered upon submersion and allows seals to conserve oxygen stores efficiently. One of the most dramatic changes is bradycardia, a profound and immediate slowing of the heart rate that can reduce beats per minute by up to 90% within the first moments of a dive.
Working in concert with the slowed heart rate is peripheral vasoconstriction, a process where blood flow is restricted from non-essential tissues, such as the limbs, digestive tract, and much of the skeletal muscle. This shunts the limited oxygenated blood supply toward the organs most intolerant of oxygen deprivation, namely the brain and the heart. The muscles that are isolated from the main blood flow must rely on their own localized oxygen reserve, greatly reducing the overall rate of oxygen consumption in the body.
This localized reserve is substantial because seals have a remarkably high concentration of the oxygen-binding protein myoglobin within their muscle tissue. The level of myoglobin in seal muscle can be 10 to 30 times greater than that found in terrestrial mammals, providing a large, ready-to-use oxygen supply for active muscles. Furthermore, the spleen acts as a specialized reservoir, contracting during the dive to release a surge of oxygen-rich red blood cells into the circulation. These three adaptations—bradycardia, vasoconstriction, and high oxygen storage capacity—provide the biological foundation for the seals’ exceptional endurance and ability to execute their complex hunting strategies in the deep ocean.