Elephant seals, the largest of all pinnipeds, are marine mammals that spend the majority of their lives in the open ocean, undertaking one of the most remarkable foraging migrations on the planet. These massive creatures, divided into Northern (Mirounga angustirostris) and Southern (Mirounga leonina) species, derive their name from the large, inflatable proboscis of the adult males. To sustain their immense size, elephant seals have developed unique and specialized feeding behaviors that involve routine deep-sea diving, allowing them to exploit resources unavailable to most other marine predators.
Diet and Preferred Prey
Elephant seals are generalist, opportunistic carnivores, but their diet is primarily composed of organisms found in the mesopelagic, or “twilight,” zone of the ocean. Their meals mainly consist of various deep-sea cephalopods, particularly squid, which they encounter at great depths. Specific squid species like the luminescent Octopoteuthis deletron and various Histioteuthidae are frequently identified in the stomach contents of foraging seals.
The seals also consume a wide variety of deep-dwelling fish, including skates, rays, eels, and small sharks. Lanternfish (myctophids), which are globally abundant in the mesopelagic zone, represent another substantial part of the diet.
Specialized Foraging Locations
The search for food drives elephant seals to undertake vast, biannual migrations, traveling thousands of kilometers from their coastal rookeries into the open ocean, or pelagic zone. These foraging trips often target specific and distant oceanic features where prey is concentrated. For Northern elephant seals, this often means traveling to the Gulf of Alaska, while Southern elephant seals range across the Antarctic and sub-Antarctic waters.
A distinct pattern of sexual segregation marks their hunting locations, with males and females often exploiting different marine environments. Males typically forage over the continental shelf and slopes, often making benthic dives to the seafloor to target prey near the bottom. Females, being smaller, generally travel further out to sea, focusing on the open-ocean mesopelagic zone, where they hunt species that undergo daily vertical migrations. This separation in hunting grounds helps reduce competition between the sexes for available prey.
The seals primarily hunt in the mesopelagic zone, which extends from about 200 to 1,000 meters below the surface. This “twilight zone” receives only faint, filtered sunlight, creating an environment where deep-sea prey are abundant. Many seals also target productivity hotspots, such as the boundaries between major ocean currents or areas around seamounts, where upwelling currents bring nutrient-rich water and concentrate food sources.
The Mechanics of Deep-Sea Hunting
The ability of the elephant seal to hunt at extreme depths is underpinned by a suite of physiological adaptations that maximize oxygen efficiency. Before a dive, the seal exhales, collapsing its lungs to avoid buoyancy and prevent the absorption of nitrogen gas, which is the cause of decompression sickness in human divers. Oxygen is instead stored primarily in the blood and muscle tissue, which have significantly elevated concentrations of hemoglobin and myoglobin, respectively.
Once underwater, the seals employ a signature “yo-yo” diving pattern, characterized by a rapid descent to foraging depth, a brief period of hunting, and a return to the surface, followed by a very short surface interval of only two to three minutes. The descent phase is often achieved by gliding for up to 90% of the time, utilizing negative buoyancy to conserve energy. This energy-saving technique allows them to spend up to 90% of their time at sea submerged and hunting.
To further manage their limited oxygen supply, elephant seals exhibit a profound dive response, or bradycardia, drastically slowing their heart rate from a resting rate of around 80 to 110 beats per minute to as low as three beats per minute during deep dives. This is coupled with peripheral vasoconstriction, a process that shunts blood flow away from the skin and non-essential organs toward the core, ensuring the heart and brain receive a constant supply of oxygenated blood. The seals use the low-light conditions of the mesopelagic zone for stealth, ambushing their prey, which they then swallow whole without needing to manipulate it with their forelimbs.