Elephant seals are marine mammals known for their exceptional diving capabilities. Two main species exist: Northern elephant seals (Mirounga angustirostris) and Southern elephant seals (Mirounga leonina). These animals are found in different parts of the world. Northern elephant seals inhabit the Pacific coast of North America, from Alaska to Baja California, while Southern elephant seals reside in sub-Antarctic and Antarctic waters. Their ability to plunge to extreme depths and remain submerged for extended periods places them among the deepest-diving air-breathing animals on Earth.
The Depths They Reach
Elephant seals spend approximately 90% of their time at sea submerged. Their average diving depth ranges from 300 to 900 meters (1,000 to 3,000 feet). Most dives last 20 to 60 minutes, though they can hold their breath for up to two hours.
Southern elephant seals have a record dive of 2,388 meters (7,835 feet). Northern elephant seals also reach impressive maximum depths, with recorded dives exceeding 1,735 meters (5,692 feet). While these are maximum depths, many routine dives are still substantial, often between 300 to 800 meters. Diving patterns vary between sexes and age groups; female northern elephant seals dive continuously for 20 hours or more per day in waters 400 to 600 meters deep.
Specialized Adaptations for Deep Diving
To endure the immense pressures and oxygen deprivation of deep dives, elephant seals possess specialized physiological and anatomical adaptations. They have a large blood volume, about 22% of their body weight, significantly more than humans. This, combined with a high concentration of oxygen-carrying red blood cells and hemoglobin, allows them to store substantial amounts of oxygen in their blood. Their muscles also contain high concentrations of myoglobin, a protein that binds and stores oxygen, further increasing their oxygen reserves.
During a dive, elephant seals exhibit a “dive response” including a slowing of their heart rate (bradycardia). Their heart rate can drop from 80-110 beats per minute at the surface to as low as 3 beats per minute underwater. Blood flow is redirected through peripheral vasoconstriction, shunting blood away from non-essential organs and extremities to prioritize the brain and heart.
Elephant seals exhale most air from their lungs before diving, which helps reduce buoyancy and prevents decompression sickness. Their lungs are designed to collapse under pressure due to flexible rib cages, pushing residual air into the trachea and bronchi where gas exchange cannot occur, protecting delicate lung tissues. This collapse is facilitated by a pulmonary surfactant that prevents lung tissues from sticking together upon re-expansion.
They tolerate low blood oxygen levels and maintain stable blood pH levels even with increased carbon dioxide. Elephant seals also possess a countercurrent heat exchange system in their bloodstreams, helping to conserve body heat in the cold deep ocean.
Why They Dive So Deep
Elephant seals dive to extreme depths primarily for foraging, seeking out prey that inhabits deeper waters. They seek prey like squid, fish, and crustaceans found in the deep, dark mesopelagic and bathypelagic zones. Accessing these food sources gives them an advantage over other marine predators that cannot reach such depths. Elephant seals use their whiskers to detect motion and locate prey in the dark depths.
Predator avoidance is another reason for deep diving. By diving to depths, they can escape predators such as killer whales and large sharks, which hunt in shallower waters. Deep dives minimize their time at the surface, where they are more vulnerable. Diving to depth during migration may also offer an energetic advantage, allowing them to glide more efficiently and conserve energy. Additionally, elephant seals may use deep-water cues, possibly geomagnetic fields, for navigation during migrations.