Northern and Southern elephant seals are champions of deep-sea diving, spending most of their lives in the open ocean. These massive pinnipeds possess a suite of physiological adaptations that allow them to routinely perform dives that would be fatal to most terrestrial creatures. Their exceptional capacity to manage oxygen stores and tolerate extreme conditions helps scientists understand the limits of mammalian diving performance.
The Maximum Breath-Hold Duration
Elephant seals spend approximately 80 to 90 percent of their time at sea submerged. Routine foraging dives typically last between 20 and 30 minutes, with seals spending only brief periods at the surface to recover between descents. These average durations allow the seals to operate within their aerobic limit, relying on oxygen already stored in their bodies.
However, when pushed to their physiological limits, their breath-holding capacity extends far beyond this routine window. The longest scientifically recorded dive for a Northern elephant seal is 119 minutes, making it the longest breath-hold of any non-cetacean mammal. Southern elephant seals have also been recorded sustaining dives of well over 100 minutes.
A noticeable difference exists in typical dive durations between sexes. While females commonly average dives around 20 minutes, adult males often sustain routine dives closer to 60 minutes. This difference is largely attributed to variations in body size, oxygen storage capacity, and the specific foraging strategies each sex employs.
Physiological Adaptations for Extreme Apnea
The elephant seal’s ability to remain underwater stems from a highly specialized system for oxygen storage and conservation. Unlike humans, who store most oxygen in their lungs, these seals carry a significantly higher proportion of their oxygen load in their blood and muscles. This is achieved through a large total blood volume and a high concentration of hemoglobin, the protein in red blood cells responsible for transporting oxygen.
The muscles are saturated with myoglobin, a protein that stores oxygen directly in the muscle tissue. This myoglobin concentration is far greater than that found in terrestrial mammals, providing a localized oxygen reserve for muscle activity during a dive. Together, these blood and muscle stores allow elephant seals to carry more than three times the amount of oxygen per unit of body mass compared to humans.
The second mechanism for maximizing dive time is the activation of the mammalian dive reflex, an involuntary response that drastically conserves oxygen. Upon submerging, the seal experiences bradycardia, a sudden and profound slowing of the heart rate that can drop from a surface rate of 80–110 beats per minute to as low as three beats per minute. This reflex is paired with peripheral vasoconstriction, the constriction of blood vessels in non-essential areas like the flippers, skin, and digestive system.
This blood shunting redirects oxygenated blood almost exclusively to the heart and the brain. By isolating circulation, the seal prioritizes the survival of its most vital tissues. Furthermore, before diving, elephant seals exhale most air from their lungs, which helps reduce buoyancy and prevents the absorption of nitrogen gas under pressure, thereby avoiding decompression sickness.
Metabolic Management
Metabolic management also plays a role in extending their time underwater. For the most extended dives, the seals tolerate anaerobic respiration in peripheral tissues temporarily cut off from the main blood supply. This process generates energy without oxygen but produces lactic acid. The seals are adapted to tolerate these low oxygen levels (hypoxemia) and the buildup of lactate, which is metabolized once they surface and normal circulation is restored.
Why Elephant Seals Dive So Deep and Long
The extreme diving behavior of elephant seals is an adaptation driven by the necessities of finding food and avoiding danger. Their primary foraging grounds are the mesopelagic and bathypelagic zones, where their preferred prey resides. This deep layer contains an abundance of squid, small sharks, skates, and various deep-sea fish.
The seals typically follow the vertical migration patterns of this deep-sea prey, which often rise closer to the surface at night but remain at depths of 400 to 800 meters during the day. The continuous diving pattern allows them to efficiently search and capture food during their months-long pelagic migrations.
The long breath-holds and deep dives also serve as an effective strategy for predator avoidance. Orcas and great white sharks, the main natural predators, primarily hunt in the shallower, sunlit waters near the surface. By spending the vast majority of their time at depths well below where these predators hunt, the seals minimize their risk of attack. This behavior provides an ecological advantage, allowing them to exploit resources unavailable to most other predators.