Seals are mammals that must breathe air to survive, despite their life in the ocean. This fundamental requirement allows them to spend extensive periods submerged beneath the water’s surface. Understanding how seals thrive in an aquatic environment while needing air reveals their biological adaptations.
Seals possess lungs and rely entirely on atmospheric oxygen, unlike fish that extract oxygen from water through gills. At the surface, seals breathe rapidly before a dive. Their nostrils close tightly underwater, preventing water entry and conserving air. This reliance on air means seals must regularly return to the surface once their internal oxygen reserves diminish.
Physiological Masterpieces: Diving Adaptations
The ability of seals to undertake prolonged dives is due to a suite of physiological adaptations, collectively known as the dive response. One adaptation is bradycardia, where a seal’s heart rate can slow dramatically, sometimes dropping to as low as 4 to 6 beats per minute during a dive, conserving oxygen. Simultaneously, peripheral vasoconstriction occurs, redirecting blood flow away from less oxygen-sensitive tissues and organs, like the limbs and digestive system, and prioritizing oxygenated blood delivery to the brain, heart, and diving muscles.
Seals also possess a greater capacity for oxygen storage compared to terrestrial mammals. They have a proportionately larger blood volume; elephant seals, for instance, have blood making up approximately 22% of their body weight, compared to about 8% in humans. Their blood contains higher concentrations of hemoglobin, the protein in red blood cells that carries oxygen. Their muscles are rich in myoglobin, a protein that stores oxygen directly within muscle tissue. Myoglobin has a higher affinity for oxygen than hemoglobin, ensuring oxygen is readily available to working muscles.
Before diving, many seals exhale a significant portion of the air in their lungs, which reduces buoyancy and minimizes nitrogen absorption, a gas that can cause decompression sickness. During deep dives, their flexible rib cages and specialized lungs allow for lung collapse, preventing nitrogen from entering the bloodstream and avoiding “the bends.” The spleen also functions as an oxygen reservoir, contracting to release a surge of oxygen-rich red blood cells into the circulation during a dive.
Underwater Endurance: How Long Seals Dive
The duration seals can remain underwater varies depending on the species, their activity, and the depth of the dive. Most seal species can hold their breath for around 30 minutes. Some species, like Southern elephant seals, frequently stay submerged for over 30 minutes, with recorded dives lasting up to two hours. These mammals can also reach depths exceeding 2,000 meters, with a recorded deepest dive of 2,388 meters.
While seals are capable of extended and deep dives, their routine dives are often much shorter. For elephant seals, average dives are around 20 minutes for females and 60 minutes for males, primarily for foraging. Dive duration is influenced by factors such as the seal’s size, overall health, and specific behaviors like foraging, resting, or evading predators.
The Importance of Surfacing
Despite their diving adaptations, seals must eventually return to the surface to breathe. This replenishes their oxygen stores and expels carbon dioxide that has accumulated in their bodies during the dive. While seals are more tolerant of carbon dioxide buildup than many terrestrial mammals, they still need to release it.
Upon surfacing after a prolonged dive, seals often hyperventilate, taking rapid, deep breaths to quickly resaturate their blood and tissues with oxygen. This recovery process prepares them for subsequent dives. The inability to surface, whether due to being trapped under ice or entangled in fishing nets, threatens seals, as it prevents them from accessing air. Research indicates that seals sense their blood oxygen levels, which helps them make decisions about dive duration and when to return to the surface.