Decompression sickness, commonly known as “the bends,” is a serious condition primarily associated with human divers who ascend too quickly from deep water. It occurs when gases, particularly nitrogen, dissolved in the blood and tissues under high pressure form bubbles as the surrounding pressure rapidly decreases. This phenomenon can cause significant pain and tissue damage in humans, raising questions about how deep-diving marine mammals, like whales, manage to avoid such a fate.
Understanding Decompression Sickness
DCS occurs when dissolved inert gases, primarily nitrogen, form bubbles in the body’s tissues and bloodstream due to a rapid reduction in ambient pressure, such as a diver ascending too quickly. Under increased pressure at depth, more nitrogen dissolves into the body’s fluids and tissues. If the ascent is too fast, the nitrogen does not have enough time to be safely exhaled through the lungs.
These expanding nitrogen bubbles can obstruct blood vessels, trigger inflammation, and damage various tissues. Symptoms can range from joint and muscle pain to more severe neurological issues, paralysis, or even death. Preventing DCS in humans involves controlled ascent rates, adherence to decompression schedules, and safety stops to allow the body to gradually release accumulated nitrogen.
How Whales Avoid the Bends
Whales possess a suite of remarkable physiological and behavioral adaptations that allow them to navigate extreme ocean depths without succumbing to decompression sickness. Their ability to manage nitrogen is fundamentally different from humans using scuba gear, as whales are breath-hold divers, meaning they descend with a finite amount of air in their lungs. This initial limited air supply inherently restricts the amount of nitrogen available to dissolve into their bodies during a dive.
A key adaptation involves the collapse of their lungs at significant depths. As pressure increases, their lungs compress, forcing air from the gas-exchange regions, known as alveoli, into the reinforced airways where gas exchange with the blood does not occur. This mechanism effectively minimizes nitrogen absorption into the bloodstream and body tissues. Furthermore, whales have flexible rib cages that can compress under immense pressure without causing damage to their vital organs.
Whales also exhibit a diving reflex that helps manage oxygen and nitrogen during deep dives. This reflex includes bradycardia, a significant slowing of the heart rate, and peripheral vasoconstriction, which redirects blood flow away from extremities and less critical tissues towards the brain and heart. This strategic blood shunting conserves oxygen for essential organs while simultaneously limiting the delivery of nitrogen to peripheral tissues. Their specialized lung architecture, which forms two distinct regions under pressure, further optimizes the exchange of oxygen and carbon dioxide while minimizing nitrogen uptake.
When Whales Experience Decompression Sickness
Despite their natural defenses, whales can experience symptoms consistent with decompression sickness, primarily when external factors disrupt their innate protective mechanisms. Research indicates a strong link between mass stranding events of whales, particularly beaked whales, and exposure to mid-frequency active sonar (MFAS) used in naval exercises. The intense sound from sonar can startle or disorient whales, causing them to panic and ascend rapidly or alter their normal, controlled diving patterns.
This abrupt change in behavior, specifically uncharacteristic rapid ascents, can prevent the proper off-gassing of nitrogen, leading to the formation of gas bubbles in their tissues, similar to the bends in humans. The stress response triggered by sonar appears to override their physiological diving reflexes, which normally regulate nitrogen uptake and release. Necropsies performed on stranded whales have provided compelling evidence, revealing gas emboli (bubbles) and tissue damage consistent with DCS.
Beyond sonar, other human-induced disturbances, such as seismic surveys for oil and gas or increased ship traffic, can also introduce significant underwater noise. These stressors can disrupt normal whale diving behavior, potentially forcing them to modify their dive profiles or increase their activity, which could elevate their risk of nitrogen accumulation. Ongoing research aims to better understand these interactions and develop strategies, such as modifying sonar usage patterns or establishing quieter zones, to mitigate the impact on whale populations.