Deadly Diving? Physiological and Behavioural Management of Decompression Stress in Diving Mammals
Authors: S. K. Hooker, A. Fahlman, M. J. Moore, N. Aguilar de Soto, Y. Bernaldo de Quirós, A. O. Brubakk, D. P. Costa, A. M. Costidis, S. Dennison, K. J. Falke, A. Fernandez, M. Ferrigno, J. R. Fitz-Clarke, M. M. Garner, D. S. Houser, P. D. Jepson, D. R. Ketten, P. H. Kvadsheim, P. T. Madsen, N. W. Pollock, D. S. Rotstein, T. K. Rowles, S. E. Simmons, W. Van Bonn, P. K. Weathersby, M. J. Weise, T. M. Williams, P. L. Tyack
DOI / Source: https://doi.org/10.1098/rspb.2011.2088
Date: 22 March 2012
Reading level: Advanced
Why This Matters for Freedivers
This paper is a useful reality-check for the freediving world: even breath-hold divers (and breath-hold animals) can, under certain patterns, accumulate enough nitrogen to form bubbles. The big lesson is that “it’s not scuba so it can’t happen” is a myth—repetitive deep diving, short surface intervals, and certain stressors can shift you from “minimising nitrogen” to “managing nitrogen.”
Synopsis
For decades, the story was simple: scuba divers get the bends because they breathe compressed air at depth, but breath-hold divers (and marine mammals) don’t—because they only take one breath and their lungs collapse at depth, stopping nitrogen uptake. This review argues that the story is not that simple.
The authors explain the core physics first: under pressure, nitrogen (N₂) can move from the lungs into blood and tissues. If tissues become supersaturated during ascent and can’t off-gas fast enough, bubbles may form. Bubbles don’t always cause symptoms (“silent bubbles” exist), but bubble formation is a central suspect in decompression sickness (DCS). They also highlight that the precise “bubble threshold” for causing problems is messy and probabilistic, even in humans.
Then comes the surprising part: there is mounting evidence that bubbles and bubble-related injury can occur in marine mammals under certain conditions. The review goes through multiple lines of evidence, including: - Severe gas and fat emboli lesions reported in some beaked whale mass strandings associated in time/place with naval sonar activity. - Chronic or acute gas-bubble lesions described in UK-stranded cetaceans (with higher prevalence in deep-diving species). - Gas bubbles observed in marine mammals that drowned in nets at depth, suggesting supersaturation plus decompression can produce bubbles. - Ultrasound-detected bubbles in some live-stranded dolphins shortly after stranding (with many animals still recovering, implying some tolerance).
Importantly, the authors don’t claim “marine mammals routinely get the bends like scuba divers.” Instead, they propose a more nuanced model: marine mammals are not simply immune—they are likely managing nitrogen load using a toolbox of adaptations, and sometimes trade-offs force them into higher-risk zones.
That leads to the key conceptual shift of the paper:
From “nitrogen minimisation” to “nitrogen management.”
Marine mammals have several protective mechanisms (lung compression/collapse, the diving response with bradycardia and vasoconstriction, altered perfusion patterns), but these are variable and influenced by behaviour and context. The review highlights that: - The diving response is not fixed; it can be stronger or weaker depending on the situation, and may be under partial voluntary control. - Lung collapse depth may be deeper than once assumed, meaning nitrogen uptake might continue longer into a dive than the classic models suggested. - Animals face trade-offs: buoyancy control, oxygen needs, sound production, thermoregulation, digestion, predator avoidance, and foraging success. Sometimes those needs may push them into dive patterns that increase nitrogen loading.
The authors also discuss modelling work suggesting certain dive profiles—especially with repetitive dives and short surface intervals—could create conditions where some tissues (often slow tissues like fat) gradually load nitrogen. In rare circumstances, a sudden behavioural change (panic/avoidance response) might worsen supersaturation and trigger bubble formation.
Finally, they lay out future research priorities, including better methods to detect bubbles in living animals, improved imaging and instrumentation, and more detailed study of how bubbles cause harm (or are tolerated) at cellular and organ levels.
Abstract
Decompression sickness is caused by inert gas uptake at pressure and subsequent supersaturation and bubble formation during decompression. Breath-hold diving marine mammals were long thought to be relatively immune due to anatomical, physiological and behavioural adaptations that reduce nitrogen loading, such as lung collapse and the diving response. However, recent observations and modelling suggest that gas bubbles may form and tissue injury may occur in marine mammals under certain circumstances. The authors review evidence for bubble incidence and injury, discuss theory and models of gas loading, and propose that marine mammal physiology should be viewed as management of nitrogen load rather than simple minimisation. They outline future research directions, including improved imaging and instrumentation to link bubble occurrence to diving behaviour and to understand pathophysiology and tolerance.