In 2016, five Navy SEAL trainees lost consciousness during pool exercises at the Navy’s Basic Underwater Demolition training. One died. All had experienced a shallow water blackout. Several world-class free-divers, diving to great depths without oxygen equipment, have met similar tragic fates. Yet seals routinely swim for comparable periods and often to greater depths. How do seals manage these long, deep dives without succumbing to the same dangers? A series of specialized physiological adaptations make them more “fit” for their aquatic environment:
- Greater oxygen storage - from a larger blood volume, hemoglobin concentration, and greater myoglobin, an oxygen-holding protein.
- More efficient oxygen utilization - from a slowing heart rate and ability to shunt blood away from non-critical organs.
- A unique perception of when their oxygen levels are getting too low.
Yet without that perception, sensing, and responding to decreasing oxygen levels, the increased oxygen storage and efficiency will not prevent them from drowning. To appreciate why this perceptual mechanism is so vital for seals, it helps to explore how humans sense oxygen and carbon dioxide in our bodies.
What Makes Us Breathe
For humans, a land-based mammal, our perception of both O₂ and CO₂ occurs primarily in the carotid body, a slight thickening of the artery as it branches to supply blood to the brain and face. It is sensitive to both oxygen and carbon dioxide. Humans can’t consciously detect changes in oxygen levels unless they’re dangerously low. Instead, we’ve evolved to perceive rising carbon dioxide (CO₂) as a warning signal. Elevated CO₂ causes unpleasant sensations like air hunger and shortness of breath, which are cognitive proxies for low oxygen. Chemoreceptors and brain regions like the cerebellum and amygdala mediate this perception. In mice and humans, the amygdala is key in triggering fear and panic responses to high CO₂.
While these CO₂-driven cues are central to human breathing, new research published in Science reveals that seals follow a very different script. For seals, the perception’s neurologic wiring and sensitivity are just the opposite – they are relatively insensitive to CO₂ and more sensitive to O₂. Evolution has multiple ways to use the same tools to create “fits” to our unique niches.
Seal Science
To investigate how seals detect oxygen, researchers conducted a novel study examining their diving behavior under controlled conditions, testing how oxygen and carbon dioxide changes affect diving behavior. Six juvenile, wild-caught grey seals swam in a simulated foraging setup, diving 60 meters to a feeding station and back to a breathing chamber, where air composition was controlled.
Before diving, seals breathed one of four gas mixtures for 5 minutes:
- Normal air (21% O₂, 0.04% CO₂)
- High CO₂ (21% O₂, 8% CO₂)
- High O₂ (50% O₂, 0.04% CO₂)
- Low O₂ (11% O₂, 0.04% CO₂)
Seals choose how long and often to dive and breathe. After analyzing 119 trials and 510 dives, the results revealed a surprising pattern in how seals react to different gas mixtures.
Increased oxygen extends dive times; decreased oxygen shortens them. An elevation of CO₂ had no impact. When taking a closer look at each trial, the changes in dive durations were due to time spent feeding not swimming to and from the food.
The time spent recovering from each dive, the surface interval, was responsive to both low oxygen and an elevated CO₂ prolonging recovery; elevated oxygen had no impact. This suggests that it was a perceived “oxygen deficit” by both factors that influenced the resting intervals.
Blood pH, which tracks with an elevated CO₂, had no impact.
As it turns out, seals carotid bodies contain more neurons detecting hypoxia exposure and more connections with those sensors to central “processing.” Those neural connections that fire together are enhanced, “wired together,” by repetitive exposure to chronic intermittent hypoxia. Seals maintain this sensitivity regardless of whether awake or asleep, indicating a robust 
and finely tuned O₂-sensing system.
In this study, gray seals adjusted their diving behavior based on oxygen availability, not carbon dioxide levels. Dives were shorter under low oxygen (hypoxia) and longer with high oxygen (hyperoxia), indicating that seals cognitively perceive and respond to circulating O₂ levels. Despite exposure to unusually high CO₂ levels—well above what seals typically encounter—there was no significant change in dive duration or any behavioral signs of CO₂ aversion. Elevated CO₂ only influenced surface recovery time, likely due to limitations in exhaling CO₂. These findings suggest that, unlike many mammals, seals show reduced cognitive sensitivity to CO₂ and rely instead on direct O₂ perception to regulate dive behavior and avoid hypoxic syncope.
“Cognitive CO2 perception alone is therefore insufficient to protect diving mammals against the risk of low O2 resulting in drowning.”
In essence, seals’ reliance on direct oxygen detection prevents the dangerous pitfalls we see in human breath-hold diving, which brings us to Goodhart’s Rule and how proxy measures can fail.
Goodhart’s Rule: "When a measure becomes a target, it ceases to be a good measure."
Humans' urge to breathe is primarily triggered by rising CO₂ levels — not falling oxygen. CO₂ is a proxy or measure of O₂ deficiency since CO₂ typically rises when oxygen is consumed and not replenished. Evolutionarily, for land-based creatures, this makes sense: CO₂ builds up quickly and reliably during breath-hold or airway obstruction, making it a good early warning signal. However, in the context of diving,
“The fallibility of cognitively sensing CO2 but not O2 is demonstratable in breath-hold diving humans. Loss of consciousness and drowning can occur in human free-divers when circulating CO2 levels are lowered through hyperventilation before diving.”
This uncouples the relationship between the proxy, CO2, and the real measure, the risk of a too-low O2. The body feels no urgency to breathe; the CO₂ “alarm” hasn’t gone off, even though hypoxia is creeping in. It is this decoupling that leads to shallow water blackouts.
Goodhart’s Rule Perfectly Explains Our Vulnerability
Under most circumstances, CO₂ as a proxy for O₂ works well. But when the system starts regulating CO₂ itself, especially in artificial contexts like hyperventilation, the signal loses meaning. The result? A biologically efficient system becomes dangerously misleading.
By comparing seals’ O₂-centered system to our reliance on CO₂, we uncover a stark lesson about the limits of proxy signals—one that resonates far beyond the ocean depths.
Seals survive deep, prolonged dives not just because they store more oxygen or use it more efficiently but because their nervous systems are tuned to the real danger: oxygen depletion. Humans, by contrast, rely on CO₂ as a stand-in—a system that usually works until it doesn’t. Shallow water blackouts are the tragic result of mistaking the measure for the mission. In a world increasingly driven by metrics, from biology to policy, Goodhart’s Rule offers a sobering reminder: optimize the wrong signal, and you might miss the real threat.
Source: Cognitive perception of circulating oxygen in seals is the reason they don’t drown Science DOI: 10.1126/science.adq4921
Image courtesy of Sea Mammal Research Unit
Chuck Dinerstein, MD, MBA
Director of Medicine
Dr. Charles Dinerstein, M.D., MBA, FACS is Director of Medicine at the American Council on Science and Health. He has over 25 years of experience as a vascular surgeon.
