In a groundbreaking study, a team of scientists from the Sea Mammal Research Unit at the University of St Andrews, alongside collaborators from the University of Exeter and the University of California, San Diego, have revealed a fascinating discovery about the physiology of gray seals. Their research, published in Science, shows that gray seals have a unique ability to monitor their blood oxygen levels to avoid drowning. This new insight adds a remarkable layer to our understanding of how marine mammals, which are known for their ability to stay underwater for long periods, manage their breathing and avoid oxygen deprivation.
The Challenge of Breathing Underwater
All mammals require oxygen to survive, and while humans and most land mammals can breathe continuously, marine mammals face a unique challenge. When underwater, they can’t simply inhale freely whenever they need to. Instead, they rely on remarkable adaptations that allow them to hold their breath for extended periods. Many marine mammals, including seals, dolphins, and whales, can stay submerged for minutes or even hours without surfacing for air. Their ability to do this is a testament to the efficiency of their respiratory and circulatory systems.
However, just like any mammal, the need for oxygen is constant. When we breathe, we inhale oxygen and exhale carbon dioxide, a byproduct of cellular respiration. In humans and other land mammals, a build-up of carbon dioxide in the blood signals the body to take a breath, alerting us to the need for fresh air. This is a basic survival mechanism that keeps us from suffocating. But for marine mammals that spend significant time underwater, the question arises: how do they know when it’s time to surface for air if they don’t have the constant sensory feedback we rely on?
The New Discovery: Blood Oxygen Monitoring in Gray Seals
The research team, led by scientists at the University of St Andrews, was determined to uncover how gray seals, in particular, manage this delicate balance between staying underwater for long periods and avoiding hypoxia (oxygen deprivation). Unlike land mammals that rely heavily on carbon dioxide as a trigger to breathe, the team speculated that marine mammals might have a different mechanism at play, one more finely tuned to their underwater environment.
To investigate, the scientists captured six adult gray seals and brought them to a controlled test pool where they could monitor the animals’ breathing responses. The goal was to explore how different mixtures of oxygen and carbon dioxide affected the seals’ behavior underwater. To ensure the seals could only come up for air at a designated chamber, the researchers set up an enclosed testing area where the seals swam between a feeding station and a breathing chamber.
In the test pool, the team carefully adjusted the air composition in the breathing chamber, exposing the seals to four different air mixtures:
- Ambient air (the normal air composition)
- Air with double the usual amount of oxygen
- Air with half the normal oxygen level
- Air with normal oxygen levels but with carbon dioxide levels 200 times higher than normal
Each air mixture presented a different challenge to the seals’ ability to manage their blood oxygen and carbon dioxide levels. By observing the seals’ behavior and timing how long they remained underwater, the researchers were able to glean valuable insights into how these animals respond to varying levels of oxygen and carbon dioxide.
The Findings: Oxygen Levels as a Key Trigger
What the researchers discovered was both surprising and enlightening. They found that the seals adjusted the length of time they spent underwater in response to the oxygen levels in the air. When the seals were exposed to air with higher-than-normal oxygen levels, they stayed submerged longer, suggesting that the seals were able to sense and respond to the increased availability of oxygen. Conversely, when the oxygen levels in the air were reduced, the seals surfaced more quickly, likely in response to a decrease in their available oxygen supply.
Interestingly, the researchers also discovered that elevated levels of carbon dioxide did not seem to have the same effect on the seals’ behavior. Despite the high concentrations of CO2 in the breathing chamber, the seals continued to stay underwater for the same duration, indicating that carbon dioxide levels alone did not play a significant role in signaling when it was time to surface. This was a critical finding, as it suggested that the gray seals rely more on monitoring their oxygen levels rather than carbon dioxide buildup to determine when to come up for air.
This ability to monitor blood oxygen levels as a form of self-regulation is a fascinating adaptation that likely evolved to help seals maximize their time underwater while avoiding the risk of drowning. Unlike land mammals, which rely heavily on carbon dioxide as a trigger, gray seals have evolved a more sophisticated and precise mechanism that allows them to avoid the dangers of low oxygen while maintaining their ability to stay submerged for extended periods.
The Implications of the Research
This study offers a profound new perspective on the physiological adaptations of marine mammals. While previous research has shown that marine mammals can hold their breath for long periods by slowing down their metabolism and redirecting blood flow to essential organs like the brain and heart, this new finding suggests that they may have an even more refined system in place. By continuously monitoring their blood oxygen levels, gray seals may be able to fine-tune their underwater behavior, balancing the need for oxygen with their desire to remain submerged for as long as possible.
Understanding how seals and other marine mammals regulate their oxygen intake could have significant implications for a range of fields, from conservation efforts to medical research. For instance, scientists might be able to study these mechanisms to better understand human respiratory health, particularly in relation to conditions that involve impaired oxygenation. Additionally, the findings could be used to inform conservation strategies for marine mammal species, especially in areas where human activity—such as noise pollution or climate change—could disrupt the seals’ natural behaviors.
Furthermore, this discovery may shed light on the broader question of how animals in extreme environments—whether deep-sea creatures, high-altitude dwellers, or those living in oxygen-deprived environments—have evolved to cope with the challenges of oxygen management. By studying marine mammals, researchers could unlock new insights into the biological mechanisms that support life in conditions that would otherwise be inhospitable to most species.
A Deeper Look at the Methodology
The methodology employed by the researchers is just as fascinating as their findings. The experiment involved the careful design of a controlled environment where all variables could be monitored and adjusted. The use of a sealed test pool allowed the team to control the air the seals breathed, ensuring they could manipulate the oxygen and carbon dioxide levels with precision.
Furthermore, the researchers took extra steps to ensure that the seals’ responses were not influenced by external factors. By using only six seals, they kept the sample size small enough to monitor each animal’s individual behavior closely. The scientists also observed the seals during the same types of tasks—swimming back and forth between the feeding station and the breathing chamber—so they could compare how each air mixture affected the seals under similar conditions.
The seals’ behavior was then measured in terms of the duration of time spent underwater and the frequency with which they surfaced. This approach allowed the team to draw clear conclusions about the relationship between oxygen levels in the air and the seals’ underwater behavior. The meticulous design of the experiment adds significant credibility to the findings, ensuring that the results are both accurate and meaningful.
Conclusion: A New Frontier in Marine Mammal Physiology
The discovery that gray seals can monitor their blood oxygen levels as a method of preventing drowning is a remarkable breakthrough in the field of marine biology. It not only deepens our understanding of how marine mammals manage their breath-holding abilities but also highlights the adaptability and sophistication of these animals in navigating their challenging underwater environment.
As researchers continue to study the physiology of marine mammals, we may uncover even more surprising adaptations that enable them to thrive in one of the planet’s most extreme habitats. This discovery opens up new avenues of research, not only in understanding the lives of seals but also in exploring how similar mechanisms might apply to other species, including humans.
The gray seal’s ability to monitor its blood oxygen levels is a testament to the wonders of evolution, offering a glimpse into the complex systems that support life on Earth. As we continue to learn more about these remarkable creatures, we are reminded of the intricate connections that exist between animals and their environments, and the ways in which nature has equipped them to survive and flourish.
References: J. Chris McKnight et al, Cognitive perception of circulating oxygen in seals is the reason they don’t drown, Science (2025). DOI: 10.1126/science.adq4921
Lucy Hawkes et al, A deep dive into oxygen sensing, Science (2025). DOI: 10.1126/science.adw1936