Marine mammals may have a secret weapon to survive long dives – an ability to directly sense their own circulating blood-oxygen levels that most mammals lack – allowing them to stay submerged longer and resurface before hypoxia leads to drowning, researchers report. Air-breathing marine mammals have developed a range of physiological adaptations to survive in aquatic environments, including thermoregulation to endure the pressures of the deep. However, one of the most critical evolutionary challenges for diving mammals is avoiding drowning. Despite adaptations for larger oxygen storage and tolerance to low oxygen levels, these animals still risk drowning if they cannot perceive when oxygen is depleted. In general, it is thought that circulating blood oxygen is cognitively imperceptible to mammals. Instead, most mammals have evolved the ability to sense elevated carbon dioxide (CO2) as a signal for low oxygen, triggering aversive feelings like "air hunger." While the cognitive perception of CO2 serves as a crucial survival mechanism, relying on it as an indicator of low oxygen while breath-hold diving, where CO2 accumulates and is retained in the body, may not be adequate to safeguard marine mammals from drowning during extended dives. To evaluate whether marine mammals can directly perceive and respond to fluctuating oxygen levels, Chris McKnight and colleagues conducted a study on wild-caught gray seals (Halichoerus grypus) to examine how controlled variations in inhaled oxygen and carbon dioxide levels influenced their diving behavior. McKnight et al. found that dive duration was strongly correlated with blood oxygen levels but remained unaffected by CO2 levels or blood pH. According to the findings, even when exposed to CO2 concentrations 200 times higher than ambient air, the seals' dive durations remained unchanged. However, altering oxygen levels – either doubling or halving the ambient concentration – significantly impacted how long the seals remained submerged. McKnight et al. argue that the study provides compelling evidence that gray seals possess the cognitive ability to perceive oxygen levels, allowing them to regulate their dive duration accordingly. And, given the widespread convergent evolution of diving-related adaptations among marine mammals, it is likely that similar oxygen perception mechanisms exist across other species. In a related Perspective, Lucy Hawkes and Jessica Kendall-Bar discuss the study in greater detail.
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