Marine mammals like the gray seal, Halichoerus grypus, are known for their remarkable diving skills. To ensure a safe dive, these mammals must be able to recognize when they are running out of oxygen – which is, not surprisingly, an uncomfortable feeling caused by the increase of carbon dioxide, a waste product, in the blood. However, there may be a delay between elevation of blood carbon dioxide and its perception, which occurs in human freedivers and can result in drowning. Therefore, it may be possible that marine mammals can directly perceive blood oxygen levels so they can quickly respond to low blood oxygen levels by returning to the surface. In fact, it has been reported that Amazonian manatees and harbor seals will shorten their dives if they are exposed to low oxygen (hypoxia) prior to diving, because inhalation of low oxygen results in low blood oxygen levels. Then, an ‘oxygen sensor’ in the blood recognizes the low blood oxygen levels and sends a signal to the brain causing the seal to return to the surface. To test whether gray seals are sensitive to low blood oxygen levels, Chris McKnight and colleagues from the University of St Andrews, UK, investigated whether changing the amount of oxygen and carbon dioxide levels can change how long the seals remain submerged for.
To do so, McKnight and colleagues set up four individual sealed breathing chambers with different air mixtures: 21% oxygen (ambient) 50% oxygen (high) and 11% oxygen (low) oxygen levels and 8% carbon dioxide, which is very high compared to normal carbon dioxide levels of 0.04%. After the six seals breathed the different air mixtures for 5 min, the researchers allowed each animal to go diving, recording how long each seal remained submerged, how much oxygen they used while diving and how acidic the seals’ blood became as its carbon dioxide and oxygen levels changed.
The team found that the seals that had inhaled high oxygen (50%) dived 6.4% longer than seals that had breathed normal air with 21% oxygen. However, the seals that had inhaled air with 11% oxygen decreased their dive time by 10%. This suggests that seals can sense whether their blood oxygen levels are adequate for long dives or whether they should stay near the surface. In addition, the team found that seals that had breathed in high levels of carbon dioxide increased the time they spent at the surface by 125.8% compared to seals that had inhaled regular air with normal oxygen and CO2 levels. The seals that had breathed in low oxygen also increased the amount of time they spent at the surface by 60.3%. So, unlike dive duration, the amount of time that the mammals spent recovering at the surface is influenced by both oxygen and carbon dioxide levels in the blood.
In summary, McKnight and colleagues have confirmed that seals decide how long to dive for by sensing the amount of oxygen carried in the blood, rather than the amount of carbon dioxide. Still, it is unknown just when these intelligent seals make the decision to continue diving or resurface. This study suggests that oxygen sensing is a safeguard that has evolved to protect species that hold their breath while diving from drowning. Altogether, it is clear that gray seals should not be challenged in a diving competition.
