FREE-DIVING IS a perilous sport. Divers, swimming underwater without oxygen tanks, frequently black out from low oxygen and put themselves at risk of drowning. Marine mammals such as seals, by contrast, can spend most of their lives below the surface without running such risks. A paper published in Science on March 20th explains why: seals can apparently sense how much oxygen they have in their blood and plan their actions accordingly.
When a mammal holds its breath, the amount of oxygen in the body begins to decrease, while the amount of carbon dioxide steadily climbs. In humans low levels of oxygen mostly do not ring any alarm bells on their own—it is high levels of carbon dioxide that eventually produce the unpleasant urge to breathe. As seals dive more frequently—and for longer—than humans do, the range of carbon dioxide in their bodies varies more widely. That led Chris McKnight, a marine biologist at the University of St Andrews, to wonder if the animals could also sense oxygen.
To test the idea, he and his team placed six wild-caught seals in a tank which included an underwater feeding station at one end and a “breathing chamber”, in which the seals could pop up to breathe, at the other. The air in that chamber was then set to one of four conditions: normal air (21% oxygen and 0.04% carbon dioxide, plus other gases); air with reduced oxygen (11%); elevated oxygen (50%); and elevated carbon dioxide (8%). If the seals were, like humans, more sensitive to carbon dioxide, their behaviour would be identical in all but the final condition.
The results suggested otherwise. When there was more oxygen, the seals extended their dives by an average of 14 seconds. When there was less oxygen, they shortened them by roughly half a minute. The high carbon dioxide condition, by contrast, produced no statistically significant effect. In other words, the seals seemed able to perceive how much oxygen they had left and then adjust their dives.
Dr McKnight believes that this ability may be common to marine mammals and other diving species, and could have evolved as an important defence against drowning. Previous studies suggest the tufted duck, snapping turtles and Nile crocodiles might be similarly sensitive.
Pinning down how widespread this ability is will take more research. It is also unclear how diving animals might perceive oxygen differently from surface-dwelling animals and what it feels like to them. Andrew Binks, a physiologist at Virginia Tech, has previously shown that expert divers can use an impending sense of black-out, caused by low oxygen, as a cue to resurface. How seals perform the same feat, however, remains a mystery. ■
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