BioEd Online

Marine mammals have an uncanny ability to remain awake and alert while holding their breath during long dives. New research shows that this might be thanks to high concentrations of oxygen-binding proteins in the brain. The proteins, called globins, could help these mammals to remain active long after blood oxygen levels sink low enough to cause a human to black out.

Some marine mammals can spend more than 90 minutes under water, altering their physiology to make the most of their limited oxygen supply. Their heartbeats slow down, and blood flow is prioritized to the brain by mobilizing oxygen stores in muscles and constricting blood vessels leading to peripheral tissues. They can also suppress their shivering instinct in cold waters, allowing their body temperature to drop, again conserving oxygen (see Seals don't shiver in chilly waters).

These changes are similar to those seen in hibernation, leading some to speculate that diving mammals enter a sort of torpor. But Terrie Williams, a marine-mammal physiologist at the University of California at Santa Cruz, has observed mammals during their dives and knows that they often remain active.

"They’re alert, their eyes are moving around watching you," says Williams. "These are not animals that appear to be asleep or hibernating. They’re on top of their game."

Brain oxygen boost

Williams decided to look for other physiological changes that could help to account for the marine mammals’ resilience. She reasoned that globins could provide an answer. Haemoglobin, a prominent oxygen-carrier found in the blood, is the best known member of the globin family, but researchers have recently uncovered more globins, including two that are found specifically in the brain.

Williams and her colleagues obtained brain samples from the carcasses of 16 mammalian species, ranging from mice to whales. They then tested each sample for its haemoglobin concentration and found that it showed a 9.5-fold range across the species. Marine mammals generally had higher concentrations of this globin and up to three times more of the two neural globins in their brains than terrestrial mammals had.

Overall, the difference was so striking that Williams says she learned to tell a terrestrial animal from a marine mammal just by looking at their brains: the haemoglobin-rich blood of marine mammals stains the brain a darker red (see picture). The results are published this week in Proceedings of the Royal Society B¹.

Brains apart

The work is an important step towards understanding the abilities of marine mammals, says Andreas Fahlman, a physiologist in Ottawa, Canada who has studied diving animals. Fahlman has also observed sea lions being very active during their long dives. "Very few people have looked at this," he says, mainly because of the logistical difficulties in working with deep-diving animals.

Lars Folkow of the University of Tromsø in Norway has also been studying oxygen deprivation, this time in the North Atlantic hooded seal, which can dive to 2,000 metres and remain submerged for an hour. He and his colleagues have found that in seals, neurons remain active after an hour of hypoxia, whereas in mice, they fall silent after five minutes.

Folkow notes that the assays Williams and her colleagues used to measure neural globin concentrations may not be capable of fully weeding out a signal from the much more abundant haemoglobin. Nevertheless, he says that he and his colleagues have also found that globin distribution differs in seal brains compared with mouse brains.

Folkow adds that subterranean mole rats, which live in underground tunnels that are sometimes flooded, also have high levels of globin in their brain. All this adds to the evidence that these proteins have some neuroprotective effect, he says.