Grey seals wait up to several hours to digest their meals, conserving oxygen which allows them to keep foraging underwater for longer.

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SYDNEY: Grey seals eat during underwater hunting trips, but conserve oxygen by waiting to digest until they're relaxing on the surface, according to researchers.

Like all marine mammals, the amount of time that grey seals can spend foraging on the bottom is limited by the amount of oxygen they can store in their blood and muscles. But digesting their food is an energy-intensive process, requiring oxygen that the seals could otherwise use to spend more time foraging.

“Digestion is costly and increases the rate at which vital oxygen stores are used up, so a digesting animal … has to come to the surface more often to breathe,” said lead author Carol Sparling, of the University of St. Andrews, in Scotland. “[This] makes diving less efficient or may mean they can't dive deep enough to reach their prey.”

In a study published in the U.K. Royal Society's journal Biology Letters, the researchers report that the seals avoid this problem - and maximise their time foraging on the bottom - by eating their fill but waiting until later to digest it. To study the seals' metabolism in a controlled setting, the team captured local wild seals and placed them in an outdoor seawater pool.

The seals were fed herrings at an underwater 'prey patch' in one corner of the pool, which the team covered with aluminium mesh panels to prevent the seals from surfacing. The only opening in the mesh was located 80 metres away, at the opposite end, so the seals were forced to swim the length of the pool underwater to find food and return to breathe. This technique simulated travel distances in the wild.

The group measured the seals' breathing, heart rates and oxygen consumption over four- to five-day simulated foraging trips. The study found that the seals' metabolic rates peaked several hours after feeding, while resting motionless on the surface, often at night. According to the researchers, the peak in metaboic rate is an indicator that digestion was occuring during these otherwise low energy periods.

“The increase in metabolic rate that normally occurs after feeding was missing when we were carrying out simulated foraging experiments,” said Sparling. “And because of the long term measurements we were making, we were picking up these overnight peaks in metabolism where the seals were apparently resting.

"For all intents and purposes they looked fast asleep, yet their metabolic rates were up to seven times resting baselines and their heart rates were extremely high.”

Grey seals are not the only marine mammals to exhibit strange digestive behaviour. Elephant seals have also been known to delay their digestion until they're drifting passively at dawn after night-long feeding trips.

And holding off on digestion isn't the only tactic used by oxygen-conscious seals to conserve energy and spend longer looking for prey underwater. Other adaptations include decreasing their heart rate and dropping their brain temperature. Both reduce the seals' metabolic rate and energy requirements.

Even shivering can affect energy usage. Arctic hooded seals can stay underwater for up to two hours, thanks in part to the way they shiver; on the surface they shake to keep warm, but they stop when submerged, thereby conserving oxygen.

The team thinks that understanding seals' complex interactions between diving, exercise and digestion is essential for conservation.

“The more we know about how an animal operates within its environment given its physiological constraints, the better we are able to understand the role of that in animal in the ecosystem,” said Sparling.