After returning to land from dives in the sea in search of food, sea lions raise their heart rate for extended periods to recover from the stress imposed on their bodies.
The discovery was made by a group of researchers led by Deakin University (Australia), in collaboration with institutions in the United States, the United Kingdom and South Africa. The work was based on studying the metabolism of animals of the species Arctocephalus pusillus pusillus and Arctocephalus pusillus doriferus, from colonies in Australia and South Africa.
In an article published in the journal Frontiers in Physiology, the scientists suggest that increasing heart rate is a way for sea lions to eliminate accumulated lactic acid and recover oxygen reserves in the muscles after foraging. The researchers explain that diving is an activity that has physiological impacts on marine animals. Long and deep dives can lead to the accumulation of lactic acid, a by-product of metabolism under conditions of high physical effort and consumption of oxygen and energy, which can be detrimental to the animals.
The elimination of accumulated lactic acid is one hypothesis among several on the table.
Between 2003 and 2008, 12 sea lions were tracked with devices that measure heart rate, record dive data and transmit location via radio. It was found that the heart rate of these marine mammals peaks at about 80 beats per minute six to eight hours after they have returned to land.
The researchers explain that, when on land, sea lions do not simply rest, but use that time to actively recover from the physiological impacts of their adventures at sea.
“One of the main advantages of these elevated heart rates on land could be the fact that seals can prioritise foraging while at sea, focusing on obtaining food and avoiding predators,” says Melissa Walker, the study’s first author. Then, on land, the sea lions can focus on recovery.
The elimination of lactic acid is only one of several possible explanations for the elevated heart rate of sea lions on land, and the team says that further studies are needed to determine what other factors might be influencing this accelerated cardiac activity.
