Seals’ Heart Rates Peak Hours After Coming Ashore — Helping Them Recover From Deep Dives

Seals are surprisingly fast underwater, outpacing even the most elite Olympic swimmers. On land, they seem to prefer to move at a slower pace.

However, according to a study recently published in Frontiers of Physiologythis time spent out of the water may not be as relaxing as it seems at first glance. Research shows that fur seals’ heart rates peak six to eight hours after returning to land, when their heart rates peak at around 80 beats per minute.

Melissa Walker, a research associate at Deakin University in Australia, described this response as a “reward for some of the physiological costs of foraging at sea.”

“Physiological recovery from oxygen debt is longer, more complex, and occurs over much longer periods than previously thought, with the elevated heart rate on land likely helping to support delayed recovery,” Walker explained in a statement.

The physiological cost of diving

Seals spend much of their time at sea hunting for food. But while necessary, diving is a physically taxing behavior, particularly because searching for food underwater requires spending time in low-oxygen environments. Activity can trigger anaerobic metabolism – energy production that occurs without oxygen – which leads to a buildup of lactic acid and other metabolic byproducts.

Seals spend a lot of time swimming near the surface, which helps them recover from the stress of diving. But that, by itself, researchers say, might not be enough to fully recover and repay the oxygen debt accumulated during dives.

Learn more: Seals’ complex, sensitive whiskers could one day help robotics navigate the dark

Tracking seals’ heartbeats

For this study, researchers measured the heart rates of six Cape fur seals (Arctocephalus pusillus pusillus) and eight Australian fur seals (A. pusillus doriferus), looking for changes as the seals moved between land and sea.

According to the study authors, heart rate profiles can offer valuable insight into how behavioral changes shape physiological demands, as heart rate tracks oxygen consumption and energy expenditure.

Heart rate measurements were taken every 10 seconds for up to 8.2 days, allowing researchers to record entire cycles between sea and land. A single cycle lasted approximately 5.5 days for Cape fur seals and 3.8 days for Australian fur seals. Most of this time was spent at sea, with the majority (60-70%) spent on the surface.

Heart rate at sea and heart rate on land

There are notable differences between the two subspecies which, although closely related, display slightly different food gathering strategies. Cape fur seals, found near the southern and southwest coasts of Africa, hunt in the open ocean. In contrast, Australian fur seals feed on the seafloor.

Deeper dives have been observed in Cape fur seals, reaching approximately 623 feet (190 meters) below the sea surface. During this time, their heart rate can drop as much as 10 beats per minute for brief periods (less than a minute).

Australian fur seals made shorter dives (276 feet or 84 meters on average), but could maintain a heart rate of 20 to 30 beats per minute for 300 seconds.

These results were largely consistent with previous research. However, the data produced on land was more unexpected. Rather than remaining stable, the heart rate fluctuated over time, peaking at 81 beats per minute (Cape fur seals) and 84 beats per minute (Australian fur seals) six to eight hours after returning to land.

Pay off an oxygen debt

Researchers suggest that the faster heart rate on land could help pay off the oxygen debt accumulated while foraging at sea.

“A key advantage of such a high heart rate on land could be that seals can prioritize their foraging at sea, focusing on acquiring food and avoiding predators, then allocating energy to processing and scavenging once back on land,” Walker said in a statement.

Many questions remain – including how various factors contribute to this delayed recovery and the mechanisms behind this physiological response – which the team hopes to explore in the future.

Learn more: These seals vocally remember their old rivals, even after a year at sea

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