Fur seals may look relaxed on shore, but their hearts reveal a hidden battle still underway after deep dives.
Diving places enormous physical stress on marine mammals. During long and deep dives, animals can switch to ‘anaerobic’ (oxygen-less) metabolism in parts of the body outside the heart and brain, which leads to a buildup of lactic acid. Although many diving species have evolved ways to reduce the risk of ‘the bends’, nitrogen bubbles can still accumulate in their bloodstream. Scientists have long believed these animals mainly recover while resting or swimming near the ocean’s surface.
A new study published in Frontiers in Physiology suggests that recovery may continue long after fur seals leave the water. Researchers found that several hours after returning to land, the animals experience major increases in heart rate, potentially helping them recover from the strain of extended foraging trips at sea.
“Here we show in Cape and Australian fur seals that there is a positive relationship between their heart rate at sea during foraging and their heart rate on land during rest. This likely means that payback for some of the physiological costs of foraging at sea is delayed and recovered later when the seal is on land,” said first author Dr. Melissa Walker, an Associate Research Fellow at Deakin University in Australia.
Scientists Track Fur Seal Recovery
Walker and her colleagues used heart rate as an indicator of oxygen use and energy expenditure. The team examined heart rate patterns during complete cycles at sea and on land in two closely related species: the Cape fur seal (Arctocephalus pusillus pusillus) and the Australian fur seal (A. pusillus doriferus).
Cape fur seals live along the southern and southwestern coasts of Africa and mainly hunt in open water. Australian fur seals, found off southeastern Australia, spend more time feeding along the seafloor.
Between 2003 and 2008, researchers monitored six female Cape fur seals at Kleinsee in South Africa and six female Australian fur seals at Kanowna Island in Australia. Each seal carried a water-tight heart rate transmitter, a dive recorder, and a radio transmitter. Measurements were collected every 10 seconds for up to 8.2 days.
Different Diving Styles in Fur Seals
The recordings showed that a complete at-sea-to-on-land cycle lasted an average of 5.5 days in Cape fur seals and 3.8 days in Australian fur seals. During those cycles, the seals spent an average of 60.4 and 96.5 hours at sea, with most of that time (between 60% and 70%) spent near the surface.
The two species also showed distinct diving behaviors.
Cape fur seals foraged mainly within the water column (68.4%). During their deepest and longest dives, lasting more than 400 seconds and reaching depths of 190 meters, their heart rates dropped to extremely low levels for short periods, reaching about 10 beats per minute for less than 60 seconds.
Australian fur seals behaved differently. They spent most of their foraging time along the seafloor (71.5%). During their longest dives, which lasted more than 400 seconds at depths of around 80 meters, they maintained higher and steadier heart rates of 20 to 30 beats per minute for as long as 300 seconds.
Unexpected Heart Rate Surges on Land
Based on earlier research, scientists expected seals resting on land to maintain relatively stable heart rates. Instead, the data revealed a surprising pattern.
Between six and eight hours after returning ashore, the seals’ heart rates surged, at times climbing to 84 beats per minute. Some animals experienced multiple spikes before their heart rates eventually settled into a stable range of 42 to 61 beats per minute, which matches rates typically seen during REM sleep.
Researchers also discovered a strong relationship between total heart activity while the seals were at sea and total heart activity after they returned to land. This finding suggests the seals may still be repaying the oxygen debt built up during intense diving and hunting.
The researchers concluded that time spent on land is not simply rest. Instead, the seals appear to actively recover after their demanding trips at sea. One possible explanation is that the elevated heart rate helps clear lactic acid from the body while rebuilding oxygen reserves that could not fully recover in the ocean. The study also discusses several other possible mechanisms.
“Physiological recovery from oxygen debt is more protracted, complex, and occurs over much longer timescales than previously understood, with the elevated heart rate on land likely helping to support a delayed recovery,” concluded Walker.
“A key benefit of such high heart rates on land may be that seals can prioritize foraging while at sea, focusing on acquiring food and avoiding predators, and then allocate energy to processing and recovery once they return to land.”
Questions About Fur Seal Recovery Remain
The researchers say more studies are needed to fully understand why the seals experience these delayed heart rate spikes after returning to shore.
“There are likely numerous factors driving the elevated heart rate that seals show on land. How factors such as dive effort, foraging success, and digestive state contribute to this response will need further investigation,” said Walker.
“Future studies could track these variables alongside onshore heart rate patterns to clarify the mechanisms behind this apparent delayed recovery.”
Reference: “Aquatic and terrestrial heart rates in fur seals: evidence for delayed metabolic processing” by Melissa J. Walker, Daniel P. Costa, Stephen P. Kirkman, Andrew J. Hoskins, W. Herman Oosthuizen, Deon Kotze, Michael A. Fedak and John P. Y. Arnould, 3 March 2026, Frontiers in Physiology.
DOI: 10.3389/fphys.2026.1755942
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