Deep dives push marine mammals to their physical limits. During long periods underwater, animals can switch to ‘anaerobic’ (oxygen-less) metabolism in organs outside the heart and brain, leading to a buildup of lactic acid. Although many diving species have evolved ways to reduce the risk of ‘the bends’, nitrogen bubbles can still form in their bloodstream. Scientists have long believed these animals recover mainly while floating or swimming at the ocean’s surface between dives.
A new study published in Frontiers in Physiology suggests that recovery may continue long after fur seals leave the water. Researchers discovered that Cape and Australian fur seals experience dramatic increases in heart rate several hours after returning to land, possibly as part of a delayed recovery process from the intense demands of foraging 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 are 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.
Tracking Fur Seal Heart Rates at Sea and on Land
The research team used heart rate as an indicator of oxygen use and energy expenditure. They monitored two closely related species: the Cape fur seal (Arctocephalus pusillus pusillus), found along southern and southwestern Africa, and the Australian fur seal (A. pusillus doriferus), which lives off southeastern Australia. Cape fur seals mainly hunt in open water, while Australian fur seals spend more time feeding along the seafloor.
Between 2003 and 2008, scientists studied six female Cape fur seals at Kleinsee in South Africa and six female Australian fur seals at Kanowna Island in Australia. Each animal was equipped with a waterproof heart rate monitor, dive recorder, and radio transmitter. Measurements were collected every 10 seconds for periods lasting up to 8.2 days.
The data showed that Cape fur seals typically completed an at-sea-to-on-land cycle in about 5.5 days, while Australian fur seals averaged 3.8 days. During these cycles, the animals spent roughly 60.4 and 96.5 hours at sea, with most of that time (between 60% and 70%) spent at the water’s surface.
Different Diving Strategies in Fur Seals
The two species also showed different hunting behaviors and heart rate patterns underwater.
Cape fur seals hunted mainly within the water column (68.4%). During their deepest and longest dives, which exceeded 400 seconds and reached depths of 190 meters, their heart rates dropped sharply but only briefly, reaching about 10 beats per minute for less than 60 seconds.
Australian fur seals, in contrast, spent most of their foraging time along the seafloor (71.5%). During dives lasting 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.
Surprising Heart Rate Spikes After Returning Ashore
Previous research suggested that seals resting on land should show relatively stable heart rates. Instead, the team observed something unexpected.
About six to eight hours after coming ashore, the seals’ heart rates surged, sometimes climbing as high as 84 beats per minute. Several spikes could occur before the heart rate eventually settled into a calmer range between 42 and 61 beats per minute, a pattern linked with REM sleep.
Researchers also found a strong connection between total heart activity during time spent at sea and heart activity after returning to land. This relationship suggests the seals may be repaying a lingering oxygen debt accumulated during diving and hunting trips.
The researchers believe the seals are not simply resting onshore. Instead, the elevated heart rates may help remove lactic acid from the body and restore depleted oxygen reserves that could not fully recover while at sea. The study discusses several additional explanations as well.
“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.”
Scientists Still Have Questions
The researchers say more work is needed to understand exactly what drives these delayed heart rate increases.
“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.”