Diving is physiologically challenging for marine animals. Long and deep dives can trigger 'anaerobic' (oxygen-less) metabolism in organs other than the heart and brain that causes lactic acid to accumulate. Even though diving animals typically have evolved tricks to avoid 'the bends', nitrogen bubbles may nevertheless build up in their blood. They tend to recover from these stresses while swimming at the surface for prolonged periods.
But what if time spent at the surface isn't enough to pay off this damaging 'oxygen debt'? This was the focus of a new study by a multinational team in Frontiers in Physiology . They showed that fur seals have a surprising response back on land: their heart rate peaks to around 80 beats per minute, six to eight hours after returning to land.
"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.
Walker and colleagues focused on heart rate as a close proxy of oxygen consumption and the burning of energy. They studied its variation over entire at-sea and on land cycles of two species, the Cape fur seal (Arctocephalus pusillus pusillus) and its close relative the Australian fur seals (A. pusillus doriferus). The first lives along the southern and southwestern coast of Africa and tends to hunt in open sea, while the latter occurs off southeastern Australia and prefers foraging at the sea bottom.
A deep dive into seal physiology
Between 2003 and 2008, the scientists studied six female Cape fur seals at Kleinsee in South Africa and six female Australian fur seals at Kanowna Island off Australia. They fitted each with a water-tight heart rate transmitter, dive recorder, and radio link, and took measurements every 10 seconds over a period of up to 8.2 days.
The instruments showed that a single at-sea-to-on-land cycle took on average 5.5 days in Cape fur seals and 3.8 days in Australian fur seals. Within each cycle, an average of 60.4 and 96.5 hours was spent at sea, mostly (between 60% and 70%) at the surface.
Cape fur seals foraged predominately in the water column (68.4%) and during their longest and deepest dives (over 400 seconds to a depth of 190 meters) maintained their lowest heart rates but only for a brief period (10 beats per minute for less than 60 seconds). In comparison, Australian fur seals foraged mainly at the bottom of the sea floor (71.5%), but during their longest and deepest dives (over 400 seconds at 80 meters) maintained a faster and more steady heart rate for prolonged periods (20 to 30 beats per minute for 300 seconds).
Change of heart onshore
Based on previous studies, the authors expected to see a mostly flat and steady heart rate in seals on land. But what they observed was different. The seals' heart rate showed clear peaks between six and eight hours after moving onshore, reaching up to 84 beats per minute. Often, there were several such peaks. Only after these peaks occurred did the rate fall to a steady state of 42 to 61 beats per minute, consistent with seals in REM sleep.
Importantly, there was a strong positive association between area under the heart rate curve at sea and area under the heart rate curve on land, suggesting a link with the total oxygen debt incurred during the seals' time at sea.
The authors concluded that far from simple resting, the seals used their time onshore to actively recover. The authors suggest that a likely explanation for seals dialing up their heart rate, and therefore metabolic rate, is to flush lactic acid from their system and rebuild oxygen stores that could not be 'paid back' while at sea. This, and other possible explanations, are explored extensively in the discussion.
"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."
Key questions remain. "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."
