Dive Depth, Not Duration, Sets Sea Lions' Recovery Clock

The team combined dive records gathered in different countries over the past 22 years, spanning marine environments from tropical to subpolar waters. Each sea lion carried a small recording device that measured pressure — and therefore depth — every four seconds. From these logs, researchers extracted maximum depth, time underwater and surface recovery time, then fed the data into statistical models that accounted for differences between individuals and species.

The result: deeper dives, where animals experience greater pressure, were followed by longer recoveries. Duration alone had little effect after shallow dives, gaining influence only at greater depths.

"While individual parts of the pattern, like the linear relationship of depth and recovery time, can actually have different explanations, the pattern as a whole demonstrates that time spent under pressure, rather than metabolic recovery alone, is the major factor determining how long sea lions need to recover between dives," Schwarz explained.

The takeaway: depth, not dive length, sets the recovery clock.

Why might pressure matter?

The researchers' leading hypothesis involves nitrogen. Sea lions inhale before diving, carrying air down in their lungs. Greater underwater pressure, they propose, may push nitrogen from that stored air into the blood and tissues, and surface intervals may let the animals offload some of the accumulated gas.

The catch: the team never measured nitrogen directly. The explanation remains hypothetical, and the authors flag it as such — a preliminary mechanism awaiting confirmation.

What does this change?

"Our work revisits a concept within diving physiology that has been present for nearly 80 years," Schwarz said. The implications reach past physiology into ecology. Recovery time directly shapes foraging efficiency, which determines which hunting strategies are viable.

Misjudge recovery costs, and you misjudge the animals. "If we underestimate the recovery time associated with deeper dives, we risk drawing the wrong conclusions about the trade-offs animals face, which foraging strategies are most profitable and how they might respond to changes in their environment," Schwarz explained.

The study covers sea lions — eared seals — and the findings may or may not transfer to other divers. Schwarz is already expanding the dataset to test that, adding fur seals, true seals and penguins. Fur seals are close relatives with similar dive physiology; true seals are more distant and differ sharply, for instance by exhaling before a dive; penguins aren't related to seals at all but share the habit of diving with air in their lungs.

That comparison doubles as a nitrogen test. Animals that dive with full lungs should show the pressure effect if nitrogen is the culprit; animals that exhale first shouldn't.

"I want to find out how widespread this recovery pattern is among marine predators and why some species might show it while others do not," Schwarz said. "By comparing animals that dive with air in their lungs with those that exhale before diving, I also hope to find further evidence for whether nitrogen is responsible for the pattern we observed."

The next dataset, in short, should reveal whether sea lions are the rule or the exception.

Publication details: Jonas F. L. Schwarz et al., "Depth sets the recovery clock: evidence for a time-at-pressure effect in sea lions," Proceedings of the Royal Society B: Biological Sciences (2026). DOI: 10.1098/rspb.2026.1690.