Deeper marine dives require longer surface recovery times for sea lions, driven by underwater pressure rather than the length of time spent submerged.
Marine mammals return to the surface between foraging runs to breathe and process physiological changes. For decades, biologists assumed that extended underwater foraging demanded longer rest periods primarily to replenish oxygen, clear carbon dioxide, and process accumulated lactate. Researchers at the University of California, Santa Cruz, Bielefeld University, and other global institutions tested this assumption by analyzing diving patterns across five distinct species, Phys.org reported.
The findings indicate that surface recovery duration depends on dive depth rather than dive duration. “Interestingly, we hadn’t originally set out to investigate what shapes the recovery time of sea lions,” Jonas F. L. Schwarz, first author of the paper, told Phys.org.
“We were comparing the diving behavior of different sea lion species to better understand how the environment and physiology influence foraging strategies,” Schwarz added. “While exploring the data, I noticed that beyond a certain dive duration, the time animals spent at the surface suddenly increased.”
Researchers Examined 800,000 Sea Lion Dives
Researchers examined records from 374 sea lions across 18 colonies, compiling data collected over the past 22 years in habitats ranging from tropical to subpolar waters. Small recording devices measured pressure and depth every four seconds across more than 800,000 individual dives. Statistical models helped isolate how depth and duration influence surface recovery intervals while accounting for individual and species variations.
“What I find most important is the overall pattern,” Schwarz told Phys.org. “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.”
Evaluating Nitrogen Management Hypotheses
The analysis revisits a diving physiology concept established nearly 80 years ago. While shallow dives showed little correlation between duration and surface recovery, longer durations became increasingly influential at greater depths. Because sea lions inhale air into their lungs before submerging, the research team hypothesized that increased underwater pressure drives nitrogen from stored air into the blood and tissues, requiring surface intervals for release.
“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 told Phys.org.
Future research will expand the dataset to include fur seals, true seals, and penguins to test whether nitrogen management explains recovery patterns across other marine predators.
