Depth sets the recovery clock: evidence for a time-at-pressure effect in sea lions

All air-breathing marine vertebrates face the same fundamental trade-off: time spent foraging underwater must be offset by surface recovery between dives. For decades, the dominant view in diving physiology has been that post-dive duration (PDD) reflects the repayment of oxygen debt and clearance of carbon dioxide and lactate. Using more than 800 000 dives from five sea lion species across 18 colonies spanning tropical to subpolar waters, we quantified how PDD scales with dive depth and duration and evaluated whether this pattern is consistent with metabolic recovery interpretations or instead points to depth-linked constraints. We found that PDD increases primarily and linearly with depth, while dive duration contributes to PDD only at greater depths, a relationship conserved across species despite pronounced differences in body mass, dive physiology and foraging ecology. Our findings conflict with predictions of metabolic recovery interpretations and point to a pressure-linked mechanism in which time at depth and resulting inert-gas loading shape surface recovery. By highlighting pressure-linked inert-gas management as a central candidate mechanism for surface interval regulation in inhalation divers, our study revises a central assumption in diving physiology, with broad consequences for understanding foraging efficiency, habitat accessibility and vulnerability to environmental change in air-breathing marine vertebrates.