# Dive-by-dive variation in the diving respiratory air volume of southern elephant seals (Mirounga leonina)

## TL;DR

Southern elephant seals adjust their diving air volume to manage buoyancy during dives, but this adjustment doesn't fully offset the costs of their dense bodies.

## Contribution

A new method was developed to estimate dive-by-dive respiratory air volume in seals using hydrodynamic glide models.

## Key findings

- Dive-by-dive DRAV was positively correlated with per-stroke effort and initial descent depth.
- DRAV increased with tissue density and maximum dive depth, but did not achieve neutral buoyancy at half the maximum depth.
- Seals adjusted DRAV to reach neutral buoyancy at around 30 m, less than 10% of their maximum dive depth.

## Abstract

The role of diving respiratory air volume (DRAV) in deep-diving phocid seals remains poorly understood, largely because of the lack of methods for measuring DRAV in free-ranging divers that exhale before diving. We developed a method to estimate DRAV using a hydrodynamic glide model applied to descent glides recorded using multi-sensor data loggers. We estimated dive-by-dive DRAV for six negatively buoyant female southern elephant seals (Mirounga leonina). During shallow descent glides, rapid compression of DRAV influenced net buoyancy and gliding speed, making this phase suitable for estimating DRAV. Our results revealed dive-by-dive variation in DRAV, which was positively correlated with root mean square (RMS) sway acceleration (a proxy for per-stroke effort) and the depth at which gliding began during the initial descent. DRAV increased with both tissue density and maximum dive depth, suggesting that seals adjusted their DRAV to stay closer to neutral buoyancy through their dives. However, the observed level of adjustment did not result in neutral buoyancy at half of the maximum dive depth, as predicted to minimise round-trip locomotion costs. Instead, the seals typically adjusted DRAV to reach neutral buoyancy at ∼30 m depth, <10% of their mean maximum dive depth. This indicates that strong negative tissue density imposes transit costs that cannot be fully compensated for by DRAV adjustment alone. Future work should explore whether other breath-hold divers show similar patterns of DRAV adjustment and quantify the associated physiological and ecological benefits.

Summary: Elephant seals adjust their diving respiratory air volume (DRAV) depending on their tissue density and maximum dive depth. Adjusting DRAV regulates their net buoyancy during dives.