Behaviour and buoyancy control in a shallow diving marine mammal, the dugong (Dugong dugon)

Shallow diving air-breathing animals face the challenge of maximising oxygen stores to extend dive duration while being subject to rapid pressure changes, especially in the first 10 m. Buoyancy control is integral to managing the physiological and physical challenges of shallow water diving. Dugongs (Dugong dugon) are shallow diving megaherbivores and must spend extended periods foraging on shallow benthic seagrass communities. Thus, balancing the locomotor activity associated with diving and limited oxygen stores is critical for dugongs, making the dugong a valuable model for investigating buoyancy control in shallow diving marine mammals. We used multi-sensor tags to investigate the buoyancy control of seven mature dugongs (three female, four male; deployments lasting 10–35 hours) in New Caledonia, 2019 (n=2) and the Exmouth Gulf, Western Australia, 2021 (n=5). Our study demonstrates that dugongs regulate their buoyancy by controlling their inhaled lung volume prior to diving, a strategy that contrasts with some other deeper diving marine mammals (e.g. phocid seals), which dive on exhalation. For the dugongs, the depth at which gliding commenced, a proxy for the animal’s buoyancy shifting from positive to negative, increased with maximum dive depth during descent and ascent. This is a strong indication for inhaled air volume increasing with maximum dive depth. Body angle, fluke beat frequency and activity during descents and ascents increased in unison. This evidence suggests that dugongs utilise lung-mediated buoyancy control to maximise the time spent at depth, while minimising the locomotory costs associated with buoyancy while diving.