How can dugongs (Dugong dugon) travel along the water column at low energetic cost? A novel hypothesis

Dugongs (Dugong dugon) spend most of their lifetime either travelling between the sea surface and shallow seafloors where they forage seagrass beds or swimming just below the sea surface. Observations indicate that these movements are carried out with a very limited or even without swimming activity. We propose the novel hypothesis that dugongs travel along the water column at low energetic cost by taking advantage of the compression of their gas-filled organs by depth-varying water pressure. This hypothesis was formulated in terms of physics as the problem of the vertical displacement through a fluid of a body, which buoyancy is related to the hydrostatic pressure. In absence of the required field measurements to solve this problem, both the total volume and the compressible volume of dugongs were modelled and approximations for their weights found in the literature were used. We examined the cases of slightly positively buoyant bodies at the sea surface that becomes neutrally buoyant just below the surface. The predicted duration of free sinking was consistent with field measurements. The energetic expenditure required to travel back from the seafloor to the surface was assessed. We found that the depths where the dugongs forage the most frequently, corresponded to where the changes in buoyancy due to the hydrostatic pressure only, were moderate.