When elephant seals embark on a long foraging trip, it can be months before they return to land, so keeping track of their antics takes a lot of ingenuity. Although modern satellite tracking devices have begun to reveal some of the mysterious animals' habits, no one had ever monitored how the seals' fat reserves vary while they roam the seas. Until Mike Fedak and his team hit on the idea of estimating the animals' lipid content, based on measurements of their buoyancy from dive records(p. 3405).

Fedak and his colleagues from Scotland and Australia travelled to Macquarie Island in Antarctica and fitted more than 80 young elephant seals with satellite linked depth meters and turbine odometers, to monitor their daily dives. Back in the lab, the team developed a mathematical model, linking the animals' body composition to their buoyancy, measured from the divers' gentle descents and ascents. But would real life and theory agree well enough to predict the animal's lipid content from the dive alone?

The team measured the elephant seal's lipid content from a tritium-labelled blood sample taken just before the youngsters departed, and compared it with the lipid value predicted by the model. The results agreed within a few percent! Measuring a seal's buoyancy as it descends or returns to the surface is enough to measure its fat stores, no matter how far from land it might be.

Biuw, M., McConnell, B., Bradshaw, C. J. A., Burton, H. and Fedak, M. (
). Blubber and buoyancy: monitoring the body condition of free-ranging seals using simple dive characteristics.
J. Exp. Biol.