Body condition, stress and reproductive success.

Time for a little theory, but I promise to keep it simple. 

Fat, well fed whales float better than thin, stressed whales because they have a higher lipid content in their blubber. When a fat, well fed whale dives it will have to beat it's tail flukes more frequently to counteract it's inherent buoyancy; while a thin, stressed whale will sink more easily and have to beat it's flukes less frequently. Conversely when swimming back to the surface; a fat, well fed whale uses it's inherent buoyancy to help it ascend; while a thin, stressed whale has to beat it's flukes more frequently. Thin, stressed whales with a poor condition are less likely to reproduce because they don't have sufficient blubber reserves to sustain themselves or their calf.

How do you measure the body condition of an animal the size and weight of a truck? Cue the Inspector Gadget of the Oceans, a whale sized breathalyser, laser photogrammetry and a biopsy dart.