Estimating Total Body Lipid Store of Free-Ranging Whales In Vivo Using Drone Photogrammetry and Biologging Tags

Lipid reserve mass is a key indicator of individual fitness and population health in wild animal populations, but no method currently exists to measure it in large aquatic species for which capture is impractical. Here, we develop and employ a new method for quantifying lipid stores in a free-ranging cetacean, the sperm whale (Physeter macrocephalus). This method combines UAV (drone) photogrammetry-based estimates of body volume with tag-derived tissue density to estimate lipid store mass to within an estimated 18% on average. Statistical analysis cross validating volume and density-based condition indicators confirmed the predicted relationships between body volume, tissue density and body composition. On average, for a whale 12 m in length, an increment in volume of 1 m³ was predicted to be accompanied by a 6.3% increase in total body lipid content, while a decrease of 1 kg m^-3 in tissue density was predicted to reflect a 5.1% increase in total body lipid content. These results support the effectiveness and encourage continued use of densitometric and morphometric methods for quantifying body condition in cetaceans and highlight the value of simultaneously employing both approaches to directly estimate lipid reserve mass. We anticipate that this method could be extended to other aquatic taxa allowing, for example, major life history events or exposure to stressors to be tied to absolute measures of lipid energy store.