How to weigh an elephant seal with one finger: a simple three-dimensional photogrammetric application

Several studies have developed photogrammetric techniques for indirect mass estimation of seals. Unfortunately, these techniques are often narrowly delineated for specific field scenarios or species. Many require sophisticated, custom-designed equipment or analytical tools, limiting their applicability. We aimed to devise a photogrammetric technique for accurate volume/mass estimation of seals under a variety of field scenarios without manipulation of the animal and with minimal equipment. We used Photomodeler Pro 3-dimensional modelling software to estimate the mass of 53 weighed southern elephant seals Mirounga leonina. The method is centred on animal volume estimation in relation to the 3-dimensional area around it, rather than features of the animal itself, an approach that liberates limitations associated with earlier studies. No morphometric body measures are required for such volume/mass estimation. We offer predictive equations that allow high confidence in mass estimates relative to measured mass (95% confidence interval of mean deviation from measured mass is from +/- 1.34 to +/- 3.83% depending on the field scenario). A single photographer with a measuring stick and non-customised digital photographic equipment can use this technique to determine the mass of an elephant seal anywhere in the field with the push of a button.