Estimating body mass of sperm whales from aerial photographs

Abstract

Body mass is a fundamental feature of animal physiology. Although sperm whales (Physeter macrocephalus) are the largest toothed predators on earth, body mass is seldom included in studies of their ecophysiology and bioenergetics due to the inherent difficulties of obtaining direct measurements. We used UAV‐photogrammetry to estimate the weight of free‐ranging sperm whales. Aerial photographs (23 calves, 11 juveniles, 55 nonmother adults, 13 mothers) were collected in the Eastern Caribbean and Mediterranean Sea during 2017–2020. Body length, widths, and heights (dorso‐ventral distance at 5% increments) were measured from dorsal and lateral photographs, while body volume was calculated using an elliptical model. Volume varied noticeably (12.01 ± 4.79 m3) in larger animals (>8 m), indicating fluctuations in body condition of adults and mothers. Volume was converted to mass, using tissue‐density estimates from catch data, animal‐borne tags, and body‐tissue composition. Average total body density ranged from 834 to 1,003 kg/m3, while the weight predictions matched with existing measurements and weight‐length relationships. Our body‐mass models can be used to study sperm whale bioenergetics, including inter‐ and intraseasonal variations in body condition, somatic growth, metabolic rates, and cost of reproduction.