Inferring sperm whale (Physeter macrocephalus) sex and developmental stage using aerial photogrammetry.

Demographic data (i.e. sex and age) are fundamental for analyzing behaviour patterns and evaluating the reproductive potential of a population. However, determining these traits in the wild can be challenging, particularly for marine animals with concealed genitals that spend most of their time underwater. Here, we developed a minimally invasive method to infer the developmental stage and sex of sperm whales (Physeter macrocephalus) off the Galápagos Islands (N = 51) using uncrewed aerial vehicle (UAV) photogrammetry. We leveraged historic whaling data on sperm whale growth and sexual dimorphism to assign developmental stages to individuals based on their body lengths. We applied Bayesian theory to estimate the probability that individuals were female based on their morphometry. Our methods allowed confident classification of the developmental stage and sex for most individuals. Moreover, an examination of the inferred developmental stage and sex of individuals participating in peduncle diving revealed patterns congruent with previous findings, which show that this behaviour is predominantly directed at females and performed by subadult individuals. Our method offers an efficient, low-cost means of obtaining demographic information from live sperm whales, contributing to a deeper understanding of their behavioural development and informing population status and viability assessments.