Integrating 3D models with morphometric measurements to improve volumetric estimates in marine mammals

Abstract

Abstract Studies of body condition are key to understanding the health, bioenergetics and ecological roles of marine mammals. Due to challenges in studying marine mammals at sea, body condition is often approximated using metrics representing the size of the dorsal surface visible from aerial imagery, but quantifying variability in body volume would enable a more holistic understanding of bioenergetics. Furthermore, the number and location of measurements needed to accurately quantify body condition has received little attention. Three‐dimensional (3D) models provide a promising tool for representing morphology and providing holistic estimates of marine mammal body condition when combined with field‐based morphometric measurements. We use humpback whales Megaptera novaeangliae to demonstrate the utility of 3D models for estimating body condition in marine mammals. We integrate morphometric measurements taken from unoccupied aerial vehicles (UAVs) with scalable 3D models to generate estimates of humpback whale body volume. We assess which and how many morphometric measurements are required to accurately estimate body volume and compare the error between volume estimates derived from 3D models and previously developed models representing volume as a series of ellipses. Using UAV measurements, we assess the contribution of each morphometric measurement to volumetric estimates, and quantify the error produced by all combinations and numbers of morphometric measurements (131,072 combinations). Error in volume estimates from 3D models generated with as few as five width measurements was <5% compared to the full models and was lower than the error produced when using five width measurements with the elliptical approach. We suggest that by conserving the external morphology of marine mammals, 3D models allow body volume and body condition to be estimated accurately with few measurements. We provide code and guidelines for creating 3D models using the open‐source software Blender and for assessing which measurements are needed to accurately capture the morphology of cetaceans. The 3D modelling approach we present will facilitate studies of intra‐ and interannual changes in body volume in marine mammals, which is vital to providing a more holistic understanding of bioenergetics and to assessing responses to environmental change and anthropogenic stressors.