How big is it really? Assessing the efficacy of indirect estimates of body size in Asian elephants

Information on an organism’s body size is pivotal in understanding its life history and fitness, as well as helping inform conservation measures. However, for many species, particularly large-bodied wild animals, taking accurate body size measurements can be a challenge. Various means to estimate body size have been employed, from more direct methods such as using photogrammetry to obtain height or length measurements, to indirect prediction of weight using other body morphometrics or even the size of dung boli. It is often unclear how accurate these measures are because they cannot be compared to objective measures. Here, we investigate how well existing estimation equations predict the actual body weight of Asian elephants Elephas maximus, using body measurements (height, chest girth, length, foot circumference and neck circumference) taken directly from a large population of semi-captive animals in Myanmar (n = 404). We then define new and better fitting formulas to predict body weight in Myanmar elephants from these readily available measures. We also investigate whether the important parameters height and chest girth can be estimated from photographs (n = 151). Our results show considerable variation in the ability of existing estimation equations to predict weight, and that the equations proposed in this paper predict weight better in almost all circumstances. We also find that measurements from standardised photographs reflect body height and chest girth after applying minor adjustments. Our results have implications for size estimation of large wild animals in the field, as well as for management in captive settings.

生物体的体型数据是解析其生活史与适合度、为保护策略制定提供关键依据的核心信息。然而，针对诸多物种——尤其是大型野生动物——精准获取体型测量数据往往颇具挑战。目前已发展出多种体型估算手段，涵盖利用摄影测量法（photogrammetry）获取身高或体长等直接测量方法，以及借助躯体形态测量学（morphometrics）指标甚至粪球大小间接预测体重的间接估算方式。但由于这些估算手段无法与客观测量结果对标，其精准度往往难以确定。本研究以缅甸境内404头半圈养亚洲象（Elephas maximus）的直接测量获得的体型数据（身高、胸围、体长、足围及颈围）为基础，探究现有估算方程对亚洲象实际体重的预测效果。随后我们基于这些易于获取的测量参数，构建了拟合效果更优的全新体重预测公式。此外，我们还针对151组样本，探究能否通过照片估算出身高与胸围这两项关键体型参数。研究结果显示，现有估算方程的体重预测能力存在显著差异，而本文提出的新公式在绝大多数场景下均能实现更精准的体重预测。同时我们发现，经小幅校准后，标准化照片获取的测量值可有效反映亚洲象的实际身高与胸围。本研究结果不仅可为野外大型野生动物的体型估算提供参考，也能为圈养种群的管理工作提供有益借鉴。