Data from: 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.生物体体型信息对于理解其生活史与进化适合度、辅助制定科学保护策略均至关重要。然而，针对诸多物种，尤其是大体型野生动物，精准获取体型实测数据往往颇具挑战。目前已开发多种体型估算手段，从利用摄影测量法（photogrammetry）获取身高或体长等直接测量方法，到通过体形态计量学（body morphometrics）指标甚至粪便团块大小间接预测体重的方法不一而足。但由于这类估算结果无法与客观实测值对标，其实际精度往往难以确定。本研究依托缅甸半圈养亚洲象（Elephas maximus）的大样本种群（n = 404）获取的直接体型测量数据（身高、胸围、体长、脚围及颈围），探究现有估算方程对亚洲象实际体重的预测效果。随后我们推导得到拟合效果更优的全新公式，可通过上述易获取的测量指标预测缅甸亚洲象的体重。此外，我们还探究了能否通过照片估算核心体型参数——身高与胸围（样本量n = 151）。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，而本文提出的新方程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，经小幅校正后，标准化拍摄的照片测量结果可有效反映真实身高与胸围。本研究结果对野外大体型野生动物的体型估算以及圈养种群管理均具有重要指导意义。