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.

生物体的体型信息对于解析其生活史与适合度、辅助制定保护措施均具有关键意义。然而，针对多数物种，尤其是大型野生动物，获取准确的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方法，以及通过其他躯体形态测量指标甚至粪团（dung boli）大小间接预测体重的间接路径。但这类方法的准确性通常难以验证，因为无法将其与客观测量值进行比对。本研究以缅甸404头半圈养亚洲象（Elephas maximus）的大种群直接测量的躯体指标（体高、胸围、体长、足围及颈围）为基础，探究现有估算方程对其实际体重的预测效果。随后，我们基于这些易获取的测量参数，构建了拟合效果更优的全新体重预测公式。此外，我们还针对体高与胸围这两项关键参数，探究了其能否通过照片进行估算（n=151）。研究结果表明，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几乎所有场景下均能实现更精准的体重预测。同时我们发现，经小幅校正后的标准化照片测量值可有效反映亚洲象的体高与胸围。本研究结果对野外大型动物的体型估算以及圈养种群管理均具有重要参考价值。