Allometric Regression of Snake Body Length from Head Image Measurements

As in many fields of wildlife research and management, camera devices and photogrammetry have become an integral part of the toolkit for exploring otherwise‐unseen aspects of the biology, behavior, and control of the invasive brown treesnake (Boiga irregularis) on Guam. Because brown treesnakes are cryptic and nocturnal, and nearly all aspects of their ecology are influenced by snake size, methods are needed to estimate snake size from images captured by infrared wildlife cameras. Unfortunately, it is difficult to capture images of an entire snake’s length at a controlled distance from a simple camera setup. Here, I describe the allometric relationships between brown treesnake body length and potential predictors: head measurements, sex, and body condition. Head length (HL) was the most important predictor of body length, alone accounting for 95.9% of the variation in brown treesnake snout‐vent length (SVL). We provide simple regression equations for predicting brown treesnake length from head measurements, an example of how to extract measurements from images, and a convenient lookup table for predicting SVL and 80% prediction intervals from HL alone. Coupled with a simple camera setup that controls subject distance and includes size standards in the image, we can estimate brown treesnake body size from images that include only the head when photographed from above. These methods have been developed to enable ongoing assessments of brown treesnake predation risk following landscape‐scale suppression efforts that could enable the reintroduction of extirpated native wildlife