Age determination based on external morphometric measurements in live raccoon dogs (Nyctereutes procyonoides): a method considering animal welfare and research efficiency

Abstract Background Accurate age determination is crucial for interpreting the behavior and ecology of raccoon dogs; however, age estimation is often overlooked due to the absence of practical, non-invasive methods or criteria. Existing approaches to age estimation tend to be overly invasive or challenging to implement in field settings, thereby limiting their utility. Results In this study, we analyzed external morphological traits of 37 raccoon dogs, whose age was verified via radiographic analysis of growth plate closure. We characterized morphological differences between adults and juveniles and developed discriminant functions to estimate the age of individuals of undetermined age. In males, adults exhibited significantly larger body weight, body length, tail length, hind foot length, ear length, head length, and neck circumference than juveniles. In females, all measured traits, except for head length, were larger in adults than in juveniles. The discriminant functions based on external morphology successfully classified adults and juveniles with 100% accuracy for both males and females. Multiple discriminant functions combining three morphological variables were generated for each sex, with body length identified as the most important predictor for age estimation in raccoon dogs. Conclusions To the best of our knowledge, this is the first study to present sex-specific, morphometry-based discriminant functions for age estimation in raccoon dogs. With perfect classification accuracy, these functions offer a reliable tool for age estimation. The non-invasive and field-applicable nature of this method offers distinct advantages regarding animal welfare and research practicality. This approach aligns with societal demands for integrating research ethics and animal welfare by minimizing the burden on study animals, thereby enabling more precise and long-term ecological studies.