Evaluating the reliability of nasal pigmentation for individual age estimation in lions: Limitations for its use in conservation practice

Nasal pigmentation has been widely used in the field as a non-invasive visual cue to estimate the age of African lions ( Panthera leo ), including in contexts such as trophy hunting, translocations, and population management. However, the reliability of this criterion for individual age classification remains unclear, particularly in situations where age estimates inform management decisions. In this study, we present a comprehensive and statistically robust evaluation of nasal pigmentation as a predictor of age, analyzing standardized photographs of 200 lions from 31 zoological institutions across 11 countries using objective image processing with FIJI software. A significant positive correlation between pigmentation and age was observed (Spearman’s rho = 0.315, p < 0.0001), confirming a general population-level trend of increasing pigmentation over time. However, the commonly used thresholds (> 50% and > 60% pigmentation) showed only moderate diagnostic performance, with sensitivity ranging from 44.1% to 69.8% and specificity from 62.2% to 78.0%. Substantial individual variability was observed, with considerable overlap in pigmentation values across age classes. This variability limits the accuracy of nasal pigmentation as a tool for estimating the age of individual lions. As a result, reliance on this trait alone may introduce uncertainty in conservation and management decisions. Nasal pigmentation may therefore be more appropriately used as a complementary indicator within broader, multi-criteria assessment frameworks, rather than as a stand-alone proxy for age. These findings have direct implications for field practices and management strategies that incorporate this visual trait into decision-making processes.