Vibrissae length as a morphological proxy for foraging behaviour in pinnipeds

Foraging behavior is a key driver of ecological and evolutionary processes, with individual specialization shaping how populations respond to environmental change. Polymorphisms in foraging strategies can both enhance and limit behavioral flexibility at the population level, making it crucial to study individual variation. However, studying foraging is notoriously difficult, and while biologging has significantly advanced our understanding of individual specializations, its limitations in sample size and potential ethical concerns highlight the need for alternatives. In this study, we introduce a complementary, non-invasive method using relative vibrissae length (RVL) measured from images as a morphological proxy for foraging strategy in Galápagos sea lions (Zalophus wollebaeki). RVL differs significantly between distinct foraging behaviours: benthic foragers have shorter vibrissae due to abrasion, compared with pelagic foragers. Our method proved highly reliable, demonstrating strong intra- and inter-observer repeatability as well as within-season consistency. Moreover, RVL remained stable across multiple years, indicating long-term persistence in individual foraging specialization. Vibrissae length thus exemplifies how behavioral specializations can shape morphological traits within an individual’s lifetime, offering a novel approach to study ecological polymorphisms. The integration of RVL assessments with existing tracking methods can enhance sample sizes and improve our understanding of individual foraging specialization at the population level. Our approach bridges the gap between high-resolution data and broader-scale ecological monitoring, providing a scalable tool for studying foraging strategies in pinnipeds.