Variation in body condition of invasive black rats ( Rattus rattus ) across an anthropogenic gradient in a Mediterranean landscape

Black rats (Rattus rattus) are among the most successful invasive mammals worldwide, occupying a wide range of natural and human-modified environments. Understanding how environmental variation influences fitness-related traits such as body condition, an integrative proxy of energetic reserves and physiological state, may help elucidate mechanisms underlying invasion success. Here, we assessed variation in body condition and body mass of R. rattus across an anthropogenic gradient in Mediterranean central Chile. We estimated body condition using the Scaled Mass Index (SMI) and compared its predictors with those of raw body mass. We analyzed 569 individuals captured across natural and rural habitats. Body condition varied with individual, seasonal, and landscape-level factors, whereas body mass was primarily driven by individual attributes. At the landscape scale, anthropogenic features emerged as key predictors of body condition, with sex-specific patterns: condition increased with urban cover in females, whereas in males it was positively associated with the anthropogenic gradient in autumn, but this relationship was weaker in spring. These findings indicate that body condition captures environmentally mediated physiological responses not reflected by body mass alone. By linking landscape context to variation in energetic condition, our results support a multiscale perspective on invasion success and suggest that anthropogenic landscapes may contribute to the physiological performance of invasive mammals across heterogeneous environments.