Spatial Variation in Heavy-Metal Bioaccumulation and Associated Physiological Stress Biomarkers in Bufo regularis Across Three Different Locations in Southwestern Nigeria

Urbanisation is a major driver of environmental contamination, yet its physiological consequences for amphibians in rapidly developing regions remain poorly understood. This study investigated spatial variation in heavy-metal bioaccumulation and associated physiological stress biomarkers in Bufo regularis along an urban–rural gradient in Southwestern Nigeria. Adult toads were sampled from Osogbo, Ile-Ife, and Modakeke sites, and concentrations of selected heavy metals (including Pb, Cd, Zn, Cu, and Cr) were quantified in liver and muscle tissues. To assess sublethal biological effects, oxidative stress biomarkers superoxide dismutase (SOD), catalase (CAT), glutathione-S-transferase (GST), reduced glutathione (GSH), and lipid peroxidation (malondialdehyde, MDA) were analysed alongside body condition indices. Heavy-metal concentrations were significantly higher in individuals from Osogbo sites than in those from Ile-Ife and Modakeke locations, with Pb and Cd showing the strongest spatial gradients. Correspondingly, Osogbo populations exhibited elevated antioxidant enzyme activities and MDA levels, indicating heightened oxidative stress, while GSH levels and body condition indices were reduced. These findings demonstrate that B. regularis effectively reflects spatial patterns of metal pollution and associated biological effects across land-use gradients. The study highlights the utility of amphibians as bioindicators of urban environmental health and underscores the potential long-term ecological consequences of metal contamination in tropical urbanising landscapes.