Does nocturnal light pollution impair immune function in a wild‐living amphibian?

Abstract Light pollution is among the most rapidly growing anthropogenic stressors on Earth. As it spreads far beyond its original source, it affects natural protected areas playing a key role in protecting biodiversity. While physiological impacts of light are extensively studied in laboratory animals and livestock, it is yet unknown what are the physiological effects of low levels of light at night on animals—such as those experienced by wildlife in natural areas exposed to light pollution. Experimental studies suggest that light pollution have deleterious effects on metabolism and immunity (among others) mainly caused by melatonin suppression. Within the One Health framework, alterations of wild animals' immunity and their capacity to combat infections due to light pollution could have far‐reaching implications for species conservation, pathogen dynamics and potential for epidemic outbreaks. The aim of this study was to assess, for the first time, whether the effects of light pollution on immunity previously reported in experimental studies, are also observable in a wild species. To do so, we measured a large set of body condition and immune traits, to obtain a comprehensive picture of individuals' health, in 15 natural populations of common toads ( Bufo bufo ) distributed along a gradient of exposure to light pollution. Although our results reveal a strong negative association between light pollution values of our sites and toads' body mass, associations with immune traits did not fully match our predictions. Toads from the most illuminated sites presented lower levels of inflammatory marker haptoglobin, a decreased albumin to globulin ratio and tended to have more eosinophils than those from the darkest sites. However, these latter associations with immune traits were not specific to light pollution only, but also related to the global urbanisation of the sites. Our study emphasises the need for more studies on the effect of light pollution on physiology in natural conditions of exposure, and its interactions with other anthropogenic perturbations, to be able to truly understand its effects on wildlife. This is critical to identify health risks for wildlife and for prioritizing conservation efforts of menaced taxa. Read the free Plain Language Summary for this article on the Journal blog.