Predators induce plastic responses in multiple prey taxa, ranging from morphological to behavioral or physiological changes. In amphibians, tadpoles activate plastic responses to reduce predation risk by reducing their activity rate and altering their morphology, specifically tail depth and pigmentation. Furthermore, there is now evidence that tadpoles’ defenses are modified when predators combine with other stressful factors such as pollutants or competitors, but our knowledge on the physiological responses underlying these responses is still scarce. Here we study physiological responses in Pelobates cultripes tadpoles exposed to a natural predator (larvae of the aquatic beetle Dytiscus circumflexus), non-lethal concentrations of herbicide (gly-phosate, 0.5 mg/L and 1.0 mg/L) or both factors combined. We measured corticosterone levels, standard metabolic rate, oxidative damage (TBARS) and activity of antioxidant enzymes, and immune response (via leukocyte count). Tadpoles reduced their corticosterone concentration by ca. 24% in the presence of predator cues, whereas corticosterone did not change in the presence of glyphosate. Two enzymes involved in antioxidant response also decreased in the presence of predators (14.7% and 13.2% respectively) but not to glyphosate. Herbicide, however, increased the number of neutrophils and reduced that of lymphocytes, and had an interaction effect with predator presence. Standard metabolic rate did not vary across treatments in our experiment. Thus we show a marked physiological response to the presence of predators but little evidence for interaction between predators and low levels of herbicide. Multiple assessment of the physiological state of animals is important to understand the basis and consequences of phenotypic plasticity.

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In recent decades, oak forests in Europe (Fagaceae family) have faced a severe multifactorial process of decline, characterized by a fast drying of the canopy and loss of tree vigour. Climate change and increased susceptibility to pests and diseases (e.g., the fungal pathogens Diplodia corticola and Phytophthora cinnamomi; Moricca et al., 2016) probably have a major role in this process (Sallé et al., 2014). These changes are registered worldwide, often without any precedents and with alarming outcomes (Hulcr & Dunn, 2011). In the Mediterranean area and mainly in Portugal, the evergreen Quercus suber (cork oak) woodlands have been showing this decline process since the 1980s, highly affecting the cork production (half of the world cork production is Portuguese; APCOR, 2020) and the biodiversity of the ecosystem. The symptoms include leaf discolouration, defoliation, small branches and sometimes exudates (Tiberi et al., 2016).