Behavioral fever decreases metabolic response to lipopolysaccharide in yellow Cururu toads (Rhinella icterica)

Ectothermic vertebrates develop behavioral fever in response to bacterial products, with potential corresponding metabolic costs associated with immune stimulation. Although behavioral fever has been described in several taxa under laboratory conditions, some important questions regarding metabolic response to bacterial products at different temperatures and effectiveness of behavioral fever remain open. Many ectotherms, such as nocturnal anurans, may be active in the field at environmental conditions that restrict thermoregulation during the immune response. How does the metabolic response to bacterial products under ecologically relevant but unfavorable thermal field conditions compare to that measured in fever thermal preferendum? Additionally, are there differences in the partitioning of metabolic costs associated with immune stimulation and Arrhenius effect (biochemical reactions rate) at normal versus fever thermal preferendum? We compared the energy expenditure untreated and LPS-treated yellow Cururu toads (Rhinella icterica) at temperature corresponding to field activity during winter nights, and at normal and fever thermal preferendum. It was hypothesized that the metabolic response to LPS would be proportionally lower at higher body temperatures. To test these hypotheses, we measured temperature in the field during night using agar models, as well as normal and fever thermal preferendum of the toads within a thermal gradient. Subsequently, we measured the toad's metabolic rates at mean agar models temperature, as well as at normal and fever thermal preferendum. Lastly, we calculated the Metabolic response to LPS as the ratio between MRLPS/MRSaline in each of these mean temperatures. Our results show that metabolic rates do not increase in response to LPS at the agar models temperature typical of the winter nights under which theses toads maintain reproductive activity. Moreover, LPS treatment increased the metabolic costs relative to Arrhenius effects at normal thermal preferendum but not at fever thermal preferendum. In this way, metabolic response to LPS was comparative lower at fever than normal thermal preferendum in yellow Cururu toads.