An emerging fungal pathogen is associated with increased resting metabolic rate and total evaporative water loss rate in a winter‐active snake

<p>1. Energy allocation tradeoffs associated with mounting metabolically costly immune responses may serve as sublethal mechanisms by which pathogens reduce host fitness. The emergence of cutaneous fungal pathogens, which invade the skin of their host and have the potential to disturb energy and water balance, highlight the importance of host physiology in determining individual- and population-level effects of disease.</p> <p>2. Snake fungal disease (SFD, ophidiomycosis), caused by the fungal pathogen <i>Ophidiomyces ophiodiicola</i> (<i>Oo</i>), is an emerging disease afflicting wild snake populations throughout eastern North America. Emaciation and dehydration are phenotypic correlates of SFD, but it is unknown if such declines in host condition occur via effects of <i>Oo</i> infection on host physiology (i.e., increased rates of metabolism and evaporative water loss, respectively).</p> <p>3. We used flow-through respirometry to assess the energetic and hydric consequences of natural <i>Oo</i> infection in winter-active pygmy rattlesnakes (<i>Sistrurus miliarius</i>). We measured resting metabolic rate (CO₂ production rate) and total evaporative water loss rate of winter-acclimatized <i>S. miliarius</i> as a function of SFD status and acute temperature (17, 25, and 32°C). We also used regression models characterizing individual variation in the thermal-sensitivity of resting metabolic rate to predict the theoretical effects of behavioral fever on daily resting CO₂ production by free-ranging <i>S. miliarius</i> with SFD in winter.</p> <p>4. Natural infection by <i>Oo</i> was associated with significant increases in resting metabolic rate (30–45%) and total evaporative water loss rate (30–40%) across all measurement temperatures. Under simulated scenarios of behavioral fever, <i>Oo</i> infection was predicted to increase daily resting CO₂ production rate by 58–102%.</p> <p>5. Our results are consistent with the hypothesis that the immune response to <i>Oo</i> infection is energetically costly and may contribute to declining host condition. Our modeling efforts combining the cumulative effects of increased immune activity and increased body temperature on metabolism represent a novel approach to quantifying the total daily energetic cost of infection in ectothermic vertebrates undergoing behavioral fever.</p>