Patterns of circulating corticosterone in a population of rattlesnakes afflicted with snake fungal disease: Stress hormones as a potential mediator of seasonal cycles in disease severity and outcomes

Abstract

Snake fungal disease (SFD) is an emerging threat to snake populations intheUnited States.Fungalpathogens are oftenassociated with a physiological stress responsemediated by the hypothalamopituitary- adrenal axis (HPA), and afflicted individuals may incur steep coping costs. The severity of SFD can vary seasonally; however, little is knownregarding (1) how SFDinfection relates toHPA activity and (2)how seasonal shifts in environment, life history, or HPA activity may interact to drive seasonal patterns of infection severity and outcomes. To test the hypothesis that SFD is associated with increased HPA activity and to identify potential environmental or physiological drivers of seasonal infection, we monitored baseline corticosterone, SFD infection severity, foraging success, body condition, and reproductive status in a field-active population of pigmy rattlesnakes. Both plasma corticosterone and the severity of clinical signs of SFD peaked in the winter. Corticosterone levels were also elevated in the fall before the seasonal rise in SFDseverity. Severely symptomatic snakeswere inlowbody condition and had elevated corticosterone levels compared to moderately infected and uninfected snakes. The monthly mean severity of SFDin the populationwas negatively related to populationwide estimates of body condition and temperature measured in the precedentmonth and positively correlated with corticosterone levels measured in the precedent month. Symptomatic females were less likely to enter reproductive bouts compared to asymptomatic females. We propose the hypothesis that the seasonal interplay among environment, host energetics, and HPA activity initiates trade-offs in the fall that drive the increase in SFD prevalence, symptomseverity, and decline in condition observed in the population through winter.