Communal Overwintering is Positively Associated with Ophidiomyces ophiodiicola Infection Load within a Southeastern Population of Timber Rattlesnakes (Crotalus horridus)

Vollständiger Abstract

Worum geht es in dieser Arbeit?

At high temperate latitudes, extreme winter temperatures and limited shelter availability often drive the evolution of communal denning behaviors in snakes. At low temperate latitudes, less is known regarding the causes and consequences of variable overwintering strategies. Ongoing research on snake populations across eastern North America has highlighted the prevalence of Ophidiomycosis (caused by Ophidiomyces ophiodiicola, Oo), commonly known as “snake fungal disease.” Despite this, little work has investigated how Oo disease dynamics might shape overwintering site selection strategies (e.g., solitary vs. communal) within populations. We studied Timber Rattlesnakes (Crotalus horridus) from middle Georgia to quantify how winter shelter class (rock feature, stream bank, stump hole) and site type (solitary, communal) were related to variation in Oo infection load (derived from quantitative polymerase chain reaction [Cq values] and clinical [lesion] scores). By integrating disease monitoring with longitudinal recording of overwintering duration, winter body mass loss, winter body temperature, and winter activity level, we tested hypotheses regarding the possible energetic costs and benefits of communal and solitary overwintering. The solitary advantage hypothesis predicted that solitary snakes would display reduced spring Oo infection prevalence and severity relative to communal snakes, given reduced conspecific contact and the possibility of environmental Oo reservoirs in communal site soils. The communal advantage hypothesis stated that communal snakes would exhibit greater fall infection loads than solitary snakes, and in turn select communal sites for greater basking opportunities to induce behavioral fever (through safety in numbers and/or more concealed basking opportunities in rock-based microhabitats). We then expected elevated activity and body temperatures among snakes with higher infection loads to carry sublethal costs of greater winter mass loss and shorter overwintering duration. Collectively, results provided mixed support for our hypotheses but generally aligned with the solitary advantage predictions. Communal snakes were observed to more frequently use rock features as winter shelters, while solitary individuals tended to use stump holes. Lesion scores and Oo fungal loads (Cq values) in spring, but not fall, were greater among communal snakes relative to solitary snakes. However, despite variation in fungal load, most snakes tested positive (80/90 samples [89%] with Cq < 33.00). Winter body temperature was positively associated with spring fungal load, which could be a signal of behavioral fever. Interestingly, body mass loss and overwintering duration were not associated with site class, site type, or fungal load measures, indicating that energetic costs of greater infection severity among communal snakes might be cryptic and/or reflected by unmeasured variables that extend across broader timeframes. Because communal snakes did not display higher fall infection loads, the potential driver of communal site selection remains unclear. Ultimately, consideration of additional selective drivers of overwintering behavior, such as kin selection and inclusive fitness benefits, is required to more fully understand the cost-benefit trade-offs that serve to maintain solitary and communal strategies.