Prey availability and snake fungal disease as drivers of timber rattlesnake habitat selection across multiple spatial scales
Annalee McColluh Tutterow, MS
Advisor: William E. Peterman, PhD
Habitat selection can be multi-scale and hierarchical, suggesting that varied environmental or resource gradients can influence habitat use patterns across different spatial scales. Snake habitat selection is influenced by microhabitat features that affect an individual’s ability to forage, avoid predation, and thermoregulate. However, it is unclear and debated whether snakes prioritize thermoregulatory needs over prey availability when foraging (i.e. opportunistically forage) or selectively forage to maximize potential prey encounters (i.e. prey-mediated habitat selection). Additionally, snakes infected with an emerging mycosis, snake fungal disease (SFD), may alter their behavior and site use to optimize physiological performance. The collective physiological and behavioral shifts associated with SFD may scale to differences in site selection patterns and home range characteristics, but these predictions on habitat use have not been robustly examined.
Understanding habitat use for individuals in various physiological and behavioral states is essential for developing comprehensive management strategies for imperiled wildlife. Rattlesnake natural history characteristics, including their stereotyped behaviors, make them ideal subjects to test hypotheses about habitat selection. My research seeks to address how distinct behaviors or physiological states, such as foraging or infection status, shape timber rattlesnake (Crotalus horridus; hereafter, TRS) space use. I used a multi-year radio-telemetry dataset to differentiate among behavior/physiology-specific site use and account for individual variation in habitat selection. The primary goals of this study were to (1) assess the influence of landscape-scale prey availability on TRS ambush site selection and to (2) quantitatively assess the effects of SFD infection on individual behavior, site selection, and space use metrics.
(1) I found that the cumulative prey landscape (i.e. the combined spatial distributions of Peromyscus spp., Tamias striatus, and Sciurus spp.) was highly predictive of snake ambush site selection. Although the spatial distributions of individual prey species did not overlap well, TRS responded to overall prey availability. My findings suggest that TRS can detect fine-scale differences in prey availability and selectively forage in prey-rich areas, making the prey landscape an important driver of habitat selection.
(2) I compared behavior, movement patterns, habitat use, and home ranges among years that snakes were asymptomatic or symptomatic of SFD and found evidence for altered space use during SFD infection. Snakes increased their maximal home range area (100% minimum convex polygon [MCP]) and doubled their core home range area (50% MCP) when infected. I also found sex-specific effects of SFD on snake behavior and habitat use, with non-gravid females typically most affected by SFD across all examined space use metrics. Symptomatic females rested more and foraged less frequently than expected, while symptomatic males rested less and foraged more frequently than expected. Females were more likely to use dry, southwestern-facing, high elevation sites in early seral stands, conditions favorable to thermoregulation, when symptomatic of SFD. During symptomatic years, snakes selected sites with greater solar exposure, but only exhibited elevated body temperatures during the fall, likely due to increased surface activity at the end of the active season.
These studies provide insight into external and internal drivers of snake habitat selection. My findings suggest that (1) optimal foraging theory may be applicable to the foraging ecology of low-energy ambush predators and (2) physiological and behavioral adjustments associated with SFD infection can scale up to effect change in habitat use.