Prairie rattlesnake vernal migration: field experimental analyses and survival value

Abstract

Many snake taxa are seasonally migratory. Wyoming prairie rattlesnakes (Crotalus viridis viridisJ exhibit lengthy seasonal migrations away from and then back to home dens. These animals have served as a model system for studies of the survival value and causation of such movement. In the field experiments reported here, radiotagged, freeranging individuals were exposed to experimentally augmented prey patches as they migrated from overwintering hibernacula in the spring of both 1987 and 1988. Once they had begun traveling, vernal migrants were exposed to: patches augmented with live, caged deer mice; patches augmented with cages containing deer-mouse-derived chemical cues but no live deer mice; or cages containing neither deer mice nor their chemicals (a control condition). Both males and females responded to patches containing live deer mice with significant reduction in activity. Females remained in patches containing caged deer mice for -24 hours before moving on, while males became virtually immobile once the caged rodents were located. Two males remained with the live deer-mouse condition for the full 21-day duration of one of the experiments. Males appear to assume ambushing tactics more than females, once prey patches are located. Males also responded to conditions containing deer-mouse chemicals alone with reduction in activity. Additional work will be needed to address the latter problem amongfree-rangingfemales. Results of this study support the following hypotheses: a primary function of prairie rattlesnake vernal migrations in Wyoming is to find food; males and females differ in vernal foraging patterns, in ways consistent with snake life history and search-time theory; and preyderived chemical cues play a role in patch selection by males. Given the alternatives, migrating prairie rattlesnakes thus exhibit a vernal episode of movement dedicated primarily to findingf ood.