Prairie rattlesnake seasonal migrations: episodes of movement, vernal foraging and sex differences

Abstract Functional study of animal migration in nature requires both description and empirical analysis of component episodes of movement dedicated to the potential solution of problems in survival. Good study systems are those exhibiting larger patterns of migration that can be partitioned empirically into functionally analysable, component episodes. This facilitates analysis of alternative explanations both for larger cycles of migration and the component episodes that comprise them. Free-ranging prairie rattlesnakes, Crotalus viridis viridis , are good subjects in this respect. Results of a long-term field study of radiotagged individuals indicate that while both males and females search for small mammal prey in the first half of the brief, 35-month summer in Wyoming, males but not females search for mates in the second half of the season. Females continue to forage for the duration of the season. Accordingly, males must search for and locate two stations in time and space (food and mates) each season, while females need search only for one (food). Given this difference, males and females exhibit predicted differences in key patterns of seasonal activity. Males exhibit significantly greater spatial searching efficiency during the vernal foraging episode than females, allowing males to concentrate foraging activities into the first half of the season, so that the second can be largely dedicated to a search for females. The mating system of prairie rattlesnakes in Wyoming is best described as prolonged mate searching polygyny.