Costs and benefits of straight versus tortuous migration paths for Prairie Rattlesnakes (Crotalus viridis viridis) in seminatural and human-dominated landscapes

Abstract

An individual's migration path shape should affect its fitness, because patchily distributed features (e.g. prey) are encountered more often on straight than tortuous paths. We hypothesized that Crotalus viridis viridis (Rafinesque, 1818) with straighter migration paths should have better body condition, because they encounter prey patches more frequently, and higher migration mortality, because they also encounter predators and hazardous human land uses more frequently, than individuals with tortuous paths. If true, then a straighter path should be favoured when the benefit (resource acquisition) outweighs the cost (mortality risk). Humans pose a significant mortality risk for migrants; thus the cost of straight-line movement should increase relative to the benefit in more human-dominated landscapes, favouring more tortuous movements. We tested these hypotheses using data on the body condition, mortality, and migration movements of 25 female rattlesnakes in one human-dominated and one semi-natural landscape. As hypothesized, we found better body condition and higher migration mortality for snakes with straighter migration paths, and that snakes followed more tortuous paths in the human-dominated landscape. Although selection for tortuous movements may reduce rates of migration mortality in human-dominated landscapes, this may ultimately contribute to population declines if poorer body condition reduces overwinter survival or reproductive success.

Snake mortality and cover board effectiveness along exclusion fencing in British Columbia, Canada

“…The impact of exclusion fencing may be greater for agile and highly active snakes. Species that are relatively active and/or undergo longer migrations may be more likely to encounter fences and other disturbances, potentially becoming isolated from crucial resources (Ferronato et al 2014;Martin et al 2017). Home range and movement data for Western Yellow-bellied Racer in BC are scant, but Brown and Parker (1976) showed home ranges for this species in Utah generally extended no further than 1000 m from den sites.…”

The physiology of movement

“…As these physiological changes are anticipated to be correlated with demographic traits and behaviours, hence forming behavioural syndromes [26,81,170] they can eventually impact equilibrium population sizes and their fluctuations [171], as mediated by costs during movement and changes in local growth rates (e.g., [172,173]). Such feedbacks can even be lagged if physiological responses are mediated through maternal effects, as for instance the case by induced hormonal effects [33].…”