Anthropogenic habitat modification often creates barriers to animal movement, transforming formerly contiguous habitat into a patchwork of habitat islands with low connectivity. Roadways are a feature of most landscapes that can act as barriers or filters to migration among local populations. Even small and recently constructed roads can have a significant impact on population genetic structure of some species, but not others. We developed a research approach that combines fine-scale molecular genetics with behavioral and ecological data to understand the impacts of roads on population structure and connectivity. We used microsatellite markers to characterize genetic variation within and among populations of timber rattlesnakes (Crotalus horridus) occupying communal hibernacula (dens) in regions bisected by roadways. We examined the impact of roads on seasonal migration, genetic diversity, and gene flow among populations. Snakes in hibernacula isolated by roads had significantly lower genetic diversity and higher genetic differentiation than snakes in hibernacula in contiguous habitat. Genetic-assignment analyses revealed that interruption to seasonal migration was the mechanism underlying these patterns. Our results underscore the sizeable impact of roads on this species, despite their relatively recent construction at our study sites (7 to 10 generations of rattlesnakes), the utility of population genetics for studies of road ecology, and the need for mitigating effects of roads.

No abstract

Individuals of many species show high levels of fidelity to natal populations, often due to reliance on patchily distributed habitat features. In many of these species, the negative impacts of inbreeding are mitigated through specialized behaviours such as seasonal mating dispersal. Quantifying population structure for species with these characteristics can potentially elucidate social and environmental factors that interact to affect mating behaviour and population connectivity. In the northern part of their range, timber rattlesnakes are communal hibernators with high natal philopatry. Individuals generally recruit to the same hibernaculum as their mother and remain faithful to that hibernaculum throughout their lives. We examined the genetic structure of Crotalus horridus hibernacula in the northeastern USA using microsatellite loci. Sampled hibernacula exhibited only modest levels of differentiation, indicating a significant level of gene flow among them. We found no significant correlation between genetic differentiation and geographical distance, but did find significant positive correlation between genetic differentiation and a cost-based distance metric adjusted to include the amount of potential basking habitat between hibernacula. Therefore, thermoregulation sites may increase gene flow by increasing the potential for contact among individuals from different populations. Parentage analyses confirmed high levels of philopatry of both sexes to their maternal hibernaculum; however, approximately one-third of paternity assignments involved individuals between hibernacula, confirming that gene flow among hibernacula occurs largely through seasonal male mating dispersal. Our results underscore the importance of integrating individual-level behaviours and landscape features with studies of fine-scale population genetics in species with high fidelity to patchily distributed habitats.

Chemicals left by organisms moving through the environment are used by other organisms to mediate interspecific interactions. Most studies of chemical eavesdropping focus on prey responding to chemical cues from predators, despite the fact that chemical cues are frequently used by predators as a source of information about prey. Crotalus horridus uses a foraging strategy that is widespread among sedentary predators: the snake chooses a site where it is likely to encounter prey and remains immobile for many hours. I investigated this ambush hunting behavior in captive-raised timber rattlesnakes and provide evidence that sit-and-wait predators may discriminate among prey chemical cues, even when they have no prior experience with the prey. Snakes explored chemical cues with chemosensory behaviors, and more frequently adopted a stereotyped ambush foraging posture toward chemical cues from prey sympatric with their population of origin than either allopatric prey or sympatric nonprey species that are eaten by other viperids. These results support the notion that intra- and interspecific variation in diet may be mediated proximally by innate recognition of cues from particular prey items. This system also describes a bioassay that may be used in the isolation and identification of prey-derived kairomones. Studies such as this can be used to determine more realistic parameters for models of predator-prey interaction and foraging behavior that involve secretive, less active predators.

Long-term studies of individual animals in nature contribute disproportionately to our understanding of the principles of ecology and evolution. Such field studies can benefit greatly from integrating the methods of molecular genetics with traditional approaches. Even though molecular genetic tools are particularly valuable for species that are difficult to observe directly, they have not been widely adopted. Here, we used molecular genetic techniques in a 10-year radio-telemetric investigation of the western diamond-backed rattlesnake (Crotalus atrox) for an analysis of its mating system and to measure sexual selection. Specifically, we used microsatellite markers to genotype 299 individuals, including neonates from litters of focal females to ascertain parentage using full-pedigree likelihood methods. We detected high levels of multiple paternity within litters, yet found little concordance between paternity and observations of courtship and mating behavior. Larger males did not father significantly more offspring, but we found evidence for size-specific male-mating strategies, with larger males guarding females for longer periods in the mating seasons. Moreover, the spatial proximity of males to mothers was significantly associated with reproductive success. Overall, our field observations alone would have been insufficient to quantitatively measure the mating system of this population of C. atrox, and we thus urge more widespread adoption of molecular tools by field researchers studying the mating systems and sexual selection of snakes and other secretive taxa.