Trade-off Between Thermal Quality and Predation Risk at Timber Rattlesnake Gestation Sites
Trade-offs can cause conservation measures designed to improve habitat for one aspect of a species life history to inadvertently decrease its suitability in other ways. We explored the potential trade-off between thermal characteristics and risk of predation in important habitat for Timber Rattlesnakes (Crotalus horridus). Although typically a woodland species, pregnant C. horridus congregate in sunny, rocky clearings known as gestation sites that provide improved thermal habitat for embryonic development. Gestation sites can become overgrown with vegetation and, thus, sites may vary in canopy openness. We hypothesized that gestation sites with more canopy openness will be warmer but leave the snakes more exposed to predators. We used hemispherical photography to calculate canopy openness and operative temperature models to quantify thermal quality. Rattlesnake foam models and time-lapse cameras were deployed to quantify the relative predator richness and risk of predation. We found that more-open gestation sites were warmer than enclosed sites. We found that potential for predator encounters was similarly common regardless of gestation site canopy openness. However, we did find that predator species richness increased with canopy openness, with the most dangerous predators (raptors, mustelids, and felids) being only present at the most-open gestation sites. These results suggest that conservation actions for C. horridus that involve opening up the canopy at existing gestation sites may have unintended fitness consequences, as more dangerous predators appear to visit these more-open sites.Abstract
A typical closed canopy Timber Rattlesnake (C. horridus) gestation site (A) contrasted with a typical open canopy gestation site (B). The yellow-highlighted area in each photo denotes the focal area of the gestation site within the clearing at which we observed pregnant C. horridus basking and where we measured canopy cover, operative temperatures, and risk of predation. Although the lower picture is a more-open area, the focal gestation area (shown in yellow) is roughly the same size as the more-enclosed gestation site in the upper picture. Gestation sites are located in central Pennsylvania.
The percent of time during the gestation season that maximum Timber Rattlesnake (C. horridus) operative temperatures at open and closed gestation sites were above preferred body temperatures (Tpref) increased as the percent canopy openness increased (A) and as incident solar radiation reaching the gestation site increased (B).
There was no relationship between the number of times per day that a potential predator visited a Timber Rattlesnake (C. horridus) gestation site and the percent canopy openness of that site (A), but the species richness of potential predators that visited a gestation site increased as canopies over those sites became more open (B).
A Red-Tailed Hawk (B. jamaicensis) is observed on game camera attacking a Timber Rattlesnake (C. horridus) foam model at one of the more open–canopied gestation sites. A yellow foam model is seen directly behind the Buteo and the shelter rock is in the background of the photo, directly to the Buteo's right.
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