Pregnant female rattlesnakes prefer to maintain an elevated body temperature (~32 °C), which allows for a more optimal development of embryos. In Pennsylvania forests, however, these warm temperatures are not very abundant. So, in order to achieve these elevated body temperatures, pregnant females seek out rare, open habitat (known as gestation sites) that receive a lot of sunlight. Sometimes, females may travel up to a mile from their den sites just to gain access to openings within the forest canopy.
However, not all open gestation sites are created equally. Last year, Mark Herr, Michaleia Mead, and I uncovered a trade-off at timber rattlesnake gestation sites of various sizes. Gestation sites that were very open provided pregnant females with more sunlight and warmer body temperatures for a longer duration of the day. But, these same sites also came with an increased risk of predation!
More predators visited these more open sites, predators like bobcats, fishers, and hawks. But, smaller, more enclosed gestation sites were so thermally poor, we observed females returning to their den at the end of the field season still pregnant! The use of thermally unfavorable gestation sites may provide the rattlesnake with a sanctuary from potential predators, but there are still terrible repercussions for choosing to use these sites. We have found that snakes using thermally unfavorable sites tend to give birth at a later date. The timing that an animal gives birth is very important. If rattlesnakes give birth to their offspring too late in the year, the small neonate offspring will have little, to no time to complete their first shed and then obtain a small meal before entering the den for hibernation. It is believed that survival for neonates unable to do these two things is close to zero. Further, some pregnant rattlesnakes that use thermally unfavorable gestation sites are known to abort their entire litter toward the end of the summer if she decides that they are developing too slowly.
So why would pregnant females continue to use small, more enclosed, thermally poor gestation sites? Possibly because there was a decreased risk of predation? Possibly, however, because of strong site fidelity? Possibly these sites were, at one time, thermally favorable, but over the years vegetation has encroached upon these open areas and shaded out the once warm, sunny rocks. Due to the rattlesnake’s fidelic response to locating a favorable gestation site, they now find themselves sitting among the shade throughout much of the day.
We’ve addressed the problem, now how do we fix it?
In order to manage timber rattlesnake populations better, forest and wildlife managers have begun to open up gestation sites, un-shading these areas from vegetation, in a process called “daylighting”. However, recall that more open sites have an increased risk of predation. So can we open up these sites just enough to let the sun in, but keep the hawks out? In an attempt to suppress the increased risk of predation that we observed at more open sites, I have begun to direct daylighting techniques to target specific trees that would increase the amount of solar radiation a site would see, without greatly increasing the risk of predation. To do this, I will use hemispherical photography (see picture below), observe the path of the sun throughout the gestation period, and then target those trees that overlap with the path of the sun. This way I can open up each of the sites just enough, but keep those trees that do not overlap with the path of the sun and could perhaps maintain some decreased risk of predation.
Of course I want to address this management idea with as much scientific rigor as possible for an ecological study. This past summer, I visited six historic gestation sites within Pennsylvania. Four out of the six sites are pretty shaded over, and I consider thermally poor. Two of the gestation sites are fairly open and should provide pregnant females with a plethora of sunlight. This summer, I surgically implanted nine pregnant timber rattlesnakes with temperature-sensitive radio transmitters, which allowed me to track these individuals throughout the summer. I followed each snake and noted their behaviors, body temperatures, and the date that they gave birth to their young. Additionally, I measured available body temperatures and risk of predation just like we did in the previous experiment that Mark, Michaleia, and I completed last summer. The catch is, this winter I will go into three of the six sites (along with US Forestry personnel), and we will remove specific trees blocking out the path of the sun. Then next summer I will repeat everything and determine if Daylighting improved these thermally poor sites. Will I see warmer available temperatures within the Daylighted sites? Will snakes within these sites maintain warmer body temperatures and for a longer duration of the day? Will these snakes give birth at an earlier date, allowing their young to shed and get a first meal before hibernation? Will I still see an increased risk of predation?
Many of these questions I won’t be able to answer until next year. But, I am collecting some interesting data thus far. As expected, the two sites that were more open did have warmer available temperatures. Snakes occupying these sites maintained warmer body temperatures, moved less often, and were the first to give birth to adorable baby rattlesnakes!
But, these sites were also visited by more potential predators. In fact, in one instance we observed a red-tailed hawk swoop down and nab a garter snake that was basking alongside our pregnant rattlesnakes!
The more enclosed sites were indeed cooler and snakes at these sites maintained cooler body temperatures and moved more often. It appeared that some of these pregnant females shuttled between nearby sites in order to track the path of the sun. In the morning the snake may be at one site, and in the afternoon that same snake would move to a nearby site.
Other snakes continued to move throughout the entire summer from one site to the next; constantly searching for a thermally suitable site where she could continue to develop her babies. Unfortunately, all of these movements brought one of our mommas too close to a nearby road where someone swerved into the shoulder in order to run her over. Although the loss of this mom was a little tough on me, it did show me just how important it is to improve these historic gestation sites.
Currently, as I am writing this blog, we are still waiting on some of our snakes from the thermally poor sites to give birth. As we find neonates (babies) at each of our sites, I will also collect data on each of them so that we can compare body condition (health) among sites.
Next year, I will continue to track snakes throughout these same gestation sites. However, following our daylighting management, we hope that all of our pregnant snakes will give birth at early dates, move less, and stay clear of predators. To be continued…