Stake Out for Serpents

Thursday, November 14, 2013 - 11:35am

If you have ever gone hiking, you may have heard the saying that any snake you might encounter on the trail is more afraid of you than you are of it. Given the chance, a snake may retreat from a human on foot, but it is much less likely to recognize the threat from a human-operated bulldozer or backhoe. As energy development moves into remote areas, research conducted by a team of wildlife biologists, herpetologists, and other field technicians led by Riparia Principal Investigator Gian Rocco, is providing clues into just how vulnerable these animals could be.

Special excerpt from the fall 2013 GEOGRAPHY newsletter commemorating 20 years of Riparia


gestation site





Photo taken by Chris Camacho at a spectacularly productive communal gestation site on July 4, 2011,a year also believed to be unusually productive for gravid females.






Rocco and his team are conducting several studies in Pennsylvania and New Jersey to determine how snakes might be affected when human activity compromises their environment. Central to answering that broader question is understanding how snakes respond to the loss of their overwintering sites. “Ethically, you don’t want to purposely go out and destroy a site to see what happens,” Rocco explains. “In New Jersey, we were offered the opportunity to observe their fate and behavior because the loss had either already occurred or was likely to occur and ‘rescued’ animals were available for long-term observation by radio-tracking. Basically, regulatory agencies decided to make the most out of an unavoidable situation resulting from utility improvement projects. And both utility companies sponsored the research, recognizing that it would benefit everyone to learn how snakes cope in such circumstances.”

"They are not random in their movements.

Snakes have mapped their world much in

the same way as we humans.”

Radio-tracking snakes is not the glamorous and exciting occupation that nature shows make it appear to be. “Truth be told, it is not for the weak-bodied or -minded as it requires physical toughness, extreme diligence, perseverance, and dedication,” Rocco warns, adding “For the passionate and truly professional wildlife biologist, it is the ultimate window into an animal’s life —there is no other experience like it—however, much will be missed unless you really, really want to be there and for all the right reasons.”

For the safety of the snakes and other reasons, specific locations and other details about these studies are not being revealed. Don’t try this at home.

The Pennsylvania study: what’s the easiest way to count rattlesnakes?
Research sponsor, the Pennsylvania Department of Conservation and Natural Resources (PADCNR) leases hundreds of thousands of acres of remote forest areas to companies for the extraction of natural gas from the underlying Marcellus Shale. These remote forest areas are also home to the timber rattlesnake (Crotalus horridus), a Pennsylvania Candidate Animal Species of Concern. (Dramatically affected by human activity, and no longer occurs in many of its former locations.)  “As human intrusion and activities will undoubtedly increase in these once remote areas, it seems reasonable as well as prudent to evaluate the potential impact of energy development on the timber rattlesnake,” Rocco notes. So, since May 2011, Riparia has been studying the long-term impact of natural gas extraction activity on timber rattlesnakes at multiple sites in two remote areas, a large treatment tract, where natural gas extraction is anticipated, and a similarly sized control tract, where no such activities are anticipated in the near future.

The timber rattlesnake is a wide-ranging, well-camouflaged, and cryptic-behaving animal, Rocco explains, so like most other snakes, they are difficult subjects to sample effectively. “We decided to focus our sampling in places that tend to attract gravid [pregnant] females because they are the easiest to detect and the most sedentary during their gestation [brooding] period,” Rocco explains. “Timber rattlesnakes give birth to live young, but doing so successfully requires them to have access to what appears to a human as a blisteringly hot, minimally shaded, rocky slope. In these communal gestation sites, up to several dozen gravid snakes absorb an abundance of warmth from sunshine during the day and radiant heat from the rocks during the night.

For them, it’s a strategic choice. For us, it means that from mid-June to late-August, most will be in the same place, allowing us repeated sampling of the same site. It’s almost as if we are monitoring places rather than the creature itself.”

It is the timber rattlesnake’s normal naturally selected behavior that puts it at risk when humans enter their environment. An ambush predator, the timber rattlesnake positions itself on the scent trail of a mouse or chipmunk. It remains immobile and relies upon is coloration and deathly still behavior to camouflage it against the forest floor. Its survival in the forest has depended upon this instinct to hide and stay motionless—no matter what. When humans enter these remote forest areas with large noisy machinery, you might think that the noise, activity, and vibration would scare away any creatures in the vicinity.  Not the timber rattlesnake. “This animal is not hard-wired to move away, even assuming it recognizes the threat. Invariably, heavy equipment will roll over the snake if it happens to be lying on the path, a one-sided game of ‘Battleship’ in the wilderness.”

In all the snake research Rocco supervises, the animals are captured by highly experienced individuals called “hunters,” and then measured for length, weighed, and permanently marked with passively induced transponder (PIT) tags. “There is no snake-wrangling here, and this is where so many nature shows again do us an injustice. Professional tools of the trade are used at all times to ensure the safety of the animals and, of course, the handlers. It is to our advantage to be as gentle as possible, following specific protocols,” Rocco notes. “This is not going out there like a bunch of cowboys on a rattlesnake round-up. You want to study healthy animals that are not injured or under stress.”

PIT tags contain a unique identification number that when energized by a scanner will read like a bar code on a grocery item. These are the same tags that your vet can place in your pet. The recapture of a PIT-tagged animal can inform researchers of its status (age, growth, breeding condition), its movement since its last capture, and can be used to infer population size, Rocco explains.  “The recapture of the same animals over and over again would suggest a small population; whereas the opposite situation – the re-capture of few previously marked animals – would suggest a very large population, among other possibilities,” he adds. Some individual snakes are equipped with radio-transmitters, a signal-transmitting device that unlike a PIT tag, allow field personnel to locate and visually confirm the location of snakes without disturbing them.

 What have we learned so far? “After two years of pre-disturbance study, we have an outstanding baseline— the study will also help us understand what type of overwintering sites timber rattlesnakes use in these parts of Pennsylvania.  This information in particular, will aid regulatory agencies striving to keep natural gas development activities as far away as possible from such critical habitat,” Rocco says.

snake map

Polygons illustrate the activity ranges (outermost locations shown) for 33 timber rattlesnakes radio-tracked in 2011 and 2012 at the treatment tract in north central Pennsylvania. Football field is scaled for comparison.


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More on the New Jersey study: if you build artificial snake dens, will they come?
What happens to snakes deprived of their overwintering sites? In the winter of 2012-2013 a utility company in the process of upgrading and repairing a high voltage electricity transmission line in northern New Jersey discovered that the stone-filled foundations of their towers were being used by several species of snakes for overwintering.

Although they do not hibernate as mammals do, snakes need shelter from freezing temperatures during the winter months when they are less active.

The snakes were captured, in some cases following hand-excavation, and held at a wildlife rehabilitator for much of the winter. Many of these snakes were released unmarked because of their small size following the arrival of warmer weather.

All other remaining snakes, those that were larger, were pit-tagged prior to their release with some of these larger ones also fitted with radio-transmitters. “You can’t put a radio collar on a snake—it’s one long neck,” Rocco observes.  Unlike PIT tags, radio-transmitters transmit a signal (a beep-like sound) at regular intervals and within a very narrow frequency to allow receiver and directional antenna-equipped field personnel to home-in on individual snakes by tuning the receiver to a particular frequency.  By this method, field biologists can see the day-to-day whereabouts of the animal. 

Copperheads (Agkistrodon contortrix), black rat snakes (Pantherophis obsoletus), and black racers (Coluber constrictor) were implanted with transmitters then released “exactly where they were found— we know from other studies that relocating them to a non-familiar area is a terrible conservation strategy—in May 2013, when weather was warm enough that they would have emerged from their overwintering dens,” he explains.

 “So far we have seen typical summer range behavior: mating, feeding, and depredation. The most interesting part is coming right now as the animals begin to move to their former overwintering sites and to where artificial dens were constructed as replacement,” Rocco says, adding “The new dens were constructed nearby the ones lost during construction of the new electricity towers— this was part of the utility company’s mostly voluntary  mitigation work in an effort to offset the loss of the previously occupied dens which were probably also inadvertently created many decades ago.”

As with the Pennsylvania study, the benefit of the New Jersey study is that the information can help regulatory agencies assess the risks when known overwintering sites are lost. The general expectation is that the snakes will retreat for the winter to the new artificial or existing natural dens. It is known that some snakes use multiple dens. “With timber rattlesnakes, females tend to have greater site fidelity whereas several of males we have observed have used different dens. We know this from other studies as well,” Rocco explains.  “But we don’t know how these snakes will view these artificial dens. They could use them or they could ignore them. They are not random in their movements. Snakes have mapped their world much in the same way as we humans.”