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Image of bull snakes.

Serpentine Science

A biologist gets up close and personal
with one of Minnesota's largest reptiles.

by Dave Crawford

I was sitting among early season prairie grasses eating lunch when a 5-foot-long bullsnake slithered past me.

I'd chosen this lunch site because another bullsnake, a female nicknamed Dottie, had retreated into a nearby burrow a few minutes earlier. Except for chewing, I was keeping my movements to a minimum as I waited for her to reappear.

Following Bullsnakes

image of a bullsnake

To keep track of individual snakes, Minnesota Biological Suvey researchers give them nicknames inspired by distinctive marks. Meet some of the bullsnakes in the study.


Updates from the Field

image of a bullsnake

Find more bullsnake photos, video, and updates from the field.

The arrival of the new bullsnake surprised me. I would have thought the presence of a 170-pound potential predator—me—would be at least a little unsettling to a snake that weighs a mere 3 pounds. But this was April, mating season, and the new arrival was male. His focus was on female bullsnakes, not on humans.

I recognized him as 4Dot. He'd gotten his nickname because of a row of four dark dots across the top of his head. I'd seen him several times before, most recently about 24 hours ago when he and Dottie had been mating next to this same burrow. Today he went to Dottie's burrow, stopped with his head over the opening, and sampled scents in the air with quick flicks of his forked tongue.

It appeared 4Dot wasn't interested in Dottie today, because he moved on after just a few seconds. I had a choice: finish my lunch and hope Dottie would emerge from her burrow, or see where 4Dot was going. I wrapped my sandwich and stuffed it in a pocket. For the next three and a half hours, I followed the bullsnake to observe what he was up to.

I had begun my work with bullsnakes in 2010 after retiring from work as a park naturalist with the Department of Natural Resources. I volunteered to work with Minnesota Biological Survey herpetologist Carol Hall on a new study. She wanted to find out where bullsnakes hibernate each winter in east-central Minnesota.

Bullsnake populations are at risk in Minnesota and elsewhere due to loss of suitable habitat. Urban sprawl, conversion of pasture into cropland, and shading of prime habitat when tree growth is unchecked by fire are major causes of habitat loss. Capture of wild snakes for the pet trade is another unfortunate difficulty bullsnakes face. Bullsnakes are listed as either protected or special-concern species in Minnesota, Wisconsin, and Iowa. Loss of bullsnakes means increases in the populations of the rodents bullsnakes prey on. A single adult bullsnake may save a farmer $400 per year by reducing rodent damage to crops. To keep bullsnake populations from vanishing, it's essential to know which habitats bullsnakes use most and at what times of year.

Life Without Cavities. Scientists know that in rocky terrain, bullsnakes often hibernate together in natural cracks and cavities deep enough to avoid winter frosts. But the study area has no natural rock cavities. There snakes might hibernate together in abandoned wells and cisterns, collapsed cellars, or abandoned mammal burrows. Snakes could also winter individually in smaller burrows of rodents such as ground squirrels and pocket gophers. By placing trail cameras at promising locations in spring and fall, we hoped to gain information on when and where bullsnakes hibernate and when they first emerge in spring.

My assignment was to spend time observing bullsnake activity in the habitats they were likely to use. I did snake surveys several times a week. I enjoyed the challenge of finding bullsnakes and documenting their locations and the types of habitat they used at different times throughout the warm months when they are active.

My strategy was to walk through potential habitat whenever the weather looked promising—that is, warm and sunny. Like all Minnesota reptiles, bullsnakes are able to reach "operating temperature" only by absorbing heat from their surroundings. On cold, cloudy days, they remain safely hidden underground, but on sunny days they move to the surface to bask in the sun and warm up.

In 2012, warm temperatures came early. By mid-March, prairies were alive with colorful sights such as eastern bluebirds and spring sounds such as the insectlike songs of grassland sparrows. I saw my first bullsnake on March 24—19 days earlier than my previous record. By April 12, the prairies were a riot of blooming violets, puccoons, and wild strawberries, and snakes were everywhere. I found and photographed 15 bullsnakes that day.

Gophersnakes. The bullsnake, a Midwestern subspecies of the gophersnake (Pituophis catenifer), ranges throughout most of the continental United States as well as parts of Canada and Mexico. Our local representative of this species is Pituophis catenifer sayi, found mostly in the southeastern quarter of Minnesota near the Mississippi, St. Croix, and Minnesota rivers. The preferred habitats of bullsnakes are sand prairies, bluff prairies, lightly wooded areas, and pastures.

Bullsnakes are nonvenomous, but they can get intimidatingly large—a bit over 6 feet long, making them the largest snake species native to the Midwest. They become even more intimidating if they feel threatened, coiling as if prepared to strike, hissing loudly, and often vibrating the tail. Confronted with this behavior, a snake researcher has to decide whether the risk of being bitten is worth taking in order to capture the snake for measurements and photographs. I like bullsnakes, so I always take the risk. As a result, in 2012 I was bitten by five different bullsnakes. The bullsnake's very short but sharp teeth can inflict bloody scratches. Even newly hatched young may bite, but they make much smaller scratches.

However, on the day of my interrupted lunch, I didn't need to capture 4Dot to identify him. I already had studied mug shots—closeup photos of his head from the top and both sides, which documented the markings that earned him his nickname. Besides visual ID, I was able to confirm his identity by waving a handheld wand over him to read the transponder tag implanted in him a few weeks earlier. The tag reader verified him as 985121024425688.

After he left Dottie's burrow, 4Dot slithered across the prairie, apparently at random, occasionally stopping to check out a burrow opening or meticulously scent-search a plot of ground with his tongue. He covered a little less than a mile in the next three and a half hours. During that time I caught, measured, and photographed two other bullsnakes that crossed 4Dot's path. I also tested 4Dot's tolerance of my presence by circling ahead and letting him come to me. He cruised past me a few feet away the first time and almost ran into me the second time. He appeared to have only one thing on his mind: finding another mate.

Snake Miles. I first started identifying individual snakes in 2012. By the end of the year, I'd nicknamed 76 bullsnakes, based on their markings. The pattern of scale shapes and color markings on each bullsnake's head is distinctive. Some snakes, like 4Dot, are recognizable at a distance. Others may have to be examined closely. But first I need to find them.

Finding bullsnakes entails a lot of walking. By the end of the season, my GPS showed I had racked up slightly over 200 miles. My search success rate was about one snake for every two and a half miles walked.

When possible, I shoot video of the snake's behavior before I approach it. Besides videos of snakes mating, searching for mates, and basking for warmth, I have videos of them biting me as I grab them, complete with a soundtrack of my expressions of surprise. After capturing, photographing, and measuring a bullsnake, I release it at the spot where I captured it.

The most fascinating behavior I documented on video in 2012 was male-versus-male combat over females during the April-May mating season. Just as two white-tailed deer bucks get into shoving contests, male bullsnakes compete for mating privileges. If you're a species with no arms or legs, how do you set up a contest with another male? You head-wrestle. Two males come together and travel parallel to each other, each trying to press the other's head to the ground with its own head.

The snakes can become so twined around each other, as they alternately are pinned and then try to pin the other, they become almost inseparable. I saw a 57-inch-long male, called Round Island, try to break off competition with 5Spot, an impressive 63 inches long. The two snakes were so tightly wound around each other, Round Island was unable to get away until, after several tries, he broke loose. I followed him as he fled the scene, so I was able to get a good look at his markings and read his transponder tag to confirm his identity.

Identification is just one part of my responsibilities. Finding wintering and egg-laying locations is critical. Mapping my observations from 2012 provided very useful insight into these two activities. It appears bullsnakes depend on certain soil and sunlight conditions for successful overwintering and for hatchling production.

Will the bullsnakes use the same locations in 2013? Will the project discover additional locations in areas we haven't had a chance to search yet? Many more years of observation will be needed to obtain a reliable understanding of the bullsnake's survival requirements. But one thing that's already clear is that snakes and roads are a deadly combination for the snakes. So, to keep bullsnakes around and let them keep rodents from becoming excessively abundant, please brake for snakes.

Another Season. I didn't see 4Dot again after the day I followed him. After mating season, bullsnakes are likely to spend more time hunting for rodents underground than roaming about on the surface. Perhaps some of the hatchlings I observed in August were 4Dot's and Dottie's offspring. Hatchlings are cute little miniatures of the adults, weighing about an ounce and measuring only 15 inches long, with a scaled-down version of an adult's ability to hiss and bite.

The onset of cooler weather and shorter days in fall sends bullsnakes back into underground shelter. As prairie grasses turn golden brown and the last few blooms of asters fade, while monarch butterflies are heading for Mexico to survive the coming winter, bullsnakes move into burrows, and my opportunity to observe their lives comes to an end. Some people wait through the winter, longing to see the first robin or flower of spring. I wait for the first bullsnake. Because bullsnakes can live for more than 15 years, I have every hope of seeing 4Dot, Dottie, Texas, Rorschach, Omega, and others in years to come.

Find more information on bullsnakes and other rare species.


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