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Iowa

Prairie Rattle Snake Study

Only on Conservancy Land

By Dan Fogell

Iowa’s Loess Hills, just a short drive from Sioux City, there are prairie rattlesnakes — grassland snakes that inhabit much of the western United States.

The Nature Conservancy’s Broken Kettle Grasslands Preserve in Plymouth County is home to Iowa’s only remaining population of prairie rattlesnakes.

In fact, the nearest Nebraskan population is nearly 90 miles to the west, while South Dakota’s closest population is at least 30 miles west of Broken Kettle.

At the suggestion of Conservancy employee Susanne Hickey, and with financial support from the Iowa Department of Natural Resources, I spent the warmer months of 2000 researching the habitat needs and population structure of this disjunct population of prairie rattlesnakes.

There are many habitat features that must exist in order for prairie rattlesnakes to thrive, and Broken Kettle has them all. At the entrance to the preserve there is a series of southwest-facing rocky hills. These hills are virtually treeless, have a limestone foundation, and are the winter refugia for Broken Kettle’s prairie rattlesnakes.

The snakes spend six months in hibernation and emerge at the onset of the warmer weather and longer days of spring. Nearly all of the snakes go through their first seasonal shedding of skin shortly after emergence. Once they have shed, they can commence with the activity that will drive their lives for the rest of the summer—feeding.

These rattlesnakes will travel short distances while continuously "tasting" the air with their forked tongues, picking up scents, and stopping at areas that have a heavy odor of mice—their preferred food. Here, they will find a "rodent runway"—a path that is consistently used by rodents—and then coil and wait for a rodent to come within striking range.

If a snake is not successful in procuring food within a few days, it will travel a little further until it finds another potential lurking site. Male and female snakes will maintain this cycle until late summer when the males decide that feeding is not as important as another activity: breeding.

Sometime around late August, male rattlesnakes will break from their feeding cycle and set out on a search to find a mate. Although they still taste the air while they are traveling, they are looking for a different scent. If they find it (sometimes they don’t), breeding takes place for about a day, then feeding resumes.

Females will not give birth until the following summer. Pregnant females emerge from hibernation just as the other snakes do, however, that’s where the behavioral similarities end. Their first move after emergence is to a mammal burrow on a south-facing hill, where they will remain until they give birth. While a pregnant female might stray a few yards or so to opportunistically search for food, she normally retreats back to the burrow within a day or so. Birthing takes place in August, and mother rattlesnakes remain with babies until they have completed their first shed. From that point on, the babies are on their own and must find food, water, and shelter while avoiding predators.

The key habitat features during the summer months are mammal burrows. Badgers and gophers dig a lot of holes, but they often abandon them and opt for new burrows. The old burrows provide excellent shelters for rattlesnakes when temperatures get too hot. The snakes also use burrows during their shedding cycles because their vision is greatly reduced during this time, thus they are more vulnerable to predation. The burrows provide security while the snakes wait out the shedding cycle. Once they shed, they continue on with regular summertime activities.

When the summer activities are over, snakes begin their migration back to their dens. The migration is gradual and snakes often stop to feed on their way back. Once they arrive at their dens, many snakes will continue to bask on the rocky ledges as they try to digest that last mouse of the season, however by mid-October, all of the snakes have retreated to their underground winter quarters where they will remain until spring.

The prairie rattlesnakes at Broken Kettle have a good chance of survival. They are considered endangered and are protected by the State of Iowa, and they live on land that is protected and managed by The Nature Conservancy. However, because they are an isolated population, a single stochastic event could destroy them all. Only continued management and monitoring will ensure their existence in Plymouth County.

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