CHAMPAIGN, Ill., 4/24/24: Illinois’ timber rattlesnakes emerge from their cold-weather dens hidden below ground and can be seen sunning themselves in the nearby forest foliage as spring temperatures warm. Scientists at the Illinois Natural History Survey (INHS) have found clues to determine when snakes will slither from their over-winter spots to inform conservation officials who manage local habitats.

Timber rattlesnakes are considered threatened and are found in most of Illinois, particularly in southern Illinois and along the Mississippi and Illinois river border counties in remote areas with sloped terrain. Once temperatures reach the low 50s, they congregate in groups of 20 or 30 in dens to hibernate through the winter. 

Although timber rattlesnakes are venomous, they are docile, slow-moving animals that are difficult to find, according to Andrew Jesper, INHS doctoral student and principal investigator of the study. They are well camouflaged in the brown and gold leaves.

The best time to see them is in the spring and fall in bluffs and outcrops where they congregate temporarily in warm spring weather and gather for winter hibernation.

For the study, Jesper used trail cameras at two sites, Jo Daviess County in northern Illinois and Jersey County in west central Illinois from 2018 to 2020 in late winter and early spring. The goal was to learn more about the emergence period and develop a model to predict when snakes would appear on the surface. 

The information is important because delays in spring emergence can shorten the time when they are active, Jesper said. In colder climates, they exhibit slower growth rates, delayed sexual maturity, smaller offspring sizes, and lower reproductive success – all affecting species survival.

Jesper observed early on a few snakes would shuttle back into their den once they emerged, but most stayed above ground after leaving their sleeping quarters. They tend to pick a spot under a rock to camp out until they migrate to summer ranges.

"Once they come out, they’re basically stranded,” said Jesper. “It’s an important and risky period. They must decide when to come out and maximize the four and a half months to forage, put on fat, and reproduce. If they come out too early, they risk exposure and mortality. It’s a fascinating dilemma.”

Previous studies have suggested temperature is the primary factor determining when snakes will surface. Jesper found a combination of active daily temperature, accumulated degree days, and latitude is most effective in predicting snake emergence. Degree days are the amount of heat available above a threshold temperature for a given day. 

On average, for every 1 degree increase in latitude, snakes emerged one week later in the spring. The trend explains some variation between the two counties, as snakes in the northern site emerged later than those in the central Illinois site.

Global warming will lengthen the active period for snakes but will make land management more difficult, according to Michael Dreslik, INHS population ecologist, and coauthor of the study. Snakes would likely surface during warm stretches in the middle of winter and move back to dens later in a warm fall.

“The snakes would be just fine,” Dreslik said. "But climate change shortens the window to manage the woodland habitats where they forage. For us, it would create more difficulty and challenges during the burning season.”

The study has implications for management practices because prescribed burns in sections of woodlands control the foliage and invasive plant species, keeping the canopy open for sunlight and preventing invasive species from taking over. Prescribed burns often occur at the same time snakes are emerging from their dens. Thus, it’s important to know when snakes are above ground.

“The model can be used to better time the management practices in areas with endangered species,” Dreslik said. “It can be used as a trigger to cease managing these denning sites because now we can predict when snakes will disperse from their over-winter sites.”

The study was funded by the Illinois Department of Natural Resources through the United States Fish and Wildlife Services’ State Wildlife Grants Program, the Illinois State Toll Highway Authority, and INHS and is published in the journal PeerJ – Life and Environment.

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Media contact: Andrew Jesper, 618-409-0984, ajesper2@illinois.edu; Michael Dreslik, 217-300-0970, dreslik@illinois.edu