The Northern Great Plains

Winter is a challenging time for most inhabitants of the Northern Great Plains. Those who live in this vast region must endure long months of harsh, sometimes sub-zero, temperatures. Only the strongest survive the long months of cold, and even the most suited have to be prepared to withstand all that the season throws their way.

In the next four installments of Rangelands Alive! we’ll be highlighting four winter survivors of the Northern Great Plains: a reptile, a rancher, an insect, and a plant. Each has its own way of thriving when the temperatures are low and the snow is high. For our first subject, we’ll be focusing on a somewhat controversial character: Crotalus viridis, a species commonly known as the prairie rattlesnake.

Since humans first encountered rattlesnakes the relationship has been difficult. In particular, rattlers like all snakes—venomous or otherwise—are often feared. And while it is true that rattlesnakes can pose a danger to people, pets, or livestock if stepped on or harassed, in general, they prefer to be left alone. However, for the purpose of this essay, we’re going to depart from those facts, and instead focus on a little-known trait of this widespread reptile: its remarkable long-distance migration and overwintering abilities.

5 min read

By Clay Bolt / WWF NGP Program

Rattlesnakes, like all reptiles, are unable to generate their own body heat. Instead, they require sunlight to warm their bodies for movement and digestion. Without warmth, reptiles can only survive for short periods of time. The prairie rattlesnake is tougher than most when it comes to tolerating the cold and can occasionally withstand several days of below freezing temperatures. However, to make it through the long, cold winters in the Northern Great Plains, they must find submerged dens at least six feet below the frost line. In the dry, rocky plains, such places are few and far between. Individual snakes often have to migrate many miles to and from these sites at the start and conclusion of each season.

A prairie rattlesnake overwintering site with scientific monitoring equipment , Alberta CA
© Dennis Jorgensen

Because these dens are so rare, it means that they are often shared between many snakes (often of various species) and reused year after year.

A prairie rattlesnake shares a den with two garter snakes, Alberta CA © Dennis Jorgensen.

A prairie rattlesnake warms itself near the entrance to its burrow, Alberta CA © Dennis Jorgensen

Prairie rattlesnakes migrate the furthest of any known terrestrial reptile from an overwintering site to feeding grounds. The longest known prairie rattlesnake migration is an impressive 33 miles. Not too bad for a species without legs! To put this into context, a marathon, which is lauded as an impressive feat of athleticism, is a mere 26.2 miles. This might leave you wondering, if the dens are so hard to come by, why do the snakes even leave? The answer comes down to the availability of food and suitable mates. Research has revealed that some individuals tend to stay close to their overwintering site. However, others will often travel long distances to find a mate and food. This keeps competition near the wintering den at a minimum and ensures that the population retains a healthy level of genetic diversity.

Prairie rattlers give birth to live young anywhere from August to October, just before the harsh winter weather hits. When temperatures begin to drop around September, they begin to migrate back to the denning site in which they were born and overwinter in each year. Ideally, they attempt to arrive before the first snowfall. Sometimes individuals don’t make it back to the nest in time in which case they either succumb to the cold or are forced to wait it out until the bad weather breaks. If it doesn’t, the stranded snakes may die.

Over the past 20 years, there have only been three deaths by prairie rattlesnake bites. They are important predators of rodents and help maintain balance in the prairie ecosystem. The next time you come across one, keep a safe distance, and take a moment to marvel at this long-distance traveler that defies the odds to survive in this challenging landscape.

For more information about prairie rattlesnakes contact Dennis Jorgensen via dennis.jorgensen at wwfus.org.

WINTER SURVIVAL IN THE NORTHERN GREAT PLAINS

PART 1: THE PRAIRIE RATTLESNAKE