Main content of the website

Fact Sheet

Manitoba's red-sided garter snake dens are world famous for being the largest concentration of snakes in the world. During the spring mating and the fall pre-denning periods in excess of 50,000 snakes congregate at the dens in the Narcisse Wildlife Management Area (WMA) which contains four major wintering dens. The dens are a network of crevasses and caverns extending to the earth’s surface which were formed by underground water that eroded and collapsed the limestone. It is only by amassing below the frost line that the snakes are able to survive the harsh Manitoba winters.

In the spring, snakes emerge from the dens and breed in the area immediately surrounding the den sites. As each female emerges from the dens they are vigorously pursued by a number of males that entwine themselves around her forming a mating ball. Adult females are easily recognized, as they are larger in both length and thickness than adult males. In late summer the young are born, a female may give birth to as many as 40-50 young with the average brood being about 20 in number. Most juvenile snakes tend to remain where they spent the summer, seeking refuge in anthills, animal burrows or any crevasse that will take them below the frost line. In their second year they will migrate to an established den site. It is thought that snakes locate their home den using a scent trail. Most adult snakes use the same den site year to year, however, mark recapture studies have shown that some snakes use alternate dens.

The movement of so many snakes to and from the winter den area has often resulted in significant mortality along Provincial Trunk Highway #17 adjacent to the Narcisse Snake Dens. Growing concern over the level of mortality led to an innovative solution to reduce the problem. In 2000, the Narcisse Snake Mortality Advisory Group forged a partnership with Manitoba Hydro, Centra Gas and a number of other businesses, organizations and volunteers to install a series of small tunnels under the highway and erect barrier fencing to guide migrating snakes into these tunnels. Mortality has been reduced by almost 75%. As a greater understanding of snake migration patterns emerges, improvements will be undertaken to increase the effectiveness of the system.

The red-sided garter snake can be found from the Northwest Territories south to Oklahoma, east to Ontario and west to British Columbia (Preston 1982). The red-sided garter snake occurs in the southern part of Manitoba north to Flin Flon. It is absent from the extreme southwestern grasslands except for Spruce Woods Provincial Park. The preferred habitat of the red-sided garter snake is near ponds in areas of moderate moisture (Preston 1982). This habitat choice is likely due to the presence of food items like frogs, leeches, tadpoles earthworms, and rodents (Preston 1982). In turn predatory mammals such as black bears, skunks and raccoons eat the snakes. Birds, however, are their most serious predators, many red-sided garters are killed each year by crows, magpies, owls and hawks.

For more information on the red-sided garter snake, please see the Conservation Data Centre Field Guide.


Preston, W. 1982 The Amphibians and Reptiles of Manitoba. Manitoba Museum of Man and Nature. 128 pp.

Fitch, H.S. 1965. An Ecological Study of the Garter Snake, Thamnophis sirtalis. University of Kansas Publications, Museum of Natural History, 15(10):493-564.

Gregory, P.T. 1977. "Life-history Parameters of the Red-sided Garter Snake (Thamnophis sirtalis parietalis) in an extreme environment in the Interlake Region of Manitoba." National Museums of Canada Publications in Zoology 13. 44 pp.