Thamnophis sirtalis parietalis reaches farther north than any other American snake, and its survival there depends on finding a hibernaculum below the frost line. Suitable spaces are rare, so snakes converge on karst sinkholes and crevices from many kilometres around. Dens at Narcisse in Manitoba have held tens of thousands of individuals in a single winter, and the site is managed with snake tunnels and seasonal viewing.
Emergence in late April and May is synchronised. Males leave first and wait at the den mouth; when a female appears, dozens may wrap around her in a mating ball. Some males release female pheromones, becoming so-called she-males, which appears to draw other males away and may help the mimic warm faster after emerging.
The aggregations are a conservation vulnerability rather than an abundance. Because so much of a regional population occupies one hole, a single event such as road mortality during migration or the destruction of a den can remove a large fraction of the snakes in the surrounding landscape.