In the fall there is snow in the high country, the leaves are mostly gone and there is a definite nip in the air. For the boreal toads (Bufo boreas) in Rocky Mountain National Park, this means that it's time to find a cozy ground squirrel burrow or well-insulated niche in a beaver dam to "hole-up" for the winter. Boreal toads may spend over half of their lives hibernating. Unfortunately, very little is known about their wintertime behavior or the microclimate of their hibernacula, the place where they hibernate.
Amphibians at more northern latitudes tend to hibernate on land rather than in the water. Overwintering in a terrestrial environment may minimize the risk of predation and does not put the animal at risk of anoxia (physiologically inadequate supply of oxygen) or hypoxia (failure of oxygen to be utilized by body tissues), a likely possibility for amphibians in pond environments where the frozen pond is also covered by snow. The challenge toads face is finding a hibernaculum that is moist and yet does not freeze. A good hibernaculum must not get too cold or dry, protect against predators, maintain oxygen levels, and still supply cues to trigger emergence from hibernation.
Boreal toads may hibernate in rock chambers near streams or in small mammal burrows or beaver dams. Hibernacula near streams provide an environment that is kept above freezing by the proximity of running water. Hibernacula in burrows or beaver dams depend on depth below the frost line and insulation to prevent freezing. Boreal toads have been seen out of their burrows during the winter on warm, sunny days, basking in the sunshine.
Boreal toads emerge from their hibernacula in the spring and make their way to communal breeding ponds. In Rocky Mountain National Park, research suggests that males return almost every year to breeding ponds while females may return to breed only once every two - five years. Breeding in the park ranges from mid-May to early July, depending on the snowpack that year.
For additional information about declining amphibian populations, click here.
Provided by Dr. Erin Muths, USGS
Last updated: March 31, 2012