There are several types of migration that occur in nature. While this term generally brings to mind the long distance flight of birds and a few species of bats, it can also refer to the seasonal movements of numerous creatures that abandon their summer domains on the surface for an environment below the frost line.
As cold air becomes more intense, and nightly temperatures more regularly drop into the teens causing water in the uppermost layer of soil to freeze, most cold-blooded organisms that reside there, particularly the red-backed salamander (Plethodon cinereus) must start to migrate down in order to prevent freezing to death.
The red-backed salamander is an exceptionally abundant amphibian that exists in and around the layer of decaying leaves and organic debris that covers the ground in our mixed and deciduous forests. Unlike nearly all other vertebrates, this salamander does not develop a set of functional lungs, or gills, during the course of its life. Rather, it relies entirely on its thin, moist skin to absorb the oxygen that it needs for respiration from the air. This forces the red-backed salamander to confine its activities to the damp environments that are conducive for the transfer of this gas through its body covering. Any setting that is either too dry or cold enough for ice crystals to develop would cease the movement of oxygen through this membrane and prove fatal.
When autumn arrives, the red-backed salamander does not produce the high concentrations of fatty substances in its body tissues that significantly depress the freezing point of water as do the wood frog, spring peeper and several other cold-hardy creatures. Consequently, the red-backed salamander must travel to some terrestrial site where the temperature remains above 32 degrees throughout the winter.
In the Adirondacks, this typically is several feet below the surface of the ground. The relative close proximity of bedrock to the forest floor, and the presence of hard pan gravel prevent this salamander from easily reaching a proper depth in numerous locations throughout the Park. Consequently, this small amphibian must first travel to a site in which it is able to by-pass such barriers before working its way down. Locations that support deep vertical cracks and fissures in the bedrock are occasionally utilized as routes to places that remain above freezing. So too are the rotting root systems of long dead trees that were able to penetrate barriers to reach deep below the surface. Mounds of dead matter, such as where several tree crowns have fallen onto one another creating a massive pile of decaying twigs, branches, bark and leaves are also sought out, as the soil beneath such massive piles of woody matter often fails to freeze regardless of how cold the winter should become.
The soil against the foundation of year-round homes may also be utilized by the red-backed salamander for a migration route downward. This ground may be quite sandy to allow for better water drainage, and often fails to freeze completely, particularly if the basement is partially heated or poorly insulated.
It is believed by some naturalists that the red-backed salamander returns to the same highly effective wintering site each year. As cold weather sets in and older adults instinctively head toward their preferred wintering sites, the younger members of the population in the immediate area may follow these elders using their exceptionally keen sense of smell and eventually learn where good wintering sites exist.
As is the case with any living entity, the severity of the winter impacts survivability. When the snow pack comes early and forms a deep layer that remains throughout the season, the soil may fail to freeze completely, regardless of the intensity and duration of the cold. Conversely, a winter lacking in adequate snow cover allows the frost line to penetrate down much further than usual.
Regardless of what type of winter we will experience, there are always multitudes of these worm-like creatures with the small, rubbery-looking legs that survive to expand the population next year. Despite its delicate appearance and fragile functioning body, the red-backed salamander is able to avoid the harsh temperatures of our long winter season, which allows it to continue as our most abundant vertebrate in the mixed and deciduous forests here in the Adirondacks.
This post was first published in the Adirondack Almanack in November 2011. Read more about Adirondack amphibians in the Almanack archives.
Photo: Red-back Salamander (Courtesy TradersCreek.com)