Black Rat Snake
A black rat snake has been living in our house. I know because whenever I climb into the attic, I find a skin it has shed. A week ago I found the latest skin. It’s about a metre and a half long.
It gives me a sense of ongoing delight to think of the snake cruising around the house — inside the walls, over the upstairs ceiling, wherever it is that it goes. Although black rat snakes are not an endangered species, they are under siege, and it’s comforting to think that at least one of them has found a safe retreat.
Besides, it’s eating the mice that love to chew on our electrical wiring.
Black rat snakes are Canada’s largest snake. They can grow to 2.5 metres and can weigh more than 2.5 kilograms. They live to a ripe old age of 25 years for males, and 20 for females. They’re constrictors, killing prey by coiling around it and squeezing. And they’re harmless to humans.
In the United States, they range over about 20 per cent of the land, from the northeastern seaboard to the Midwest. But in Canada they exist only in isolated pockets within Ontario.
Four of those pockets are so small and embattled there’s a good chance they won’t survive. They’re in the Carolinian region along Lake Erie in southwestern Ontario. The fifth pocket is what’s called the Frontenac Axis, that finger of the Canadian Shield, containing about 3,200 square kilometres, that dips southward from the Rideau Canal waterway around Smith’s Falls to form the Thousand Islands in the St. Lawrence River. We live in that finger.
By now, our snake has departed. In late September or early October, black rat snakes gather in hibernacula — crevices in granite outcroppings that descend well below the frost level. There they winter, fifty or so in each hibernaculum, at a constant temperature of about 3° Celsius. They emerge in late April.
Even in the Frontenac Axis, large as it is, however, the snake is under attack. Highways and roads cut it off from other parts of its range. Housing and commercial expansion gobble up its territory. Being run over on a road is a major hazard.
According to Prof. Patrick Weatherhead, professor of biology at Carleton University, it is vulnerable to a catastrophic decline if there is an increase in the number of juvenile and adult snakes killed or cut off from their hibernacula.
That’s because the snakes don’t breed until late in life — after about ten years of age. And since the proportion older than ten is not great, any significant reduction in their numbers can threaten an entire population.
As Jeff Leggo, senior park warden at St. Lawrence Islands National Park, notes, no one knows at what point a population gets too small to sustain itself, and crashes.
Even though there are lots of black rat snakes in the United States, those in Canada are probably genetically different because of their isolation. And in the Frontenac Axis, they are an integral part of an ecosystem that has the second highest reptile and amphibian diversity in all of Canada. The highest, says Leggo, is in the Georgian Bay islands.
Black rat snakes have have a wide range of about 9.5 hectares, with around four snakes per hectare. And they require a very specific mixture of rock, and forest, and open space.
That specific mixture is also suitable to other wildlife and to certain plant forms, says Leggo. And so the snakes make an excellent indicator of the condition of the ecosystem — one of ten indicators used by park staff. If the snakes falter, then something is going wrong with the ecosystem.
« The snake is quite important to us,’’ Leggo says.
About a year ago, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was asked to designate the black rat snake as « vulnerable,’’ meaning that it should be the subject of special concern because its limited range in Canada and because it is particularly sensitive to human activities or natural events.
COSEWIC has not yet reached a conclusion. For what it’s worth, I don’t see how it can refuse.