Just because you can’t see them, it doesn’t mean they are not there!
So it turns out that I was hearing bullfrogs as well as green frogs at the cottage and we actually have both. The baritone call of the bullfrog mooing is so distinctive, but I only ever saw green frogs. This is partly because bullfrogs are nocturnal predators. Our bullfrogs like to hide, as you can see, but I can just see enough detail to tell it apart from a green frog. The dorsolated ridge on a bullfrog starts at the eye and hooks around the tympanum, looking a little like glasses hooked around ears.
The green frog has a dorsolated ridge that runs down the sides of the back, which the bullfrog doesn’t have. It is also smaller than a bullfrog, only reaching about 4 inches in length.
Bullfrogs are the largest frogs in North America and can grow up to 8 inches in length. Bullfrog numbers declined in Ontario during the 1980’s and 1990’s because they were harvested for food and dissection in school. Their numbers are now increasing in areas where they were depleted, but in some parts of their range in eastern Ontario there is still a problem with too few bullfrogs. They are native here, but where they were introduced in British Columbia and Vancouver Island they have become an invasive species. Bullfrogs will eat just about any animal they can swallow, including other bullfrogs. They are threatening the local ecosystem, eating not only other species of frogs, salamanders, garter snakes, hatchling turtles, but also songbirds and baby ducks.
It takes male bullfrogs three years to reach maturity, but females take five years and they can live up to nine years. Predators of bullfrogs are herons, water snakes, turtles, egrets, raccoons and Kingfishers. Given that they will also predate their own species and the amount of bullfrog predators we have, it explains why they are not invasive here.