BY Jessica Tartaglia
ON May 19, 2018
It’s spring! And the last thing you probably want to think about is the cold winter weather we just left behind. But as the temperatures get warmer there are recognizable signs that spring is here. Most people are familiar with how mammals and birds survive through winter, by either migrating, hibernating, or just by using the resources available to them in the frigid conditions of Canadian winters. But as we start to hear the emergence of frogs in the still cool spring evenings you may wonder how an animal that doesn’t have the ability to regulate its own body temperature can survive such extreme weather. Some frogs spend their winters at the bottom of ponds where the water doesn’t freeze, other species burrow underground to below the frontline, and some freeze solid! One of these freezing species is the wood frog (Lithobates sylvaticus).
The wood frog may seem like an unassuming typical frog, it is small to medium size with colour variation from tan to reddish or dark brown, distinguished from other native species by the dark mask and light stripe on the upper lip. However, the wood frog is truly an incredible animal that demonstrates the ability of a species to adapt and overcome hardship. It is the most northerly reaching reptile or amphibian species in North America with its range stretching throughout the northeastern United States, all of Canada and into Alaska. They are remarkably adaptated to allow for them to withstand the temperatures that they experience this far north. Wood frogs are able to freeze up to 45% of their body in hibernation during the winter months. But how is this possible? How can they survive being frozen alive? Unlike other frogs who stay at the bottom of ponds or burry themselves underground wood frogs will dig down a little ways into the leaf litter and as the environment around them starts to freeze they do too. With the cooling temperatures come a couple of physiological changes for these frogs. As ice starts to form the liver of the wood frog metabolizes glycogen being stored, turning it into glucose and sending it around the body. This glucose along with a compound called urea are moved into the cells of the frogs organs and into its blood where they limit the amount of ice that can form in the cells and reduce the amount the cells are able to shrink. Their heart stops beating and they stop breathing, but come spring time this death like state is reversed. As temperatures start to warm wood frogs start to thaw and although their bodies take a little time to fully recover they have essentially come back to life.
These remarkable little frogs are able to undergo multiple freezing events over the course of a winter as the environment naturally cools and warms throughout the season. There is however a limit to the temperature they can withstand, and wood frogs found further north have a higher tolerance to the cold than those found in southern Canada and further south in their range. Like other species of frogs, wood frogs are facing many obstacles set in their paths by human development and industrialization. Factors such as road mortality, agricultural run off, habitat loss are having a large impact on all our native frog species. Climate change is a major factor that may have a detrimental effect on wood frogs. Although they are listed as a species of Least Concern on the IUCN list, studies in Ontario have shown these frogs are now absent in places they once were heard. As seasonal temperature changes occur with climate change we can only hope that these magnificent frogs are able to adapt and thrive.