Larry Lyons: Frogs will soon be frogsiclesPublished 3:50pm Wednesday, October 7, 2009
With winter bearing down us the wife had an interesting question – how, exactly, do frogs over winter? We’ve all heard the stock answer, they bury into the mud. That’s true for aquatic turtles but an incorrect assumption for frogs. How frogs, and while we’re at it we might as well include toads, manage to survive the frozen winter months depends on the species.
Toads, being land dwellers, have a quite simple strategy. They are excellent diggers and they simply dig a deep burrow extending below the frost line. There they go into a mild state of hibernation where their metabolism slows down and they no longer need food or water. Things may be chilly but there’s still air to breathe and no risk of freezing.
Aquatic frogs such as leopard frogs, green frogs and bullfrogs don’t bury into the mud. That would cut off their oxygen and they would quickly suffocate. Turtles get away with the minimal oxygen in mud by going into a deep state of hibernation where they need almost no oxygen. What little is required they absorb through the skin of their mouth, throat and vent.
Aquatic frogs don’t go into true hibernation. They just slow down a lot, which is called a state of torpor. As winter approaches they go to oxygen rich deep water and find a place to simply hang out. To avoid being eaten while napping they snuggle up next to a log or other concealing debris or structure. They absorb oxygen from the water through their skin and may even slowly swim around a bit during the winter months.
The real wintertime marvels are land dwelling frogs like wood frogs and spring peepers. They can’t dig underground like toads and can’t stay submerged in water for long periods. The best they can do is shuffle down under a few inches of leaf litter or find a suitable crack in a log or rocks. Obviously, they are going to freeze, which in most animals is guaranteed death. Ice forming in body tissues punctures blood vessels and squeezes and deforms cells causing them to rupture. Even if the cells don’t rupture the ice scrambles their structure so when thawed the organs are severely damaged. Ice crystals also draw moisture out of the cells causing fatal dehydration.
Northern terrestrial frogs like wood frogs and peepers, and many insects for that matter, have some unique ways of dealing with these ice issues. Rather than waiting for extreme cold temperatures to rapidly freeze them, they have bacteria and proteins that promote freezing. They begin slowly freezing as soon as temperatures reach the freezing point.
Freezing takes place over a longer period of time which allows the frog’s body to make metabolic adjustments. Eventually blood stops flowing, lungs, heart and muscles stop functioning, a mass of ice fills the body cavity, large, flat ice crystals run between layers of skin and muscle and the eyes turn white as the lenses freeze. We now have a frogsicle in suspended animation.
But amazingly, only 65 percent of their body water turns to ice. The frog has manufactured huge amounts of blood sugar to serve as antifreeze. Somehow, and to the dismay of diabetics we don’t yet know how, they can tolerate blood sugar levels 100 times higher than normal without suffering massive injury like humans when their blood sugar raises just 2-10 percent. Ice forms around the outside of the frog’s cells but this glucose syrup keeps the inside of the cells from freezing. This prevents ice damage to the organs. Ain’t that somethin’?
As back up insurance blood clotting proteins are highly elevated to quickly stop any bleeding as the frog thaws. When spring temperatures are consistently above freezing they thaw out and are good to go. That’s why in early spring the peepers and wood frogs are happily going about their business way before the other frogs and toads even realize spring has sprung.
Larry Lyons writes a weekly outdoor column for Leader Publications. He can be reached at firstname.lastname@example.org