Creature Feature: Frogs on Ice

They're the soundtrack of early spring: the piercing calls of spring peepers and the soft quacking of wood frogs, or as locals call them, croakers.

We hear them when they've reached their breeding ponds and vernal pools, and we may see them springing across roads on rainy nights. You can easily see wood frogs paddling about as they look for mates, while those peepers are devilishly hard to see.

But did you know that only days before, these frogs were frozen solid? Each species has evolved to be like an ice cube to survive the winter, and then thaw out and emerge for mating as soon as the weather warms.

 Wood frog or "croaker"

Wood frog or "croaker"

The wood frog, Rana sylvatica, is a brownish amphibian about 2 -3 inches long with a distinctive black mask. They are usually the first frog we see in early spring. Well-adapted to cold, they are the only frog species living above the Arctic Circle. They enjoy a few days in water breeding, and most of their lives are spent on the forest floor. 

 

The spring peeper, Pseudacris crucifer, is tiny – an inch and a half at most – with a tan body and a dark brown “X” marking its back.  You’re far more likely to hear spring peepers than see them; the shrill mating call of the males can be deafening.  They are most active at night, though early in the breeding season they also call on warm days.

Like all amphibians, croakers and peepers need water to breed, but they avoid permanent lakes and ponds where fish could eat their young.  Instead, they often use temporary bodies of water called vernal pools – small areas of low land in the forest that collect rain water and melting snow.  Because vernal pools dry up in the summertime, they can’t support fish.  This makes them the perfect tadpole nursery.

 A male spring peeper enlarges his vocal sac to call for a female

A male spring peeper enlarges his vocal sac to call for a female

Laying eggs in water with no fish is one way to avoid getting eaten, but how else do wood frogs and peepers elude predators?

These frogs’ most obvious adaptation is their camouflage, or cryptic coloration.  While green frogs and bullfrogs blend in with the algae and vegetation in a pond, wood frogs and spring peepers have a brown, mottled color that helps them hide in the leaves on the forest floor.

It is their timing, though, that is their most fascinating adaptation.  Wood frogs and spring peepers breed earlier than any other frog in our area, allowing them to take advantage of the vernal pools that dry up later in summer and contain fewer predators.  In contrast, other frogs and toads that hibernate longer also breed later, which means they have to use permanent water bodies like ponds and lakes, where they risk predation from fish and turtles.

So how do these cold-blooded creatures manage to emerge from hibernation so early in the spring?

In order to spend the winter exposed to the elements, wood frogs and spring peepers have evolved the ability to freeze solid for days or weeks at a time and survive in a state of suspended animation.  In humans and most other animals, freezing kills tissue because the water inside expands as it freezes and bursts open the cells.  Wood frogs and spring peepers have the ability to move water out of their body’s organs and replace it with sugars produced by their liver that act as antifreeze.  Meanwhile, the water that was removed from the cells freezes in the spaces between the organs.  The result is a rock-solid frozen frog that will come back to life within hours of being thawed!  The dormant frog may have no heartbeat and no brain activity for months, but can hop away within hours of warming up.

The ability to survive while frozen gives wood frogs and spring peepers an advantage, since they can emerge and move to their breeding grounds on the first warm days of spring.  The males call out to attract females, but in the frenzy of frog breeding sometimes males may clasp onto other males in cases of mistaken identity.  Either the suitor will notice that his mate is the wrong size and shape, or the surprised object of his affection will let out a wail of disapproval.  Spend enough time at the edge of a breeding pond, and you’ll see some pretty interesting behavior! But go soon, as even at this writing many wood frogs have already returned to the forest, and the ponds are taken over by peepers.

Female wood frogs and spring peepers lay over 1,000 eggs attached in clumps to aquatic plants, and the males fertilize them as they are laid.  It may take only days or up to a month for these eggs to hatch into tadpoles, depending on the water temperature.  Tadpoles feed on algae and decaying plants for about 2 months before undergoing metamorphosis.  Their tail is absorbed into their body, they grow legs, and they climb out of the water to spend the rest of the year living in the forest, where they will eat a variety of insects and small invertebrates.

 These spring peeper tadpoles are almost ready to emerge as frogs

These spring peeper tadpoles are almost ready to emerge as frogs

Vernal pools and their surrounding forest are an essential habitat for our native amphibians, but are threatened by land development, fragmentation, polluted runoff from farms and roads, and careless logging practices.  The Ammonoosuc Conservation Trust works to protect forest and wetland habitats.  Management plans on all of our properties ensure that water and wildlife habitat are protected.  Read about our land projects and learn more about our land management practices.

What You Can Do

If you have vernal pools on your property, keep native vegetation growing around them and avoid land-use activities that could pollute the water with pesticides, fertilizers, or sediment.  You can become involved in mapping vernal pools by contacting New Hampshire Fish & Game

 Vernal pools are excellent habitat for breeding amphibians like the wood frog and spring peeper.

Vernal pools are excellent habitat for breeding amphibians like the wood frog and spring peeper.