During these dark days of winter, it is heartening to know that spring is just around the corner.
Some associate the new season with the return of migrant birds such as American robins, red-winged blackbirds and various sparrows. An even earlier sign of spring, however, will be the choruses of frogs such as the spring peeper and wood frog at local breeding pools. We often admire the migratory abilities of birds en route to warmer southern climates, sometimes with envy as we stick out winter in New York. Yet, it is often forgotten that our native frogs and toads have amazing survival techniques that allow them to remain in northern climates throughout the winter.
In New York State, frogs and toads (anurans) need to overwinter for up to five months. Perhaps the most crucial period in a temperate anuran's life is the winter. Cold-blooded (ectothermic) animals such as reptiles and amphibians do not generate their own internal body heat and are usually about the temperature of the surrounding environment. If exposed to freezing temperatures, these creatures might freeze to death. However, being ectothermic has its advantages because lower metabolic rates at lower temperatures decreases the energy an animal needs to survive.
Most frogs accumulate enough fat to make it through a long winter and even lay eggs in the spring. To survive the winter frogs must maintain water balance, avoid predators and have enough oxygen for respiration. Consequently, frogs and toads need to find sites that are protected from freezing.
Many of us were indoctrinated with the idea that frogs hibernate in the mud at the bottom of ponds. Drawings in children's literature and popular magazines show scenes of the "pond in winter" where much of the aquatic pond fauna is hovering close to the bottom with frogs and turtles nestled into the mud substrate.
Although pond and lake bottoms would be suitable areas for frogs to avoid freezing, one very serious, life-threatening problem exists in many ponds and lakes. During the growing season, nutrient-rich ponds and lakes produce large amounts of organic matter from plants and algae. When the plants die off, they settle to the bottom of the pond where bacteria start the decomposition process that can consume most of the oxygen in the water. During winter, the problem is compounded because thick ice often curtails gas exchange between the water and the atmosphere, preventing additional oxygen from dissolving in the pond water. The pond water can become so deprived of oxygen that it is lethal to many animals. Therefore, shallow, organically-rich ponds and lakes, which are typically great breeding sites for flogs and toads, are deadly sites for overwintering.
Many northern anurans spend the winter sheltered in terrestrial sites that have plenty of oxygen, but where freezing and dehydration are greater concerns. Toads (Bufo sp) are adept at burrowing into loose soils and many a farmer or gardener has been startled to find a toad while turning over the garden during the early spring. Toads will also use the woodland burrows of mice, chipmunks, and groundhogs to reach areas below the frost line, thereby avoiding freezing temperatures. Although little is known about burrow microenvironments, it appears that they remain reasonably humid and oxygenated throughout the winter.
Other anurans that use terrestrial sites include the wood frog (Rana sylvatica), spring peeper (Pseudacris crucifer), western chorus frog (Pseudacris triseriata), and gray treefrogs (Hyla versicolor and Hyla chrysoscelis). These species are poor at burrowing and apparently lack efficient mechanisms for voiding excess water if submerged for long periods. They resort to burying themselves in thick, moist, leaf litter on the forest floor, often with other individuals. These sites are not well buffered against extreme cold when there is shallow snow cover, so these five species are sometimes subjected to freezing temperatures. Remarkably, all five of these frogs have freeze tolerance mechanisms that allow them to freeze solid and thaw without harm.
Normally, freezing causes the expansion of cells and results in the destruction of cell components and compartments. To counteract this, freeze-tolerant frogs allow freezing to occur in their extracellular spaces, Preventing freezing inside the cells. They accomplish this by evacuating much of their intracellular water and replacing it with solutions of glycerol or glucose, which act like antifreeze and help prevent intracellular freezing.
In extreme cases, up to 65 percent of their bodies can be frozen and they can apparently remain in this frozen state for several weeks. With warmer temperatures, the frogs thaw and life support systems resume. Like a package of hamburger left on the counter to thaw, a frozen item may be thawed on the outside, but there still might be frozen core. This would be very problematic for a living creature. If a frog started to thaw from the outside inward, the brain would thaw before the heart and the brain would be deprived of oxygen. Freeze-tolerant frogs control the process so that the entire body thaws evenly. The entire process is being studied by scientists because it could improve our ability to preserve human organs for longer periods.
Although terrestrial overwintering is common among toads and the five freeze-tolerant frogs, the majority of frogs employ a different strategy. To escape freezing temperatures these flogs search for aquatic or semi-aquatic sites that do not freeze. Small streams often remain unfrozen because they are flowing and turbulent, which also keeps oxygen levels high. Stream bottoms are often covered by large, flat rocks that are perfect hiding places. Deep lakes can be suitable overwintering sites because they remain unfrozen and typically have more oxygen than shallow ponds. Semi-aquatic sites include undercut banks of streams and caves where humidity is high, freezing does not occur, and frogs can remain in contact with the air.
The true frogs (family Ranidae) represent the largest group in New York and include the bullfrog, green, pickerel, leopard, wood, and mink frogs. Very little is known about overwintering of these frogs and most reports are anecdotal. Most ranids can remain submerged for long periods without surfacing because their skin can absorb oxygen form the water. They also can manage the influx of excess water into their bodies during submergence, so they commonly overwinter underwater. Leopard frogs (Rana pipiens) have been reported in deep lakes and streams. Sections of rivers and streams that remain open under dam overflows have also been reported as leopard frog overwintering locations. However, some ranids, including leopard frogs, green frogs (Rana clamitans), bullfrogs (Rana catesbeiana) and pickerel flogs (Rana palustris) can be found overwintering in semi-aquatic sites such as caves. Frogs can be found wedged into rocky, wet crevices or semi-active at the edges of cave streams. Pickerel frogs are by far the most common species in caves.
Temperate anurans combine amazing adaptations with specific types of overwintering sites to survive the long, harsh northern winters. As we become increasingly more aware of the crucial role frogs and toads play in our environment, we also realize the great need to protect them and their habitats. Landscapes that contain suitable overwintering sites adjacent to breeding areas will be critical to the stability of temperate frog and toad populations.
Why did the frog cross the road?
Green frogs (Rana clamitans) are one of the most common and widespread frogs in the Northeast. They are one of New York State's larger frogs, reaching lengths of 90 mm with some females weighing as much as 100 grams.
They breed in ponds and small lakes and it has often been assumed that they rarely stray far from the pond edge. With the approach of winter it was presumed that they moved into the pond and buried themselves in the mud at the bottom. For the past three years I have been using surgically implanted radio transmitters to study the overwintering habits of green frogs. I have found that green frogs do not overwinter in shallow breeding ponds, but use a variety of alternative sites for overwintering, including small streams, springs, and even a beaver dam. All of these overwintering sites have flowing water, so they remain unfrozen and well oxygenated.
Green frogs undergo extensive migrations, some as far as 500 meters, to reach these overwintering sites. During migration they are highly secretive, remaining hidden by day under leaves and thick grasses. When spring arrives, they leave their overwintering sites and return to the breeding pond. At the DEC Wildlife Center in Delmar, Albany County, this rite of spring is observed with the posting of a cautionary road sign: "Frog Crossing."
Victor S. Lamoureux is a doctoral student in biology at SUNY Binghamton.
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|Title Annotation:||how frogs survive the winter|
|Author:||Lamoureux, Victor S.|
|Publication:||New York State Conservationist|
|Date:||Feb 1, 1999|
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