It was Sept. — the day after the autumnal equinox. I stood at the edge of my garden pond, counting the frogs that had recently been drawn to my very small shore; adult and juvenile green frogs, and juvenile leopard frogs. This day I counted six.
Every fall I have observed that the green frogs seem to get browner this time of year.
The curious naturalist in me wanted to know why this occurs, physiologically. Why would it be beneficial to the survival of some frog species to change their coloration in response to seasonal changes? I did some research and learned that many frog species do change color in response to changes in light, temperature or humidity. The reasons are two-fold: for camouflage and for thermo-regulation.
Animal adaptations are about survival and reproduction — both of which are essential to the perpetuation of a species. When we observe physical and behavioral characteristics, we ask how that trait supports this. If you were a frog, why would it be beneficial to change your coloration seasonally? What are your vulnerabilities? How do you avoid predation? How do you find food?
Camouflage is a common adaptation for predator avoidance among prey species; for predatory animals, it reduces detection by their prey. In the summer, green frogs camouflage quite nicely with verdant vegetation, often appearing as just another leaf of an arrowhead, or the pad of a lily! But come autumn, as the incident angle of the sun increases and evening temperatures drop, those very adaptations that supported survival in the summer are now a liability. Thus, the frogs’ green now fades to brown accordingly, blending with autumn leaves and pond bottoms.
Color change can also help ectothermic (so-called “cold-blooded”) animals regulate their body temperatures in response to changing environmental conditions. Frogs are ectothermic — their body temperature is dependent on their surrounding environment.
Darkening of skin pigments in autumn can serve to increase absorption of sunlight (and therefore warmth) as day length shortens, and as the incident angle of the sun increases.
As I write this, it is the winter solstice, Dec. 21. My little pond is now covered with a thin layer of ice crystals and snow. I contemplate what has happened to all the frogs that were active a few months ago. While investigating frog color-change adaptations, I came upon fascinating research about hibernation patterns in the frogs and toads of the Northeast.
Aquatic species — such as green frogs, leopard frogs and bullfrogs — descend to pond or lake bottoms and lie on top of the mud, or under debris and enter a state of torpor or inactivity, characterized by sluggishness and a dramatic slowdown of metabolic processes. Exposure to oxygen-rich cold water enables aquatic frogs to continue to absorb dissolved oxygen through their skin.
Deep in torpor, their metabolism in rest mode, aquatic frogs are oblivious to the state of the pond surface above. Is there a deep layer of snow upon the ice from a winter nor’easter? Are there red fox or raccoon tracks upon the snow, predators that would surely hunt frogs on warm summer eves? The winter challenges of active mammals do not beset the frogs; instead they have settled into a long, winter’s nap! Who could resist?
Toads, which are terrestrial except during spring mating season, will burrow below the frost line in soft soil, using their hind legs as spades. Like the aquatic frogs, they will enter a state of torpor until spring.
By far the most research has been done on the freeze-tolerant physiology of wood frogs. (In New England this strategy is also shared by the spring peeper, and gray tree frog.) The wood frog overwinters under the leaf litter on the forest floor. As soon as ice crystals develop and contact the surface of the frog, a chain reaction begins that results in freezing of up to 65 percent of the frog tissue.
Prior to freezing, water is drawn out of cells, while high levels of glucose accumulate inside the cells, acting as a natural antifreeze or cryoprotectant. Ice crystals form in extracellular tissue, under the skin, and in body cavities; however, vital organs are protected from freezing. There is no breathing, no heartbeat and no kidney function.
The wood frog is frozen in a state of suspended animation until spring thaw, when it comes back to full physiologic function within hours.
Unlocking the mysteries of wood frog anti-freeze could have helpful applications for human health, particularly regarding organ preservation and donation. This is a perfect example of biomimicry — using information from nature’s adaptations to solve human problems.
In the meantime, next time you stand at the icy edge of a pond, or tromp over frozen woodlands buried in snow, remember that there are amphibians underwater, or underfoot, deep in torpor, or cryogenically frozen until the spring thaw … while we retreat to our warm abodes and hot chocolate.
Patty O’Donnell is an environmental educator at the Hitchcock Center for the Environment. To see an amazing video of wood frogs freezing, visit www.pbs.org/wgbh/ nova/nature/frozen-frogs.html.
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