At this time of year one would expect to find Leopard Frogs lying on the bottom of a pond, partially but not completely covered with leaves or mud as they hibernate their way through winter. Because of the depth of a pond, and the fact that in winter the water temperature is around 39°F., ice isn’t an issue at the bottom of a pond, and the frogs and turtles that overwinter there don’t usually freeze.
However, sometimes ponds freeze over before amphibians or reptiles that overwinter in them arrive at their hibernacula. Apparently this is what happened to these Leopard Frogs, and they took refuge in the only open body of water they could find – a large but shallow puddle about 10’ wide by 20’ long in a dirt road. Shortly after they arrived temperatures dropped and the frogs were trapped under (and eventually will be encased in) the ice. Unlike Wood Frogs, Spring Peepers and Gray Treefrogs, Leopard Frogs are not freeze tolerant, so their demise is inevitable. (Thanks to Kelly Maginnis for photo, and Jim Andrews for his herpetological expertise.)
The migration of amphibians from the woodlands where they overwinter to their breeding pools in March is a familiar spring phenomenon to most. However, some of these amphibians engage in a fall migration as well. Amphibians (and reptiles) need to find a good overwintering spot, where they won’t freeze. In some cases, that might be a few feet upstream or into a seepage area, and in others it is a few hundred yards uphill. The extent of the migration and the species participating vary in different parts of the state.
In western Vermont, Jim Andrews, Director of the Vermont Reptile and Amphibian Atlas(http://vtherpatlas.org), has observed Blue-spotted and Four-toed Salamanders moving down into wetlands in the spring, staying close to the wetland for moisture and feeding during the summer, and then moving uphill back to wintering areas in the fall. On the other hand, he has found that most Spotted and Jefferson Salamanders, as well as Wood Frogs, move down in the spring to breed, and then head back uphill in stages soon after they are done breeding.
Eastern Red-backed Salamanders (pictured) migrate as needed during the summer to maintain their body moisture, and in the fall, search for an animal burrow or crevice to hibernate in. Migration often occurs at night, while it’s raining, so as to prevent their skin from drying out. (Amphibians need to have moist skin because a large portion of the frog’s “breathing” occurs via oxygen diffusing through the skin. Oxygen will not diffuse through a dry membrane.) After the very dry summer we’ve had, a large number of amphibians were observed taking advantage of the recent rain and migrating.
Despite their name, Green Frogs are not always green. They can be brown or tan, as well as many shades of green. Usually Green Frogs in the Northeast are a combination of these colors, but occasionally one sees greenish-blue coloring on a Green Frog. An understanding of what causes a frog’s green color sheds light on why sometimes all or part of a Green Frog may be close to turquoise than green.
Basically there are three types of pigment cells (chromatophores) which stack up on top of each other in a frog’s skin. The bottom layer (melanophores) of pigment cells contain melanin, a pigment that appears dark brown or black. On top of these cells are iridopores, which reflect light off the surface of crystals inside the cells. When light hits these cells, they produce a silvery iridescent reflection in frogs, as well as other amphibians, fish and invertebrates. In most green frogs, sunlight penetrates through the skin to the little mirrors in the iridophores. The light that reflects back is blue. The blue light travels up to the top layer of cells called xanthophores, which often contain yellowish pigments. The light that filters through the top cells appears green to the human eye.
The pictured turquoise-headed Green Frog most likely lacks some xanthophores in the skin on its head, and thus we see reflected blue light there.
Congratulations to Naturally Curious readers for their familiarity with Eastern Gray Treefrog (Hyla versicolor) toe pads! This is the easiest time of year to find these amphibians, mainly because of their habits. During the fall, winter and spring treefrogs are usually silent, unlike the summer, when males can be heard calling their melodic trill from bushes and trees near bodies of water after the evening air temperature rises above 59°F. Because they are nocturnal, well camouflaged, and hibernate in the winter, you don’t often come across one except for the warmer months when males are calling. The colors of an Eastern Gray Treefrog (brown, green and pearl-gray) vary with the colors of its background and environmental factors such as season and humidity, but shades of gray are most common. Their green color is more prominent during the breeding season and in young frogs.
When hunting insects, or when disturbed, treefrogs can leap great distances and, thanks to advanced toe pads, when they land they can cling to practically any surface, including vertical branches and leaves that are wet. A very low angle between the toe pads and substrate as well as mucous glands located in channels between the hexagonal pattern on a treefrog’s toe pads have inspired the design for treads on car tires.
You can listen to an Eastern Gray Treefrog’s call by going to http://langelliott.com/calls-of-frogs-and-toads-of-the-northeast/ (Sound recording © Lang Elliott – langelliott.com)