Amphibians
Gray Tree Frog
Hyla versicolor · also called grey tree frog, Cope's gray tree frog
A gray tree frog is a small, warty frog that spends the summer pressed against bark, so well matched to grey lichen that it disappears — until it leaps and flashes bright orange from under its legs. It changes colour between grey and green, it winters frozen almost solid under the leaf litter, and there are actually two of these frogs, identical to the eye and separated only by the pitch of their trill.
Overview
The gray tree frog is Hyla versicolor (with its identical twin Cope’s gray tree frog, Hyla chrysoscelis), a small tree frog of woodland and wooded suburbs across the eastern United States and southeastern Canada. It is a night-hunting insectivore of moths, beetles, and spiders, sitting still in trees and shrubs and ambushing prey, and it has large adhesive toe pads that let it cling to smooth leaves and windowpanes.
Colour, and a flash of orange
It shifts over minutes between grey, brown, charcoal, and green to match its background, temperature, and mood. The hidden inner thighs are bright orange, normally covered, and the sudden flash when the frog jumps is thought to startle a predator before vanishing on landing.
Two frogs in one
The eastern gray tree frog and Cope’s look identical and overlap widely, but the eastern species has twice the chromosomes (it arose by genome doubling) and a slower, more musical trill, while Cope’s gives a faster, harsher one. They are told apart by ear, not eye.
Frozen for winter
Gray tree frogs hibernate on land under bark and leaf litter and tolerate up to ~40% of their body water freezing, flooding their cells with glycerol and glucose. The heart stops while frozen and restarts on the thaw.
Status
Both species are Least Concern and stable, common across their range and tolerant of rural and suburban life wherever there are trees and fish-free breeding pools.
Gray Tree Frog: common questions
Why is it called the gray tree frog if it is often green?
Because grey, matching pale lichen-covered bark, is the colour it wears most of the time as an adult sitting on a tree trunk, and it was named from typical specimens. The frog is actually able to change colour over minutes to an hour across a range that includes near-white, several shades of grey and brown, and a bright leaf-green, and the shift is driven by temperature, humidity, light, the background it is sitting on, and its stress level. A frog that has just been active or is sitting on green vegetation is often green, while a cool, resting frog on bark is usually grey. Juveniles are frequently bright green and become greyer as they mature.
How do you tell the eastern gray tree frog from Cope's gray tree frog?
Not by looking. The two species are effectively identical in size, colour, pattern, warty skin, and the orange flash under the thighs, and their ranges overlap broadly. The reliable differences are the call and the chromosomes. The eastern gray tree frog, Hyla versicolor, has a slower, lower, more musical trill and is tetraploid, meaning it has four sets of chromosomes. Cope's gray tree frog, Hyla chrysoscelis, has a faster, harsher, higher-pitched trill and is diploid, with two sets, the ancestral number. The tetraploid eastern species arose from the diploid one, probably more than once, through the doubling of the genome, which is unusually common in this group of frogs.
Can a gray tree frog really survive being frozen?
Yes, within limits. Gray tree frogs spend winter on land under leaf litter, logs, and loose bark, where they are exposed to freezing temperatures, and they are among the frogs that tolerate their bodies partially freezing. As ice starts to form, the frog rapidly produces large amounts of glycerol and glucose, which act as cryoprotectants inside the cells and control ice formation so that it happens in the body cavity and under the skin rather than within cells. Up to around 40 percent of the frog's body water can turn to ice. While frozen the frog does not breathe and its heart does not beat, and both restart within hours once it thaws in spring.
Are gray tree frogs good pets, and are they poisonous?
They are commonly kept and are relatively hardy, needing a tall planted terrarium with branches, moderate humidity, a temperature drop at night, and a diet of appropriately sized live insects, and captive-bred animals are preferable to wild-caught ones. Their skin secretions are mildly irritating rather than dangerous. Handling a gray tree frog and then rubbing your eyes, nose, or mouth can cause burning and discomfort, and the secretions can be more of a problem for people with sensitive skin, so washing your hands before and after handling is important. As with all amphibians, they absorb substances through their skin, so hands should be wet and free of soap, lotion, and repellent, and handling kept brief.