Amphibians
Cope's Gray Treefrog
Hyla chrysoscelis · also called Cope's grey treefrog, southern gray treefrog
There are two kinds of gray treefrog trilling from the pond edges of the eastern United States on a warm June night, and no one can tell them apart by sight. One is the common gray treefrog; the other is Cope’s gray treefrog. They look the same, live in the same woods, and differ mainly in the speed of their song and in the number of chromosomes in every cell.
Overview
Hyla chrysoscelis is a small, warty tree frog, barely 5 cm long, that shifts colour between grey, charcoal, and green to match bark or moss. It has a white spot under each eye and a hidden flash of bright orange-yellow on the thighs. It lives in wooded country across the central and south-eastern US, hunting insects in the canopy and coming down to shallow ponds to breed.
The cryptic twin
Cope’s gray treefrog and the common gray treefrog (Hyla versicolor) are cryptic species — separate species that look identical. The common gray treefrog is tetraploid, carrying four chromosome sets, and it arose when the two-set genome of a Cope’s-type ancestor doubled. Cope’s kept the original two sets. The one reliable field difference is the call: Cope’s trills faster and harsher, the common gray treefrog slower and more musical, a difference driven by cell size that now keeps the two from interbreeding.
Built for winter
Like its twin, Cope’s gray treefrog tolerates freezing. In winter it shelters under bark and leaf litter and lets ice form between its cells while flooding them with glycerol and glucose antifreeze; heartbeat and breathing stop until the thaw. Big sticky toe pads let it climb vertical trunks and cling under leaves.
Status
Cope’s gray treefrog is Least Concern, common and widespread, with declines only where woodland and its breeding wetlands are lost together.
Cope's Gray Treefrog: common questions
What is the difference between Cope's gray treefrog and the common gray treefrog?
In the field, only the call. The two are so alike in size, warty skin, grey-green bark camouflage, the white mark under the eye, and the flash of orange-yellow on the thighs that even experts cannot tell a silent frog apart. Cope's gray treefrog trills faster and more harshly; the common gray treefrog trills slower and sounds more musical. The deeper difference is genetic — the common gray treefrog has four sets of chromosomes and Cope's has two, and the four-set species originated from the doubling of a two-set ancestor. Their ranges overlap widely across the eastern United States.
How can two frog species look identical?
Because the trait that separates them, the number of chromosome sets, does not change the outward body much. The common gray treefrog is a polyploid, meaning its cells carry extra complete chromosome sets, and it formed directly from a Cope's-type ancestor whose genome doubled, possibly more than once and possibly involving hybridisation. The new polyploid could no longer breed successfully with the two-set form, so it became a separate species, but it inherited essentially the same appearance and habits. What did shift was the call, because polyploid cells are larger and change the timing of the muscles that drive the trill, and that call difference now keeps the two from interbreeding.
Where does Cope's gray treefrog live and when do you hear it?
It lives in and around wooded country across the central and south-eastern United States, spending most of the year high in trees, where its camouflage makes it very hard to spot. It comes down to breed from late spring into midsummer, and that is when it is obvious, with males trilling loudly from shrubs and grass at the edges of shallow, fish-free ponds, ditches, swamps, and flooded fields, often on warm, humid, or rainy nights. Outside the breeding season the frog is mostly silent and stays in the canopy, though individuals sometimes give a short call from a tree after rain.
Can Cope's gray treefrog really freeze solid?
Not completely solid, but it survives having a large share of its body water turn to ice. As temperatures drop it moves under leaf litter, loose bark, or logs and lets ice form in the spaces between its cells while pumping its cells full of glycerol and glucose, which keep them from dehydrating and collapsing. Breathing and heartbeat stop, and the frog is stiff and cold to the touch. When spring warmth thaws it, the heart restarts and normal function returns within hours. This freeze tolerance lets the species winter farther north than most frogs, without needing to burrow below the frost line.