Twice the Trills

Telling gray treefrogs apart takes a good ear.

Illustration by Robert Meganck

One of these is not like the other. But when it comes to Virginia’s two gray treefrogs, don’t count on being able to tell the difference. The Cope’s gray treefrog (Hyla chrysoscelis) and the common gray treefrog (Hyla versicolor) are so visually alike that only an expert—or a high-powered microscope—can discern the distinction. “If you are looking at one in the wild, there is no difference,” says Mike Clifford, chairman of the education committee for the Virginia Herpetological Society.

In fact, only recently did it become clear that the two are, well, two. Although researchers had noted as far back as the 1930s some variation in the calls between apparently identical treefrogs, says Clifford, it took laboratory analysis to determine that there were indeed two species masquerading in the same frog form.

To be specific, Cope’s, or H. chrysoscelis, is a “diploid” organism; it has 24 chromosomes, or two complete sets, in each cell. The common gray treefrog, or H. versicolor, however, is “tetraploid,” with 48 chromosomes—four complete sets—in each cell. And what does one frog do with twice as many chromosomes as the other? It’s theorized, according to Clifford, that chrysoscelis came first, but that versicolor “found some evolutionary advantage” in the greater number of chromosomes. 

That advantage may have played a role in shaping what territory each inhabits in eastern North America. Versicolor, says Clifford, “is a much more northern and higher-elevation species,” with a range that extends into southern Canada. Chrysoscelis, by contrast, “is far more common throughout the southeast,” he says. 

“You can think of Cope’s as a southern treefrog and versicolor as a more northern treefrog.” In winter, both species are capable of producing a sugary fluid—glycerol—that protects their bodies against freezing temperatures, but the more northern-ranging versicolor appears to be the more cold-tolerant of the two, explains Clifford.

Because these two frogs are otherwise so similar, they often are simply referenced collectively as the “gray treefrog complex”—which, though it may sound like a condominium neighborhood in a nature-themed ski resort, nevertheless makes for a less cumbersome way of talking about this treefrog twosome. And as it happens, the ranges of both members of the complex overlap in Virginia. As a result, one or the other gray treefrog can be found throughout the Commonwealth. “They are by far the most commonly heard and seen treefrogs in the state,” says Clifford. 

“Heard” may be the more likely. As Clifford points out, like birders, frog enthusiasts can identify species by their call, and when it comes to frogs, that strategy might be preferable, he suggests, to looking for them by wading through a swamp. What’s more, either gray treefrog, just to further confuse you, might not necessarily be gray at all. These frogs are capable of changing color rapidly—across a range from dark brown to solid green to varied shades of gray. “It can be extremely close to the natural habitat around them,” Clifford says. “Their standard color is gray with a darker lichen-like pattern, which is unbelievably close to that of a white oak.” 

If you want to test your talent at telling chrysoscelis from versicolor this spring, go to the “Frogs and Toads of Virginia” page at VirginiaHerpetologicalSociety.com and listen to audio of their individual calls. “The common gray treefrog has a more musical trill as compared to Cope’s, which has a more raspy, grainy type of a call,” says Clifford. 

In most parts of Virginia, the frogs start calling in April, when the breeding season begins. During that time of year, you can find them by vernal pools, small ponds, water-filled roadside ditches—and even sometimes your backyard pool. When Clifford had one in his yard, “They would set up calling stations all along the pool deck and actually lay their eggs,” he says. “And then I’d have a swimming pool full of tadpoles.”

Outside of the breeding season, however, and as their name would suggest, gray treefrogs spend their time up high, typically in trees (they prefer deciduous, but “I can’t say they would never be found on a pine tree,” says Clifford), although they can also be found around houses and porches. Decorative shutters framing your windows apparently make a great treefrog hiding place. “They spend the daytime out of sight, then come out at night to feed,” says Clifford.

The frogs will continue to call into the summer—particularly when it’s very humid or when rain is coming on. These “rain calls” are more relaxed, “like they are enjoying the weather,” says Clifford. 

But you can listen now for the first of the gray treefrogs calling out in the darkness.It’s the sound of spring, spreading across the Commonwealth.


This article originally appeared in our April 2018 issue.

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