Wild Ideas: The treefrogs are calling, and I must go   

Much of the spring, I have felt like a private eye specializing in divorce cases — stalking animals, camera in hand, to try to catch them in flagrante delicto. One evening last week, it was the calling of gray treefrogs that put me on the hunt for mating herps once again.

I had heard the slow, deep trills of at least two treefrogs calling, so I grabbed my camera, cell phone and a flashlight and walked down the hill from my house toward the garage. With the recent deluge of rain and the direction of the calls, I was pretty sure the frogs were breeding in my landlord’s small fishing boat down there, which had collected water in its bow.

It was a warm night, with the humidity rising, a sign of temps also on the rise, reaching the 90s by mid-week. A slight breeze carried the perfume of blooming blackberry, Japanese honeysuckle and multiflora rose.

The light was almost gone by the time I reached the boat. As I crept up to it, the calling stopped. Shining my flashlight around inside, I saw a gray treefrog, about two inches long, sitting quietly on some plywood at the bottom, near the water-filled bow. I took a few shots in the dark, trying to hold my flashlight on the frog while focusing my camera, which did not result in the best shots.

By Pam Owen
A small, if fat, gray treefrog sits on a piece of plywood near water that collected in a small fishing boat.

I stepped back, listening in the growing darkness for the two sets of calls to start again, to try to pinpoint the other frog’s location, recording the calls with an app on my phone. When the calls restarted, I checked the boat again, finding the frog I had photographed was now sitting in the water at the shallow edge of the water. Although I could see the air sac around in its throat fill and empty, no sound came out.

By Pam Owen
The frog’s air sac, which males use to call, fills as it waits for potential mates.

This frog also looked a bit fat, and only males call, so could it have been a gravid (egg-filled) female instead? I finally determined that one of the frogs that was calling was hidden under a plastic cup holder in the bow, with too small a gap between the frog and the water for me to see or photograph it. Giving up, I decided to let the little herps get on with their business and wandered home, savoring the soft night air and the sweet smells wafting through it.

In checking out the boat daily since that night, I didn’t find any eggs. By last Saturday night (May 20), the temperature had dropped into the 50s, and the treefrogs had gone silent. My landlord finally needed to use the boat, so I asked if he could put some water in a large, shallow, plastic tub that I use for gardening and place it in the forest edge near the boat, which he did. As I’m writing this on Monday (May 22), the frogs are still silent, and there’s no sign of breeding activity at the tub or boat, which has now collected more water from today’s rain. Perhaps only males had shown up and, failing to attract females, had quit for now. Or perhaps, the fat frog I had seen had been a female and she had decided not to mate, for whatever reason.

In the meantime, I worked on expanding my skimpy knowledge of treefrogs, finding that we have two species here in Virginia that are gray: gray treefrog (Hyla versicolor) and Cope’s gray treefrog (Hyla chrysoscelis). The former is more common and the only one I’ve ever heard calling where I live, in Rappahannock County. This jibes with Virginia Fish and Wildlife Information Service distribution maps for the commonwealth’s six native treefrog species.

H. versicolor has a slower, deeper trill than Cope’s, as can be heard on the recording I made.

This species has a white mark under each eye, which shows clearly on the frog I photographed. Another visual cue is a bright orange patch on the concealed surfaces of this species’ hind legs, which barely shows in one photo I took and which Cope’s does not have. H. versicolor also has twice the chromosomes of Cope’s, which keeps the two species from interbreeding. Like our native wood frog, the gray treefrog has glycerol, a kind of antifreeze, in its blood, enabling it to survive freezing. Virginia’s four other native treefrog species, which are not gray, are all either further east, south or both.

By Pam Owen
A gray treefrog sits on some plywood in a boat (left), then moves into water in the bow (right), barely showing the orange on the underside of its leg, which is an identification point.

While gray treefrogs are often heard in our area, “this species is not often seen on the ground or near the water’s edge except during the breeding season,” VaFWIS explains. Instead, it “tends to forage while in small trees or shrubs near to or standing in water.” Special sticky pads on the frog’s feet enable it to cling easily to even vertical surfaces, such as tree trunks. And its mottled coloring (light gray with dark gray, brown or black patterns) camouflages it. It can also change color rapidly, tending to darken when its surrounding environment is dark or cold.

By Pam Owen
The “love boat,” where Pam’s dog, Mollie, tries to track down gray treefrogs that have been calling at night.

The other amazing thing about H. versicolor is the loudness of its voice, considering its size (1.25-2.5 inch). From a few feet away, it can be almost deafening. As Robert Stebbins and Nathan Cohen write in “A Natural History of Amphibians,” the high volume of one male can drive off another male. They also note that this species expends more energy than other cold-blooded vertebrates to generate a remarkably loud call to attract mates. Perhaps this is because gray treefrogs are terrestrial, and widely scattered, so a male must try hard to attract a mate to whatever small pool he finds that is suitable for breeding. This species also has an “explosive bark,” the authors write, which in one study was shown to have “a repellent effect” on northern tailed shrews, one of the frog’s predators.

The gray treefrog’s breeding season is long. While sources vary on this, breeding may start as early as March and run as late as August. Up here on the mountain, I recorded the first sustained calling on May 5. The frogs should still have plenty of time to breed, and I’m hoping that the new “pool” we’ve offered will suit them and soon fill up with eggs.

© 2017 Pam Owen

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Pam Owen
About Pam Owen 275 Articles
Writer, editor, photographer, and passionate nature conservationist living in Rappahannock County, in the Blue Ridge Mountains of Virginia. Two favorite quotes: By E.O. Wilson, who coined the term "biodiversity," "Nature holds the key to our aesthetic, intellectual, cognitive and even spiritual satisfaction”; by Douglas Adams, “I love deadlines. I love the whooshing sound they make as they pass by.”