Gray, but not really

Gray Treefrog

The weather cooperated for me, and I was able to get some mothing in on all three nights I was at my parents’. The nights were relatively warm, and there was quite a bit of activity at the blacklights, so I took my time browsing over the sheets looking for species I hadn’t seen yet. I have the one sheet set up on the clothesline, which is not far from my mom’s water garden, an old watering trough, not used for that purpose for many, many years and now filled with rocks and aquatic plants. While I was standing there, a frog started calling from the water. So I thought I’d have a peek and see if I could spot him.

After circling the garden a couple times I determined he was in one particular corner. I checked all the spots I thought a frog should be, along the water’s edge, in between the rocks, in the flower bed, couldn’t see him. I finally decided he must be up inside the water spout feature, an old hand pump that, decades ago, had been used to pump water up from the property’s well. The pump perches at the corner of the trough, with the pipe set in the water, such that one edge of what used to be secured to the ground now forms an overhang over the water. I figured he’d crawled under there. So I returned to the moths. On my next trip back from the house, my headlamp just happened to pass across the top of the pump – and there he was. Such a great ventriloquist!

Gray Treefrog

He’s a Gray Treefrog, though he’s not a very gray treefrog. The species’ name is Hyla versicolor, the latter part being a reflection on the frogs’ variable colouration – some individuals are the gray that gives the species its name, while others are bright green, brown or yellowish, and there’s a range of colouration between them all. In addition to this natural variation, Gray Treefrogs are able to change their colour, like a chameleon, though the process is not nearly as fast as in chameleons. They are covered in black mottling, the extent of which varies also according to individual and surroundings. A frog on a tree trunk can be nearly impossible to detect as it adjusts its colour and mottling to blend in. And, as if all that wasn’t enough, the frog’s colour is also influenced by the ambient temperature, with more or less black being shown according to whether they need to absorb the sun’s heat or not. Males will generally have darker throats than the females, but of course I didn’t have a female to compare to.

The treefrog’s normal “base” colouration is gray – dead frogs and those in unnatural surroundings will usually be this colour. So I’m not sure why this one’s bright green – perhaps he’d just crawled up from the grass and hadn’t had time to change yet.

Gray Treefrog

In all colour variations, there are three things that make this species of treefrog distinguishable from other frog species (other than the fact that no other frogs climb trees). The first is the black mottling on the back, usually in somewhat linear patterns to create borders to slightly darker areas (though some individuals may not show these markings). The second is the wide sticky pads at the end of each toe that allows the frog to grip the tree (or other surface) when climbing (all treefrogs show this feature, of course). And the third is that both Hyla versicolor and its nearly identical sister species H. chrysoscelis show yellow on the inside of their thighs, usually not really visible unless the legs are extended or one looks closely, like here. The two sister species share much of the same geographic range and are really only separable by call.

Websites also indicate that the species has a pale spot under the eye, and it took me a while to figure that out – I was looking for an actual white or pale dot, but they were using the word “spot” as in region or area, and were referring to the patch of skin bordered on each side by darker patches. Really, the “pale spot” is exactly the same colour as the rest of the frog, it just looks pale because it’s a small area bordered on by dark markings. In the case of my individual, his dark markings aren’t even very dark, they’re barely different than the light areas.

Gray Treefrog Gray Treefrog

He seemed strangely unperturbed by my being there. He’d pause for a moment as I moved from one side to the other, perhaps assessing the sound, but wouldn’t stay quiet for long. It’s prime breeding season, after all! It probably also helped that I had the headlamp on, and as I shone it in his face it hid my silhouette from view.

It’s amazing how much their throat can inflate. When not calling it’s just a loose pocket of skin, held close to the body, deflated and loose like a dewlap. When he fills it with air, it balloons to the size of his head. The throat sac isn’t involved in sound production directly, but rather acts as a resonating chamber, amplifying the sound so it broadcasts across a much greater distance. I read at one spot that this call can be heard up to a kilometer away (a little over half a mile).

The Gray Treefrog has a short, moderately high-pitched trill, which in my area is only really easily confused with the much longer, sustained trill of the American Toad. The trills of H. versicolor and H. chrysoscelis are distinguishable by differences in speed and pitch (length is the same; speed refers to the separation between the individual notes that make up the trill – usually in milliseconds). I rather suspect this would require some degree of experience with the two species to feel confident about labeling it one or the other. To complicate matters, the speed of the trill varies with ambient temperature, and is slower in cooler weather (which makes sense, being a cold-blooded creature).

Gray Treefrog

A close look at his eye, and ear. A frog’s ear is not a hollow tube like that of vertebrates, but a tympanic disc at the skin’s surface. It’s a little like having our eardrum right on the side of our head, instead of tucked well inside. The same general structures apply to convert the vibrations of the membrane into signals to the brain. However, a frog can also “hear” with its lungs. The change in air pressure, particularly with loud noises, will also create vibrations in the lung linings, which are sent to the brain much the same way as from the ear. Also, the lungs have a direct link between them and the ear, equalizing pressure, which is presumed to serve as a protection to the eardrum against the frog’s own incredibly loud calls. An average frog’s calls are 90-95 decibels – about as loud as a lawnmower or jackhammer. Eight hours of sound this loud can permanently damage your hearing. Unless you’re a frog!

Gray Treefrog

He certainly was loud, especially when I got right up close to him. He was also very alone. The nearest other calling treefrogs were at least 50 meters/yards away. Treefrogs tend to be more solitary than other frogs, not being as confined to an area of habitat the way wetland frogs are, so it wasn’t too surprising that there weren’t other males nearby. However, it seemed his efforts hadn’t yet attracted a female. He’s in a good spot, with a handy predator-free water body right there for her. She’ll lay potentially up to 2000 eggs, singly or in small clusters, over the course of the breeding season. I haven’t seen any eggs in the little water garden, but will have to pay attention for them now, or tadpoles if I’m not around for a stretch (the eggs will hatch in a remarkably fast 3-6 days after being laid, depending on water temperature). How neat would it be to have treefrog tadpoles in your water garden?