Where Ranges Overlap

Day 306: Where the ranges of two similar species overlap, it is always advisable to take photos from multiple angles in order to capture as many field characteristics as possible. This philosophy holds true for plants as well, but it is particularly important when trying to make the distinction between frogs. Two species occur between 2500' and 6000' with Cascades Frog (Rana cascadae) having a range of 2500' to 6000' and Spotted Frog (Rana pretiosa) from near sea level to 8000'. Red-Legged Frog (Rana aurora) also occurs up to 4700', but only rarely is found above 3000'. R. aurora was not a consideration in the case of this specimen from Ghost Lake at 4400' near Cayuse Pass, but the identification points discussed here will also help separate it.

It's often impossible to capture a froggie for examination and certainly not the method the frog would prefer. A few simple observations will allow the amphibian to retain its dignity while assuring the observer of a good possibility of a correct ID. First, does the subject have strong dorsolateral folds extending from immediately behind the eye and continuing to the hip? If distinct, you can rule out Spotted Frog. But suppose the object of your attention is almost entirely submerged in water? Note the orientation of the eye. Does it look out toward the side, or does it seem to have an upward tilt? Best determined by looking directly down on the frog in question, an upward angle denotes Spotted Frog and rules out the other two options. Likewise, presence of full toe webbing indicates R. pretiosa; partial webbing is present in both R. cascadae and R. aurora, but of course this requires a view of the toes.

Now it gets a little more technical. Often, Cascades and Red-Legged Frogs can be distinguished by the presence or absence of reddish pigment on the legs. D'uh! But since the skin of frogs contains photoreceptor cells which respond to light levels by changing colour over a protracted period of time, this is not always obvious. If observable, dark spots on the back will be distinct in Cascade Frog, somewhat blurry in Red-Legged, but these can be difficult to distinguish when your froggie friend is in "mud camo" mode. The colour of the tummy and throat can be helpful, but not alway. Absence of mottling distinguished Cascades Frog, but either species may exhibit varying degrees of mottling. Likewise, presence of a facial mask may or may not be easy to determine, but if it is distinct behind the eye and not speckled with light patches, your frog is R. cascadae. The light yellow, unmarked belly and chest on this specimen add to the evidence that it is a Cascades Frog.

There are other points to consider as well, and while it's always best to note each one, three should suffice in all but the most difficult situations. Happy frogging!