The Quick Summary:
The “Gray Treefrog Complex” consists of two species: the gray treefrog, Hyla versicolor, and Cope’s gray treefrog, Hyla chrysoscelis. These two species are visually indistinguishable from one another and their ranges overlap considerably. In the field, the best way to differentiate the two are by the male’s advertisement (mating) call. Otherwise, they can be separated by performing a cytological and/or cytogenetic analysis. Therefore, if a sound file of the call(s) (or an actual specimen) does not accompany a report or observation, even though there are clear photos, the best and most accurate identification is “Gray Treefrog Complex”.
The “Gray Treefrog Complex” consists of two species: the gray treefrog, Hyla versicolor, and Cope’s gray treefrog, Hyla chrysoscelis. Some list Dryophytes as the generic name. As there is some debate, I will keep using Hyla for now. Common names for H. versicolor include gray treefrog, eastern gray treefrog, northern gray treefrog, common gray treefrog, and tetraploid gray treefrog. The currently accepted proper name is gray treefrog, though I typically use the former name eastern gray treefrog to avoid confusion with Cope’s gray treefrog. Common names for H. chrysoscelis include Cope’s gray treefrog and southern gray treefrog; the currently accepted proper name is Cope’s gray treefrog.
Although called “gray” treefrogs, both of these species can quickly change their background coloration from gray to brown to bright green in only a matter of minutes.
The two species of gray treefrogs are indistinguishable based on external morphology; they are identical in appearance (more on this later). The most readily identifiable characteristic to separate the two species in the field is the male’s advertisement (mating) call. Briefly explained here, H. versicolor produces a slower more bird-like trill consisting of ~16 to 34 pulses per second, while H. chrysoscelis has a faster, more harsh metallic-like trill consisting of ~34 to 60 pulses per second. Keep in mind that pulse rate varies with temperature which can affect both species (faster/slower calls due to warmer/colder weather). After some practice hearing the two species call, one can learn to identify each species even if calling in warmer or colder weather. There are many resources online to listen to the calls of both species.
I’ll also briefly cover the cellular differences as well. Some of these differences have not been tested everywhere in their range. Basically, H. versicolor is a tetraploid (four sets of chromosomes), has more and larger red blood cells, and larger toe pad epithelial projections with three or four nucleoli in each cell, and H. chrysoscelis is a diploid (two sets of chromosomes), has less and smaller red blood cells, and smaller toe pad epithelial projections with one or two nucleoli in each cell. These characteristics cannot be used to distinguish the two species unless a cytological and/or cytogenetic analysis is performed, except perhaps using a magnification lens to examine the toe pad epithelial projections, but there are some issues associated with the latter method. None of these characteristics are useful for an identifiable voucher photograph. Therefore, the specific species cannot be determined using typical photographs.
Some researchers have listed other morphological characteristics such as presence/absence of a pattern on the back, the intensity of the pattern, whether or not the pattern is outlined in black, pattern and/or color of the insides of the thighs, smoothness/roughness of the skin, presence/absence of a light squarish spot under the eye, etc. to tentatively identify the two species. Additionally, H. versicolor usually attains a larger size than H. chrysoscelis. There may be correlations and basis for these “more often” or “less often” characteristics, but the bottom line is that currently none of these morphological characteristics have shown to reliably distinguish these two treefrog species.
Some have listed differences in habitat. Just like with the morphological characteristics above, there may be correlations and preferences, but habitat, arboreality, etc. are not a reliable means of identification as there is too much overlap in both species.
Some have also listed range as a way to distinguish between the two. In general, the two overlap broadly throughout their entire geographic ranges. While more study is certainly needed, there appear to be large gaps with H. versicolor apparently absent from a large portion of the southeastern US where H. chrysoscelis is found and a large portion of northeastern US where H. versicolor is present but H. chrysoscelis appears absent. Therefore, the following applies to the Minnesota, Iowa, Wisconsin area. In Iowa, range is not useful – both species occur statewide, and there are many places where I have found both species calling from the same wetland. I have also found both species calling from the same wetland in Minnesota as well. In Minnesota, people often use range to try to identify a species. For example, in northeastern Minnesota in the arrowhead region, H. versicolor has been documented, but H. chrysoscelis has not (yet), so some people identify gray treefrogs from this region as H. versicolor based upon range. The reverse is true for western Minnesota. However, climate change, habitat alteration, and other factors may cause gray treefrog ranges to expand into areas where they were previously not documented. For this reason, range alone should not be used to identify the two species.
iNaturalist – The Quick Summary:
If you only have a photograph/image of a gray treefrog, use “Gray Tree Frog Complex” as the ID. This option may not automatically show up in your suggested IDs. However, if you start to type in “gray treefrog”, it is usually the first option that appears. A photo/image cannot be used to identify gray treefrogs to species, and no one can justifiably confirm or “agree” to species with only a photo/image. If you hear them calling, it’s best to take an audio or video recording. You can attach an audio file to an observation, but currently you cannot attach a video file to an observation. If you took a video, there are programs available to take the audio from a video and you can then post that audio to your observation.
For photos, you could use a higher taxonomic ID such as “Hyline Tree Frogs” or “Holarctic Treefrogs” etc., but if you already know it’s one of the two gray treefrogs, (and there are only two Hyla in MN, IA, and WI) then it’s most accurate to use “Gray Tree Frog Complex”.
This page replaces the rather lengthy explanation I used to post on these iNaturalist observations:
It may be that species, unfortunately there is not enough evidence provided to definitively confirm the identification, which can only be accomplished with a sound file of the advertisement call (and/or a cytological and/or cytogenetic analysis). “Gray Treefrog Complex” covers both species and is the most accurate ID for these observations.