Telling the Gray Treefrogs Apart

There is a problem with Gray Treefrogs...there are actually two species disguised as one.  The two species are physically identical in external appearance but have different numbers of chromosomes and have different calls.
This has created a historical problem in defining their ranges.  Typically, the ranges of an animal are determined by looking at where specimens have been found.  Unfortunately, Gray Treefrogs in museum jars don't call so most older collections have large numbers of unidentifiable Gray Treefrogs.

They are both grayish frogs with a mottled "tree bark" pattern and they both vary.

Here is the Eastern Gray Treefrog (Hyla versicolor) from Grimes County, Texas -

and another from Houston County, Texas -

and here is the identical Cope's Gray Treefrog (Hyla chrysoscelis) from central Arkansas -

and another Cope's Treefrog from central Texas -

Both species can show varying amounts of green coloration, like this Hyla chrysoscelis from south-central Texas.

Some of these frogs have very little visible mottling on their backs and can appear almost solid green backed or solid silvery white.  
They can be told from other solid green frogs by the yellow/orange mottling on their hind legs.  Here is a pretty green-backed Hyla chrysoscelis from Karnes County, Texas.  You can see the yellowish-orange color on the hind legs  -

So if they look identical, how does one tell them apart?

Easy....sort of....

Hyla versicolor has twice as many chromosomes as Hyla chrysoscelis and so if you karyotype one you will have your identification.  But karyotyping isn't a very useful character for identifying them in the field.  
Because they have more chromosomes, H. versicolor have slighty larger cells as well.  Supposedly this can be seen to the "trained eye" with magnifying glass by looking at the cells on the toe pads.

The easier way to tell them apart is their calls.  Hyla versicolor's call is a pretty, bird-like whistled trill.  Here's one from Lavaca County, Texas. 

Gray Treefrog calls - Lavaca County

The whistled trill of Hyla chrysoscelis is similar, but it is drier and less bird-like.  This is due to the fact that the trill is actually faster.

Cope's Gray Treefrog call

The trill rate differences not only make the two species sound different, you can actually count the rate of trills per second to help distinguish between the two.  Of course, with trill rates of 25-65 pulses per second, you can't count them as you hear them, but you can see the difference in a spectrogram.

These two sections of spectrogram represent 0.25 seconds of each species call.  Hyla versicolor is on the left and Hyla chrysoscelis is on the right.  

Each of these vertical blue lines represents on pulse of the trill.You can see that the H. versicolor made 7 pulses in this time (28 pulses per second) while the H. chrysoscelis made 11 pulses (44 pulses per second).  The threshhold for distinguishing the two species is often quoted as greater or less than 30 pulses per second although I think 33 pulses per second is a better separator.  (But see the information below about temperature and pulse rate!)

When you do hear them together in areas where there ranges overlap, you can hear the difference pretty clearly.  Here is a spectrograph of a recording I made in Davy Crockett National Forest in East Texas.  What you see is a Hyla versicolor being answered by a Hyla chrysoscelis (maybe 2?).  I have labeled each species calls (v for versicolor, c for chrysoscelis).  

So listening to the recording, you will hear (in order):
- Hyla versicolor
- Hyla chrysoscelis
- Hyla versicolor
- Hyla versicolor
- Hyla chrysoscelis
- Hyla versicolor
- Hyla chrysoscelis (different individual)
- Hyla versicolor

And here is the recording.  See if you can "follow along" the spectrograph and hear the difference in their calls.  (The higher pitched upward trilling call in the background is the Cajun Chorus Frog - Pseudacris fouquettei).

Both Gray Treefrogs together at Davy Crockett National Forest

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Of course, the pulse rate of frog calls varies with body temperature (warm frogs call faster).  So how could you tell the slower pulse rate of a cold H. chrysoscelis from the faster pulse rate of a warm H. versicolor?  Fortunately, there isn't much overlap.

Here's a potentially confusing call from Kendall County, Texas.  This is Hyla chrysoscelis, but its pulse rate is around 36 pulses per second, giving it an almost Hyla versicolor sounding call.  However the versicolor-like nature of this call is due to the relatively low temperatures while this frog was calling (58°F).

http://www.birdsandherps.com/frog_calls/HYCH_Ranger_Creek_Road.mp3

This graph shows the pulse frequency for a number of H. chrysoscelis and H. versicolor calls at different temperatures that I have recorded them (or in a few cases, where I have received recordings with temperatures from others).  
The blue dots represent Hyla chrysoscelis and the red dots represent Hyla versicolor.

Notice that the trend lines for each data set are roughly parallel (I'm not sure how MS Excel calculates the "linear trend line" mathematically).  This does help reinforce the idea that the increase in call rate versus temperature is roughly equivalent in each species.

From these data, you can see that Hyla versicolor and Hyla chrysoscelis do overlap in pulse rate, but only at the opposite ends of their temperature ranges. 

Just by chance, the lowest pulse rate I have been able to document for Hyla chrysoscelis is 34 pulses per second for a recording made in Cavalier County, North Dakota at 60°F.   The highest pulse rate I have ever documented for Hyla versicolor is also 34 pulses per second for a frog calling in Colorado County, Texas at 81°F.   So even though I have documentation of both species producing a pulse rate of 34 pulses per second, the Hyla chrysoscelis produced that pulse rate at an ambient temperature over 20°F colder.

Therefore we can expect that there would never be a situation where the two species would be exhibiting the same call rate in the same area at the same time.   The pulse rate of Hyla chrysoscelis is roughly twice that of Hyla versicolor at all temperatures.

(There is a published formula that has been used to standardize the call rates of the two species for temperature.  I will dig that out of my readings and post and cite it here soon.)

To help even further, there is research to suggest that in areas where their ranges overlap (in Texas at least) the pulse rate of Hyla chrysoscelis calls is even higher, helping to further distinguish their calls.

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So since museum specimens don't call, the ranges of these two species are still being worked out.  In many states that used to assume they had both species, it has turned out that only one species occurs.  But there are areas where there is some overlap and some uncertainty still.  Texas is one place with some question marks since both species do occur here.

You can get a rough idea of their ranges by looking at the excellent National Amphibian Atlas.  You can search for any species range.  They show a county by county range map for the species although they acknowledge there is some uncertainty in some of the records for which species of Gray Treefrog it is.

But if you look at their maps, this is the range map for Hyla chrysoscelis -

and here is their map for Hyla versicolor -

What you see on these maps is that Hyla versicolor doesn't appear in the Southeastern US.  In the Northeast, it is the only species present.  

But Texas has an interesting distribution map for these species.  Here is their map for Hyla chrysoscelis in Texas -

You see the big "hole" in SE Texas around the Houston area where the only gray treefrogs are Hyla versicolor.  This is approximately what I have heard, although I think the "reappearance" Hyla chrysoscelis doesn't take place as far east or south (i.e. I have sampled several of the counties that this map implies have chrysoscelis but I have only been able to document Hyla versicolor).  There are also areas of overlap east of Houston not shown on this map.

Here are records by county for the last several years of my recording these species as well as a few records from the Inaturalist Herps of Texas project. (updated Dec 2021)

Obviously, I have a lot of counties to fill in on the map, but the data follow the general pattern shown in the Patuxent maps.  Hyla versicolor are generally found in the SE part of the state and west and north of there they appear to be replaced by Hyla chrysoscelis.  I have found that the boundary/area of overlap is a bit further west than the Patuxent map implies and it is quite narrow.  When you find one species calling, you can drive 20 miles east/west and you find only the other species.

Many field guides imply that their ranges overlap extensively in Texas.  This is due to their reliance on older maps that didn't distinguish the two species very well.  My experience is that the two species don't overlap in range very much (at least in the areas between San Antonio and Houston that I have sampled well).  I have only heard the two species together on a few places in the yellow shaded counties.  They don't overlap throughout those counties, just in a narrow band within those counties. I don't have enough data yet for the northern boundary of their range (it's a long drive from my home in San Antonio!).

I will acknowledge here that the absence of data is not data, but my conclusions in these areas of overlap area based on numerous samples at various times, not just single points.  Here's a map of my recording locations going east of San Antonio 150 miles or so.  Hyla chrysoscelis (green pins) is the only species heard until you get about 75 miles east of San Antonio then suddenly you hear nothing but Hyla versicolor (pink pins).  The few places I have heard the two species calling together are marked with yellow pins.

The broad red line is just there to help show my interpretation of the contact zone between the species.  Yes, the two species do occur together, but the zone of overlap is fairly abrupt in this part of Texas in my experience.  That line represents a contact zone that is less than 15 miles wide.  This isn't intended to be authoritative, but these records are all backed up by recordings available to anyone wanting to check them at Inaturalist, NaHerp.com and Herpmapper.org. 

As you move into far east Texas, Hyla chrysoscelis does occur in the counties east of Houston as well.  In Louisiana, Hyla chrysoscelis seems to take over again.  So there appears to be a little "pocket" of versicolor in Texas. My general feeling is that if you are within 70 miles of Sealy, Texas you are going to find Hyla versicolor.  If you go east or west of that, you are likely to find Hyla chrysoscelis.  The range of versicolor appears to extend further up into the forests of east-central Texas though.  I look forward to the opportunity to fill in some more gaps on this map.

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So even more recent range maps appear to be less than accurate.  Part of the problem seems to be the way the data are reported and collected.  I have seen well-meaning people report species in citizen science databases (like Herpmapper, NAHerp, or Inaturalist) where they made their identification based on:

1. "my field guide shows both species present in my area, and we saw hundreds of them so I am assuming we saw both species" 

- The problem is that the field guides are based on historical records that weren't correctly identified to species.

2.  "we heard mostly species X, but there was one that sounded a bit different" 

- The problem with this argument is that the calls of each species vary between individuals and with temperature.  And sometimes a frog will make a random weird call!  You have to listen to a series of calls to be sure what you are hearing.

3. "I've heard lots of species X.  These were slower/faster" 

- Many people don't know about the difference in their calls based on temperature and how a cold Hyla chrysoscelis can sound like a Hyla versicolor.

These species can be identified by call, but it takes a little patience.  Hopefully with time we will gather a more precise view of their distributions in the Lone Star State.

© Chris Harrison 2013 (updated 2016, 2021)