Cope’s Tree Frog or Gray Tree Frog?

The Question: I found this little guy in my backyard. It’s smooth and green on top and gray/tan on the bottom. It has striped legs and horizontal pupils. What is it? Submitted by: Elizabeth, southern Indiana, USA The Question: It’s 9:00 p.m. and I live next to a small woods. I hear a noise outside and was wondering if it was a bug or bird. I hear this noise regularly. It’s a repetitive trill same note. Here’s my recording:  click here. Submitted by: Diane, Ohio, USA

The Short Answer: I’m combining Elizabeth’s question and Diane’s question because they are both dealing with the same species … maybe. Elizabeth’s picture is of either a gray tree frog (Hyla versicolor) or a Cope’s tree frog (Hyla chrysoscelis).  They are pretty much impossible to tell apart by appearance, especially since both are so highly variable. In fact, despite the “gray” in its name, the gray tree frog can be any color from green to brown or gray, as this collage shows. And the Cope’s has the same variation. To make it even more difficult, any one individual frog can change color to some extent depending on its surroundings. What both species have in common is yellow markings on the underside of their hind legs, so look for that.

The way experts tell the two species apart is by their call. Carl Gerhardt, at the University of Missouri, who studies communication in frogs, says that above 75 degrees, the difference between the two species is that you can hear distinct pulses in the call of a gray tree frog, whereas the Cope’s sounds more like a buzz. (As the temperature drops, however, the trill of the Cope’s slows down and begins to sound more like the gray.) Listen to these recordings of the two species:

Gray Tree Frog                        Cope’s Tree Frog

Now listen to Diane’s recording again and try to guess which frog Diane’s is. Click here to see if your answer agrees with Dr. Gerhardt.

Another way to tell which frog you have is by location. As the range maps at these links show, if you’re looking at one of these frogs in the most southeastern states of the United States (Florida, Georgia, Mississippi, Alabama, South Carolina), you probably have a Cope’s tree frog. If you are in the northeast, from New York to New England, you almost certainly have a gray tree frog. In all the other states, it’s possible to find both. Where Diane lives in Stark County, Ohio, only the gray tree frog has been recorded, so that’s confirmation that Dr. Gerhardt’s identification from the frog’s song is correct.

Gray tree frog range map        Cope’s tree frog range map

Elizabeth, by the way, said she took her picture in southern Indiana, which means she probably has a Cope’s tree frog.

The Weird Genetics: It’s clear that the Cope’s tree frog and the gray tree frog are very closely related. Just how closely was discovered in the 1970s when scientists realized that the gray tree frog is basically a Cope’s tree frog with a doubled set of chromosomes.

If you remember your high school biology, in most organisms every cell has two copies of each chromosome. When cells in the reproductive organs undergo meiosis to create eggs and sperm, the chromosomes are duplicated, resulting in four copies of each chromosome. Then the reproductive cells divide twice, each time halving the number of chromosomes. The end result is egg and sperm cells with one of each chromosome. When you put and egg and sperm together you get back to having pairs of chromosomes, one from the egg and one from the sperm, just in time to make an embryo.

Sometimes, this process goes awry and an egg or sperm cell will end up with two sets of chromosomes or even three or four, instead of one. When it joins with its partner, the resulting embryo will have more than the usual number of chromosomes. Usually when this happens, the embryo doesn’t develop, or it dies after developing.

But sometimes the embryo develops normally and results in an organism with more than the usual number of chromosomes. For some reason, this is relatively common in amphibians. And the gray tree frog is an example. It is essentially a Cope’s tree frog with a doubled set of chromosomes. Having twice the number of chromosomes means that the Cope’s tree frog and gray tree frog are generally unable to interbreed, which is why they remain separate species.

But the fact that a gray tree frog genome is essentially the same as a Cope’s tree frog genome, only doubled, explains why they are so similar and so hard to tell apart. In fact, careful analysis of the DNA of these frogs shows that some gray tree frogs are more closely related to certain populations of Cope’s tree frogs than they are to each other, suggesting that while the gray tree frogs all have the same doubled set of chromosomes and look and act the same, they may actually represent three separate events where the chromosomes of Cope’s tree frogs were doubled. And because the doubled chromosomes of the three separate gray tree frog populations match, they are able to interbreed readily.

What’s interesting is that the three populations of gray tree frogs sound very much the same when they sing – and sound similarly different from Cope’s tree frogs. This suggests that the doubled chromosomes themselves lead to the different song. In fact, Dr. Gerhardt and one of his students have been able to artificially create frogs with three sets of chromosomes (midway between the two of a Cope’s and the four of a gray tree frog). These frogs sing a song whose trill rate is between the Cope’s and the gray.

How this happens is not well understood. But having extra genetic material does have effects. For one thing, it makes the cells of the gray tree frog larger than those of the Cope’s tree frog. It seems likely that the larger cells cause the slower trill rate in the artificially created frogs and in the gray tree frog.

Which is good for us, because otherwise, the only way to tell the two species apart would be to put their cells under a microscope and count chromosomes.

Sources:

Thanks to Dr. Gerhardt for the identification of Diane’s recording.

Keller, M J & Gerhardt, H C. (2001). Polyploidy alters advertisement call structure in gray tree frogs. Proceedings – Royal Society. Biological sciences, 268(1465), 341-5.

Amphibiaweb summary of scientific research on Hyla chrysoscelis and Hyla versicolor: http://amphibiaweb.org/cgi/amphib_query?where-genus=Hyla&where-species=chrysoscelis.

Dr Gerhardt believes this to be a Gray tree frog (Hyla versicolor). Click to return.