Bull Session

I say potato, you like potahtos. You wear pajamas, I wear PJs. And a rose by any other name, we’re told, would smell equally sweet. So does it really matter that we all agree on what to call an American bullfrog? 

“HELL, YEAH!” 

That’s the collective cry of taxonomists around the globe raising their voices in indignant protest. (Yes, these are men and women of strong, science-based convictions.) You see, to a biologist who studies the classification of organisms, names are not at all trivial… but they should all be binomial.

Swedish botanist Carl Linnaeus is credited with introducing this ubiquitous classification system, based on bestowing a unique two-word Latin name upon each species, precisely to avoid the kind of misunderstandings that arise when you say ersters and I say oysters.

Or, for example, when one scientist is talking about a fish using one of its common names, “dolphin” (Coryphaena hippurus, aka mahi-mahi, dorado, pompano), and another scientist hears “dolphin” and thinks of a perpetually-smiling bottlenose marine mammal (Tursiops truncatus).

And yet, despite Carl’s best efforts, disagreements persist. As in the case of the American bullfrog, whose official Latin name (Rana catesbeiana or Lithobates catesbeianus, depending on whom you ask) is more likely to be disputed and cause confusion, ironically, than its common name.

The quibble over nomenclature began about 10 years ago and quickly became a quarrel. Darrel Frost, Herpetology Curator at the American Museum of Natural History, suggested a conceptual leap that would divide members of the genus Rana, which includes bullfrogs, into nearly a dozen new genera. Many of Frost’s colleagues, unconvinced that his argument held water, refused to jump into the newly proposed systematics pond.

In response, feelings, opinions, and counter-claims have been aired publicly in peer-reviewed journals. Several years ago, a group of international researchers created a consortium to promote their own preferred adaptation of the froggy family tree. The taxonomy community still hasn’t managed to harmonize this chorus, which is why she says Rana and he says Lithobates.

But hold on… let’s not call the whole thing off just yet.Because, of course, a bullfrog doesn’t need a taxonomist to know exactly who he or she is… once s/he reaches a certain age, anyway.

Sure, there may be some gender ambiguity early on but that’s common among young amphibians. Their sex is determined genetically, although research suggests that for many frog species, exposure to environmental estrogen or variations in water temperature during tadpole-hood can induce male-to-female or female-to-male transitions. Self awareness doesn’t always come easily, and it can take some time for those gender identity questions to work themselves out. Bullfrog development is relatively slow—one to three years from egg to adult, and another two years to reach sexual maturity.

By the time they’re ready to procreate, however, males and females have definite, discernible physical differences. Males are smaller than females, their tympana (external eardrums) are larger than their eyes, and when in breeding condition their throats are yellow; female tympana are equal or smaller in size than their eyes, and their throats are white. There are behavioral distinctions as well—male bullfrogs are territorial over the summer mating season, and quite vocal about it, too; females are relatively silent, although older gals have been known to sing along with the guys. (I have a sneaking suspicion their favorite tune is If I Were A Boy.)

Ok, ok… life, in all it forms, is full of uncertainty and differences of opinion, at the laboratory bench and the water’s edge. But can we all come together on this much, at least? That the creatures featured throughout this post are:

a. Amphibians

b. Bullfrogs

c. Cool.

Everything else is neether/nyther here nor there.


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© 2018 Next-Door Nature. Reprints welcomed with written permission from the author. Thanks to the following photographers who made their work available through the Creative Commons license: Mark Beckemeyer, LadyDragonflyCC, Greg Schechter, Rick Cameron, and Kaibab National Forest.

Vert-de-Gris

Cope’s gray treefrog on a pepper plant. Photo: Alan Howell © 2017, Star Path Images, Used with Permission.

It isn’t easy being green. Kermit the Frog said it, so you know it has to be true.

He’s always seemed a reluctant celebrity, so my guess is that being the most famous Muppet-amphibian on the planet isn’t always a picnic. I wonder whether life would be a little less stressful if, like some of his cousins, Kermit could change from green to another color when he’d rather not be so conspicuous.

North American gray treefrogs know how to be seen and when to blend into their surrounding, shifting the spectrum from bright emerald to peridot, to gold, copper, platinum, silver, and even gunmetal. Since the places they hang out — trees and shrubs in woodlands, meadows, prairies, swamps, suburbs, and cities — tend toward palettes awash with green and gray hues, these arboreal amphibians can keep their sartorial choices simple. 

What looks like a costume change is actually a rather high-tech, cellular-level special effect created by chromatophores. These pigment-containing and light-reflecting cells, or groups of cells, are present in the skin of certain frogs, as well as some reptiles (including chameleons), cephalopods (such as octopi), fish, and crustaceans. Mammals and birds, on the other hand, have melanocytes, a different class of cells for coloration.

Chromatophores are classified based on their hue under white light:

  • melanophores (black/brown)
  • cyanophores (blue)
  • erythrophores (red)
  • xanthophores (yellow)
  • leucophores (white)
  • iridophores (reflective or iridescent)

The process, known as physiological color change, can be controlled by hormones, neurons, or both. Most studies have focused primarily on melanophores because they are the darkest and, therefore, the most visible. When pigment inside the chromatophores disperses throughout cell the skin appears to darken; when pigment aggregates in cells the skin appears to lighten. Either way, you’ve got an amazing makeover, or a convincing disguise, in no time flat! 

There’s even more to the gray treefrog’s ability to assume an alter-ego than meets the eye —what appears to be a single species is actually two very close relatives: Hyla versicolor, aka Eastern, although there’s a long list of pseudonyms from which to choose (there are more ways to hide than blending into the substrate); and Hyla chrysoscelis, aka Cope’s (which also has a couple of nicknames but not as many as H. versicolor).

Cope’s and Eastern are equally skilled at clinging to and climbing slick surfaces using large toe pads that secrete mucous, creating surface tension. It’s not uncommon to find one plastered against a window pane (allowing for an interesting view of their nether-regions). Both species prefer a diet of small insects, spiders, and snails. Both hibernate under leaves, bark, or rocks. Well, “hibernate” sounds a lot more cozy than the reality… which is that their bodies pretty much freeze and their lungs, heart, brain, and other organs stop working until they thaw out and reanimate in the spring. It’s a pretty nifty chemical trick, on par with being able to transform your complexion at will. 

Kermit has an unmistakable personal brand, but distinguishing H. versicolor from H. chrysocelis visually is just about impossible. Both Eastern and Cope’s are relatively small (1.5—2.0 in/3.5—5 cm), and the adults are often mistaken for younglings. Both wear a sweep of bright citrus-orange along the inseam of their hind legs (a signature shade is all the rage, you know).  Females of both species are usually (but not always) larger, with a ladylike white throat. During the breeding season, males have a macho (make that hipster, since it resembles a beard) black, gray, or brown throat. Guys sing. Gals swoon, but don’t sing along.

The ranges of these two species overlap, but Cope’s are more widely distributed. If you find a gray treefrog in North Carolina or Georgia, you can be reasonably certain it’s a Cope’s, but if you’re in Iowa or Pennsylvania, all bets are off. The only risk-free way to know for sure is to do a DNA test — Cope’s are a diploid species, meaning they have two sets of chromosomes, one from each parent (for a total of 24); Easterns are a tetraploid species, meaning they have four sets of chromosomes (for a total of 48).

Now, some frog call connoisseurs claim they can detect variations in breeding calls, and that Cope’s treefrogs have a faster and slightly higher-pitched trill than Easterns. But the call rates of both species change with ambient temperature… so color me skeptical.

[Postscript:  What kind of frog is Kermit? While I’ve never found a definitive answer to this question, I always assumed that because he was created/discovered by Jim Henson, he was probably Hyla muppetalis or Rana hensonii… something like that. However, in 2015 Brian Kubicki of the Costa Rican Amphibian Research Center discovered a frog who, if not the same species, must at least be a first cousin, or possibly Kermit’s doppleganger.]

Diane’s bare-hearted glass frog (Hyalinobatrachium dianae). Photo: © Brian Kubicki, Costa Rican Amphibian Research Center, Used with Permission

[Thanks to the photographers who granted permission to use their photos, and to those who made their work available through the Creative Commons license: Dave Huth(composite of color variations), , Chiswick Chap & CheChe, (melanophores schematic), Alan Wolf, Andrew Hoffman, and Douglas Mills© 2017 Next-Door Nature. Reprints welcomed with written permission from the author.]