Thursday, November 5, 2015

Canyon Tree Frogs and Cryptic Species

"Trust not too much in appearances" -Virgil

A Canyon Tree Frog calling to a potential mate, photo by Jessica Phelps

What does "home" sound like? For me, the sound of home is the call of a Canyon Tree Frog. A rather odd call which could be described as a cross between the sound of a domestic sheep and a duck. I recall one night camping alone on a remote, burned over plateau in the Great Basin Desert. I went there to explore and for perspective. Sometimes nothing gives you perspective better than getting as close as possible to the natural forces that shaped us. This is probably why I love the deserts so much, as it seems here those forces are especially apparent even to casual observers. I remember the sun setting and feeling perhaps unsettled; similar to how people often do when they are alone after dark. A few minutes later I heard the call of a single frog, easily identifiable as a Canyon Tree Frog. Eventually several joined, then more, and even more. Eventually I could hear the calming call of hundreds of "Tree Frogs", though there was no living tree in site. I've since returned and camped here several times and have never found the frogs, though their incessant calls tell me they are there. Though I admittedly haven't tried very hard, perhaps worried that gazing upon frogs that no human has yet seen will somehow diminish them.

Okay. I'm getting a little too poetic and emotional for a science blog right?

You get the point. I am sentimental about these frogs. I love them. 

A "tree" frog where there are no trees, the Canyon Tree Frog has found a way to make an arboreal living in an often terrestrial landscape. These semi-aquatic frogs climb on canyon walls instead of vegetation, their coloration often varying depending on the color of the canyon they inhabit (pinkish brown if sandstone, grey if granite). These wall-climbing frogs also seem to be fairly intelligent, moving large distances between suitable bodies of water in patterns not typical of random dispersal. In other words, they KNOW where they are going.

This widespread "species" also allows us to delve into another paradox in evolution. Sometimes two species can look the same but can be quite different. These are called "cryptic species", species that look the same morphologically but cannot interbreed; hidden species. The Canyon Treefrog as we know it might actually be several cryptic species. 

Canyon Tree Frogs are widespread but their range is also choppy and intermittent in nature because of the desert springs and streams they occupy. They are found throughout the American Southwest and into Mexico but these regions have diverged quite substantially based on genetic evidence. 

In addition to this genetic differentiation, there are also subtle differences in the calls of these lineages which may not be enough for a human ear to pick up on. To a female frog though, these differences mean the world. A frog found in the southern lineages strongly prefer the calls of her own locality, and don't like the calls of other Canyon Tree Frog localities. Even something as simple as not speaking the same "language"; could be considered a barrier to reproduction and thus an example of speciation. These data are supported by the genetic data which supports strong divergence between geographical localities of Canyon Tree Frogs.

Sadly, these frogs are threatened by many of the same forces threatening amphibians worldwide. A changing climate, extreme variability and drought, and disease are all threats to this and other amphibian species. For my sake though, I hope that every time I return to that camp site I will be able to hear my frogs.

Further reading and references:

  • Kay, David W. "Movements and Homing in the Canyon Tree Frog (Hyla Cadaverina)." The Southwestern Naturalist 34.2 (1989): 293-5. Web.
  • KLYMUS, KATY E., SARAH C. HUMFELD, and H. CARL GERHARDT. "Geographical Variation in Male Advertisement Calls and Female Preference of the wide‐ranging Canyon Treefrog, Hyla Arenicolor." Biological Journal of the Linnean Society 107.1 (2012): 219-32. Web.
  • Barber, PAUL H. "Phylogeography of the Canyon Treefrog, Hyla Arenicolor (Cope) Based on Mitochondrial DNA Sequence Data." Molecular Ecology 8.4 (1999): 547-62. Web.
  • Klymus, Katy E. "Phylogenetic and Behavioral Differentiation in the Canyon Treefrog, Hyla Arenicolor." ProQuest Dissertations Publishing, 2011. Web.
  • Bradley, GA, et al. "Chytridiomycosis in Native Arizona Frogs." Journal of Wildlife Diseases 38.1 (2002): 206-12. Web.

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