Canyon Treefrog (Hyla arenicolor)
Canyon Treefrog, La Sal, Sierra Los Ajos, Sonora. Photo by Jim Rorabaugh
Canyon Treefrogs in amplexus, Sierra San Luis, Sonora. Photo by Jim Rorabaugh
Late stage Canyon Treefrog tadpole, Sonora, MX. Photo by Jim Rorabaugh
Canyon Treefrog calling, Cochise County, AZ. Photo by Jim Rorabaugh
Canyon Treefrog, Photo by Warren Savary
Canyon Treefrog eggs attached to the rock surface of a tinaja, Big Casa Blanca Cyn, AZ. A few very small tadpoles are present, as well. Photo by Jim Rorabaugh.
To hear the call of the Canyon Treefrog, follow this link:
The Canyon Treefrog (Hyla arenicolor) is probably the most common frog in the Santa Catalinas, Rincons, Santa Ritas, and other Sky Island mountain ranges surrounding Tucson. Half-true to their name, Canyon Treefrogs live in the ephemeral waterways that spill out of these mountains. But they seem to prefer rocks over trees, and typically can be found adhered to boulders and cliffs above the pools of water where they breed and lay their eggs. This species is not confined to the Tucson area, or even to desert foothills. It is a wide-ranging frog that occurs as far south in Mexico as Oaxaca, as far north as southern Utah and southeastern Colorado, and there are isolated populations in Texas and Nevada. Canyon Treefrogs have a very wide elevational range, from sea level in Mexico to nearly 10,000 feet (3048 m) in Arizona’s Sky Islands (Stebbins 2003). DNA evidence indicates 3 deeply divergent lineages, which suggests that the taxonomy of this species may need to be revised (Barber 1999).
These little treefrogs are about an inch or two in length (2.5-5 cm) from snout to vent, with rough skin, large and distinctive circular pads on their toes, and cryptic dorsal coloration (Fig. 1). Underneath, they are creamy white, with bright yellow or orange on the undersides of their legs. These frogs are seldom seen, yet I find that nearly every serious hiker and naturalist in Tucson knows they are out there and remembers their first experience with them. For who expects to find a treefrog in the desert? And who expects it to suddenly appear from the rock background it so closely matches? Where one moment there is just a granitic boulder, the next moment there’s both a boulder and a frog – and the two are so similar it is not hard to believe that they were formed by the same geological event.
Areni – Color Change
Much of what we know about color change in Canyon Treefrogs and other amphibians comes from the work of Dr. Joseph Bagnara, Mac Hadley, and graduate students at the University of Arizona beginning in the 1970s (Bagnara and Hadley 1972, Bagnara and Fernandez 1993; for a detailed discussion of amphibian color see Dr. Bagnara’s  article in the Sonoran Herpetologist). Color change in amphibians is governed by a hormone called melanocyte-stimulating hormone (MSH), which stimulates color pigment cells to change skin color. In the Canyon Treefrog pigment cells can be black (melanophores), yellow-red (chromatophores) or silvery (iridophores), all working together in what Hadley and Bagnara call a Dermal Chromatophore Unit. The white color of treefrogs in bright sun is due to reflecting platelets in the iridophores, which contain crystalline deposits lined up parallel to the skin surface. When the treefrog turns dark, it is because melanin has been dispersed into the epidermis above the iridophores from the melanophores that are located beneath them (Bagnara 1994).
Canyon Treefrogs can change more quickly from light to dark than vice-versa. In light-to-dark change, MSH is quickly dispersed into the blood stream, whereas in a dark-to-light change the process of producing and releasing MSH from the pituitary into the blood occurs more slowly. For a class project at the University of Arizona I once tested this by acclimatizing a series of treefrogs in black and white bowls until the frogs had changed color; then, I transferred each frog to the opposite bowl and recorded the color change with a Chromameter colormeter (an electronic probe used for quantifying color). Within 10 minutes the Munsel color value units (MCVs) of the white frogs had dropped from about 6 to 4 and the skin had become dark brown. In contrast more than 20 minutes passed before the dark frog’s skin changed 2 MCVs to become light.
When I exposed the frogs to a checkerboard pattern of white and dark, the frogs usually chose dark no matter what their initial color was, even in trials when a Black-necked Gartersnake (Thamnophis cyrtopsis) was visible to the frog behind glass. However, I ran all the trials at about 26oC. When I showed the results to Dr. Bagnara, he told me that Canyon Treefrogs tend to choose a light or dark colored background based on temperature, and then match the background – so if I had exposed the frogs to the same checkerboard at a higher temperature, presumably they would have consistently chosen white.
More than just light and dark, color in Hyla arenicolor is extremely complex, with individuals in different areas exhibiting color highlights that match the colors of the rocks in the canyon where they live. We can only assume that these colors are the result of selective pressures over time, as in the classic case of black and white moths on black and white trees in England (Kettlewell 1955).
But different frogs also exhibit different landscapes of color; that is, individuals that can change from all-white to dark brown usually show a mottled pattern in between. This pattern is the result of patterns in the distribution of dermal color units. The match between the richness of the skin-color landscape of frogs in our local canyons and the richness of the rock landscape can be very striking (Fig. 2). These patterns must also be evolutionarily selected for, although the frog’s ability to change from light to dark influences how the pattern is expressed at any given time. The pattern on each individual frog, moreover, appears to be fixed. That is, you can probably photograph individual frogs and use this mottled pattern to keep track of them over time. It is this complex pattern, in combination with color change, that makes Canyon Treefrogs so difficult to see on rocks in the field.
The species name arenicolor means “sand-colored.” This seems to be such a boring name for an animal with such incredible color complexity! Yet the dorsal colors are simple – there are no brilliant greens or oranges in this treefrog. And desert souls know how infinite the depth and variety of color in sand can be.
Like Water for Frogs
Desert frogs are water conservation specialists. Lowland Leopard Frogs (Lithobates yavapaiensis) need permanent surface water, but not much, to reproduce and survive. Barking Frogs (Eleutherodactylus augusti) live in damp crevices or caves and are terrestrial only during summer rains. Canyon Treefrogs are usually found near water, but have a series of adaptations that make them extremely persistent in our arid region. They occur and breed in very ephemeral streams during wet years, populations probably die out during very dry years, only to expand back to these areas when it is wet again.
When it’s really hot and dry Canyon Treefrogs aggregate, often in large groups of 100 or more, on rock surfaces over pools of shrinking water on vertical rock walls. Individuals periodically climb down to the pool for a 10-15 minute soak in the water, then climb back up again. Due to highly specialized ventral cells, they take in water through their ventral surface and can sometimes be found sitting on rock cracks that are slowly seeping water. Even on hot summer days these frogs bask in full sun with their internal temperatures rising as high as 35oC. Why do they do it? In his PhD dissertation on this species, Stephen Wylie (1981) suggests that these exposed rock faces are very difficult for predators – mainly snakes – to reach.
At any rate, the need for water in this species is clearly less than for strictly aquatic frogs. At night and during the summer rains, they forage away from stream pools. In winter, while searching for Desert Tortoises (Gopherus morafkai), famed local herpetologist Roger Repp and I have found them tucked into rock crevices more than 100 meters from water. Like other terrestrial treefrogs, they do not seem to need standing water at all in wet periods. For example, who would expect to encounter Hyla arenicolor in talus slopes at high elevations in the Sky Islands? But there they are, far from any obvious water, climbing around on the rhyolite with Mountain Spiny Lizards (Sceloporus jarrovii) and Twin-spotted Rattlesnakes (Crotalus pricei).
Reproduction and Survival
In Tucson, Canyon Treefrogs reproduce in spring (I heard one calling today, on 12 March, 2005) but continue during and after the summer rains. With their large elevational range it is probably fair to say that Canyon Treefrogs can be found breeding somewhere near Tucson just about anytime from February to October. Their eggs are hard to find and hatch into tadpoles very quickly. The tadpoles are relatively small, transforming into froglets when they are about 38 mm in length (Degenhardt et al. 1996). Zweifel (1961) found that the period from oviposition to metamorphosis in the Chiricahua Mountains was 50-60 days. Wylie (1981) found the period to be 70-74 days in the Pinaleños, and varied depending on water temperature and other factors. Sometimes, the tadpoles overwinter (D. Swann and E. Wallace, personal observation).
Canyon Treefrogs eat a variety of invertebrates including beetles, ants, true bugs, and caddis flies (Degenhardt et al. 1996). They are susceptible to predation by Giant Water Bugs and gartersnakes. One warm day in May several years ago I came across a series of drying pools, each filled with a mass of teaming Canyon Treefrog tadpoles. Nearly every pool also contained a Black-necked Gartersnake gorging itself on the tadpoles. The contents of every gartersnake captured by Wylie (1981) in his study site contained tadpoles, metamorphs, and juveniles of Canyon Treefrogs – and no other food items. As males were calling on 22 July 1997, Dennis Caldwell (pers. comm.) observed Giant Water Bugs attacking any frog that entered a pool in Brown Canyon, Baboquivori Mountains (Fig. 3).
Chytrid fungus, which has been implicated in recent die-offs of amphibians world-wide, does affect this species although the extent is unknown. Individuals collected from Catalina State Park in 1999 showed characteristic lesions associated with this infection (Bradley et al. 2002), and the Tucson Herpetological Society’s Southeastern Arizona Winter Amphibian Monitoring Program also documented an infected individual in Tanque Verde Canyon in 2000. Several local populations have experienced wide fluctuations or declines in the past decade, which may be related to disease, drought, or a combination of these factors. Nevertheless, in many places at both high and low elevations, Canyon Treefrogs remain abundant. Fortunately Coronado National Forest and Saguaro National Park preserve a large amount of canyon habitat.
By the time you read these words, Canyon Treefrogs will be calling and breeding in the mountains and foothills surrounding Tucson. Their song sounds something like a bleating sheep, but to desert amphibian aficionados it signals spring as surely as Mexican gold poppies and the first rattlesnake stretched across the trail. If the distribution and volume of Hyla arenicolor song follows El Niño patterns of winter rainfall, then this month should be a good time to go for a sunset hike in search of this under-appreciated Sonoran Desert frog.
Bagnara, J.T., and M.E. Hadley. 1972. Chromatophores and Color Change: the ComparativePhysiology of Animal Pigmentation. Prentice-Hall, Inc. Englewood Cliffs, N.J. 202 pp.
Bagnara, J.T. 1994. Vertebrate pigmentary phenomena from a herpetological point of view. Sonoran Herpetologist 7:96-102.
Bagnara, J.T. and P.J. Fernandez. 1993. Hormonal influences on the development of amphibianpigmentation patterns. Zoological Science 10:733-748.
Barber, P. H. 1999. Phylogeography of the canyon treefrog Hyla arenicolor (Cope) based on mitochondrial DNA sequence data. Molecular Ecology 8:547-562.
Bradley, G.A., P.C. Rosen, M.J. Sredl, T.R. Jones, and J. E. Longcore. 2002. Chytridiomycosis in native Arizona frogs. Journal of Wildlife Diseases 38:206-212.
Degenhardt, W.G., C.W. Painter, and A.H. Price. 1996. Amphibians and Reptiles of New Mexico. University of New Mexico Press, Albuquerque. 431 pp.
Kettlewell, H.B.D. 1955. Selection experiments on industrial melanism in the Lepidoptera. Heredity 9:323-342.
Stebbins, R.C. 2003. Western Reptiles and Amphibians. Houghton-Mifflin Company, Boston, Mass. 533 pp.
Wylie, S.R. 1981. Effects of basking on the biology of the Canyon Treefrog, Hyla arenicolor Cope. PhD. dissertation, Arizona State University, Tempe. 121 pp.
Zweifel, R.G. 1961. Larval development of the treefrogs Hyla arenicolor and Hyla wrightorum. American Museum Novitates 2056: 1-19.
Author: Don E. Swann. Originally published in Sonoran Herpetologist 2005 18(4):39-42
For additional information on this species, please see the following volume and page in the Sonoran Herpetologist: 2008 Sep:98.