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NATURE HAPPENINGS - May 2006


Bleating Canyon Treefrogs
by Damian Fagan

As I reach Sipapu Natural Bridge in the pre-dawn darkness, I hear the far-off bleating of sheep. Or what seems to sound like sheep. Though I am here to conduct early morning breeding bird surveys, I can not ignore these sounds.

As I round a bend in the canyon, the bleating stops. When I stop, the bleating continues. Eventually the trail crosses the boulder-strewn creek and I catch the trailing segment of sound coming from a nearby pool. Upon closer inspection of the pool I find frogs ─ canyon treefrogs that leap from the banks or boulders into the safety of the pool.

Though the frogs are difficult to locate because of their cryptic skin color, which closely resembles the sandstone wall, they are easy to locate when they jump. Their aquatic escapes are brief as the frogs haul themselves out of the pool and perch on a rock or cling to the canyon wall. There, they continue to call.
The males are calling to advertise their availability for mating, and from the sounds of things, there are quite a few bachelors in the hood.

When the males chorus, their vocal sacs (located below their chins) inflate like a dusky, opaque balloon. This vocal sac enables the call to resonate and project beyond the pool. The “community chorus” draws the females to the pools, but I do not know if the females select the best singers in the bunch.
In Larry Hyslop’s Beeplants and Whiptails he describes an encounter with treefrogs in Zion National Park, “Across the pond, the now familiar call of a male frog is a sound much too loud for such a small frog. It is nothing humans would call beautiful although I assume it sounds good to a female frog.” Others describe this call as “the bleating of a sheep with a cold” or “…sounds like brrurt-brrurt-brrurt, like a rivet gun coming from inside a tin can.”

What these frogs lack in size, they make up in voice. Adults are 1¼-2¼” long and have an ash-gray to dark brown skin. There are spots or darker blotches on the upperside of the plump body, while the underside is white or cream colored.

Their toe pads are shaped like suction cups to provide a better grip on vertical surfaces. The pads have tiny divisions that spread apart and enable the frog to gain a better grip on surfaces. The last two bones of each toe have extra cartilage segments between them, enabling the frog to swivel their toe and place it flat against a surface. Their long legs also allow them to make sizeable leaps.

The breeding season runs from March to August, depending upon temperature and elevation. Generally, evening water temperatures should be above 55°. The warmer the water the faster the eggs develop.

The tadpoles mature into adults in about 70 days. They are vulnerable to predation by fish, aquatic invertebrates, small mammal predators, and salamander larvae. An estimated 2% of the eggs will mature into adults, and of this amount, around 10% will survive the first year.

The treefrog’s scientific name is Hyla arenicolor. Although Hyla means, “tree” this species spends little time in trees or shrubs. Arenicolor means, “sand color or tone” and refers to the skin color. The remarkable thing about their skin is that it can change color to match its surroundings.

Like many amphibians, treefrogs occur in aquatic locations such as streams, potholes or stocktanks. Their unique respiratory system includes lungs for breathing but may also utilize a cutaneous respiration system to supply oxygen to their body. Air is drawn in through the moist skin via blood vessels that lie just below the skin’s surface. This enables the frog to stay submerged while avoiding predators or curious biologists.

Though the frogs had quieted down long before I ascended out of the canyon, I was pleased to hear their choruses in the early morning. Even if they did sound like a flock of rivet-popping, runny-nosed sheep.

 

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