frogs

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"Nature Guide Journal"

22 February 2001

I heard them the other evening: heralds of spring, singing in the wooded gully behind my house.  Frogs!

Even young children are familiar with these favorite amphibians, hopping frogs and their swimming tadpole young.

"Amphibian" is from the Greek, generally translated to "double life."  The name derives from the animals' two-phase life: a water-based form (larvae and tadpoles) that metamorphoses into a land-based, or terrestrial form (salamanders and frogs).  (In addition to salamanders and frogs, there is a third group of amphibians, the caecilians, that live only in the tropics.)

While their body temperature is wholly dependent on environmental conditions (sometimes said to be "cold-blooded"), amphibians do move about their environment to manage their body temperature.  Here, they seek cool, damp burrows and crannies in summer, and search out well-insulated resting spots in winter.

The native amphibian most heard in our region is the Pacific tree frog (Hyla regilla), now called the Pacific chorus frog (Pseudacris regilla) by some.  The tree frog comes in a variety of colors, from bright green to mottled brown, but always sports a broad dark stripe from the tip of the nose, through the eye and ear, to the shoulder.

Our smallest (up to 2" long) and most common frog, the tree frog is rather tolerant of drying out, and may be found quite a distance from water.  They return to water to lay their eggs in shallow ponds and pools.  The soft clusters of tightly packed clear eggs are attached to submerged plants near the bottom.  The tadpoles hatch from the eggs and develop into juvenile frogs in relatively short order—just under three months total—which allows them to use temporary ponds as breeding sites.

Several people have mentioned to me recently that they used to hear more frogs in the spring than they do now.  Quite likely true:  Many species of amphibians are declining world-wide, including in Oregon.  Most of the decline is attributed to loss of critical habitat:  the boggy wetlands and swales people often undervalue.

The increase in UV light has also been identified as a possible frog-killer, especially in higher elevations.  In our region, frogs are subject to infection by a fungus that kills the egg masses.  The eggs of the red-legged frog seem particularly vulnerable. Interestingly, some have speculated that the effect of the fungus is spurred or enhanced by the increase in UV light.

Another major threat to our native frogs are the bull-frogs that were introduced to the west in the 1920's and 1930's as a food source for people.  While the tadpoles of the bull-frog (Rana catesbeiana) are vegetarians, the adults are voracious carnivores whose diet includes other frogs, crayfish, and young turtles—even small birds and mammals.

So what's all the singing about?  Male frogs apparently sing to identify their species and sex, as well as to attract female frogs.  Further, it appears that the frogs measure their population density by group singing: the volume or number of voices of the males' mass song somehow affects the number of offspring, thereby adjusting the overall population.

One excellent guide to the amphibians of our region, which includes the water-based stages, is:  Amphibians of Oregon, Washington, and British Columbia: a Field Guide; written by Charlotte Corkran and Christina Thoms; published by Lone Pine Publishing in Alberta, Canada. 

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visit our other pages on related topics:

salamanders

ponds and lakes

wetlands

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