Wild Thing: Hitchhiking Herp
The Rio Grande chirping frog rides tropical plants to new homes across the state.
By Lee Ann Linam
Somewhere deep in a bush on this warm, humid night in College Station there is a frog hiding, but for the life of me, I can’t find it. I check the ground, where most frogs and toads hang out, but the high-pitched chirp is coming from up in the branches of the shrubbery. I cup my ears to try to pinpoint the call, but this frog is a ventriloquist. I try listening from all sides, pulling back the greenery, trying to triangulate its location, and then, finally, there it is — the Rio Grande chirping frog, a half-inch-long, darkly mottled frog that’s supposed to be found only in the most southerly counties in Texas.
The Rio Grande chirping frog (Eleutherodactylus cystignathoides) breaks all the rules. This tiny Texas amphibian doesn’t go to water to breed, and it’s been hitchhiking all over the state in recent years. Though the frogs are native to the Lower Rio Grande Valley, folks all over the eastern half of Texas have heard their tiny nocturnal chirps, described as sounding like a bird or the squeak of tennis shoes on a gym floor. Urbanization, the bane of many wildlife species, somehow seems to be beneficial for this one.
Chirping frogs are a part of the free-toed frog group — the largest genus of vertebrates in the world, with more than 700 members, found primarily in tropical areas. Two other chirping frogs are found in Texas: the cliff chirping frog of the Texas Hill Country and the spotted chirping frog, found only in the Trans-Pecos. All share the unique characteristic of direct development of the young. While most frogs go to ponds to lay hundreds or thousands of eggs and the young larvae develop as tadpoles in the water before metamorphosing, chirping frogs lay a dozen or so eggs in pockets of moist soil. The young frogs actually go through the larval development stage in the egg and emerge from the egg as tiny little froglets.
That unique attribute explains why Rio Grande chirping frogs have been showing up all over Texas, from San Antonio to Dallas to Houston to College Station. As tropical plants have been shipped out of the Lower Rio Grande Valley, biologists suspect that they have carried cargos of chirping frogs and their eggs. Encountering irrigated landscapes and the warmth retained in concrete structures in cities, Rio Grande chirping frogs have flourished, expanding their range into nearby natural areas as well.
Data submitted by volunteers in TPWD’s Texas Amphibian Watch (www.tpwd.state.tx.us/amphibians) have confirmed an increasing presence of the species in many areas, especially in coastal counties.
Little is known about how the chirping frogs cope in these new habitats — whether they compete with other species, whether predators exist in these environs or whether they are simply a welcome addition as a predator of small insects. We do know that the squeak of Rio Grande chirping frogs will be heard in new habitats this spring. Volunteers who report hearing and seeing them can help us learn more about the little frog that breaks all the rules.