HYLA CHRYSOSCELIS ALSO CROSSES THE BLUE RIDGE

Richard L. Hoffman

Virginia Museum of Natural History

Martinsville, Virginia 24112

The known distribution of Cope’s gray treefrog, Hyla chrysoscelis, has undergone an interesting evolution since the earliest recognition of two call types of what was then called Hyla versicolor by Francis Harper in 1935. The introduction of audiogram procedures quantified discrimination of call rates subject to variability induced by both geographic an thermal factors, but even prior to that innovation, a single observer (listener) could readily separate two (in the eastern States, at least) kinds of calls.

That some kind of allopatry was involved in the situation was first implied by noble & Hassler (1936), based on their findings in estuarine Maryland. In brief, what seemed to be a more southern “harsh-voiced” (“fast call”) form was gradually replaced by a more northern “mellow-voiced” (“slow call”) form. A rather comparable displacement was noted by C.F. Walker (1946) for Ohio, where the “harsh” call type seemed confined to the southern, unglaciated part of that state (this pattern was immediately confirmed for neighboring Indiana Mittleman [1947]).

Later in 1946, I published a summary of my observations in Virginia, which indicated that the “harsh/fast” call was confined to the Coastal Plain and upper Tennessee River drainage basin, also that the males associated with such a call averaged about 10 mm less in body length than those of the “mellow/slow” call type which occurred in the piedmont and mountain regions.

In succeeding years the distribution and taxonomic status of the two “call races” were clarified in papers by W.F. Blair (1958) and Clifford Johnson (1959, 1963, 1966), in the last of which Cope’s early name Hyla fermoalis chrysocelis (1880) was revived to denominate the southern race as a valid sibling species reproductively isolated from the structurally identical northern form Hyla versicolor Leconte (1825). In a distribution map (1966, Fig. 1) based on a sonogram data, Johnson showed that the range of Hyla chrysoscelis extended from eastern Virginia south to Florida, west to central Texas, thence northward to Kentucky and Missouri. The distribution of Hyla versicolor was depicted as basically north of 40° N. Lat., with a southward extension down through the Appalachians to northern Georgia. In essence, the map confirmed and supplemented the ranges defined by previous authors. Most recently, the distribution of Hyla chrysoscelis in the northeastern end of its range was discussed by Zweifel (1970), who recorded the species from extreme southern New Jersey, its then and still northernmost known locality.

Subsequent to 1946, I continued to accumulate distributional records for the two species in Virginia, and in 1950, also in eastern Tennessee and Kentucky in company with James A. Fowler. In 1948, syntopic calling of the two was reported (Hoffman & Kleinpeter 1948:607) at one site in Burkes Garden, Tazewell County, Va. As long ago as 1952, a transect of southern Virginia along US 58, on a late spring day following rain, confirmed the previously reported pattern, e.g., it was necessary to drive as far east as Emporia before the first “fast/harsh” calls were detected.

However, major changes in our perception of the ranges of the two species have occurred during the past several decades. Recordings made by J.C. Mitchell and C.A. Pague during the 1980s revealed extensive overlaps, and some sympatry, of the two forms in the Virginia Piedmont. Populations of Hyla chrysoscelis were located as far west as a line connecting Martinsville and Fredricksburg, a change confirmed by my own field experience between 1988 and the present. By 1991, it was the gray treefrog commonly heard in Henry County. In 1992, males were calling along Va. 40 between Ferrum and Woolwine in western Franklin County, essentially at the base of the Blue Ridge.

In June 1993, I heard a large chorus of Hyla chrysoscelis in an alder swamp beside the Blue Ridge Parkway, ca 4.5 km (AL) ESE of Floyd, in Floyd Co. This site, enclosed by the Parkway and Va. Rt. 637, is over a kilometer west of the Blue Ridge crest (watershed), at an elevation of 810 meters. The chorus was active again in late May and early June of 1994. I was unable to obtain specimens from this site but on 10 June 1004, Dr. Carl Gerhardt (who has been investigating this species-complex for several years) made sonograms of a male calling from a site less than a kilometer away, and confirmed my belief that the species is indeed Hyla chrysoscelis.

However, records for “upcountry” populations of Hyla chrysoscelis were made even earlier. In 1991, J.C. Mitchell, K.A. Buhlmann, and M.W. Klemens heard two males calling in a wet site about 5 km south settlement of Meadows of Dan (Patrick Co.), and shortly after midnight on 13 July located a chorus at Mile 183.6 on the Blue Ridge Parkway (also in Patrick Co.). At this site they obtained three males and a female (deposited in the AMNH), both during calling and as road-kills.

I had visited the Floyd county alder swamp on summer nights in 1991 and 1992 for the purpose of light-trapping aquatic insects, without hearing any gray treefrogs. Yet the population must have been there for some time: so many could hardly have migrated in almost simultaneously. On 5 June 1994, I heard for the first time advertising males along the entire length of the Shooting Creed ravine, in south-westernmost Franklin County. This feature is traversed by Va. Rt. 860 on which I have driven several times a month each summer since 1989 without ever hearing gray treefrogs. Most of the calling groups were small, in places very difficult to access, but I was successful in locating several frogs in a seepage area right beside the road, ca. 2 km east of the junction of Va. Rt. 860 with 635 in Floyd County (about a hundred meters west of the Floyd-Franklin county line). Males and a mated pair (VMNH 6549-50) were found in flowing water so shallow it did not cover their bodies, a preposterous breeding site for a frog which normally frequents small bodies of standing water.

During the calling season of 1995, on repeated visits to the sites just described (some of them on virtually the same day as in 1994), not a single call of Hyla chrysoscelis was heard despite generally similar climatic conditions! This was as true for lowland sites (along Va. 40, at 300 m.) as for those along Shooting Creek and in Floyd Co.

I am at a loss to explain these observations, which have occurred over just a few years in a part of the state through which I travel at nearly weekly intervals every summer: always driving slowly and often stopping to operate a UV light trap. It seems unlikely that I would miss hearing calling treefrogs prior to 1993. Yet it is also implausible that simple westward migration up and over the Blue Ridge escarpment (nearly 300 meters vertically in the 8 kilometers from Va. Rt. 40 to the Blue Ridge Parkway) could have been accomplished so quickly, and allow time for a substantial population to build up in the Parkway alder swamp site. Why did I not hear calling along Shooting Creek for several years prior to 1993, nor during 1995?

An almost mercurial spread of Hyla chrysoscelis across the Piedmont of Virginia and over the Blue Ridge strains one’s credulity. If such hop-by-hop spread was not involved, one is forced to the conclusion that the species was well-established in the mountains but simply not calling when I happened to drive past. The negative experience of 1995 (and somewhat similar experiences with hylids elsewhere in Virginia) suggests that species may inhabit a particular locality but be overlooked for years because of some inexplicable suppression of normal male advertisement during the mating season.

Acknowledgements

I am indebted to Drs. Joseph C. Mitchell and Carl Gerhardt for providing information about their finds of Hyla chrysoscelis on the Blue Ridge, and for readings over early drafts of the manuscript.

References

Blair, W.F. 1958. Mating call in the specimen of anuran amphibians. Amer. Nat. 92:27-51

Harper, Francis. 1935. Records of amphibians in the Southeastern

States. Amer. Midl. Nat. 16(3):275-310.

Hoffman, R.L. 1946. The voice of Hyla versicolor in Virginia.

Herpetologica 3:141-142.

Hoffman, R.L. and H.I. Kleinpeter. 1948. Amphibians from Burkes

Garden, Virginia. Amer. Midl. Nat. 39(3):602-607.

Johnson, C. 1959. Genetic incompatibility in the call races of Hyla

versicolor LeConte. Copeia 1959: 327-335.

----, 1963. Additional evidence of sterility between call-types in the

Hyla versicolor complex. Copeia 1963: 139-143.

----, 1966. Species recognition in the Hyla versicolor complex. Texas

J. Sci. 18:361-364.

Mittleman, M.B. 1947. Miscellaneous notes on Indiana amphibians and

Reptiles. Amer Midl. Nat. 38:466-487.

Noble, G.K., and W.G. Hassler, 1936. Three Salienta of geographic

Interest from southern Maryland. Copeia 1936:63-64.

Walker, C.F., 1946. The amphibians of Ohio. Part I. The frogs and toads (order Salienta). Ohio State Mus. Sci. Bull. 1(3): 1-109.

Sweifel, R.G., 1970. Distribution and mating call of the treefrog Hyla

chrysoscelis, at the northeastern edge of its range. Chesapeake Sci. 11:94-97.

Footnote to title

 ”How pleasant it is to cross the mountains”. This was the motto of the Spotswood Expedition, which surmounted the Blue Ridge at Swift Run Gap in 1716, and was struck on a medallion given to each member of the group.

Figure caption

Fig. 1. Map of southeastern Floyd Co. with adjoining parts of Patrick and Franklin counties, showing localities mentioned in the text. (1) alder swamp at jct. Va. Rts. 860 and 637, (2) Shooting Creek, Va. Rt. 860, (3) near junction of Va. Rts. 8 and 714, (4) Meadows of Dan, (5) Mile 183.6, Blue Ridge Parkway.

For most of its length, the Floyd County line follows both the Atlantic-Mississippi drainage divide and the Blue Ridge Parkway closely.