Speciation in Chorus Frogs. I. Reproductive Character Displacement in the Pseudacris Nigrita Complex

Fouquette, M. J., Jr. (Department of Zoology, Arizona State University, Tempe, Arizona 85281) 1975. Speciation in Chorus Frogs. I. Reproductive character displacement in the Pseudacris nigrita complex. Syst. Zool. 24:16-22.-Analysis of mating calls of two cognate species of Pseudacris reveals divergence in two components, pulse rate and pulse number, which in at least one species seems best interpreted as reproductive character displacement. Pulse rate is apparently the critical parameter upon which female discrimination of mating call is based, but pulse number appears to be involved too. [Anura; bioacoustics.] Brown and Wilson (1956) introduced the term "character displacement" to describe the phenomenon where two (usually cognate) species which are similar in allopatry, diverge in one or more characters where their ranges overlap. Grant (1972) redefined and broadened the term; the aspect confined to the original definition he called "divergent character displacement." Such divergence in zones of sympatry may result from a) reinforcement of incipient premating reproductive isolating mechanisms (Blair, 1955), or b) ecological displacement, or c) some other cause. On theoretical grounds both reproductive and ecological character displacement should be expected to occur frequently. However, while reports of the latter are rather abundant, well-documented examples of the former effect are scarce. Among anuran amphibians, mating call is of overriding importance in premating isolation among sympatric species of many groups (Mecham, 1961; Littlejohn, 1969; numerous others). Hence we might.expect to find abundant examples of reproductive character displacement represented in call differences; however, few amphibian studies have produced a compelling case for that effect. The best documented example is in Australian Litoria (= Hyla; Littlejohn, 1965). Other oft-cited examples among Gastrophryne (= Microhyla; Blair, 1955; Brown and Wilson, 1956) and Acris (Blair, 1958) may be criticized as readily explained as a result of other factors, or as supported by insufficient data. Some cognate species-pairs of chorus frogs (Pseudacris) differ markedly in mating call where they occur sympatrically, but in allopatry the calls may be similar, suggesting possible examples of reproductive character displacement. To examine this critically, a study was undertaken of forms comprising the Pseudacris nigrita complex in the southeastern U.S. An important aspect of the study was analysis of mating calls sampled from populations of Pseudacris n. nigrita and Pseudacris triseriata feriarum (nomenclature of Schwartz, 1957), from areas of allopatry and sympatry. The two forms are hereinafter referred to as nigrita and feriarum, respectively. They are discrete biological species and, although morphologically similar, are readily separable (Crenshaw and Blair, 1959). Ranges of the two do not overlap extensively except along the Chattahoochee River drainage (forming the border between Alabama and Georgia), extending southward into western Florida as the Apalachicola River drainage. Broader analysis of the systematics of these forms and of the nature and significance of variation in their calls throughout their ranges will be reported elsewhere, as will similar findings for other species of the complex.