Mating Call in the Speciation of Anuran Amphibians

Analysis of the mating call is useful for the study of evolutionary problems in anuran amphibians because characteristics of the call are chiefly related to the functions of attracting a mate and of species identification in breeding congresses. Use of the sound spectrograph in recent years has made possible the measurement and objective comparison of calls. Variations in call occur among individuals in local populations and between the calls of the same individual under different conditions. Frequency and repetition rate tend to increase with increased environmental temperature and to decrease with increased body size. The call may vary in relation to sexual excitement. The variation in call of a single Hyla versicolor recorded on nine different nights and the variations among 17 Pseudacris streckeri recorded from a breeding population on a single night are discussed. Variations not explainable in terms of environmental or ontogenetic factors or degree of sexual excitement are deemed to have a genetic basis. Patterns of geographic variation in call are discernible in species which have been investigated. In Pseudacris nigrita, on present information the call races tend to correspond to subspecies described on morphological bases. In Hyla versicolor, the relationships of three call races seem best explained by the hypothesis of secondary interbreeding after Pleistocene separation of three segments of the population. Each species of anuran has a call that differs from the call of every other species. Groups of species within a genus, however, show basic similarities in call structure, which, with other evidence, are indicative of evolutionary relationship. Species groups in the genera Scaphiopus, Bufo, and Hyla of the United States are suggested on the basis of call structure and other evidence. Various allopatric and sympatric species pairs are discussed in relation to their bearing on the question of purely geographic speciation versus geographic separation with subsequent reinforcement of isolation mechanisms. Some allopatric species (Pseudacris streckeri-P. ornata, Scaphiopus holbrooki-S. hurteri) show little differentiation in call. Others (Hyla femoralis-H. arenicolor and various members of the Bufo americanus species group) show strong differentiation. Some allopatric species show as much differentiation in call as do sympatric species. Among sympatric species, Acris gryllus and A. crepitans, along with previously reported species pairs, show reinforcement of call differences where they occur together. Some sympatric species pairs (Bufo compactilis-B. cognatus, Hyla cinerea-H. gratiosa, Hyla versicolor-H. phaeocrypta) have strongly differentiated calls but show no evidence of reinforcement where they occur together. The evidence from the allopatric and sympatric species pairs is interpreted as indicating that speciation has occurred either through the evolution of effective isolation mechanisms under geographical isolation or through reinforcement of partially effective mechanisms after ranges have become sympatric. The mating call is weak or lacking in some anuran species. All instances of this in the United States are found in far western species of Bufo, Rana and Ascaphus and in which the identification function of call may be relatively unimportant because of the scarcity of species there. Natural hybridization sometimes occurs between species with well differentiated calls. All well authenticated natural hybrids among U. S. toads involve crosses between Bufo woodhousei with representatives of the B. americanus group or with B. valliceps. This indicates that call differences are not necessarily sufficient to prevent interspecific hybridization when other isolation mechanisms are weakened or eliminated. Hybrids are also known in Scaphiopus. The calls of hybrids between closely related species are intermediate in character between those of the parental species. The calls of hybrids between two distantly related species (Bufo woodhousei and B. valliceps) are imperfect and tend to resemble the calls of the male (B. valliceps) parent.