Vocalization as an Isolating Mechanism in Frogs

Abstract

In laboratory tests, female Hyla crucifer selected the call of their own species in preference to that of Pseudacris triseriata, but not in preference to that of P. ornata. Female triseriata selected the call of their own species instead of that of the allopatric leptodactylid Eleutherodactylus portoricensis. On the other hand, only about 8 out of 10 succeeded when given a chance to respond to the calls of sympatric hylids-P. ornata, P. brachyphona and H. crucifer. Local P. triseriata females were unable to discriminate be- tween a call of a member of their own population and that of a frog recorded in New York; however, when given a chance to respond to a call recorded in Manitoba, 8 out of 9 reacted to the locally recorded call. Thus, for some species, vocalization is a major isolating mechanism, for others it is partially effective, and :for still others, it is ineffective. In those species in which vocalization is not a complete isolating mechanism, other factors-such as body size, rather small and subtle differences in behavior, habitat preference and timing-prevent interspecific matings. As is well known, optimum conditions for amphibian reproduc- tion are generally associated with rainfall. At such times, numerous individuals of several species often aggregate in the same pond. In these situations, it is important that interspecific matings be avoided. The latter result in gross biological inefficiency. Special secretions produced by the hedonic glands of males contribute greatly in pre- venting interspecific matings from occurring among most caudate amphibians. Among the Anura, such glands are absent. Numerous workers (Noble, 1931; A. P. Blair, 1941a, 1942; W. F. Blair, 1955, 1956; and many others) have stated that the calls of anurans serve to prevent crossing of the various species utilizing the same breeding site. However, there are no experimental data to verify this conclu- sion. With the recent observations that Pseudacris triseriata under- goes reproductive behavior in the laboratory and that the mating call alone guides a female to a particular male (Martof and Thomp- son, 1958), a method was provided for the testing of frog calls as an isolating mechanism.