PHYSIOLOGICAL EVIDENCE FOR NATURAL HYBRIDIZATION OFBUFO AMERICANUSANDBUFO FOWLERI

Abstract

Members of the two toad species, B4fo aniericanus and Bufo fowleri,2 apparently hybridize in areas where their ranges are in contact or overlap. Although naturalists have been aware of the existence of hybrids since the turn of the century, little intensive study of hybridization in nature has been undertaken until recent years. Hybridization was assumed by early workers (Miller and Chapin, 1910; Hubbs, 1918; Myers, 1927; Pickens, 1927; and others) largely from the presence of morphological intermediates in areas of intergradation. Blair (1941, 1942) confirmed earlier suggestions by demonstrating that laboratory crosses between americanus and fozuler-i give rise to viable hybrids, which resemble intermediate individuals collected in the field. The inference was reasonably made that the intermediate specimens found in nature owe their origin to interspecific crosses. Offspring of several experimental crosses were reared through metamorphosis, and male fowleriamericanus were backcrossed to femiiale americanus and found fertile. Even though the hybrids were viable and apparently fertile, one cannot rule out the possibility that such hybrids are at a selective disadvantage to non-lhybrids in nattire. The relative survival valtue of the hybrids and non-hybrids can only be assessed 1 y colmlbilning experilmlenital investigations ancl field observations on alleged hybrid an(l parental populations. Uniless characters involving somiie aspect of viability or plhysiological efficiency are employed, little selective advantage can be attached to any of the morphological types encountered in the field. Studies by Moore (1949a, b and earlier papers) of various species of the genus Rana have indicated that a number of emnbryonic characters, such as rate of development and temperature tolerance, are closely correlated with geographical distribution of the species and represent adaptations on the part of each species to environmental temperature during breeding. These physiological characters are highly speciesspecific. In a series of experiments designed to reveal the adaptive natture ol emblryological characters in several species of toads, it has been deimonstrated (Volpe, in preparation) that these characters are of value as a species-differential between amiltericanus and fowleri. Consequently, these physiological characters would furnish a useful criterion of hybridization, as well as reveal any adaptive differences between the hybrid and putative parents. With this view in miind, an investigation was undertaken along three lines. First, to explore the possibility of regional differentiation within each species, individuals from several populations of each species along the eastern coast were studied experimentally. This was necessary to avoid attributing an expression of geographical variation to hybridization. The second phase of the investigation involved laboratory crosses between the two species anid observations oln the rate of develop1 This work was completed durinig teniure as a University Fellow (1951-1952), Columbia University, New York. 2 Considerable confusion exists in the classification of toads. The two toad groups under consideration are classically treated as species. Recently, Netting and Goin (1946) have listed the American toad as Bitfo terrestris avi-ericanus, and Fowler's toad is widely referred to as Bufo woodhousi fozuleri.