Experimental and Natural Hybridization between Bufo terrestris and Bufo fowleri

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In analysing populations in which interspecific hybridization is suspected, the systematist seeks out the similarities and dissimilarities between the parental species. The alleged hybrids are then evaluated in terms of the differential characters of the two species. Determining which and how many of the character differences are to be selected as criteria of hybridization constitutes the critical phase of the procedure. Two species may differ so widely that individuals of one group are readily separable from the other. The differences may relate to the presence or absence of several qualitative characters. Some of the quantitative characters may display no or little overlapping of values when the measurements are plotted. In this case, a index method (Anderson, 1936, 1949; Hubbs and Hubbs, 1943) may be employed and it may be safely inferred that individuals with intermediate values represent products of hybridization. In other cases, the differences between two species may be such that there is considerable overlap when the values for any single character are plotted. Bufo terrestris and Bufo fowleri are two such entities. Few of the morphological differences between them are absolute, in the sense that one species possesses a characteristic that the other entirely lacks. A unique morphological feature of B. terrestris is the knob-like swellings on the posterior ends of the interorbital cranial crests. For the most part, however, the differences between B. terrestris and B. fowleri are ones of degrees. The two toad species differ in the size and distribution of warts, the extent of ventral spotting, the shape and coloration of the parotoid glands, the configuration of the snout, and other variable quantitative traits. These characters overlap broadly. The paucity of well-defined morphological differentials between the two species renders difficult the detection of hybridization in sympatric areas. Convincing data requires the demonstration that experimental hybrids between the two species resemble individuals collected in mixed breeding aggregations. The data reported herein deal with the success attained in reconstructing experimentally the suspected natural hybridization between B. terrestris and B. fowleri in southeastern Louisiana and adjacent Mississippi. Controlled cross-breeding experiments revealed the morphological features of hybrid individuals, as well as provided the