Contributions to Zoology, 77 (1) – 2008Johan van Rooijen; Gernot Vogel: An investigation into the taxonomy of Dendrelaphis tristis (Daudin, 1803): revalidation of Dipsas schokari (Kuhl, 1820) (Serpentes, Colubridae)

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Statistical analyses


Fig. 1. Ordination of species A and species B along the first two principal components, based on a PCA of the characters VENT, SUBC, TAIL and EYED.

TwoStep Cluster analysis split the sample into two clusters and validated the initial assignment of specimens to the two species (henceforth: species A and species B). The results of a PCA are shown in Figure 1. A plot of the number of ventral scales against eye-diameter is shown in Figure 2. Both figures demonstrate a clear separation between the two species.


Fig. 2. Plot of the number of ventral scales against the eye-diameter for species A and B.

The most prominent results of univariate confirmatory analyses are summarized in Table 2. Species A has a higher number of ventral scales, a higher average number of subcaudal scales and a smaller average eye-diameter. The relation between SVL and eye-diameter for the two species is depicted in Figure 3. With regard to coloration, a conspicuous character of species A is a bright interparietal spot which is absent in species B. However, several specimens constituted exceptions to this rule. In one specimen of species A (from Bombay, India) and two specimens of species B (from Sri Lanka), an interparietal spot was present but in a rudimentary (i.e. barely visible) form. Finally, species A has a substantially shorter vertebral stripe than species B.


Table 2. Descriptive statistics with regard to the most diagnostic characters of species A and B. Eye-diameter is SVL-adjusted (see Materials and Methods). Significance-levels are provided in the last column.

Sexual dimorphism in the diagnostic characters was not detected in either species, a finding that is largely in line with results regarding several congeneric species (How et al., 1996; Vogel and Van Rooijen, 2007; Van Rooijen and Vogel, 2008).


Fig. 3. Eye-diameter relative to SVL in species A and B.

Eye-diameter, number of ventrals and number of subcaudals were subjected to analysis of geographical variation. This analysis was restricted to OTU’s from which both species had been sampled, thus excluding OTU 3 and OTU 4. Only the number of ventral scales was found to exhibit significant geographic variation (P = 0.01), although this variation is negligible in the context of interspecific differences (P < 0.000001). With regard to this character, the two species were found to differ slightly in their patterns of geographic variation based on a significant interaction OTU x species (P = 0.02).