Sexual dimorphism in two species of Sibynomorphus (Squamata, Dipsadidae) from Brazil

Tatiane Parnazio, Davor Vrcibradic


Snakes usually present sexual dimorphism in both morphology and pholidosis. Members of the Neotropical tribe Dipsadini (Dipsadidae) commonly have unusual patterns of sexual dimorphism regarding the numbers of ventral scales (higher in males or non-dimorphic). The northern (trans-Andean) species of Sibynomorphus apparently deviate from the typical dipsadine pattern, but there is little data currently available on the southern (cis-Andean) members of the genus. We tested for differences in some morphological and meristic characters between males and females of two cis-Andean species of Sibynomorphus [S. mikanii (Schlegel, 1837) and S. neuwiedi (Ihering, 1911)]. Preserved specimens from Brazilian institutional collections were analyzed with respect to snout-vent length (SVL), relative length of the tail and ventral and subcaudal scale counts. Both species presented patterns of sexual dimorphism typical of most snakes with regard to SVL (larger in females), relative length of the tail and subcaudal scale counts (greater in males). Regarding ventral counts, however, S. neuwiedi showed no sexual dimorphism (as is common among Dipsadini), whereas in S. mikanii females had a significantly greater number of ventrals than males (as in most snakes, including trans-Andean Sibynomorphus).

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