Friday, June 24, 2016

Black-headed Snake, Tantilla melanocephala (Family Colubridae)

Wilson and Mena (1980) state that Tantilla melanocephala has the widest range of any species in the genus, occurring from Guatemala south to southern Peru, Bolivia, northern Argentina, and Uruguay. They also note it inhabits a wide range of habitats. They write, “Tantilla melanocephala is a widely-distributed snake and, as such, shows a wide range of variation in pattern and scale characteristics. What follows is a summary of those characteristics: (1) head cap cream to dark brown, usually pale brown to dark brown; (2) pale nuchal collar complete, divided medially, divided medially and laterally or reduced to two small spots centered on the parietals (usually not complete); (3) dorsal ground color tan to brown; (4) dark subocular blotch completely separating pre- and postocular pale spots or not; (5) lateral extension of dark head cap present or not; (6) brown to dark brown dark nape band present or not; (7) pale neck band following dark nape band present or not, if present, occupying '/2-2 dorsal scale lengths, sometimes reduced to small spots on either side of the dark middorsal stripe; (8) pale lateral band present or not, if present, usually occupying from as little as the upper portion of row 3 to as much as the upper portion of row 3, all of row 4, and the lower portion of row 5, bounded above and/or below with dark pigment or not. Variation in scales may be summarized as follows: prefrontal in contact with second supralabial on one or both sides or not (in contact on both sides in 66.2% of specimens); first pair of infralabials in contact medially or not (in contact in 80.7% of specimens); ventrals in males 125-174, in females 134-177; subcaudals in males 41-92, in females 41-75. Total length in snakes of all ages ranges from 95-486 mm and tail length ranges from 15-140 mm. Relative tail length ranges from 15.8-35.9%. Range. —From near sea level to intermediate elevations along both versants from Guatemala throughout the length of Central America into South America as far south as southern Peru, Bolivia, northern Argentina, and Uruguay; also on the islands of Trinidad and Tobago.”

Writing about variation in ventrals in the West Indies they state, “Trinidad is 18 to 19 kilometres from the nearest point on the mainland of Venezuela in the north and = 13 kilometres in the south (across the delta of the Orinoco River) and Tobago is 28 to 29 kilometres from the nearest point on Trinidad (near Toco). The differences in ventral counts between specimens from Trinidad and adjacent Venezuela, compared to specimens from Trinidad and Tobago, are striking. Mean ventral numbers in Trinidad are lower by approximately 8 to 15 scales than specimens from adjacent Venezuela. On Tobago, ventral numbers again increase markedly over those in Trinidad and are even higher than those from mainland Venezuela. In fact, only specimens from upper Central America have higher ventrals than those of specimens from Tobago. The mean for ventrals for Trinidad males is 20.2 scales lower than that for Tobagan males. Comparable figures for females are not available, but the count for the single Tobagan female is 9 scales above the highest count for Trinidad females. Ventral numbers decrease in the Guianas (material available only from Guyana and Surinam) as compared to adjacent areas in Venezuela, as well as in Brazil, Uruguay and Paraguay, but increase again in Argentina (the pattern in the latter 4 countries is poorly understood due to the paucity of material).”

Thus, it seems highly unlikely that Tantilla melanocephala is a single, widespread species. The ventral counts given by Wilson and Mena for Trinidad (137- 152 in males and 147-159 in females) do not even overlap with their numbers for Tobago (157-160 for males and 168 for a single female).

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