This page aims to make available in a more widely accessible format the conclusions of a recent study which were published by Wüster et al. (2001) in the Herpetological Journal.

The study was initiated as a result of finding a freshly road-killed specimen of Drymarchon during fieldwork in the state of Falcón, NW Venezuela. The specimen could not be assigned to one of the previously known taxa of Drymarchon found in Venezuela, which are Drymarchon corais corais and D. c. melanurus. This prompted the authors to initiate a study of the systematics of the South American representatives of the genus.

The principal approach taken was the use of multivariate analysis of morphological (scalation and colour pattern) characters. Basically, these methods assess variation across a broad range of characters simultaneously, instead of getting hung up on individual characters, as is often the case in conventional systematics.

The results (see below) show the existence of three highly distinct taxa of Drymarchon in mainland South America: the populations conventionally assigned to Drymarchon corais corais, those conventionally assigned to D.c. melanurus, and those representing the species of the mysterious road-killed specimen.

In particular, there is no evidence whatsoever of intergrades between the three main clusters. This applies despite the fact that both the melanurus and the corais samples included a number of specimens from NW Venezuela, where their ranges meet. This suggests that there are no natural hybrids between these forms, and that they should therefore be considered to be separate species, Drymarchon corais and Drymarchon melanurus.

The third species, including the mysterious roadkill from Falcon, is as distinct from D. corais and D. melanurus as they are from each other, and differs from them in a number of colour pattern and scalation characters. It was consequently described as a new species:
 

Type locality: near the village of Las Dos Bocas (11°18' N, 69°24' W), in the eastern foothills of the Sierra de San Luís, Municipio Colina, Estado Falcón, Venezuela.

Drymarchon caudomaculatus is readily distinguishable from D. corais and D. melanurus in a number of characters.

Dorsal coloration. Drymarchon caudomaculatus retains the same ground colour along the entire length of its body. Especially on the posterior part of the dorsum, there are isolated, cream-coloured scales. In D. corais, the anterior part of the body  is darker (often black) than the posterior body and tail. The opposite applies in D. melanurus. Both the latter lack isolated, cream-coloured scales on the dorsum.

Ventral coloration. In D. caudomaculatus, the throat and anterior venter is extensively suffused with dark pigment, whereas the remainder of the ventral side is creamish white. In D. corais, the venter is normally uniformly light along the entire length of the animal, whereas D. melanurus has a light anterior and dark posterior venter.

Tail coloration. In D. caudomaculatus, the tail is covered in individual scales that are creamy-white, contrasting strongly with other scales that retain the brown dorsal colour. This results in a strongly mottled effect. There is no such effect in D. corais, which has a uniformly yellow tail, or in D. melanurus, which has a black tail. The underside of the tail of D. caudomaculatus is creamy-white, but with isolated dark brown subcaudal scales.
 
 

The results of the study of Wüster et al. have implications for the classification of the entire genus. Since the entire genus requires a full systematic revision, we can only offer suggestions based on the available data.

In the case of the Florida indigo snake, Collins (1991) proposed that this should be recognised as a separate species, D. couperi, on account of consistent differences in head scalation. We agree with this arrangement for the time being.

The taxon margaritae displays a unique combination of pattern characters, including a dorsal colour change similar to that of D. corais (dark anteriorly, paler markings posteriorly), but a ventral pattern change more akin to that of D. melanurus (darker posteriorly, compared to uniformly pale throughout in D. corais). The only known specimen has 76 subcaudal pairs, compared to a documented maximum of 64.5 in D. caudomaculatus. In the light of these clear, categorical differences, and pending further studies, we consider this form to be a separate species, D. margaritae, as originally proposed by Roze (1959).

The fact that D. melanurus is a separate species from D. corais raises the question of the status of the Central and North American indigo snakes. There is absolutely no evidence whatsoever associating any of them with the South American D. corais. Smith (1941) noted the existence of hybrid zones between melanurus on one hand and the taxa erebennus, orizabensis, and rubidus on the other. Additionally, juveniles and some adults of erebennus, rubidus, and unicolor display oblique dark bars on the sides of the neck, which appear to be homologous with the marks seen in D. melanurus. This makes the classification of the taxa erebennus, rubidus, unicolor, and, presumably, orizabensis, as subspecies of D. melanurus a tenable provisional arrangement.

The recognised taxa of Drymarchon are thus as follows:

- Drymarchon caudomaculatus
- Drymarchon corais
- Drymarchon couperi
- Drymarchon margaritae
- Drymarchon melanurus, with the following subspecies:
        D. m. melanurus
        D. m. erebennus
        D. m. orizabensis
        D. m. rubidus
        D. m. unicolor

The systematics of this genus require extensive further revision, especially in order to clarify the status of the various Central American/Mexican taxa. It would not be surprising if future studies found some of these to represent further separate species.
 

COLLINS, J.T. (1991). Viewpoint: a new taxonomic arrangement for some North American amphibians and reptiles. Herpetological Review 22, 42-43.

ROZE, J.A. (1959). Una nueva especie del género Drymarchon (Serpientes: Colubridae) de la Isla de Margarita, Venezuela. Novedades Científicas - Contribuciones Ocasionales del Museo de Historia Natural La Salle 25, 1-4.

SMITH, H.M. (1941). A review of the subspecies of the indigo snake (Drymarchon corais). Journal of the Washington Academy of Science 31, 466-481.

WÜSTER, W., J.L. YRAUSQUIN & A. MIJARES-URRUTIA. (2001***). A new species of indigo snake from northwestern Venezuela (Serpentes: Colubridae: Drymarchon). Herpetological Journal, 11: 157-165. Adobe Acrobat file available here

*** the journal issue concerned actually appeared in March 2002
 

Head of living specimen. Note light internasals and anterior prefrontals, typical of this species.

Side view of head of holotype of Drymarchon caudomaculatus.
Note very small upper anterior temporal scale, which fails to contact the posterior temporals.
This is a common condition in this species, but rare in other Drymarchon.

Ventral side of holotype of D. caudomaculatus. Note the extensive dark mottling under the throat,
whereas the rest of the venter is light with occasional dark scales.  Also, note occasional dark subcaudals.
All these are characteristic elements of the colour pattern of this species.

Type locality of D. caudomaculatus, in Estado Falcón, Venezuela.
The holotype was found DOR in the middle foreground, in an area of dry woodland.

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Last modified 27 February 2003.