Amphibian & Reptile Conservation



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048

Amphib. Reptile Conserv. | http://redlist-ARC.org

June 2013 | Volume 7 | Number 1 | e63

Cantils (genus Agkistrodon) are some of the most feared snakes in Mesoamerica, as their bite and powerful venom have caused 

numerous human fatalities. Equipped with a large and strikingly-marked head, a stout body, and a nervous attitude that often is mis-

taken for aggression, these snakes usually are killed on sight. Cantils primarily are found in tropical forests that undergo a prolonged 

dry season, but occasionally inhabit savannas and areas that flood seasonally after heavy rains. Pictured here is a cantil from Parque 

Nacional Santa Rosa, in northwestern Costa Rica. Photo by Louis W. Porras.



 

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Amphib. Reptile Conserv. | http://redlist-ARC.org



June 2013 | Volume 7 | Number 1 | e63

Amphibian & Reptile Conservation 7(1): 48–73.

A taxonomic reevaluation and conservation assessment of 

the common cantil, 

Agkistrodon bilineatus (Squamata: 

Viperidae): a race against time

1

Louis W. Porras, 

2

Larry David Wilson, 

3,4

Gordon W. Schuett,

and 

4

Randall S. Reiserer

1

7705 Wyatt Earp Avenue, Eagle Mountain, Utah, 84005, USA 

2

Centro Zamorano de Biodiversidad, Escuela Agrícola Panamericana Zamorano, Francisco 

Morazán, HONDURAS; 16010 S.W. 207th Avenue, Miami, Florida, 33187, USA 

3

Department of Biology and Center for Behavioral Neuroscience, 



Georgia State University, Atlanta, Georgia, 30303, USA 

4

The Copperhead Institute, P.O. Box 6755, Spartanburg, South Carolina 29304, USA



Abstract.—Several lines of evidence suggest that numerous populations of cantils (Agkistrodon bi-

lineatusA. taylori), New World pitvipers with a distribution in Mesoamerica, are in rapid decline. We 

examined the IUCN conservation status for 

A. bilineatus, assessed for the entire range of the spe-

cies, as well as the Environmental Vulnerability Scores (EVS) provided for certain countries along 

its distribution. Because of pronounced disparities in these conservation assessments and notable 

phenotypic differences that coincide with the geographic distribution of certain cantil populations, 

we conduct a taxonomic reassessment of the common cantil, 

Agkistrodon bilineatus (Günther 

1863), to determine if the recognized subspecies of 

A. bilineatus merit specific status. Based on 

our morphological assessment, biogeographical evidence, and the results of previous DNA-based 

studies, we elevate the three previously recognized subspecies of 

A. bilineatus to full species (A. 

bilineatusA. russeolus, and A. howardgloydi). Given this taxonomic reassessment, we examine the 

conservation status of the newly elevated taxa, suggest avenues for future studies within this com-

plex of pitvipers, and provide conservation recommendations.

Key words. Character evolution, evolutionary species, Mesoamerica, subspecies concept 

Resumen.—Varias líneas de evidencia sugieren que numerosas poblaciones de cantiles (Agkistrodon 

bilineatusA. taylori), víboras de foseta del Nuevo Mundo con una distribución en Mesoamérica, es-

tán en rápido declive. Examinamos los resultados sobre el estado de conservación propuestos por 

la UICN para 

A. bilineatus, que fueron evaluados para la distribución total de la especie, así como 

los resultados de los Índices de Vulnerabilidad Ambiental (en inglés, Environmental Vulnerability 

Scores [EVS]) que fueron determinados para esta especie en algunos países a lo largo de su distri-

bución. Por haber disparidades pronunciadas en estas evaluaciones de conservación y diferencias 

fenotípicas notables que coinciden con la distribución geográfica de ciertas poblaciones de can-

tiles, en este trabajo realizamos una reevaluación taxonómica del cantil común, 

Agkistrodon biline-

atus (Günther 1863), para determinar si las subespecies reconocidas de A. bilineatus merecen el 

estatus de especie. Basado en nuestro análisis morfológico, evidencia biogeográfica y los resulta-

dos de anteriores estudios basados en ADN, elevamos las tres subespecies de 

A. bilineatus previa-

mente reconocidas al nivel de especie (

A. bilineatusA. russeolus y A. howardgloydi). Tomando en 

cuenta esta nueva evaluación taxonómica, examinamos el estado de conservación de los taxones 

aquí elevados, hacemos sugerencias para estudios futuros dentro de este complejo de víboras de 

foseta y ofrecemos recomendaciones para su conservación.

Palabras claves. Evolución de caracteres, especies evolutivas, Mesoamérica, concepto de subespecies 

Citation: Porras LW, Wilson LD, Schuett GW, Reiserer RS. 2013. A taxonomic reevaluation and conservation assessment of the common cantil, 



Agkistrodon bilineatus (Squamata: Viperidae): a race against time. Amphibian & Reptile Conservation 7(1): 48–73 (e63).

Correspondence. 

Emails: 

1

empub@msn.com (Corresponding author) 

2

bufodoc@aol.com 

3,4


gwschuett@yahoo.com

4

rreiserer@gmail.com

Copyright: © 2013 Porras et al. This is an open-access article distributed under the terms of the Creative Commons 

Attribution–NonCommercial–NoDerivs 3.0 Unported License, which permits unrestricted use for non-commer-

cial and education purposes only provided the original author and source are credited.


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June 2013 | Volume 7 | Number 1 | e63

Porras et al.



Although the restoration of tropical dry forest is still pos-

sible, humanity will not give the globe back to its wild-

land denizens, and old-growth tropical dry forest will 

never again cover large areas.

Janzen 2004: 80.



Introduction 

The  common  cantil  (Agkistrodon bilineatus) is a poly-

typic species of North American pitviper with a variably 

fragmented distribution extending from extreme south-

western Chihuahua and southern Sonora, Mexico, to 

northwestern Costa Rica, on the Pacific versant, and parts 

of  the Yucatan  Peninsula,  northern  Belize,  Guatemala, 

and extreme western Honduras on the Atlantic versant; 

it also occurs in Las Islas Marías, an archipelago of four 

islands located about 100 km west of the state of Nayarit, 

Mexico (Gloyd and Conant 1990; Campbell and Lamar 

2004; Lemos-Espinal and Smith 2007; Babb and Dugan 

2008;  García-Grajales  and  Buenorostro-Silva  2011; 

McCranie 2011). With few exceptions, the dominant 

vegetation zones occupied by A. bilineatus are dry for-

est, deciduous forest, thorn scrub, and savanna, primarily 

areas of low relief that have been exploited heavily for 

irrigated agriculture and where this species mostly has 

become a rare snake; the elevational range of A. bilinea-

tus extends from near sea level to about 1,500 m (Gloyd 

and Conant 1990; Conant 1992). Along the Pacific coast 

of Mesoamerica, tropical dry forests were reported as the 

most endangered of the major tropical ecosystems, with 

only 0.09% of that region afforded official conservation 

status (Janzen 1988). A quarter of a century after Janzen’s 

elucidative paper, aside from protected areas, dry forests 

throughout this region have continued to deteriorate.

In a monographic study of the Agkistrodon complex, 

Gloyd and Conant (1990) provided an extensive review 

of the cantils, including information on their taxonomy, 

morphology, distribution, and aspects of their natural 

history. Based on multiple lines of evidence, Parkinson 

et al. (2002) conducted a phylogeographic analysis of 

the cantils and elevated A. b. taylori to the rank of full 

species, emphasizing that the loss of forested areas in 

the habitat of this species underscored the need for its 

conservation. More recently, Wilson et al. (2010) com-

piled an extensive conservation assessment for the en-

tire Mesoamerican herpetofauna, in which numerous 

authorities provided information on the status of can-

tils. Although the methodological approaches of these 

authors varied, it was clear from the outcome that the 

conservation status of A. bilineatus showed dramatic 

differences when analyzed on a country by country or 

regional basis, since the reported or estimated IUCN 

rankings for this species extended the gamut from Least 

Concern to Critically Endangered (Lavin-Murcio and 

Lazcano 2010; Sasa et al. 2010). Some authors also 

provided Environmental Vulnerability Scores (EVS; a 

conservation measure developed and used by Wilson and 

McCranie 1992, 2004, and McCranie and Wilson 2002) 

for certain countries, and their results were more infor-

mative. This measure provides a rough gauge of the theo-

retical degree that herptofaunal species are vulnerable to 

environmental degradation; the scores at the upper end 

of the scale (ranging from 14 to 20) indicate a greater de-

gree of concern (Wilson et al. 2013), and the EVS for A. 



bilineatus was reported as 15 for Honduras, Nicaragua, 

and Costa Rica, and as 16 for Belize (Sasa et al. 2010; 

Stafford et al. 2010; Sunyer and Köhler 2010; Townsend 

and Wilson 2010).

Based  on  our  field  experiences,  recent  discussions 

with several colleagues working in regions where cantils 

occur, and information from the published literature, we 

echo the statements of several of the aforementioned au-

thorities that in many regions A. bilineatus has declined 

significantly, largely as a result of human activities.

Our principal goal in this paper is to reexamine the 

conservation status of A. bilineatus, inasmuch as the 

available information suggests that certain populations 

are declining or imperiled. In conservation biology the 

threat status of an organism typically is evaluated at the 

species level, so first we reevaluate the taxonomic status 

of the three subspecies of A. bilineatus (bilineatusrus-

seolus, and howardgloydi) to determine if any (or all) of 

them shows sufficient lineage divergence to warrant full 

species recognition. Accordingly, our conservation as-

sessment develops from our taxonomic conclusions.



Morphological Assessment

Gloyd  and  Conant  (1990)  and  Campbell  and  Lamar 

(2004) provided an extensive amount of biological in-

formation on cantils, including excellent drawings of the 

scalation and pattern of the relevant taxa discussed in 

this paper, so we relied largely on these sources for our 

morphological assessment. Unlike previous views (see 

Gloyd and Conant 1990), the genus Agkistrodon now is 

restricted to the New World (see Molecular Assessment).

As in other pitviper genera, Agkistrodon (sensu 

stricto) is characterized by the presence of a deep fa-

cial pit, a vertically elliptical pupil, a large venom 

gland in the temporal region, and a canaliculated fang 

on the maxilla followed by a series of smaller replace-

ment fangs. In Agkistrodon, however, the scales on the 

crown generally are large and plate-like, although often 

they are fragmented or contain partial sutures, and the 

skull is relatively broad and equipped with short fangs. 

Other characters include a pronounced canthus rostra-

lis, the presence of a loreal scale in all members except 

A. piscivorus, a robust (or relatively robust) body, and a 

moderate to long tail. Scale characters such as supralabi-

als, infralabials, and dorsal scale rows at midbody show 

little variation among the species, although the last of 

these characters is slightly higher in A. piscivorus. The 


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Amphib. Reptile Conserv. | http://redlist-ARC.org

June 2013 | Volume 7 | Number 1 | e63

number of ventral scales is lower in A. bilineatus and A. 



taylori than in A. contortrix and A. piscivorus, and the 

number of subcaudals is slightly lower in the latter two 

species. In Agkistrodon, some or most of the subcaudal 

scales are divided, and the terminal spine on the tail tip 

is turned downward in all the taxa except A. piscivorus

Moderate hemipenial differences have been reported 

among the taxa, but the similarities are more pronounced 

when comparing A. contortrix and A. piscivorus to A. bi-



lineatus and A. taylori (Gloyd and Conant 1990; Malnate 

1990). The tail tip of neonates and juveniles of all spe-

cies of Agkistrodon is brightly colored and typically is 

yellow,  white,  or  pink  (Gloyd  and  Conant  1990).  The 

coloration of the tail tip changes as animals mature, to a 

faded yellow, green, gray, black, or sometimes to match 

the color of the dorsum. Young individuals often use their 

tail to lure prey (e.g., anurans, lizards) by way of vertical 

undulations and waving, a behavior termed “caudal lur-

ing” (reviewed by Strimple 1988, 1992; Carpenter and 

Gillingham 1990).

1. The cantils

Commonly known as cantils, A. bilineatus and A. taylori 

are thick-bodied pitvipers (Serpentes: Viperidae) with a 

large head and a moderately long and slender tail, and 

their maximum total lengths are similar. As in the other 

species of Agkistrodon, the scale characters of cantils 

only show a moderately low range of variation (Table 1).

A wide range of color pattern variation is evident in 



Agkistrodon, and these characters were used to diag-

nose the three subspecies of A. bilineatus  (Burger  and 

Robertson 1951; Gloyd 1972; Conant 1984), as well to 

elevate A. taylori to the rank of full species (Parkinson 

et al. 2000). The coloration of the head is distinctive, as 

cantils  are  adorned  with  five  conspicuous  pale  stripes, 

one vertically on the front of the snout and two later-

ally on each side of the head. The dorsal color pattern 

consists of crossbands, at least in juveniles, and this char-

acter shows a notable degree of geographic and ontoge-

netic variation. The chin color and ventral coloration also 

demonstrate considerable geographic variation.



2. Color and pattern characteristics of the 

ornate cantil

Among the cantils, the color pattern of A. taylori is the 

most vivid (Fig.1). The lower facial stripe is broad and 

extends to cover the lower edge of the supralabials, the 

dorsal pattern is composed of pronounced black cross-

bands separated by gray or pale brown areas that often 

contain yellowish brown or orange, the chin is patterned 

with bold markings with wide white or yellow elements, 

and the venter contains dark gray or black markings 

Fig. 1. Adult female Agkistrodon taylori from Aldamas, Tamaulipas, Mexico. The ornate cantil often is vividly marked. 

Photo by Tim Burkhardt.

Taxonomy and conservation of the common cantil



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Amphib. Reptile Conserv. | http://redlist-ARC.org

June 2013 | Volume 7 | Number 1 | e63

arranged in a somewhat checkerboard pattern. In contrast 

to  juveniles,  adults  exhibit  a  subdued  pattern  that  con-

tains brighter colors, but older individuals of both sexes 

tend to become melanistic, and sexual color dimorphism 

is present in all age classes (Burchfield 1982). The tail tip 

of young individuals has been reported as sulphur yel-

low, ivory white, or salmon pink (Burchfield 1982; Gloyd 

and Conant 1990); the tail tip of most young individuals, 

however, is sulphur yellow (LWP, GWS, pers. observ.; 

Fig. 2).

3. Color and pattern characteristics of the  

common cantil

In A. b. bilineatus, both the upper and lower facial stripes 

are relatively broad, and the lower stripe is continuous 

and bordered below by dark pigment along the mouth 

line. From a frontal view, the vertical stripe along the 

rostral and mental and the lateral head stripes often meet 

on the tip of the snout. In adults, the dorsal ground color 

ranges from very dark brown to black, and if crossbands 

are  present  often  they  are  difficult  to  distinguish.  The 

dorsal pattern consists of small white spots or streaks. 

The chin and throat are dark brown or black with a pat-

tern of narrow white lines or markings, and the venter is 

dark brown or black with pale markings. The coloration 

of neonates and juveniles is some shade of brown, and 

consists of brown or chestnut crossbands separated by a 

paler ground color, with the lateral edges of the cross-

bands flecked with white. The crossbands gradually fade 

with maturity, however, as the overall dorsal coloration 

darkens (Fig. 3). The tail tip of neonates and juveniles 

has been reported in numerous publications as bright yel-

low (e.g., Allen 1949; Gloyd and Conant 1990). Sexual 

color dimorphism has not been reported in any age class.

In  A. b. russeolus, the upper facial stripe is narrow 

and sometimes is intermittent posterior to the eye, and 

the lower stripe is broader and continuous and separated 

from the commissure by a band of dark pigment. From a 

frontal view, the vertical stripe along the rostral and men-

tal and the two upper lateral head stripes typically meet 

on the tip of the snout. The dorsal ground color of adults 

generally is pale reddish brown, and the pattern consists 

of broad, deep reddish brown to brown crossbands that 

are separated dorsally by areas of paler coloration, and 

often are edged irregularly with white. The crossbands 

remain apparent, even in older adults. Laterally, the cen-

ters of the crossbands are paler and usually contain one 

or two dark spots. The pattern on the chin and throat of-

ten is reduced, with small whitish spots or lines present 

on a darker background. Approximately the median third 

of the venter lacks a pattern or contains a few markings. 

The  coloration  of  a  neonate  (150  to  175  mm TL)  col-

lected near Mérida, Yucatán, was described from life 

by Howard K. Gloyd (Gloyd and Conant 1990: 83) as 

showing a velvety appearance, and its pattern consisted 

of rich chestnut-brown crossbands with rufous brown 

interspaces, which were edged with blackish brown and 

interrupted lines of white, “and the tip of the tail gray.” 

A neonate from Dzibilchaltún, Yucatán, showed a similar 

coloration except that the banding was edged intermit-

tently only with white, and the tail tip was pale gray with 

faint white banding (Fig. 4). This individual was main-

tained in captivity and by the time it had grown to a total 

Character

A. b. bilineatus

A. b. russeolus

A. b. howardgloydi

A. taylori

Total length

1,090 mm


1,050 mm*

960 mm


960 mm

Ventrals

127–143 (134.5)

131–141 (136.1)

128–135 (131.1)

127–138 (133.7)

Subcaudals

52–71 (61.6)

46–62 (55.4)

54–61 (58.8)

40–56 (48.3)

Supralabials

5–9 (8.1)

8–9 (8.0)

7–9 (8.0)

7–9 (8.0)

Infralabials

9–13 (10.7)

8–12 (10.8)

9–12 (10.9)

9–12 (10.4)

Dorsal scale rows 

(midbody)

21–25 (22.9)

23–25 (23.1)

23–25 (23.4)

21–23 (22.9)

Table 1. Maximum total length and selected scale characters in the three subspecies of Agkistrodon bilineatus and in A. taylori

Min-max values are followed by the mean (in parentheses). Data derived from Gloyd and Conant (1990).

*Specimen with an incomplete tail.

Fig. 2. Neonate female Agkistrodon taylori born in captivity 

from adults collected in the state of Tamaulipas, Mexico. Sexual 

color pattern dimorphism is evident in all age classes, except in 

very old individuals that sometimes darken with age. In young 

males, the rhombs on the dorsum tend to form bands and the 

interstitial pattern is reduced. Photo by Breck Bartholomew.

Porras et al.


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Amphib. Reptile Conserv. | http://redlist-ARC.org

June 2013 | Volume 7 | Number 1 | e63

Fig. 3. Young adult Agkistrodon b. bilineatus from Apatzingán, Michoacan, Mexico, at an elevation of 330 m. Adult individuals 

from much of the west coast of Mexico often lose the dorsal banding (see cover of this issue). Photo by Javier Alvarado.





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