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Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
074
Dr. Daniel D. Beck (right) with Martin Villa at the Centro Ecologia de Sonora, in Hermosillo, Mexico. Dr. Beck is holding a near-
record length Río Fuerte beaded lizard (Heloderma horridum exasperatum). Photo by Thomas Wiewandt.
075
Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
Amphibian & Reptile Conservation 7(1): 74–96.
Taxonomic reassessment and conservation status
of the beaded lizard, Heloderma horridum
(Squamata: Helodermatidae)
1Randall S. Reiserer, 1,2Gordon W. Schuett, and 3Daniel D. Beck
1The Copperhead Institute, P. O. Box 6755, Spartanburg, South Carolina 29304, USA 2Department of Biology and Center for Behavioral Neuro-
science, Georgia State University, 33 Gilmer Street, SE, Unit 8, Atlanta, Georgia, 30303-3088, USA 3Department of Biological Sciences, Central
Washington University, Ellensburg, Washington 98926, USA
Abstract.—The beaded lizard (Heloderma horridum) and Gila monster (H. suspectum) are large,
highly venomous, anguimorph lizards threatened by human persecution, habitat loss and degrada-
tion, and climate change. A recent DNA-based phylogenetic analysis of helodermatids (Douglas et
al. 2010. Molecular Phylogenetics and Evolution 55: 153–167) suggests that the current infraspecific
taxonomy (subspecies) of beaded lizards underestimates their biodiversity, and that species status
for the various subspecies is warranted. Those authors discussed “conservation phylogenetics,”
which incorporates historical genetics in conservation decisions. Here, we reassess the taxonomy
of beaded lizards utilizing the abovementioned molecular analysis, and incorporate morphology by
performing a character mapping analysis. Furthermore, utilizing fossil-calibrated sequence diver-
gence results, we explore beaded lizard diversification against a backdrop of the origin, diversifica-
tion, and expansion of seasonally dry tropical forests (SDTFs) in Mexico and Guatemala. These for-
ests are the primary biomes occupied by beaded lizards, and in Mesoamerica most are considered
threatened, endangered, or extirpated. Pair-wise net sequence divergence (%) values were greatest
between H. h. charlesbogerti and H. h. exasperatum (9.8%), and least between H. h. alvarezi and H. h.
charlesbogerti (1%). The former clade represents populations that are widely separated in distribu-
tion (eastern Guatemala vs. southern Sonora, Mexico), whereas in the latter clade the populations
are much closer (eastern Guatemala vs. Chiapas, Mexico). The nominate subspecies (Heloderma h.
horridum) differed from the other subspecies of H. horridum at 5.4% to 7.1%. After diverging from a
most-recent common ancestor ~35 mya in the Late Eocene, subsequent diversification (cladogen-
esis) of beaded lizards occurred during the late Miocene (9.71 mya), followed by a lengthy stasis of
up to 5 my, and further cladogenesis extended into the Pliocene and Pleistocene. In both beaded
lizards and SDTFs, the tempo of evolution and diversification was uneven, and their current distribu-
tions are fragmented. Based on multiple lines of evidence, including a review of the use of trinomi-
als in taxonomy, we elevate the four subspecies of beaded lizards to full species: Heloderma alvarezi
(Chiapan beaded lizard), H. charlesbogerti (Guatemalan beaded lizard), H. exasperatum Río Fuerte
beaded lizard), and H. horridum (Mexican beaded lizard), with no changes in their vernacular names.
Finally, we propose a series of research programs and conservation recommendations.
Key words. mtDNA, ATPase, nuclear genes, character mapping, genomics, seasonally dry tropical forests, reptiles
Resumen.—El escorpión (Heloderma horridum) y el monstruo de Gila (H. suspectum) son lagartijas
grandes, anguimorfas, y muy venenosas que están sufriendo diversas amenazas como resultado de
la persecución humana, degradación y pérdida del hábitat y el cambio climático global. Un análisis
filogenético reciente basado en ADN de este grupo (Douglas et al. 2010. Molecular Phylogenetics
and Evolution 55: 153–167) sugiere que la actual taxonomía intraespecífica (subespecies) del es-
corpión está subestimando la diversidad biológica, y el reconocimiento de especies es justificable.
Estos autores discuten la utilidad del enfoque denominado “conservación filogenética”, que hace
hincapié en la incorporación de la genética histórica en las decisiones de conservación. En este
estudio, reevaluamos la taxonomía del escorpión utilizando el análisis molecular antes mencionado
e incorporamos la morfología en un análisis de mapeo de caracteres. Así mismo, con los resultados
de la secuencia de divergencia calibrada con fósiles, se explora la diversificación del escorpión en
forma yuxtapuesta al origen, la diversificación y la expansión de los bosques tropicales estacional-
mente secos (SDTFs) en México y Guatemala. Estos bosques son los principales biomas ocupados
por los escorpiones, y en Mesoamérica la mayoria son considerados amenazados, en peligro o
Copyright: © 2013 Reiserer et al. This is an open-access article distributed under the terms of the Creative Com-
mons Attribution–NonCommercial–NoDerivs 3.0 Unported License, which permits unrestricted use for non-com-
mercial and education purposes only provided the original author and source are credited.
076Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
Reiserer et al.
The century-long debate over the meaning and utility of
the subspecies concept has produced spirited print but
only superficial consensus. I suggest that genuine con-
sensus about subspecies is an impossible goal ... the sub-
species concept itself is simply too heterogeneous to be
classified as strict science.
Fitzpatrick 2010: 54.
Introduction
The beaded lizard (Heloderma horridum) is a large, high-
ly venomous, anguimorph (Helodermatidae) squamate
with a fragmented distribution in Mesoamerica that ex-
tends from northwestern Mexico (Sonora, Chihuahua) to
eastern Guatemala (Bogert and Martín del Campo 1956;
Campbell and Vannini 1988; Campbell and Lamar 2004;
Beck 2005; Beaman et al. 2006; Anzueto and Campbell
2010; Wilson et al. 2010, 2013; Domínguez-Vega et
al. 2012). Among the reptilian fauna of this region, the
beaded lizard (in Spanish, known as the “escorpión”) is
well known to local inhabitants, yet its natural history
is surrounded by mystery, notoriety and misconception.
Consequently, it is frequently slaughtered when encoun-
tered (Beck 2005).
Adding to this anthropogenic pressure, beaded lizard
populations, with rare exceptions (Lemos-Espinal et al.
2003; Monroy-Vilchis et al. 2005), occur primarily in
seasonally dry tropical forests, SDTFs (Campbell and
Lamar 2004; Beck 2005; Campbell and Vannini 1988;
Domínguez-Vega et al. 2012), the most endangered
biome in Mesoamerica owing to persistent deforesta-
tion for agriculture, cattle ranching, and a burgeoning
human population (Janzen 1988; Myers et al. 2000; Trejo
and Dirzo 2000; Hoekstra et al. 2005; Miles et al. 2006;
Stoner and Sánchez-Azofeifa, 2009; Williams-Linera
and Lorea 2009; Beck 2005; Pennington et al. 2006;
Wilson et al. 2010, 2013; Dirzo et al. 2011; De-Nova et
al. 2012; Domínguez-Vega et al. 2012; Golicher et al.
2012). Furthermore, drought and fires escalate the above
threats (Beck 2005; Miles et al. 2006), and recent predic-
tive models of climate change show that the persistence
of SDTFs in this region is highly dubious (Trejo and
Dirzo 2000; Miles et al. 2006; Golicher et al. 2012).
Despite its large size and charismatic nature, our
knowledge of the ecology, geographical distribution,
and status of populations of H. horridum remains lim-
ited (Beck and Lowe 1991; Beck 2005; Ariano-Sánchez
2006; Douglas et al. 2010; Domiguez-Vega et al. 2012).
Furthermore, based on multiple lines of evidence, a taxo-
nomic reevaluation of this group of lizards is long over-
due (Beck 2005; Douglas et al. 2010).
Here, we continue the dialogue concerning the infra-
specifc (subspecific) taxonomy and conservation status
of beaded lizards. We reviewed recent publications by
Beck (2005) and Domínguez-Vega et al. (2012), and aug-
ment their conclusions based on personal (DDB) field re-
search in Mexico. We reassess the taxonomic status of
the populations of H. horridum using morphology, bio-
geography, and a recent molecular-based (mtDNA,
nDNA) analysis conducted by Douglas et al. (2010).
Although Douglas et al. (2010) commented on the mo-
extirpados. Los valores de la secuencia de divergencia neta por pares (%) fueron mayores entre H.
h. charlesbogerti y H. h. exasperatum (9,8%) y menores entre H. h. alvarezi y H. h. charlesbogerti
(1%). El primer grupo representa a poblaciones que están muy distantes una de la otra en su distri-
bución (este de Guatemala vs. sur de Sonora, México), mientras que las poblaciones en el segundo
grupo están mucho más relacionadas (este de Guatemala vs. Chiapas, México). La subespecie de-
nominada (Heloderma h. horridum) difirió de las otras subespecies de H. horridum entre un 5,4% a
7,1%. Después de la separación de un ancestro común más reciente, ~35 mda a finales del Eoceno,
ocurrió una diversificación (cladogénesis) posterior de Heloderma a finales del Mioceno tardío (9,71
mda), seguida de un estancamiento prolongado de hasta 5 mda, con una cladogénesis posterior
que se extendió hasta el Plioceno y Pleistoceno. En ambos grupos, escorpiones y bosques tropi-
cales estacionalmente secos, los procesos de evolución y diversificación fueron desiguales, y su
distribución fue fragmentada. Hoy en día, el escorpión está distribuido de manera irregular a lo
largo de su amplio rango geográfico. Basándonos en varias líneas de evidencia, incluyendo una re-
visión del uso de trinomios taxonómicos, elevamos las cuatro subespecies del escorpión al nivel de
especie: Heloderma alvarezi (escorpión de Chiapas), H. charlesbogerti (escorpión Guatemalteco),
H. exasperatum (escorpión del Río Fuerte), y H. horridum (escorpión Mexicano), sin cambios en los
nombres vernáculos. Por último, proponemos una serie de programas de investigación y recomen-
daciones para su conservación.
Palabras claves. ADNmt, ATPasas, genes nucleares, mapeo de caracteres, genómica, bosque tropical estacionalmente
seco, reptiles
Citation: Reiserer RS, Schuett GW, Beck DD. 2013. Taxonomic reassessment and conservation status of the beaded lizard, Heloderma hor ridum
(Squamata: Helodermatidae). Amphibian & Reptile Conservation 7(1): 74–96 (e67).
077Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
Taxonomy and conservation of beaded lizards
lecular diversity of Heloderma, especially in H. horri-
dum, they did not provide explicit taxonomic changes.
In this paper, therefore, we reevaluate and expand upon
their conclusions. To gain insights into phenotypic (mor-
phological) evolution of extant Heloderma, with em-
phasis on H. horridum, we conduct a character mapping
analysis (Brooks and McLennan 1991; Harvey and Pagel
1991; Martins 1996; Maddison and Maddison 2011), uti-
lizing the phylogenetic information (trees) recovered by
Douglas et al. (2010).
Overview of Morphology and Molecules
in the genus Heloderma
1. Morphological assessment
Published over half a century ago, Bogert and Martín del
Campo’s (1956) detailed and expansive monograph of
extant and fossil helodermatid lizards remains the defini-
tive morphological reference (reviewed in Campbell and
Lamar, 2004; Beck, 2005), and it contains the diagno-
ses and descriptions of two new subspecies (Heloderma
horridum alvarezi and H. h. exasperatum). Thirty-two
years later, Campbell and Vannini (1988) described a
new subspecies (H. h. charlesbogerti), from the Río Mo-
tagua Valley in eastern Guatemala, in honor of Charles
Bogert’s pioneering work on these lizards. With few ex-
ceptions, such as Conrad et al. (2010) and Gauthier et al.
(2012), who examined higher-level relationships of the
Helodermatidae and other anguimorphs, a modern phy-
logeographic analysis of morphological diversity for ex-
tant helodermatids is lacking. However, as we illustrate
in our character mapping analysis, the morphological
characters used by Bogert and Martín del Campo (1956)
in diagnosing and describing the subspecies of beaded
lizards, though somewhat incomplete, remains useful in
analyzing phenotypic variation.
2. Diagnosis, description, and distribution
of Heloderma horridum
Diagnosis and description. —Bogert and Martín del
Campo (1956) and Campbell and Vannini (1988) pro-
vided diagnoses and descriptions of the subspecies of
Heloderma horridum. Recent information on the biol-
ogy, systematics, and taxonomy of H. horridum and H.
suspectum is summarized and critiqued by Campbell and
Lamar (2004) and Beck (2005), and Beaman et al. (2006)
provided a literature reference summary of the Heloder-
matidae. Presently, four subspecies of H. horridum are
recognized (Figs. 1–5).
Mexican beaded lizard: H. h. horridum (Wiegmann
1829)
Río Fuerte beaded lizard: H. h. exasperatum Bogert
and Martín del Campo 1956
Chiapan beaded lizard: H. h. alvarezi Bogert and Mar-
tín del Campo 1956
Guatemalan beaded lizard: H. h. charlesbogerti
Campbell and Vannini 1988
The four subspecies of H. horridum were diagnosed
and described on the basis of scutellation, color pattern,
and geographical distribution, and we refer the reader to
the aforementioned works for detailed descriptions and
taxonomic keys. The characters used by Bogert and Mar-
tín del Campo (1956) and Campbell and Vannini (1988)
to diagnose the subspecies have been reevaluated as to
their stability, albeit informally (Campbell and Lamar
2004; Beck 2005). Poe and Wiens (2000) and Douglas
et al. (2007) discussed the problem of character stabil-
ity in phylogenetic analyses. Kraus (1988), for example,
commented that reasonable evidence for character stabil-
ity, and thus its usefulness as a shared-derived character
(apomorphy), was the occurrence of a discrete trait in
adults at a frequency of 80% or greater. In our character
mapping analysis using published morphological char-
acters (discussed below), character stability was a major
assumption. Consequently, further research is warranted
for substantiation.
Geographic distribution. —The geographic distribu-
tion of Heloderma horridum extends from southern So-
nora and adjacent western Chihuahua, in Mexico, south-
ward to eastern and southern Guatemala (Campbell and
Lamar 2004; Beck 2005; Anzueto and Campbell 2010;
Domiguez-Vega et al. 2012).
The Río Fuerte Beaded Lizard (H. h. exasperatum) in-
habits the foothills of the Sierra Madre Occidental, with-
in the drainage basins of the Río Mayo and Río Fuerte of
the Sonoran-Sinaloan transition subtropical dry forest in
southern Sonora, extreme western Chihuahua, and north-
ern Sinaloa (Campbell and Lamar 2004; Beck 2005). Its
distribution closely matches the fingers of SDTFs within
this region, but it has also been encountered in pine-oak
forest at 1,400 m near Alamos, Sonora (Schwalbe and
Lowe 2000). Bogert and Martín del Campo (1956) com-
mented that as far as their records indicated, a consider-
able hiatus existed between the distribution of H. h. exas-
peratum (to the north) and H. h. horridum (to the south),
but owing to the narrow contact between the supranasal
and postnasal in H. h. horridum from Sinaloa, intergra-
dation might be found in populations north of Mazatlán.
Based on this information, Beck (2005: 24) stated, “…in
tropical dry forest habitats north of Mazatlan, Sinaloa, H.
h. exasperatum likely intergrades with H. h. horridum.”
Definitive data on intergradation remains unreported,
however, and published distribution maps have incorpo-
rated that assumption (e.g., Campbell and Lamar 2004;
Beck 2005). Campbell and Lamar (2004, p. 104) show
a single example of H. suspectum from El Dorado in
west-central Sinaloa, Mexico (deposited in the American
Museum of Natural History [90786]), a locality 280 km
south from northern records in Río del Fuerte, Sinaloa.
078Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
Reiserer et al.
Fig. 1. A. Adult Río Fuerte beaded lizard (Heloderma horridum exasperatum) in a defensive display (Alamos, Sonora). B. Adult
Río Fuerte beaded lizard raiding a bird nest (Alamos, Sonora). Photos by Thomas Wiewandt.
Fig. 2. Adult Mexican beaded lizard (H. h. horridum) observed on 11 July 2011 at Emiliano Zapata, municipality of La Huerta,
coastal Jalisco, Mexico. Photo by Javier Alvarado.
079Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
Fig. 3. Adult Chiapan beaded lizard (Heloderma horridum alvarezi) from Sumidero Canyon in the Río Grijalva Valley, east of
Tuxtla Gutiérrez, Chiapas, Mexico. Photo by Thomas Wiewandt.
Fig. 4. Adult Guatemalan beaded lizard (Heloderma horridum charlesbogerti) from the Motagua Valley, Guatemala.
Photo by Daniel Ariano-Sánchez.
Taxonomy and conservation of beaded lizards
080Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
Owing to this unusual location, we suggest a re-examina-
tion of this museum specimen to verify its identity. Neo-
nates and juveniles of H. h. exasperatum resemble adults
in color pattern (Fig. 5a), but they show greater contrast
(i.e., a pale yellow to nearly white pattern on a ground
color of brownish-black). Also, their color pattern can be
distinguished from that of adults (e.g., no yellow speck-
ling between the tail bands), and an ontogenetic increase
in yellow pigment occurs (Bogert and Martín del Campo
1956; Beck 2005).
The Mexican beaded lizard (H. h. horridum), the
subspecies with the most extensive distribution, occurs
primarily in dry forest habitats from southern Sinaloa
southward to Oaxaca, including the states of Jalisco,
Nayarit, Colima, Michoacán, and Guerrero, and inland
into the states of México and Morelos (Campbell and
Lamar 2004; Beck 2005). Monroy-Vilchis et al. (2005)
recorded an observation of this taxon at mid eleva-
tions (e.g., 1861 m) in pine-oak woodlands in the state
of México. Campbell and Vannini (1988), citing Álva-
rez del Toro (1983), indicated the probability of areas
of intergradation between H. h. horridum and H. h. al-
varezi, in the area between the Isthmus of Tehuantepec
and Cintalapa, Chiapas. Nonetheless, Álvarez del Toro
(1983) stated that individuals of beaded lizards with yel-
low markings (a coloration character present in H. h.
horridum) are found in the region from Cintalapa to the
Isthmus of Tehuantepec, as well as in dry areas along the
coast from Arriaga (near the Isthmus of Tehuantepec) to
Huixtla (near the Guatemalan border). Literature infor-
mation on intergradation between these two subspecies
is inconclusive and, therefore, will require further inves-
tigation. Neonates and juveniles of H. h. horridum, like
those of H. h. exasperatum, resemble adults in color pat-
tern (Fig. 5b), but their color contrast is greater (Bogert
and Martín del Campo 1956; Beck 2005).
The Chiapan beaded lizard (H. h. alvarezi) inhab-
its dry forests in the Central Depression (Río Grijalva
Reiserer et al.
Fig. 5. A. Juvenile Heloderma horridum exasperatum (in situ,
Álamos, Sonora, Mexico). Photo by Stephanie Meyer.
B. Neonate Heloderma h. horridum (wild-collected July 2011,
Chamela, Jalisco). Photo by Kerry Holcomb.
C. Neonate Heloderma horridum alvarezi (Río Lagartero
Depression, extreme western Guatemala).
Photo by Quetzal Dwyer.
D. Neonate Heloderma horridum charlesbogerti (hatched at
Zoo Atlanta in late 2012). Photo by David Brothers, courtesy
of Zoo Atlanta.
A
B
C
D
081Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
Depression) of central Chiapas and the Río Lagartero
Depression in extreme western Guatemala (Campbell
and Lamar 2004; Beck 2005; Johnson et al. 2010; Wil-
son et al. 2010: p. 435). This taxon is unique among the
subspecies in that it undergoes an ontogenetic increase
in melanism, whereby it tends to lose the juvenile color
pattern (Bogert and Martín del Campo 1956; Beck 2005).
Neonates and juveniles often are distinctly marked with
yellow spots and bands, including on the tail (Fig. 5c),
whereas the color pattern of adults gradually transforms
to an almost uniform dark brown or gray. Black individu-
als, however, are uncommon. Yellow banding on the tail,
a characteristic typical of the other subspecies of beaded
lizards, (Fig. 2), is essentially absent in adults (Bogert
and Martín del Campo 1956; Beck 2005).
The Guatemalan beaded lizard (H. h. charlesbogerti)
inhabits the Río Motagua Valley, in the Atlantic versant
of eastern Guatemala (Campbell and Vannini 1988). Re-
cently, however, Anzueto and Campbell (2010) reported
three specimens from two disjunct populations on the
Pacific versant of Guatemala, to the southwest of the
Motagua Valley. Neonates resemble adults in color pat-
tern, though they tend to be paler (Fig. 5d).
In summary, the distribution of H. horridum is frag-
mented throughout its extensive range and corresponds
closely with the patchy distribution of SDTFs in Mexico
and Guatemala (Beck 2005; Miles et al. 2006; Domín-
guez-Vega et al. 2012). The distribution of the Guate-
malan beaded lizard (H. h. charlesbogerti) is distinctly
allopatric (Campbell and Vannini 1988; Beck 2005;
Ariano-Sánchez 2006; Anzueto and Campbell 2010).
3. Molecular assessment
Douglas et al. (2010) provided the first detailed molec-
ular-based (mtDNA, nDNA) analysis of the phylogeo-
graphic diversity of helodermatid lizards, which is avail-
able at www.cnah.org/cnah_pdf.asp. Two authors (GWS,
DDB) of this paper were co-authors. Specifically, Doug-
las et al. (2010) used a “conservation phylogenetics”
approach (Avise 2005, 2008; Avise et al. 2008), which
combines and emphasizes the principles and approaches
of genetics and phylogeography and how they can be ap-
plied to describe and interpret biodiversity.
Methods. —Douglas et al. (2010) sampled 135 locality-
specific individuals of Heloderma (48 H. horridum, 87 H.
suspectum) from throughout their range (their ingroup).
The outgroup taxa included multiple lineages of lizards
and snakes, with an emphasis on anguimorphs. Based on
both morphological and DNA-based analyses, all author-
ities have recognized the extant helodermatid lizards as
monotypic (a single genus, Heloderma), and as members
of a larger monophyletic assemblage of lizards termed
the Anguimorpha (Pregill et al. 1986; Estes et al. 1988;
Townsend et al. 2004; Wiens et al. 2010, 2012; Gauthier
et al. 2012). This lineage includes the well-known va-
ranids (Varanus), alligator lizards and their relatives
(Anguidae), as well as such relatively obscure taxa as the
Old World Lanthanotidae (Lanthanotus) and Shinisauri-
dae (Shinisaurus), and the New World Xenosauridae (Xe-
nosaurus). The mtDNA analyses in Douglas et al. (2010)
were rooted with the tuatara (Sphenodon punctatus), and
Bayesian and maximum parsimony (MP) analyses were
conducted using Mr. Bayes (Hulsenbeck and Rohnquist
2001).
Douglas et al. (2010) used sequence data from both mi-
tochondrial (mt) DNA and nuclear (n) DNA as molecular
markers in their phylogenetic analyses. Specifically, they
discussed reasons for selecting mtDNA regions ATPase
8 and 6, and the nDNA introns alpha-enolase (ENOL)
and ornithine decarboxylase (OD). The utility of com-
bining mt- and nDNAs (supertree) in recovering phylo-
genetic signals has been discussed (Douglas et al. 2007,
2010), yet each of these markers and the procedure of
combining sequence data have both benefits and pitfalls
(Wiens 2008; Castoe et al. 2009). Long-branch attraction
and convergence, for example, can result in misleading
relationships (Bergsten 2005; Wiens 2008; Castoe et al.
2009). The tools for detecting and potentially correcting
these problems have been discussed (e.g., Castoe et al.
2009; Assis and Rieppel 2011).
Results and discussion. —Douglas et al. (2010) recov-
ered the genus Heloderma as monophyletic (Heloder-
matidae), with H. horridum and H. suspectum as sister
taxa. In a partitioned Bayesian analysis of mtDNA, He-
lodermatidae was recovered as sister to the anguimorph
clade (Shinisaurus (Abronia + Elgaria)), which in turn
was sister to the clade Lanthanotus + Varanus. Recent
molecular studies of squamates by Wiens et al. (2012, see
references therein) recovered a similar topology to that
of Douglas et al. (2010). However, an extensive morpho-
logical analysis by Gauthier et al. (2012) supported a tra-
ditional topology of Heloderma as sister to varanids and
Lanthanotus borneensis (see Estes et al. 1986; Pregill et
al. 1988). In Douglas et al. (2010), a partitioned Bayes-
ian analysis of the nuclear marker alpha-enolase (intron 8
and exon 8 and 9), however, recovered Heloderma as sis-
ter to a monophyletic Varanus. Using a combined analy-
sis of morphology (extant and fossil data), mitochondrial,
and nuclear markers, Lee (2009) recovered Varanidae as
sister to the clade Helodermatidae + Anguidae. In a com-
bined approach, Wiens et al. (2010) recovered results that
were similar to those of Lee (2009). A recent DNA-based
analysis of Squamata by Pyron et al. (2013) examined
4151 species (lizards and snakes), and they recovered
Helodermatidae as sister to the clade Anniellidae + An-
guidae. Moreover, they recovered the clade Varanidae +
Lanthanotidae as sister to Shinisauridae.
How do systematists deal with this type of incon-
gruity (discordance) in studies that use different types
(e.g., morphology vs. molecular) of phylogenetic mark-
ers? Recently, Assis and Rieppel (2011) and Losos et al.
(2012) discussed the common occurrence of discordance
between molecular and morphological phylogenetic
Taxonomy and conservation of beaded lizards
082Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
analyses. Specifically, with respect to discordance, As-
sis and Rieppel (2011) stated that, “...the issue is not to
simply let the molecular signal override the morphologi-
cal one. The issue instead is to make empirical evidence
scientific by trying to find out why such contrastive
signals are obtained in the first place.” We concur with
their opinions, and thus further research is warranted to
resolve such conflicts in the phylogeny of anguimorph
squamates.
Relationships among the four subspecies of H. hor-
ridum recovered in the analysis by Douglas et al. (2010,
p. 158–159, fig. 3a, b) are depicted in Fig. 6. This topol-
ogy was derived from a partitioned Bayesian analysis of
the mtDNA regions ATPase 8 and 6. The Gila monster
(H. suspectum) was the immediate outgroup. Two sets of
sister pairs of beaded lizards were recovered: H. h. exas-
peratum (HHE) + H. h. horridum (HHH), and H. h. al-
varezi (HHA) + H. h. charlesbogerti (HHC). The current
subspecific designations for H. horridum were robustly
supported (concordant) by these genetic analyses. Un-
like results obtained for Gila monsters (H. suspectum),
haplotype and genotype data for H. horridum were both
diverse and highly concordant with the designated sub-
species and their respective geographic distributions.
Douglas et al. (2010) generated pair-wise net sequence
divergence (%) values based on their recovered relation-
ships (Table 1, Fig. 6). The greatest divergence was be-
tween HHE and HHC (9.8%), and the least between HHA
and HHC (1%). The former pair represents populations
widely separated in distribution (southern Sonora, Mex-
ico vs. eastern Guatemala), whereas the latter are much
more closely distributed (Chiapas, Mexico vs. eastern
Guatemala). The nominate subspecies (Heloderma h.
horridum) differed from the other three subspecies of
beaded lizards, from 5.4% to 7.1%.
Reiserer et al.
Fig. 6. Character mapping analysis. Tree topology and node dates based on Douglas et al. (2010). Morphological characters (Table
2) were mapped via parsimony and outgroup methods using the software program Mesquite (Maddison and Maddison 2011). Node
1 = Late Eocene (~35 million years ago, mya); Node 2 = 9.71 mya; Node 3 = 4.42 mya; and Node 4 = 3.02 mya (see Table 1). See
text for details of the analysis.
HHA HHC HHE HHH
HHA —
HHC 1% (3.02) —
HHE 9.3% 9.8% —
HHH 5.4% 6.2% 7.1% (4.42) —
Table 1. Pair-wise net sequence divergence (%) values between
the four subspecies of the beaded lizard (Heloderma horridum)
derived from a partitioned Bayesian analysis of the mtDNA re-
gions ATPase 8 and 6 (modified from Douglas et al. 2010, pp.
157–159, 163; fig. 3a, b, tables 1 and 3). Values in parentheses
denote evolutionary divergence times, which represent mean
age. Mean age is the time in millions of years (mya) since the
most-recent common ancestor (tree node) and is provided for
the sister clades HHE-HHH and HHA-HHC (Fig. 6). Beaded
lizards and Gila monsters (H. suspectum) are hypothesized
to have diverged from a most-recent common ancestor in the
late Eocene ~35 mya (Douglas et al. 2010, p. 163). Percent se-
quence divergence was greatest for HHC-HHE, and was lowest
for HHA-HHC. See text for further details.
HHA = H. h. alvarezi; HHC = H. h. charlesbogerti; HHE = H. h. exas-
peratum; HHH = H. h. horridum.
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Taxonomy and conservation of beaded lizards
Table 2. Morphological characters used for the character mapping analysis (see Table 1, Fig. 6). See text for details.
Character State Designation
Tail length 41–55% of snout-to-vent length A0
≥65%ofsnout-to-ventlength A1
Number of caudal vertebrae 25–28 B0
40 B1
Number of transverse rows of ventromedial absent C0
caudal scales (vent to tail tip) greater than 62 present C1
Usually one pair of enlarged preanal scales present D0
absent D1
First pair of infralabials usually in contact with present E0
chin shields absent E1
Number of maxillary teeth 8–9 F0
6–7 F1
Upper posterior process of splenial bone overlaps inner surface of coronoid G0
does not overlap coronoid G1
Number of black tail bands (including black 4–5 H0
terminus on tail of juveniles) 6–7 H1
Adult total length < 570 mm I0
> 600 mm I1
Tongue color black or nearly so J0
pink J1
Supranasal-postnasal association in contact K0
separated by first canthal K1
Association of second supralabial and in contact L0
prenasal/nasal plates separated by lorilabial L1
Shape of mental scute shield-shaped (elongate and triangular) M0
wedge-shaped (twice as long as wide) M1
Dominant adult dorsal coloration orange, pink N0
black or dark brown N1
yellow N2
Adult dorsal yellow spotting absent O0
extremely low O1
low O2
med O3
high O4
Mental scute scalloped edges absent P0
moderately scalloped edges P1
Enlarged preanal scutes in some females absent Q0
present Q1
Ontogenetic melanism absent R0
present R1
Spots on tail in adults absent S0
present S1
Bands on tail black T0
yellow T1
absent T2
084Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
Reiserer et al.
4. Character mapping analysis
A character mapping analysis (CMA) is one of several ro-
bust tools used in comparative biology to comprehend the
distribution of traits (e.g., morphology), often by explic-
itly utilizing molecular phylogenetic information (Brooks
and McLennan 1991; Harvey and Pagel 1991; Martins
1996; Freeman and Herron 2004; Maddison and Mad-
dison 2011; for a critique, see Assis and Rieppel 2011).
Specifically, the CMA aims to provide insights to the ori-
gin, frequency, and distribution of selected traits formally
expressed onto a tree (e.g., Schuett et al. 2001, 2009; Fen-
wick et al. 2011). These procedures also are potentially
useful in disentangling homology from homoplasy (Free-
man and Herron 2004). Furthermore, the CMA provides
a framework for testing hypotheses of adaptive evolution
and the identification of species (Harvey and Pagel 1991;
Futuyma 1998; Freeman and Herron 2004; Schuett et al.
2001, 2009; Maddison and Maddison 2011). However,
CMA does not replace a strict phylogenetic analysis of
morphological traits (Assis and Rieppel 2011).
Here, we used character mapping to investigate the
morphological traits of the four subspecies of H. horri-
dum, to gain insights on the distribution, divergence, and
homology (e.g., shared-derived traits, such as possible
autapomorphies) of these traits.
Methods. —We used published morphological data on
Heloderma (Bogert and Martín del Campo 1956; Camp-
bell and Vannini 1988; Campbell and Lamar 2004; Beck
2005) and selected 20 morphological characters for the
CMA (Table 2). All characters were coded as binary (i.e.,
0, 1) or multi-state (e.g., 0, 1, 2). Non-discrete multi-state
characters (e.g., color pattern) were ordered from low-
est to highest values. Character polarity was established
by using H. suspectum as the outgroup. The CMA traced
each character independently by using the outgroup anal-
ysis and parsimony procedures in Mesquite (Maddison
and Maddison 2011), and we combined the individual
results onto a global tree.
Results and discussion. —The CMA results (Fig. 6)
show that multiple morphological traits are putative apo-
morphies or autapomorphies (traits unique to a single
taxon) for the various H. horridum clades (subspecies)
delimited in the molecular tree recovered by Douglas et
al. (2010). Although we had a priori knowledge of spe-
cific and unique traits (presumptive autapomorphies)
used to diagnose each of the subspecies, the CMA pres-
ents them in a phylogenetic and temporal framework.
Our results show trends in scutellation (e.g., presence-
absence, relative positions), relative tail length, and body
color pattern, including ontogenetic melanism. Are the
characters we used in the CMA stable in the subspecies?
That question remains for future investigation; however,
we have no evidence to the contrary. Indeed, we antici-
pate that these characters, and others likely to be revealed
through detailed studies, will exhibit stability.
Importantly, each of these traits is amenable to further
investigation and formal tests. For examples, what is
the evolutionary and ecological significance of tongue
color differences in beaded lizards (always pink) and
Gila monsters (always black), the extreme differences in
adult dorsal color pattern in H. h. exasperatum (yellow
is predominant) vs. H. h. alvarezi (dark brown and pat-
ternless predominate), and ontogenetic melanism in H. h.
alvarezi? As we discussed, beaded lizards occupy similar
seasonally dry tropical forests, yet each of the subspe-
cies exhibits pronounced molecular and morphological
differentiation.
Similar types of questions concerning adaptation have
used a CMA to explore social systems and sexual dimor-
phisms in lizards (Carothers 1984), male fighting and
prey subjugation in snakes (Schuett et al. 2001), types
of bipedalism in varanoids (Schuett et al. 2009), and di-
rection of mode of parity (oviparous vs. viviparous) in
viperids (Fenwick et al. 2011).
Subspecies and the Taxonomy of Beaded
Lizards
Introduced in the late 19th century by ornithologists to de-
scribe geographic variation in avian species, the concept
of subspecies and trinomial taxonomy exploded onto the
scene in the early 20th century (Bogert et al. 1943), but
not without controversy. The use of subspecies has been
both exalted and condemned by biologists (see perspec-
tives by Mallet 1995; Douglas et al. 2002; Zink 2004;
Fitzpatrick 2010). Thousands of papers have been pub-
lished in an attempt to either bolster the utility and prom-
ulgation of subspecies, or to denounce the concept as
meaningless and misleading in evolutionary theory (Wil-
son and Brown 1953; Zink 2004). What is the problem?
One common critical response is that the subspecies con-
cept lacks coherence in meaning, and hence is difficult
to comprehend (Futuyma 1998; Zink 2004). Moreover,
the use of subspecies often masks real diversity (cryptic
species, convergence) or depicts diversity that is non-ex-
istent or only trivial (e.g., lack of support in DNA-based
analyses; Zink 2004). Indeed, as John Fitzpatrick attests
(2010, p. 54), “The trinomial system cannot accurately
represent the kind of information now available about ge-
netic and character variation across space. Instead, even
more accurate tools are being perfected for quantitative,
standardized descriptions of variation. These analyses—
not subspecies classifications—will keep providing new
scientific insights into geographic variation.”
Even with the identification of a variety of problems,
many authors recommend that complete abandonment
of the trinomial category in taxonomy is not necessary
nor advised (e.g., Mallett 1995, Hawlitschek et al. 2012).
Unfortunately, a consensus among biologists concerning
the use of subspecies is not likely to emerge (Fitzpatrick
2010). In step with Fitzpatrick’s (2010) comments, we
085Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
contend that the plethora of variation detected in organ-
isms must be approached in a modern sense that does
not rely upon a cumbersome and outdated taxonomic
system. Indeed, we anticipate that the description of
geographic variation in organisms, once emancipated
from infraspecific taxonomy, will actually accelerate our
understanding of variation and its complexities. In our
view, the confusion in recognizing subspecies can also
mislead conservation planning, and it has on more than
one occasion (e.g., the dusky seaside sparrow, see Avise
and Nelson 1989). We thus agree with Wilson and Brown
(1953), Douglas et al. (2002), Zink (2004), Fitzpatrick
(2010) and others in their insightful criticisms leveled at
the subspecies concept and the use of trinomials in taxon-
omy. Other authors have echoed similar views (Burbrink
et al., 2000; Burbrink 2001; Douglas et al. 2007; Tobias
et al. 2010; Braby et al. 2011; Hoisington-Lopez 2012;
Porras et al. 2013).
Given our reassessment of molecular (mt- and
nDNAs), phylogeographic, morphological, and biogeo-
graphic evidence, we elevate the subspecies of Heloder-
ma horridum to the rank of full species (Wiley, 1978;
Zink 2004; Tobias et al. 2010; Braby et al. 2011; Porras
et al. 2013). Indeed, Douglas et al. (2010, p. 164) stated
that, “… unlike H. suspectum, our analyses support the
subspecific designations within H. horridum. However,
these particular lineages almost certainly circumscribe
more than a single species … Thus, one benefit of a con-
servation phylogenetic perspective is that it can properly
identify biodiversity to its correct (and thus manageable)
taxonomic level.” Accordingly, based on multiples lines
of concordant evidence, we recognize four species of
beaded lizards. They are:
Mexican beaded lizard: Heloderma horridum (Wieg-
mann 1829)
Río Fuerte beaded lizard: Heloderma exasperatum
(Bogert and Martín del Campo 1956)
Chiapan beaded lizard: Heloderma alvarezi (Bogert
and Martín del Campo 1956)
Guatemalan beaded lizard: Heloderma charlesbogerti
(Campbell and Vannini, 1988)
In the above arrangement, we do not recognize subspe-
cies and vernacular names remain unchanged. The geo-
graphic distribution of the four species of beaded lizards
is presented in Fig. 7. Locality data for the map were
derived from Bogert and Martín del Campo (1956),
Campbell and Vannini (1988), Schwalbe and Lowe
(2000), Lemos-Espinal et al. (2003), Campbell and La-
mar (2004), Beck (2005), Monroy-Vilchis et al. (2005),
Ariano-Sánchez and Salazar (2007), Anzueto and Camp-
bell (2010), Domiguez-Vega et al. (2012), and Sánchez-
De La Vega et al. (2012). The “?” on the map (coastal
Oaxaca, municipality: San Pedro Tututepec) denotes a
jet-black adult specimen photographed by Vicente Mata-
Silva (pers. comm.) in December 2010. The validity of
Taxonomy and conservation of beaded lizards
this record is questionable owing to its striking coloration
resemblance to H. alvarezi from the Central Depression
(Río Grijalva Depression) of Chiapas and extreme west-
ern Guatemala, rather than to H. horridum. Although the
individual might represent an isolated population of H.
alvarezi, further study in this area of Oaxaca is required
to rule out human activity as an agent (e.g., displace-
ment).
Beaded Lizards and Seasonally Dry
Tropical Forests
The key to understanding the evolution and biogeogra-
phy of beaded lizards and the prospects for implementing
meaningful conservation measures is through a recogni-
tion of the biomes they occupy, which we emphasize are
the widely but patchily distributed low elevation season-
ally dry tropical forests (SDTFs; see Trejo and Dirzo
2000; Campbell and Lamar 2004; Beck 2005; Ariano-
Sánchez 2006; Miles et al. 2006; Pennington et al. 2006;
Dirzo et al. 2011; Domiguez-Vega et al. 2012).
The evolution of SDTFs in Mesoamerica is a complex
evolutionary scenario (Stuart 1954, 1966), and our un-
derstanding of their origin and temporal diversification
is in its infancy (Janzen, 1988; Becerra 2005; Pennington
et al. 2006; Dirzo et al. 2011; De-Nova et al. 2012). One
approach to grapple with complex issues such as the ori-
gin and historical construction of SDTFs in Mesoamerica
has been to examine a single but highly diverse plant tax-
on within a phylogenetic (phylogenomic) backdrop. This
approach, accomplished by Becerra (2005) and more re-
cently by De-Nova et al. (2012), uses the woody plant
(tree) Bursera (Burseracae, Sapindales), a highly diverse
genus (> 100 species) with a distribution in the New
World and emblematic of most dry forest landscapes
(De-Nova et al. 2012). Owing to this diversity, coupled
with extensive endemism, this taxon has yielded valuable
information that serves as a reasonable proxy for diver-
sification and expansion of the SDTF biomes (Dick and
Pennington 2012). Hence, plant (angiosperm) species
richness and expansion of SDTF biomes in Mesoamerica
is hypothesized to parallel the diversification of Bursera
(Dick and Wright 2005).
Based on both plastid and nuclear genomic markers
that were analyzed using fossil-calibrated techniques and
ancestral habitat reconstruction, the origin of Bursera in
Mesoamerica is hypothesized to be in northwestern Mex-
ico in the earliest Eocene (~50 mya), with subsequent ex-
tensive diversification and southern expansion along the
Mexican Transvolcanic Belt in the Miocene, especially
~7–10 mya (De-Nova et al. 2012). Accelerated clade di-
versification of Bursera and its sister genus Commiphora
occurred during the Miocene, a period of increased arid-
ity likely derived from seasonal cooling and rain shadow
effects (Dick and Wright 2005). Although causal con-
nections are complex, they include global tectonic pro-
086Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
cesses, orogenic activities (uplifting of the Sierra Madre
Occidental and Sierra Made Oriental) and local volca-
nism (Dick and Wright 2005; De-Nova et al. 2012). De-
Nova et al. (2012) concluded by emphasizing that their
phylogenomic analysis of Bursera points to high species
diversity of SDTFs in Mesoamerica that derives from
within-habitat speciation rates that occurred in the enve-
lope of increasing aridity from the early Miocene to the
present. Furthermore, they stated (p. 285), “This scenario
agrees with previous suggestions that [angiosperm] lin-
eages mostly restricted to dry environments in Mexico
resulted from long periods of isolated evolution rather
than rapid species generation....”
Beaded Lizard Evolution and Diversification
The phylogenetic analyses of Heloderma horridum
(sensu lato) by Douglas et al (2010) provided fossil-
calibrated estimates of divergence times, which allow us
to draw connections to the origin and diversification of
SDTFs in Mesoamerica (Table 1, Fig. 6). Based on those
analyses, H. horridum (sensu lato) and H. suspectum are
hypothesized to have diverged from a most-recent com-
mon ancestor in the late Eocene (~35 mya), which cor-
responds to the establishment of Bursera in northwestern
Mexico. Subsequent diversification (cladogenesis) of the
beaded lizards occurred during the late Miocene (9.71
mya), followed by a lengthy period of stasis of up to 5
my, with subsequent cladogenesis extending into the
Pliocene and Pleistocene. Of particular interest is that
this scenario approximately parallels the diversification
and southern expansion of SDTFs (Dick and Wright
2005; De-Nova et al. 2012). Accordingly, based on the
above discussion of SDTFs and phylogenetic analyses,
we suggest that beaded lizard lineage diversification
resulted from long periods of isolated (allopatric) evo-
lution in SDTFs. Douglas et al. (2010) referred to the
fragmented tropical dry forests of western Mexico as
“engines” for diversification. The extralimital distribu-
tion of H. exasperatum and H. horridum into adjacent
pine-oak woodland and thorn scrub biomes appears to be
relatively uncommon (Schwalbe and Lowe 2000; Beck
2005; Monroy-Vilchis et al. 2005).
Reiserer et al.
Fig. 7. The distribution of beaded lizards in Mexico and Guatemala. Colored dots represent verified sightings (populations) and
museum records. Note the fragmented populations of all four species, which closely approximates the patchy distribution of sea-
sonal dry tropical forests (see map in Brown and Lowe [1980]). See text for explanation of question marks (“?”) and other details.
087Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
Conservation of Beaded Lizards
A primary aim of this paper is to provide a useful and
accurate synthesis of information on the taxonomy of
beaded lizards that will lead to informed decisions re-
garding their conservation (see Douglas et al., 2010).
Until recently, H. horridum (sensu lato) was designated
as Vulnerable on the World Conservation Union (IUCN)
Red List. In 2007, that designation was changed to Least
Concern based on more stringent criteria (Canseco-
Marquez and Muñoz 2007; categories and criteria ver-
sion 3.1). The 2007 IUCN Red List also determined that,
“Additional research is needed into the taxonomic status,
distribution and threats to this species” (Canseco-Mar-
quez and Muñoz 2007). The critically endangered status
of H. h. charlesbogerti (sensu lato) in Guatemala (Ari-
ano-Sánchez 2006; Ariano-Sánchez and Salazar 2007)
has not altered the current IUCN Red List designation
of this taxon, because population trends of other beaded
lizards in Mexico remain “unknown” (www.iucnredlist.
org/search; see International Reptile Conservation Foun-
dation, IRCF; www.ircf.org). As more information on the
population status of the newly elevated beaded lizards
becomes available, in view of their fragmented distribu-
tions and threats to their habitats, the IUCN likely will
designate these taxa as Vulnerable or a higher threat cat-
egory (see our EVS analysis below). For example, H. ex-
asperatum, H. alvarezi, and H. charlesbogerti all occupy
limited areas of SDTF (Beck 2005).
In Mexico, helodermatid lizards are listed as “threat-
ened” (amenazadas) under the Mexican law (NOM-
059-SEMARNAT-2010), legislation comparable to that
in the United States Endangered Species Act. The threat-
ened category from Mexican law coincides, in part, with
the “Vulnerable” category of the IUCN Red List. This
document defines “threatened” as species or populations
that could become at risk of extinction in a short to me-
dium period if negative factors continue to operate that
reduce population sizes or alter habitats. Heloderma h.
charlesbogerti (sensu lato) is listed on the Guatemalan
Lista Roja (Red List) as “endangered,” with approxi-
mately 200–250 adult individuals remaining in under
26,000 ha of its natural habitat of SDTF and thorn scrub,
(Ariano-Sánchez 2006).
Furthermore, H. h. charlesbogerti (sensu lato) is listed
on CITES Appendix I, a designation that includes spe-
cies threatened with extinction (see CITES document
appended to Ariano-Sánchez and Salazar 2007). Trade
in CITES Appendix I species is prohibited except under
exceptional circumstances, such as for scientific research
(CITES 2007). The remaining taxa of Heloderma hor-
ridum (sensu lato) (H. h. alvarezi, H. h. exasperatum,
and H. h. horridum) are listed on Appendix II of CITES
(CITES 2007). International trade in Appendix II species
might be authorized under an export permit, issued by
the originating country only if conditions are met that
show trade will not be detrimental to the survival of the
species in the wild. The United States Fish & Wildlife
Service issues permits only if documentation is provided
proving legal origin, including a complete paper trail
back to legal founder animals. This procedure allows the
importation of beaded lizards into the United States to
be tightly regulated (in theory), and also subjects such
imports to provisions of the Lacey Act that control com-
merce in illegally obtained fish and wildlife (Beck 2005).
Beaded Lizards: Denizens of Endangered
SDTFs
Although occasional sightings of beaded lizards have
been reported from mid elevation pine-oak woodlands, all
four species primarily inhabit lowland SDTFs and rarely
in associated thorn scrub, in both Mexico and Guatemala
(Schwalbe and Lowe 2000; Lemos-Espinal et al. 2003;
Campbell and Lamar 2004; Beck 2005; Monroy-Vilchis
et al. 2005; Ariano and Salazar 2007; Domiguez-Vega et
al. 2012). Thus, the optimal measure to reduce threats to
beaded lizards is to maintain the integrity of their tropi-
cal dry forest habitats. Current threats to beaded lizards
throughout their range include habitat loss, road mor-
tality, poaching, and illegal trade (Beck 2005; Miles et
al. 2006; Golicher et al. 2012). Habitat loss takes many
forms, from the conversion of SDTFs to areas of agricul-
ture and cattle ranching, to forest fragmentation owing
to roads and other forms of development (Pennington et
al. 2006). Degradation from human-introduced invasive
(exotic) organisms and fire also are contributing factors
(Beck 2005).
When the Spaniards arrived in the Western Hemi-
sphere, Mesoamerican SDTFs covered a region stretch-
ing from Sonora (Mexico) to Panama, an area roughly the
size of France (~550,000 km2). Today, only 0.1% of that
region (under 500 km2) has official conservation status,
and less than 2% remains sufficiently intact to attract the
attention of conservationists (Janzen 1988; Hoekstra et
al. 2005). Of all 13 terrestrial biomes analyzed by Hoek-
stra et al. (2005), the SDTF biome has the third highest
conservation risk index (ratio of % land area converted
per % land area protected), far above tropical wet forest
and temperate forest biomes (Miles et al. 2006).
Mexico ranks among the most species rich countries
in the world (García 2006; Urbina-Cardona and Flores-
Villela 2010; Wilson and Johnson 2010; Wilson et al.
2010, 2013). Nearly one-third of all the Mexican herpe-
tofaunal species are found in SDTFs (García 2006; De-
Nova et al. 2012). Neotropical dry forests span over 16
degrees of latitude in Mexico, giving way to variation
in climatic and topography that results in a diversity of
tropical dry forest types, and a concurrent high propor-
tion of endemism of flora and fauna (García 2006; De-
Nova et al. 2012; Wilson et al. 2010; 2013). Mexican
seasonally tropical dry forest, classified into seven ecore-
gions that encompass about 250,000 km2, has enormous
conservation value and has been identified as a hotspot
Taxonomy and conservation of beaded lizards
088Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
for conservation priorities (Myers et al. 2000; Sánchez-
Azofeifa et al. 2005; García 2006; Urbina-Cardona and
Flores-Villela, 2010; Wilson et al. 2010, Mittermeier et
al. 2011). The vast majority (98%) of this region, how-
ever, lies outside of federally protected areas (De-Nova
et al. 2012). With few exceptions, most of the protected
areas in Mexico occur in the states of Chiapas and Jalis-
co, leaving much of the region (e.g., Nayarit and Sinaloa)
without government (federal) protection (García 2006).
In Guatemala, less than 10% of an estimated 200,000
ha of original suitable habitat have been established as
protected critical habitat in the Motagua Valley for the
endangered H. charlesbogerti (Nájera Acevedo 2006). A
strong effort led by local citizens, conservation workers,
biologists, government officials, NGOs, and conserva-
tion organizations (e.g., The Nature Conservancy, Inter-
national Reptile Conservation Association, Zoo Atlanta,
and Zootropic) negotiated to have H. h. charlesbogerti
(sensu lato) placed on CITES Appendix I, to purchase
habitat, conduct research, employ local villagers in mon-
itoring the lizards, and promote environmental education
(Lock 2009). Similar efforts for beaded lizards have been
underway for many years in Chiapas (Mexico), spear-
headed at ZooMAT (Ramírez-Velázquez 2009), and in
Chamela, Jalisco (www.ibiologia.unam.mx/ebchamela/
www/reserva.html). Such efforts will need to expand in
the years ahead and will doubtless play a crucial role if
we hope to retain the integrity of existing SDTFs inhab-
ited by beaded lizards throughout their range.
Discussion
In this paper, we reassessed the taxonomy of Heloderma
horridum (sensu lato) using both published information
and new analyses (e.g., CMA). We concluded that diver-
sity in beaded lizards is greater than explained by infra-
specific differences and that the recognition of subspecies
is not warranted, as it obscures diversity. Our decision to
elevate the four subspecies of H. horridum to full species
status is not entirely novel (Beck 2005; Douglas et al.
2010). Furthermore, our taxonomic changes are based on
integrative information (i.e., morphology, mt- and nDNA
sequence information, biogeography) and changing per-
spectives on the utility of formally recognizing infraspe-
cific diversity using a trinomial taxonomy (Wilson and
Brown 1953; Douglas et al. 2002; Zink 2004; Porras et
al. 2013). This decision not only adds to a better under-
standing of the evolution of helodermatids, but also pro-
vides an important evolutionary framework from which
to judge conservation decisions with prudence (Douglas
et al. 2002).
Below, we delineate and discuss prospective research
and conservation recommendations for beaded lizards
based on our present review. Borrowing some of the
guidelines and recommendations for future research and
conservation for cantils, also inhabitants of SDTFs, by
Porras et al. (2013), we outline similar ones for the four
species of beaded lizards (H. alvarezi, H. charlesbogerti,
H. exasperatum, and H. horridum).
Future Research and Conservation
Recommendations
1. Throughout this paper we emphasized the importance
of SDTFs in the distribution of beaded lizards, yet most
SDTFs within their distribution are not Protected Natural
Areas (PNAs; Beck 2005; Urbina-Cardona and Flores-
Villela 2009; Domiguez-Vega et al. 2012). Accordingly,
emphasis should be placed on those areas of SDTFs for
prospective research, new conservation projects, and for
establishing new PNAs. The protection of beaded liz-
ards must be placed into a larger context of conservation
planning. Proper stewardship of SDTFs and other biomes
must include meaningful (scientific) protective measures
for all of the flora and fauna, rather than piecemeal (e.g.,
taxon-by-taxon) approaches that lack a cohesive conser-
vation plan (Douglas et al. 2010).
We applaud the efforts of Domíguez-Vega et al.
(2012) in identifying conservation areas for beaded liz-
ards; however, we do not agree with all of their conclu-
sions. In particular, based on field experiences by one
of us (DDB), we contend that the potential (predicted)
range of H. exasperatum in Sonora (Mexico) based on
the results of their habitat suitability modeling, appears
exaggerated and thus may be misleading. In our opin-
ion, their distribution maps (figs. 2 and 3) overestimate
the extent of true SDTFs in Sonora, showing their occur-
rence in a type of biome that is more accurately classi-
fied as Sinaloan Thorn Scrub (see the excellent maps in
Brown and Lowe 1980; Robichaux and Yetman 2000).
In Sonora, beaded lizards (H. exasperatum) are rarely
found in association with pure thorn scrub, while Gila
monsters, in contrast, are frequently encountered in that
type of habitat (Schwalbe and Lowe 2000; Beck 2005).
2. With few exceptions, the population viability of beaded
lizards is largely unknown (Beck 2005; Ariano-Sánchez
2006; Ariano-Sánchez et al. 2007; Domíguez-Vega et al.
2012). We highly recommend that modern assessments
of the four species occur at or near localities where they
have been recorded (e.g., Jiménez-Valverde and Lobo
2007). Whereas H. charlesbogerti, and to a lesser degree
H. alvarezi (Ramírez-Velázquez 2009), are receiving in-
ternational conservation attention, we feel that similar
consideration is necessary for H. exasperatum owing
to its relatively limited geographic range (Sonora, Chi-
huahua, Sinaloa), the large extent of habitat destruction
and fragmentation (Fig. 8), and limited areas receiving
protection (Trejo and Dirzo 2000; Domíguez-Vega et
al. 2012; see http://www.conanp.gob.mx/regionales/). In
1996, about 92, 000 hectares in the Sierra de Álamos and
the upper drainage of the Río Cuchujaqui were declared
a biosphere reserve by the Secretary of the Environment
and Natural Resources (SEMARNAT 2010), called the
Reiserer et al.
089Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
Área de Protección de Fauna y Flora Sierra de Álamos
y Río Cuchujaqui (Martin and Yetman 2000; S. Meyer,
pers. comm.). Efforts continue in Sonora to set aside ad-
ditional habitat for conservation, but, other than Alamos,
no other areas with true SDTFs presently exist (Robich-
aux and Yetman 2000; S. Meyer, pers. comm.).
3. Conservation management plans for each of the spe-
cies of beaded lizards should be developed from an
integrative perspective based on modern population
assessments, genetic information, and ecological (e.g.,
soil, precipitation, temperature) and behavioral data
(e.g., social structure, mating systems, home range size).
Such a conservation plan is in place for the Guatemalan
beaded lizard (H. charlesbogerti) by CONAP-Zootropic
(www.ircf.org/downloads/PCHELODERMA-2Web.
pdf). Also, aspects of burgeoning human population
growth must be considered, since outside of PNAs these
large slow-moving lizards generally are slaughtered on
sight, killed on roads by vehicles (Fig. 9), and threatened
by persistent habitat destruction primarily for agriculture
and cattle ranching (Fig. 10). For discussions on conser-
vation measures in helodermatid lizards, see Sullivan et
al. (2004),
Beck (2005), Kwiatkowski et al. (2008), Doug-
las et al. (2010), Domíguez-Vega et al. (2012), and Ariano-
Sánchez and Salazar (2013).
In Mexico, the IUCN lists
Heloderma horridum (sensu
lato) under the category of Least
Concern. Recently, Wilson et al.
(2013) reported the Environmen-
tal Vulnerability Score (EVS)
for H. horridum (sensu lato) as
11. Briefly, an EVS analysis as-
sesses the potential threat sta-
tus of a given species based on
multiple criteria and provides a
single score or index value (Wil-
son and McCranie 2004; Porras
et al. 2013; Wilson et al. 2013).
High EVS scores (e.g., 17), for
example, signify vulnerability.
With the taxonomic changes we
proposed for beaded lizards, an
EVS assessment is thus required
for each species. Using the new
criteria developed by Wilson et
al. (2013; see Porras et al. 2013),
we recalculated the EVS for the
species of beaded lizards, which
are presented below:
H. horridum: 5 + 4 + 5 = 14
H. exasperatum: 5 + 7 + 5 = 17
H. alvarezi: 4 + 6 + 5 = 15
H. charlesbogerti: 4 + 8 + 5 = 17
These recalculated values fall into
the high vulnerability category
(Wilson et al. 2013; Porras et al.
2013), underscoring the urgency
for the development of conserva-
tion management plans and long-
term population monitoring of all
species of beaded lizards. These
values thus need to be reported
to the appropriate IUCN commit-
tees, so immediate changes in sta-
tus can be made and conservation
actions implemented.
Taxonomy and conservation of beaded lizards
Fig. 8. Destruction of seasonally dry tropical forest near Alamos, Sonora, Mexico.
Photo by Daniel D. Beck.
Fig. 9. A dead-on-the-road (DOR) H. exasperatum (sensu stricto) near Álamos, Sonora,
Mexico. Vehicles on paved roads are an increasing threat to beaded lizards, Gila monsters,
and other wildlife. Photo by Thomas Wiewandt.
090Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
4. We recommend the establishment of zoo conservation
(AZA) educational outreach programs, both ex situ and
in situ, such as those currently in progress for H. charles-
bogerti (www.IRCF.org;www.zooatlanta.org) and for
H. alvarezi in Chiapas (Ramírez-Velázquez, 2009, see
Fig. 11). Because of its limited range, destruction of its
natural habitat, small population
size (200–250 adults) and endan-
gered status, H. charlesbogerti is
currently listed as CITES Appen-
dix I (Ariano-Sánchez and Sala-
zar 2007). Given the taxonomic
elevation of these taxa, conserva-
tion agencies can use these char-
ismatic lizards as flagship species
in efforts to publicize conserva-
tion efforts in their respective
countries at all levels of interest
and concern, including educa-
tion and ecotourism (Beck 2005).
Eli Lilly Co., Disney Worldwide
Conservation Fund and The Na-
ture Conservancy support the
conservation of H. charlesboger-
ti (Ariano-Sánchez and Salazar
2012). Such corporate involve-
ment provides funds and positive
public exposure (e.g., social net-
work advertising) that otherwise
would not be possible.
5. One of the major conclusions
of this paper is that our knowl-
edge of the taxonomy and phy-
logeography of beaded lizards
remains at an elementary level.
As discussed, a robust phylogeo-
graphic analysis using morpho-
logical characters is not avail-
able. Our character mapping
exercise, for various reasons, is
not a substitute procedure for
detailed phylogenetic analyses
using morphology (Assis 2009;
Assis and Rieppel 2011). Other
authors have made similar pleas
concerning the importance of
morphology, including fossils, in
phylogenetic reconstruction (Poe
and Wiens 2000; Wiens 2004,
2008; Gauthier et al. 2012).
Moreover, further studies on the
historical biogeography of he-
lodermatids (e.g., ancestral area
reconstruction) are needed (e.g.,
Ronquist 1997, 2001; Ree and
Smith 2008). Detailed morpho-
logical analyses can be conducted with new tools such as
computed tomography (CT) scans of osteological char-
acters of both extant and fossil specimens (Gauthier et al.
2012), and geometric morphometric approaches to exter-
nal characters (Davis 2012). Furthermore, in the expand-
ing field of “venomics” new venom characters in beaded
Reiserer et al.
Fig. 10. Agave cultivation in Mexico results in the destruction of seasonally dry tropical
forests. Photo by Thomas Wiewandt.
Fig. 11. Antonio Ramirez Ramírez-Velázquez, a herpetologist, discusses the beauty and
importance of beaded lizards (H. alvarezi, sensu stricto) to a group of enthusiastic children
and their teacher at Zoo Miguel Álvarez del Toro (ZooMAT) in Tuxtla Gutiérrez, Chiapas,
Mexico. The zoo was named in honor of its founding director, Señor Miguel Alvarez del
Toro, who had a keen academic and conservation interest in beaded lizards. He collected
the type specimen of H. alvarezi (described in Bogert and Martín del Campo, 1956), which
was named in his honor. ZooMAT offers hands-on environmental education programs to
schoolchildren and other citizens of southern Mexico. Photo by Thomas Wiewandt.
091Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
lizards will likely be discovered, which might prove use-
ful in phylogenetic analyses (Fry et al. 2009, 2010).
As we progress into the “Age of Genomics” with
ever-growing computational advancements (e.g., bio-
informatics; Horner et al. 2009), new and exciting meth-
ods to explore organismal diversity are opening, includ-
ing such next-generation approaches as pyrosequencing
(microsatellite isolation), establishing transcriptome
databases, and whole-genome sequencing (Wiens 2008;
Castoe et al. 2011; Culver et al. 2011). Currently, plans
are underway to apply pyrosequencing methods to helo-
dermatids to generate a nearly inexhaustible supply of
microsatellite markers for a variety of proposed analy-
ses (W. Booth and T. Castoe, pers. comm.). Standing on
the shoulders of The Human Genome Project (Culver et
al. 2011), and reaping the success of genome projects in
other reptilian taxa (Castoe et al. 2011), it is now possible
to establish a “Helodermatid Genome Project.” Beaded
lizards and the Gila monster are especially good candi-
dates for such an investment, especially given the impor-
tance of their venom components in medical research and
recent pharmaceutical applications (Beck 2005; Douglas
et al. 2010; Fry et al. 2009, 2010).
6. An important take-home message from Douglas et al.
(2010) is that future conservation efforts will require a
robust understanding of phylogenetic diversity (e.g.,
conservation phylogenetics) to make sensible (logical)
and comprehensive conservation plans. For example, the
range of H. horridum (sensu stricto) is the most expansive
of the species of beaded lizards and has not been fully
explored with respect to genetic diversity. Accordingly,
sampling throughout its range may yield cryptic genetic
diversity, perhaps even new species. We emphasize that
viable conservation planning must incorporate all intel-
lectual tools available, including those that incorporate
old methods (e.g., paleoecological data) but viewed
through a new lens (Douglas et al. 2007, 2009; Willis
et al. 2010). Wisely, Greene (2005) reminds us that we
are still grappling with understanding basic and essential
issues concerning the natural history of most organisms.
To that end, we must continue in our efforts to educate
students and the public of the need for and importance of
this branch of science.
7. The new taxonomic arrangement of beaded lizards
we proposed will affect other fields of science, such as
conservation biology and human medicine (Beck, 2005;
Douglas et al., 2010). In Fry et al. (2010, p. 396, table 1),
toxins are matched to the subspecies of beaded lizards
and Gila monsters. Yet as noted by Beck (2005) and
Douglas et al. (2010), the banded Gila monster (H. s.
cinctum) is not a valid subspecies, which is based on
several levels of analysis (i.e., morphology, geographic
distribution, and haplotype data). Individuals assigned to
H. s. cinctum based on color and pattern, for example,
have been found in southwestern Arizona near the Mexi-
can border and in west-central New Mexico (Beck 2005).
Furthermore, most venom researchers, including those
who study helodermatids, often obtain samples from cap-
tive subjects in private collections and zoological institu-
tions. Many of these animals have been bred in captivity
and result from crossing individuals of unknown origin
or from different populations (D. Boyer, pers. comm).
Among other negative outcomes, such “mutts” will con-
found results of the true variation of venoms. Geographic
and ontogenetic variation in venom constituents is well
established in other squamates (Minton and Weinstein
1986; Alape-Girón et al. 2008; Gibbs et al. 2009), which
is apparently the case in helodermatids (Fry et al. 2010).
Thus, we strongly encourage researchers investigating
helodermatid venoms for molecular analysis and phar-
maceutical development to use subjects with detailed lo-
cality information, as well as age, gender, and size, and
to provide those data in their publications.
8. Owing to problems that many scientists, their stu-
dents, and other interested parties from Mesoamerica
have in gaining access to primary scientific literature,
we highly recommend that authors seek Open Access
peer-reviewed journals as venues for their publications
on beaded lizards, an important factor in our choice for
selecting the present journal (www.redlist-ARC.org) as a
venue for our data and conservation message.
Acknowledgments.—We thank Larry David Wilson
for inviting us to participate in the Special Mexico Is-
sue. A Heritage Grant from the Arizona Game and Fish
Department and a Research Incentive Award/Scholarly
Research and Creative Activities Award (Arizona State
University) awarded to GWS funded parts of this re-
search. Zoo Atlanta (Dwight Lawson, Joe Mendelson III)
and Georgia State University (Department of Biology)
provided various levels of support. Warren Booth, Donal
Boyer, Dale DeNardo, Andrés García, Stephanie Mey-
er, and Tom Wiewandt were always willing to discuss
beaded lizard and tropical dry forest biology with us. We
thank Brad Lock, Louis Porras, and Larry David Wilson
for their suggestions and valuable insights in improving
an earlier version of this manuscript. Also, three review-
ers, including Daniel Ariano-Sánchez, provided key
information and sharpened our focus, though we bear
the burden of any blunders. We thank Javier Alvarado,
Daniel Ariano-Sánchez, David Brothers, Quetzal Dwyer,
Kerry Holcomb, Vicente Mata-Silva, Stephanie Meyer,
Adam Thompson, and Tom Wiewandt for graciously
supplying us with images. Vicente Mata-Silva kindly as-
sisted us in preparing the resumen and locating literature
on Heloderma.
Taxonomy and conservation of beaded lizards
092Amphib. Reptile Conserv. | http://redlist-ARC.org July 2013 | Volume 7 | Number 1 | e67
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Reiserer et al.
Randall S. Reiserer is an integrative biologist whose research focuses on understanding the interrelationships
among ecology, morphology, and behavior. Within the broad framework of evolutionary biology, he studies
cognition, neuroscience, mimicry, life-history evolution, and the influence of niche dynamics on patterns of
evolutionary change. His primary research centers on reptiles and amphibians, but his academic interests
span all major vertebrate groups. His studies of behavior are varied and range from caudal luring and thermal
behavior in rattlesnakes to learning and memory in transgenic mice. Randall established methods for study-
ing visual perception and stimulus control in is studies of caudal luring in snakes. He commonly employs
phylogenetic comparative methods and statistics to investigate and test evolutionary patterns and adaptive
hypotheses. Dr. Reiserer is an editor of the upcoming peer-reviewed book, The Rattlesnakes of Arizona.
Gordon W. Schuett is an evolutionary biologist and herpetologist who has conducted extensive research on rep-
tiles. His work has focused primarily on venomous snakes, but he has also published on turtles, lizards, and
amphibians. Among his most significant contributions are studies of winner-loser effects in agonistic encoun-
ters, mate competition, mating system theory, hormone cycles and reproduction, caudal luring and mimicry,
long-term sperm storage, phylogeographic analyses of North American pitvipers, and as a co-discoverer of
facultative parthenogenesis in non-avian reptiles. He served as chief editor of the peer-reviewed book Biology
of the Vipers and is presently serving as chief editor of an upcoming peer-reviewed book The Rattlesnakes of
Arizona (rattlesnakesofarizona.org). Gordon is a Director and scientific board member of the newly founded
non-profit The Copperhead Institute (copperheadinstitute.org). He was the founding Editor of the journal
Herpetological Natural History. Dr. Schuett resides in Arizona and is an adjunct professor in the Department
of Biology at Georgia State University.
Daniel D. Beck is an ecologist and herpetologist who has conducted research on the ecology, physiology, and be-
havior of rattlesnakes and helodermatid lizards. He has pioneered many of the field studies on helodermatid
lizards in the past 30 years, including topics ranging from energy metabolism and habitat use to combat and
foraging behaviors in locations ranging from the deserts of Utah, Arizona, and New Mexico, to the tropical
dry forests of Sonora and Jalisco, Mexico. His book, Biology of Gila Monsters and Beaded Lizards (2005),
presents a synthesis of much of our knowledge of these charismatic reptiles. Dr. Beck is Professor of Biology
at Central Washington University, in Ellensburg, Washington, where he lives in a straw bale house with his
wife, biologist Kris Ernest, and their two teenage children.
