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Aspidoscelis sexlineata (Sauria: Teiidae) in Mexico: distribution, habitat, morphology, and taxonomy.

The six-lined racerunner (Aspidoscelis sexlineata = Cnemidophorus sexlineatus; Reeder et al., 2002) is a small whiptail lizard with a vast geographic range. Not only is it the most widely distributed species of the genus in the United States, it also extends the range of the group well east of other congeners in Texas and Oklahoma (Conant and Collins, 1998). Over most of the range, A. sexlineata is a relatively well-known species. Although local and continental distributional limits of this species in most areas result from conspicuous physical features or ecological conditions, factors that exclude it from the Rio Grande Valley in Webb, Zapata, Starr, Hidalgo, and mainland parts of Cameron counties, Texas, are unresolved. Of the aforementioned counties, A. sexlineata has been recorded only in Starr (ca. 57 km from the Rio Grande; Trauth, 1992) and Webb counties (as close as ca. 7 km to the Rio Grande; Walker et al., 2001). At all sites investigated in the immediate vicinity of the Rio Grande in these counties, and in bordering areas of Coahuila, Nuevo Leon, and Tamaulipas, Mexico, habitats suitable for whiptail lizards are occupied by some combination of A. laredoensis clonal-complex A (parthenogenetic), A. laredoensis B (parthenogenetic), A. gularis (gonochoristic), and A. tigris (gonochoristic; Walker, 1987a, 1987b). Based on these data, a note by Jimeenez-Ramos et al. (1999) reporting A. sexlineata in Tamaulipas was regarded by some of us as doubtful pending further evidence.

Data obtained in our study revealed that the report of the six-lined racerunner in Tamaulipas by Jimenez-Ramos et al. (1999) was in fact based on correctly identified and well-preserved voucher specimens, which we reference herein. Purposes of this report are to provide a significant range extension for A. sexlineata in Tamaulipas, a description of habitat occupied by the species therein, a comparison of morphological variation between the Mexican sample and one of the closest known populations in the United States on South Padre Island, Texas, and a discussion of the subspecific status of the Tamaulipan population.

Materials and Methods--Field Studies--During 19842008, J. E. Cordes, M. A. Paulissen, J. M. Walker, and associates searched for A. sexlineata without success along both sides of the Rio Grande from Del Rio, Texas, and Acuna, Coahuila, to the Gulf of Mexico (Walker, 1987a, 1987b; Paulissen et al., 2001; Walker et al., 2004), although numerous new sites were discovered for A. laredoensis A and B, and A. gularis, A. tigris, or both. Records reported for A. sexlineata in northern Webb County ca. 7 km from the Rio Grande not only represent the nearest sites to the river known for the species in Texas, but also identify the only area known in which it has been observed in syntopy with both A. laredoensis A and B and A. gularis (Walker et al., 2001). These studies also revealed that A. sexlineata is the only species of whiptail lizard on Brazos and South Padre islands, Cameron County (Trauth, 1992; Paulissen et al., 1997), making them the nearest to populations of the species in Tamaulipas, Mexico. Herein, we reference 20 vouchers of A. sexlineata from South Padre Island from the University of Arkansas Department of Biological Sciences Zoology (UADZ) collection.

In 1997-1999, D. Jimenez-Ramos, E. Perez-Ramos, and J. A. Vargas-Contreras conducted field work in the herpetologically poorly known northeastern coastal part of Tamaulipas, in areas located east and southeast of Matamoros, from which they collected the first vouchers of A. sexlineata in Mexico (Jimenez-Ramos et al., 1999). These specimens are permanently stored in ethanol in the collection of the Museo de Zoologia "Alfonso L. Herrera" (MZFC) in Mexico City.

Size, Color, Scutellation, and Meristic Characters--We measured snout-vent length of A. sexlineata to the nearest 1.0 mm. Descriptions of color pattern (i.e., names of longitudinal pale stripes and intervening dark fields) include terminology used by Burt (1931), Duellman and Zweifel (1962), and Trauth (1992). We recorded relative size of mesoptychial scales along the edge of the posterior gular fold and relative size of postantebrachial scales on the posterior surfaces of forearms in each voucher of A. sexlineata and comparative material of A. laredoensis A, B, and A. gularis (Burt, 1931; Duellman and Zweifel, 1962). Meristic characters and ratios analyzed in samples of A. sexlineata included number of granular scales around midbody counted from the lateral longitudinal row of ventral scales on the left side of the body to these scales on the right side of the body, number of granular scales between the paravertebral stripes at midbody, combined number of right and left femoral pores, ratio of number of granular scales between paravertebral stripes at midbody:number of granular scales around midbody X 100, and ratio of length of vertebral (=middorsal) stripe:length of vertebral field X 100.

Statistical Treatments--We used JMP software (Version 7; SAS Institute, Inc., Cary, North Carolina, 1987-2007) to generate mean [+ or -] 1 SE and range for each character and ratio analyzed. Means were compared (P < 0.05) using Student's t-tests in JMP.

Results--Distribution--Jimenez-Ramos et al. (1999) reported a single coordinate and two beaches as sources of 10 specimens of A. sexlineata (MZFC 11178-11187) collected on 9-10 June 1997 and 29-30 April 1998 from northeastern Tamaulipas, for which we provide additional information. At present, the species is known only from the municipality of Matamoros. Therein, it has been recorded at Playa Bagdad (25[degrees]51.424'N, 97[degrees]09.015'W, 0-5 m elevation), which is 300 m west of the Gulf of Mexico, 38 km east of Matamoros, 11.4 km south of the mouth of the Rio Bravo (= Rio Grande), and 30 km south of the population of A. sexlineata in the United States on South Padre Island, Cameron County, Texas (all straight-line distances based on http://earth.google.com). Aspidoscelis sexlineata also occurs at Playa Lauro Villar (25[degrees]50.64'N, 97[degrees]09.26'W) at 1.04 km south of Playa Bagdad, and Playa Washington (25[degrees]50.90'N, 97[degrees]09.26'W) at 50 m from Playa Lauro Villar. Historically, both formation and principal natural influences and threats to terrestrial habitats at Playa Bagdad, and the other two sites, have been the maritime climate and hurricanes from the Gulf of Mexico (Williams, 1951). Recently, devastation of habitat caused by large numbers of recreational vehicles on sand dunes at Playa Bagdad has placed resident reptiles in a threatened, if not endangered, state (www.jeeperos.com/foros/showthread.php?t=49630).

Subsequent to the report of Jimenez-Ramos et al. (1999), E. Perez-Ramos and associates also collected two A. sexlineata at Mezquital (25[degrees]14.448'N, 97[degrees]26.466'W, sea level) in June 1999, which represent a significant range extension for the species southward in coastal Tamaulipas to 84 km southeast of Matamoros, 74.3 km south of Playa Lauro Villar, and 103 km south of South Padre Island. Aspidoscelis sexlineata can be expected south of Mezquital in Tamaulipas on the coast, on barrier peninsulas, or both. Mezquital is located at Puerto Mezquital, a gap that connects the Gulf of Mexico to Laguna Madre through a barrier peninsula ca. 2 km in width. This area was severely impacted by Hurricane Emily in 2005.

Habitat--Aspidoscelis sexlineata occurs in coastal Tamaulipas, a barrier peninsula of that state, and on Brazos and South Padre islands, Texas, in habitats that lack both the natural complexity and stability of more inland areas of both states. Consequently, structure of reptilian communities consists of few species at these sand-dune sites. In the dunes of Tamaulipas, the reptile fauna includes four lizards (six-lined racerunner, Texas horned lizard Phrynosoma cornutum, glass lizard Ophisaurus incomptus, and northern keeled earless lizard Holbrookia propinqua), a snake (western ribbon snake Thamnophis proximus), and a turtle (Texas tortoise Gopherus berlandieri). Aspidoscelis sexlineata is a small, moderately secretive, fast-moving, and wary ground-dwelling inhabitant of these sand dunes, some of which have ridges to 5 m in elevation and intervening narrow flats arranged parallel to the Gulf of Mexico and its prevailing winds. These lizards mostly were observed in openings near moderately thick vegetation on leeward sides of dunes, occasionally within 30-50 m of the littoral zone. At Playa Bagdad, some A. sexlineata were collected around the lighthouse in the mouth of the Rio Bravo. A variety of halophytic plants were at Tamaulipan sites, including bunchgrasses, prostrate creepers, succulents, and thorny plants (e.g., water hyssop Bacopa, legume Mimosa, legume Sophora, sea oats Uniola paniculata, beach morning glory Ipomoea pes-caprae, sea purslane Sesuvium portulaca, dropseed Sprobolus pyramidatus, Calliandria biflora, Cassia gregii, Cassia texana, golden dyssodia Dyssodia pentachaeta, shoregrass Monanthochloe littoralis, silverscale saltbush Atriplex argentea, Cortez's croton Croton cortesianus, and lamb's quarters Chenopodium murale).

Collecting--Such was the wariness, nearly constant movement, low densities of A. sexlineata, or a combination of these at four sites in Tamaulipas that success in collecting was low (i.e., 9 June 1997, two specimens; 10 June 1997, one; 29 April 1998, five; 30 April 1998, two; June 1999, two), with a mean of only two specimens/trip. On South Padre Island, results from two of three visits reflected moderate densities and extreme wariness for A. sexlineata (i.e., 5 September 1985, 1 specimen; 27 May 1986, 8; 3 September 1993, 11), with a mean of 7 individuals collected/trip.

Scutellation and Meristic Characters--Specimens from South Padre Island and Tamaulipas possess characters that are diagnostic for A. sexlineata. Mesoptychial scales bordering the edge of the posterior gular fold are enlarged. Postantebrachial scales on forearms are essentially granular (i.e., only slightly enlarged) in each individual and represent a distinct discontinuity compared to sizes of these scales in parthenogenetic A. laredoensis A and B (moderately enlarged) and in A. gularis (greatly enlarged). No significant difference (P > 0.05) between samples of A. sexlineata from South Padre Island and Tamaulipas was present in two meristic characters (number of granular scales around midbody and number of granular scales between paravertebral stripes at midbody) and one ratio (number of granular scales between paravertebral stripes at midbody:number of granular scales around midbody; Table 1). However, significant differences (P < 0.05) were noted for one meristic character (combined number of right and left femoral pores) and a ratio based on dorsal striping pattern (length of vertebral stripe:length of vertebral field; Table 1).

Color--Six males (MZFC 11179, 11181-11182, 11184, 11186-11187) and four females (MZFC 11178, 11180, 11183, 11185) of A. sexlineata from Tamaulipas exhibit the features of color pattern typical of A. sexlineata (Fig. 1). Ventrally, females are white with a hint of pale blue in adults; adult males are more distinctly bluish. Absence of three features in life, pinkish or reddish on throats, dark suffusion on chests and abdomens of males, and orange-red on tails of both sexes, distinguished these specimens from A. gularis, which is widely distributed elsewhere in Tamaulipas, including some coastal areas. We excluded A. inornata (little striped whiptail) as the species present at sites in Tamaulipas based on dorsal color pattern, number of granular scales around midbody, size of mesoptychial scales, and distance from known populations of the species in Mexico and Texas (Wright and Lowe, 1993). Dorsally, all specimens of A. sexlineata in the MZFC are vividly striped (Fig. 1). Typical of the A. sexlineata species group (Burt, 1931; Duellman and Zweifel, 1962), each of these lizards has six distinct white to cream-white primary stripes, which were likely cream to pale yellow anterior to the forelimbs in life. From ventral to dorsal on each side, the stripes include a lateral (suborbital region to hind limb), dorsolateral (superciliary scale onto the tail), and paravertebral (parietal scale to past the base of tail where fusion with stripe of opposite side occurs). Paravertebral stripes are narrowly separated by 8-10 scales in 10 specimens, nine of which possess an indistinct to moderately distinct and partial to complete vertebral stripe. Dark-colored fields between stripes are mostly brown to nearly black, except for the paler-colored field near the ventral scales on each side. Lack of pale spots in all dark fields of these specimens also rules out the possibility that any of them represent the spotted parthenogenetic species A. laredoensis A or B, both of which are limited to the vicinity of the Rio Bravo in parts of Tamaulipas.

[FIGURE 1 OMITTED]

Life colors recorded by E. Perez-Ramos and associates for specimens of A. sexlineata from Tamaulipas, some of which no longer are apparent in ethanol, included a distinctive lemon-yellow coloration on sides of the head above and below the lateral stripe, white mental and gular scales in most specimens, blue-hued chest and abdominal scales (most apparent in adult males), and gray-brown tails. In general, hues noted were more brilliant in males than females of A. sexlineata from Tamaulipas. Juveniles from Tamaulipas possessed a navy blue tail.

Discussion--Trauth (1992) described Cnemidophorus sexlineatus stephensi from the plains of southern Texas (Starr, Jim Hogg, Brooks, and Kenedy counties) and Brazos and South Padre islands (Cameron County). The trinomen was emended to C. sexlineatus stephensae by Trauth (1995) to rectify a gender issue. The species, along with all members of the C. cozumela, C. deppii, C. sexlineatus, C. tesselatus, and C. tigris species groups of North and Central America, were reallocated to the genus Aspidoscelis by Reeder et al. (2002) to partially resolve paraphyly in Cnemidophorus. These authors recognized only two subspecies of the six-lined racerunner, the nominal subspecies and A. sexlineata viridis, which would include populations herein allocated to A. sexlineata stephensae based on geographical provenance. Liner (1994) listed A. sexlineata viridis in the fauna of Mexico; however, Liner (2007) used the name A. sexlineata along with "subspecies undetermined" in reference to the taxon in the herpetofauna of the country. More recently, Liner and Casas-Andreu (2008) mentioned that specimens reported from Mexico by Jimenez-Ramos et al. (1999) were collected nearest to the range of the subspecies under discussion herein. Our recognition of the Texas yellow-headed racerunner, A. sexlineata stephensae is in concurrence with a checklist provided by the Society for Study of Amphibians and Reptiles (Crother, 2008) and that of the Center for North American Herpetology (Collins and Taggart, 2009).

Despite searches conducted over many years, we have never observed a population of A. sexlineata stephensae in syntopy with either clonal complex A or B of parthenogenetic A. laredoensis (Walker, 1987a, 1987b; Trauth, 1992; Paulissen et al., 1997). Although this subspecies is syntopic with A. gularis at some sites in the plains of southern Texas, the latter species occurs with no other congener near the Texas highway 100 bridge in Port Isabel, Cameron County (based on UADZ 1400-1403), but it is not present with A. sexlineata stephensae on South Padre Island at the opposite end of the bridge.

Whether the population of A. sexlineata in Tamaulipas is relictual, and is ancestral to A. sexlineata stephensae, or is a recent arrival and derived from it, is a question of interest. We endorse the latter explanation based on probable speciation of A. inornata and A. sexlineata in the western United States from an ancestor that occurred on the Mexican Plateau (Wright and Lowe, 1993; Reeder et al., 2002), apparent spread of A. sexlineata eastward across the country (Conant and Collins, 1998), and penetration of it into Tamaulipas via a coastal corridor, a barrier-island corridor, or both. From comparisons presented herein, it is apparent that populations of A. sexlineata from South Padre Island and Tamaulipas are weakly differentiated (i.e., in maximum snout-vent length, one meristic character, and subtle details of color; Table 1). Trauth (1992) diagnosed A. sexlineata stephensae as follows: maximum snout--vent length 68 mm; absence of a vertebral stripe (vestige may be present), and yellow coloration on the face and lateral surfaces of the neck. We examined 20 specimens of A. sexlineata from South Padre Island; the largest male (snout-vent length 57 mm) and female (snout-vent length 56 mm) provide further evidence that this insular population is of dwarf size compared to mainland populations (Trauth, 1992). Among the 10 specimens of A. sexlineata examined from Tamaulipas, the largest male (snout-vent length 64 mm) and the largest female (snout-vent length 66 mm) also are smaller than the maximum snout-vent length reported for A. sexlineata stephensae by Trauth (1992). Based on specimens available to us from Brooks and Kenedy counties, South Padre Island, and Tamaulipas, expression of the vertebral stripe is geographically variable. We noted a trend from the usual absence of a vertebral stripe in lizards on the mainland, to a less-frequent completely absent condition in lizards on islands, and when the stripe is present, it is only weakly expressed (Table 1). The vertebral stripe rarely is absent (10%) in lizards from south of the Rio Bravo in Tamaulipas and when present it is moderately strongly expressed (Table 1).

We have observed the striking basis for the aptly applied common name of A. sexlineata stephensae, Texas yellow-headed racerunner (=Texas yellow-faced racerunner of Paulissen et al., 1997), to be characteristic of freshly collected specimens, particularly males, from Webb County (e.g., Bruni along unpaved road from Texas highway 359), Starr County (e.g., 2.0 km W La Gloria by F-M 1017, then 1.4 km N on unpaved F-M 1017), Brooks County (e.g., junction U.S. highway 281 and Business 281 to Encino, 29.8 km S Falfurrias), Kenedy County (e.g., along U.S. highway 77, 59.1 km S junction of highway 77 and Texas highway 285), and Jim Hogg County (e.g., vicinity of Hebbronville). Freshly killed specimens from South Padre Island did not have either a yellow head or face per se; this color was limited to the lateral stripe and bordering scales on each side of the head.

In summary, only two of the three characters used by Trauth (1992) to diagnose A. sexlineata stephensae, yellow color of head and neck and either an absent or vestigial vertebral stripe, can be used to identify live or recently killed specimens, and only the absent or vestigial vertebral stripe is applicable to preserved specimens (yellow color fades in ethanol). Nevertheless, based on our examination of live specimens, freshly killed specimens, or both from counties included in the range in Texas, we recommend retention of this subspecific name for the variant of the six-lined racerunner in the ecologically distinctive plains of southern Texas. It also is reasonable to retain the population on South Padre Island in the subspecies at this time. Notes on color recorded for UADZ 5367-5377 from the island prior to storage in ethanol indicated only a small amount of green-yellow ventral to orbits in five specimens of 38-48 mm snout-vent length, more green than yellow in the same area in three females of 57-58 mm snout-vent length, and slightly more yellow than green on the area anterior and ventral to the eyes in three males of 46-53 mm snout-vent length. Trauth (1992) also observed less yellow on the head and neck of specimens of A. sexlineata stephensae from Kenedy County in which is located sites on the mainland that are nearest to South Padre Island. One feature of color in the population of six-lined racerunners in Tamaulipas that exceeds the diagnosis of this subspecies is development of the vertebral stripe. Among the 10 specimens from Tamaulipas, 5 have an essentially complete, although indistinct, stripe (Fig. 1). Nevertheless, we provisionally assign the population in Tamaulipas to A. sexlineata stephensae pending examination of additional specimens.

Specimens Examined--United States, Texas, Cameron County, South Padre Island: electrical substation W of Ocean Boulevard ca. 500 m S of South Padre Island Convention Center, 26[degrees]08.186'N, 97[degrees]10.331'W (5 September 1985, UADZ 1399; 27 May 1986, UADZ 1798-1802; 3 September 1993, UADZ 5367-5377); north end of Ocean Boulevard, 26[degrees]14.786'N, 97[degrees]11.185'W (27 May 1986, UADZ 1803-1805). Mexico, Tamaulipas, municipality of Matamoros: Playa Bagdad, 25[degrees]51.424'N, 97[degrees]09.015'W; Playa Washington, 25[degrees]50.90'N, 97[degrees]09.26'W; and Playa Lauro Villar, 25[degrees]50.64'N, 97[degrees]09.26'W (9 June 1997, MZFC 11178-11179; 10 June 1997, MZFC 11180; 29 April 1998, MZFC 11181-11185; 30 April 1998, MZFC 11186-11187); Mezquital at Puerto Mezquital, 25[degrees]14.448'N, 97[degrees]26.466'W (June 1999, two uncatalogued specimens).

Specimens of Aspidoscelis from Texas referenced in this report were collected under authority of permits issued to JEC and MAP by Texas Parks and Wildlife Department. Specimens from Tamaulipas were collected under authority of permits issued by appropriate agencies of government in Mexico. We are grateful to S. E. Trauth for introducing us to A. sexlineata stephensae in Webb, Jim Hogg, and Starr counties, Texas, and for other courtesies that hastened completion of the manuscript. We thank J. A. Lemos-Espinal for advice that led to collaboration on this paper.

Submitted 1 October 2008. Accepted 18 July 2009.

Associate Editor was Geoffrey C. Carpenter.

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Edmundo Perez-Ramos, Adrian Nieto-Montes de Oca, Jorge A. Vargas-Contreras, James E. Cordes, Mark A. Paulissen, and James M. Walker *

Museo de Zoologia, Facultad de Ciencias, Universidad National Autonoma de Mexico, Apartado Postal 70-399, Mexico, Distrito Federal 04510, Mexico (EP-R, AN-MO)

Facultad de Ciencias Quimico Biologicas, Universidad Autonoma de Campeche, Avenida Agustin Melgar s/n Colonia Buenavista, 24039 Campeche, Campeche, Mexico (JAV-C)

Division of Sciences, Louisiana State University at Eunice, Eunice, LA 70535 (JEC)

Department of Natural Sciences, Northeastern State University, Tahlequah, OK 74464 (MAP)

Department of Biological Sciences, University of Arkansas, Fayetteville, AR 72701 (JMW)

* Correspondent: jmwalker@uark.edu
Table 1--Descriptive statistics of Aspidoscelis sexlineata stephensae
from sites on South Padre Island, Cameron County, Texas, and sites in
the municipality of Matamoros, Tamaulipas, Mexico (numbers are mean 6
1 SE with range and size of sample beneath); * P < 0.05.

                                  South Padre          Municipality
Character                            Island            of Matamoros

Number of granular scales      88.7 [+ or -] 1.1    91.2 [+ or -] 1.4
  around midbody                   82-98 (20)           85-98 (10)

Number of granular scales       8.2 [+ or -] 0.5     9.5 [+ or -] 0.2
  between paravertebral            3-12 (20)            8-10 (10)
  stripes at midbody

Combined number of right       29.7 [+ or -] 0.3    31.8 [+ or -] 0.6
  and left femoral pores *         27-32 (20)           29-35 (10)

Number of granular scales       9.2 [+ or -] 0.5    10.4 [+ or -] 0.3
  between paravertebral          3.8-12.5 (20)        9.3-11.8 (10)
  stripes at midbody: number
  of granular scales around
  midbody X 100

Length of vertebral            48.8 [+ or -] 8.4    88.9 [+ or -] 4.5
  stripe:length of vertebral      7.8-71.8 (9)        69.4-100.0 (9)
  field X 100*

Snout-vent length              55.3 [+ or -] 0.4    58.9 [+ or -] 1.7
                                   53-57 (9)            52-66 (9)
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Author:Perez-Ramos, Edmundo; de Oca, Adrian Nieto-Montes; Vargas-Contreras, Jorge A.; Cordes, James E.; Pau
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:1MEX
Date:Sep 1, 2010
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