Diet of the Texas yellow-faced racerunner, Cnemidophorus sexlineatus stephensi (Sauria: Teiidae), in southern Texas.
The six-lined racerunner, Cnemidophorus sexlineatus, is one of the most widely distributed lizards in the United States. Its geographic range extends from the foothills of the Rocky Mountains in Colorado eastward to the Atlantic coast (Conant & Collins 1991). Nearly the entire range of this species is occupied by two subspecies (and their intergrades): Cnemidophorus sexlineatus sexlineatus in the eastern U.S. and C. sexlineatus viridis from the central U.S. westward. Both subspecies occur in Texas forming a poorly defined intergrade zone in eastern and central Texas (Dixon 1987). Recently, Trauth (1992) described a third subspecies of C. sexlineatus from the South Texas Sand Plains and adjacent areas of Kenedy, Brooks, Jim Hogg, Starr and Webb counties and South Padre Island, Cameron County. Cnemidophorus sexlineatus stephensi, or Texas yellow-faced racerunner, is distinguished from the other subspecies in Texas by smaller adult size (<70 mm snout-to-vent length as opposed to 70 mm or greater for the other subspecies), frequent absence of a vertebral stripe (typically present in the other subspecies), and yellowish coloration on the face and neck (Trauth 1992).
There has been surprisingly little study of the ecology of Cnemidophorus sexlineatus in Texas. Laughlin (1958) studied the ecological relationships between C. sexlineatus and C. gularis (referred to as C. sacki) in San Patricio County; Hoddenbach (1966) reported on reproduction in a population of C. sexlineatus in Lubbock County; and Clark (1976) studied ecology and demography of a C. sexlineatus population in Brazos County. Based on the locations of these studies, it appears that none involved forms presently allocated to C. sexlineatus stephensi. Furthermore, only Laughlin (1958) analyzed lizard stomach contents; that study found that the diet of C. sexlineatus consisted almost exclusively of arthropods, especially orthopterans and spiders. Similar studies conducted in Kansas and Oklahoma report that grasshoppers, spiders, and various types of planthoppers constitute the bulk of the diet of racerunners (Fitch 1958; Hardy 1962; Paulissen 1987a). However, a study in Florida showed that termites were the numerically dominant prey in the diet of C. sexlineatus sexlineatus, although beetles and various orthopterans made a greater contribution to the total volume of prey consumed (Punzo 1990). Studies conducted in the Rio Grande Valley of Texas have shown that termites comprised the majority of the diet of both species of the parthenogenetic Cnemidophorus laredoensis complex (Paulissen et al. 1988). This may be indicative of the diet of C. sexlineatus stephensi since the geographic range of the C. laredoensis complex in Cameron, Hidalgo, Starr, and Webb counties lies only 45-50 km south of the southern limit of the known range of C. sexlineatus stephensi as reported by Walker (1987a; 1987b). Furthermore, the species of the C. laredoensis complex arose from hybrids between C. sexlineatus and C. gularis (cf. McKinney et al. 1973; Bickham et al. 1976). The purpose of this report is to provide the first description of the diet of C. sexlineatus stephensi and to compare the diet of this form to that of other C. sexlineatus populations as well as to deep south Texas populations of the C. laredoensis complex.
The three sites from which lizard diet data were obtained were as follows: (1) South Padre Island (Cameron County): low dunes, sand flats, and dirt roads adjacent to an electrical substation about 500 meters south of the South Padre Island Convention Center; (2) U.S. Highway 77 (Kenedy County): sandy roadside with patches of sunflowers and sandburs between the highway and a fenced cattle pasture along U.S. Highway 77, 50.1 km south of the junction of Highway 77 and state highway 285; (3) U.S. Highway 281 (Brooks County): sandy roadside with clumps of sandbur, patches of dense grass along a fence row, and mesquite trees in the center of the highway island at the junction of U.S. Highway 281 and Business 281 (to Encino), 29.8 km south of Falfurrias. The South Padre Island site was sampled on 27 May 1986 and on 3 September 1993 (this sample included four young-of-the-year). The Highway 77 site was sampled 4 September 1993; the Highway 281 site was sampled 27 May 1994. Lizards were collected with BB guns or by noosing, and were preserved; the snout-to-vent length (SVL) was determined to the nearest mm as a measure of lizard size. Later, the stomach contents of each lizard were removed, counted, and identified to the lowest possible taxon (usually family). The volume of intact prey items was estimated either by volumetric displacement of water in a small, calibrated cylinder or by measuring the length and width of prey items and using the following formula (Vitt et al. 1993) for the volume of a prolate spheroid:
volume = 4/3 [pi] (0.5 length)(0.5 width)[.sup.2]
The former method was used for the South Padre Island samples, the latter for the Highway 77 and Highway 281 samples. Volume estimates of selected prey items computed by the two methods were very close (e.g., for termites, volumetric displacement gives an estimate of 1.6 [mm.sup.3]; the prolate spheroid formula gives an estimate of 1.4 [mm.sup.3]). Correlations between lizard SVL and prey measures were computed using Spearman rank correlations for the pooled samples.
RESULTS AND DISCUSSION
A summary of the diet of Cnemidophorus sexlineatus stephensi is presented in Table 1. For brevity, the insects are listed by taxonomic order; a complete breakdown of the insect prey listed by family is available from the senior author upon request. Orthopterans and spiders (Araneae) were the dominant prey items in most of the samples in this study. Among the orthoptera, crickets (Gryllidae) were the most important prey items in the May 1986 South Padre Island sample and the September 1993 Highway 77 sample, but short-horned grasshoppers (Acrididae) comprised the bulk of the prey consumed by lizards in the May 1994 Highway 281 sample. By contrast, lizards collected September 1993 from South Padre Island consumed few orthopterans, but one lizard in that sample did consume 10 dictyopharids (a large planthopper: Homoptera). Two lizards in the Highway 77 sample consumed large leaf-footed bugs (Hemiptera: Coreidae); three others consumed several large (12 mm) alate ants (Hymenoptera). Termites (Isoptera) comprised a substantial numerical percentage of lizard diet in three of the samples, but constitute only a minor percentage volumetrically. This is because termites are small and because few lizards actually consumed termites. Only one lizard in each of the two South Padre Island samples and three lizards in Highway 77 sample consumed termites (and in the latter sample, two of the three lizards had eaten only one termite). Termites are generally patchily distributed; apparently if a racerunner finds a patch of termites, it will eat many of them at a time. However, this obviously happens infrequently given that only 3 of the 34 lizards analyzed for this study had consumed more than one termite. This contrasts with the situation for the Cnemidophorus laredoensis complex of unisexual lizards which consume huge numbers of termites (Paulissen et al. 1988). Presumably the difference arises from differences in the availability of termites in C. sexlineatus stephensi versus C. laredoensis habitats rather than differences in feeding preferences given the close genetic relationship of these lizard species.
There was no significant correlation between lizard SVL and the number of prey consumed (r = +0.08; P > > 0.05). However, there was a significant positive correlation between lizard SVL and total volume of prey consumed (r = +0.44; P = 0.03) and a marginally non-significant positive correlation between lizard SVL and mean volume of individual prey items consumed (r = +0.33; P = 0.11). These relations hold because the September 1993 South Padre Island sample included four small, young-of-the-year individuals which consumed small prey and the lizards in the Highway 281 sample (which were all adult-sized) consumed large grasshopper prey (Table 2). When the four young-of-the-year are removed from the analysis, both the correlation between lizard SVL and total prey volume and the correlation between lizard SVL and mean volume of individual prey items consumed become statistically non-significant (P > > 0.05).
Overall, the results presented here suggest the diet of Cnemidophorus sexlineatus stephensi in south Texas is more similar to the diet of other C. sexlineatus populations in the central U. S. (San Patricio County of Texas, Kansas and Oklahoma) than to either the diet of C. sexlineatus in Florida, or the closely related unisexual species of the C. laredoensis complex in the Rio Grande Valley. This similarity may be due to similarities in food preferences and feeding behaviors among populations of C. sexlineatus in the central U. S. However, it may be that C. sexlineatus populations everywhere are basically generalists, feeding on whatever arthropods they encounter (Paulissen 1987b). If so, diet similarities may reflect nothing more than similarities of the prey base among different areas. Study of other populations of C. sexlineatus in Texas is needed to fill in gaps in the understanding of the ecology of this species as well as to clarify the status and distribution of the subspecies in the state.
Table 1. Diet of the lizard Cnemidophorus sexlineatus stephensi in southern Texas as determined from analyses of stomach contents of four samples (see text for locations and descriptions of the sites). The numerical and volumetric percentage respectively of the prey taxon in the stomach contents sample is given. Site S. Padre S. Padre Hwy 77 Date 27 May 1986 3 Sept. 1993 4 Sept. 1993 Lizards Sampled (N = 8) (N = 6) (N = 8) Prey Percent Percent Percent Percent Percent Percent Taxon by by by by by by Number Volume Number Volume Number Volume ISOPODA 1.9% 1.1% 2.6% 3.5% AMPHIPODA 1.9% 1.1% ARANEAE 28.8% 26.3% 23.4% 28.7% 7.2% 10.1% CHILOPODA 0.9% <0.1% INSECTA Orthoptera 18.8% 57.7% 1.3% 6.3% 29.7% 42.2% Isoptera 13.4% 0.8% 46.7% 7.2% 38.7% 3.4% Hemiptera 3.8% 1.8% 2.7% 20.9% Homoptera 15.6% 21.7% 0.9% <0.1% Neuroptera 1.9% 1.4% Coleoptera 7.6% 4.2% 2.6% 2.1% (Adult) Coleoptera 5.8% 3.6% 2.6% 9.1% 1.8% 0.3% (Larvae) Lepidoptera 1.3% 12.1% 4.5% 5.2% (Larvae) Hymenoptera 1.9% 0.1% 2.6% 0.2% 11.7% 17.6% Insect Pupae 1.3% 8.8% Unidentified 3.8% 2.1% 1.8% <0.1% Site Hwy 281 Date 27 May 1994 Lizards Sampled (N = 12) Prey Percent Percent Taxon by by Number Volume ISOPODA AMPHIPODA ARANEAE 25.9% 3.7% CHILOPODA INSECTA Orthoptera 37.0% 85.1% Isoptera Hemiptera Homoptera 11.1% <0.1% Neuroptera Coleoptera (Adult) Coleoptera (Larvae) Lepidoptera (Larvae) Hymenoptera 7.4% 0.6% Insect Pupae 3.7% 10.6% Unidentified 11.1% <0.1% Table 2. Mean [+ or -] standard deviation of lizard snout-to-vent lengths (mm), number of prey per lizard, total volume of prey per lizard ([mm.sup.3]), and mean volume of individual prey items ([mm.sup.3]) from each of the four samples of Cnemidophorus sexlineatus stephensi. Numbers in parentheses represent minimum and maximum values. S. Padre S. Padre Hwy 77 May 1986 Sept. 1993 Sept. 1993 (N = 8) (N = 6) (N = 8) Snout-to-Vent 56[+ or -]1.8mm 47[+ or -]8.2mm 60[+ or -]3.8mm Length (mm) (54-59mm) (37-60mm) (53-65mm) Number of 7[+ or -]4.0 13[+ or -]13.3 14[+ or -]17.2 Prey Items (3-13) (2-38) (1-53) Total Volume 203[+ or -]113.7 141[+ or -]103.3 287[+ or -]179.5 of Prey [mm.sup.3] [mm.sup.3] [mm.sup.3] [mm.sup.3]) (68-415[mm.sup.3]) (13-309 (18-514[mm.sup.3]) [mm.sup.3]) Volume of 25.6[+ or -]10.1 13.7[+ or -]6.8 20.4[+ or -]11.8 Individual [mm.sup.3] [mm.sup.3] [mm.sup.3] Prey (11.5-38.7 (4.6-20.6 (4.3-34.8 ([mm.sup.3]) [mm.sup.3]) [mm.sup.3]) [mm.sup.3]) Hwy 281 All May 1994 Samples (N = 12) (N = 34) Snout-to-Vent 59[+ or -]3.7mm 56[+ or -]6.3mm Length (mm) (53-66mm) (37-66mm) Number of 2[+ or -]1.5 8[+ or -]11.0 Prey Items (1-6) (1-53) Total Volume 169[+ or -]172.8[mm.sup.3] 200[+ or -]146.2[mm.sup.3] of Prey (46-515[mm.sup.3]) (13-515[mm.sup.3]) ([mm.sup.3]) Volume of 136.1[+ or -]190.6[mm.sup.3] 48.0[+ or -]101.0 Individual [mm.sup.3] Prey (11.5-515.0[mm.sup.3]) (4.3-515.0[mm.sup.3] ([mm.sup.3])
Specimens were obtained under the authority of Scientific Collecting Permits issued to the authors by the Texas Parks and Wildlife Department (1986: number 61; 1993-1994: number SPR-0691-408). Support for travel to Texas was provided by a University of Arkansas, J. William Fulbright College of Arts and Sciences Research Incentive Grant to JMW in 1986 and by the McNeese State University Miller Endowed Professorship of Science awarded to MAP in 1993.
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Mark A. Paulissen, James M. Walker and James E. Cordes
Department of Biological and Environmental Sciences, McNeese State University
Lake Charles, Louisiana 70609; Department of Biological Sciences
University of Arkansas, Fayetteville, Arkansas 72701; and Division of Sciences
Louisiana State University at Eunice, Eunice, Louisiana 70535
MAP at: email@example.com
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|Author:||Paulissen, Mark A.; Walker, James M.; Cordes, James E.|
|Publication:||The Texas Journal of Science|
|Date:||May 1, 1997|
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