STUDIES ON THE NATURAL HISTORY AND ECOLOGY OF SYMPATRIC WHIPTAIL LIZARDS (CNEMIDOPHORUS MARMORATUS AND C. TESSELATUS) FROM MADERA CANYON (BREWSTER COUNTY, TEXAS).
There have been extensive studies focusing on various aspects of the life histories and ecology of the whiptail lizards, Cnemidophorus marmoratus ( = C. tigris marmoratus of Mitchell 1979) and C. tesselatus throughout much of their range in the southwestern United States (Pack 1923; Zweifel 1965; Knopf 1966; McCoy 1968; Pianka 1970; 1986; Parker 1972; Asplund 1974; Parker & Selander 1976; Vitt & Ohmart 1977; Schall 1978; Cuellar 1979; Mitchell 1979; Leuck 1982; Case 1983; Anderson & Karasov 1988; Anderson 1993; Paulissenet al. 1993; Taylor & Buschman 1993; Vitt & Breitenbach 1993; Taylor et al. 1992; 1994; 1997; Walker et al. 1998), including the Big Bend region of Texas (Tinkle 1959; Degenhardt 1966; Milstead 1957a; 1957b; 1965; Pianka 1970; Saxon 1970; Scudday & Dixon 1973; Walker et al. 1991).
The marbled whiptail of Trans-Pecos Texas (C. marmoratus), considered by some to be a subspecies of C. tigris (C. tigris marmoratus) sidered by some to be a subspecies of C. tigris (C. tigris marmoratus) (Dessauer & Cole 1991), has received little attention. This whiptail, as well as the checkered whiptail C. tesselatus, and the southwestern earless lizard Cophosaurus texanus scitulus, are the three most common lizards found in Madera Canyon (Punzo 2000a), a site located near the southwestern boundary of Big Bend Ranch State Park (BBRSP; Brewster County, Texas). The ecology of C. texanus scitulus at this location has been described in detail (Punzo 2000a). Cnemidophorus marmoratus is found throughout Trans-Pecos Texas (Chihuahuan Desert) and is most commonly associated with open, sandy areas with sparse vegetation (Bartlett & Bartlett 1999). Cnemidophorus tesselatus (part of the C. tesselatus complex of Taylor et al. 1996) ranges from Trans-Pecos Texas north to southeastern Colorado (Conant & Collins 1998), and prefers the more rocky areas of floodplains, canyons and foothill uplands (Degenhardt 1966; Garnett & Barker 1987; Walker et al. 1998). The purpose of the present study was to describe various aspects of the natural history and behavioral ecology of C. marmoratus and C. tesselatus in Madera Canyon (MC) in order to determine the ways in which these two teiids interact with one another.
Madera Canyon (29[degrees]17'30" N; 103[degrees]55'04" W) is approximately 3.1 km in length and is located at an elevation of approximately 900 m. The canyon floor contains areas with loose, sandy soils, compact adobe soils, and gravel hardpan, with an abundance of exposed rocks of varying size. This canyon lies within the creosote-lechuguilla-cactus association of the Chihuahaun Desert (Punzo 1998; 2000b). For a detailed description of the vegetation of this region the reader should consult Tinkham (1948) and Powell (1988).
MATERIALS AND METHODS
Cnemidophorus marmoratus and C. tesselatus are strictly diurnal and were abundant at this site, where their foraging activities and interactions were monitored throughout most of the day. Preliminary collections of lizards indicated that C. marmoratus seemed to prefer sandy substrates with reduced vegetation, while C. tesselatus was more typically observed on adobe soils with varying amounts of scattered pebbles and rocks. Nevertheless, these two lizards were frequently observed foraging within close proximity to one another ([less than] .5 in). In view of this, the potential for competitive interactions was very real. These lizards were observed at close distances through binoculars or through a Vivitar 400 mm lens attached to a Nikon Fe2 35mm camera mounted on a stationary tripod. Observations were made over the course of several months (April through August, 1998), between 0830 - 1600 hr CST, and their behavioral responses toward one another and foraging behaviors were recorded. In addition, it was possib le to record their tongue-flicking rates (TFR = number of TF / min) when engaged in basking, active foraging, and during periods when they came within close proximity of one another.
All observations and lizard collections were conducted on a 2.2 ha portion of the canyon. This area was gridded with wooden stakes and thoroughly censused between April through August, 1998. Additional sites along the canyon walls were also sampled 2 - 3 times / wk. Lizards were collected by noosing or through the use of pitfall traps (Punzo 2000a) and provided with a unique identification number via toe clipping and paint marks as described by Punzo (1982). The paint marks allowed identification of individuals at a distance through binoculars. Pitfall traps were checked on a daily basis. Voucher specimens have been deposited in the University of Tampa Vertebrate Collection.
For each lizard the snout-vent length (SVL) was measured to the nearest 0.1 mm with vernier calipers. Cloacal temperatures were recorded with a Schultheis quick-reading thermometer. The soil type, air temperature, time of capture, and physical features of the microhabitat were also recorded. Data on time of capture were used to determine diel periodicity. All lizards were released within 48 hr of capture at the original capture site.
The diet composition for these two lizards was determined through the use of a stomach flushing technique as described by Legler & Sullivan (1979). Stomach contents were collected from 90 adult lizards for each species. Only lizards captured for the first time were used in stomach content analyses. Following stomach flushing, lizards were kept in temporary holding cages for 48 hr and then released at their capture sites. Prey items were identified to Order and Family wherever possible. Diet composition was expressed as the number of prey found in the stomachs, the percentage prey occurrence by volume, and the percent frequency as described by Punzo (1990).
For trophic niche estimates, the length (a) and width (b) of each prey item (exclusive of the legs and antennae) was measured to the nearest 0.01 mm by using a Unitron Model 81A dissecting microscope fitted with an ocular micrometer. The volume of each prey item was then determined by using the equation for the volume of a prolate spheroid as described by Dunham (1983) and Punzo (1992): V = 4/3 [pi] (a / 2) [(b / 2).sup.2].
Levin's measure of niche breadth (B) was calculated from data based on size category (percent volume, PV) of food items as described by Krebs (1989). Values can range from 0 (prey species in only one resource category) to 1.0 (species represented equally in all resource categories). Dietary and temporal niche overlap (D) were estimated using the method described by Floyd & Jenssen (1983), based on the following equation:
D = 1 - 1/2 [[[sigma].sup.n].sub.i=1] [[P.sub.xi] - [P.sub.yi]]
where [[P.sub.xi] and [P.sub.yi]] are frequencies of prey categories or time intervals for periods of activity, for species x and y, respectively, for the I th category. The index D, can range from 0 (no overlap) to 1.0 (complete overlap). Frequencies (%) of occurrence for various prey taxa (trophic niche), or time intervals of foraging activity (temporal niche) were used for the determination of D (Schoener 1968; Floyd & Jenssen 1983).
All statistical procedures followed those described by Sokal & Rohlf (1995).
RESULTS AND DISCUSSION
The SVL for C. marmoratus collected in pitfall traps or by noosing ranged from 73 - 97 mm (mean: 83.1 [+ or -] 0.58 SE; n = 133). For C. tesselatus, values ranged from 68 - 101 (mean: 85.1 [+ or -] 0.44; n = 108). The average air temperature at time of capture for both species was 27.3 [+ or -] 0.13 SE. There were no significant differences in the cloacal temperatures between C. marmoratus (mean: 38.5 [+ or -] 0.03 SE) and C. tesselatus (mean: 37.8 [+ or -] 0.04) (t = 0.94, P [greater than] 0.5). Similar cloacal temperatures have been reported (Parker 1972; Cuellar 1979; Mitchell 1979; Pianka 1986; Anderson & Karasov 1988; Walker et al. 1991) in foraging individuals of C. tigris, C. tesselatus, C. uniparens, C. inornatus and C. gularis.
The diet composition for these lizards is shown in Table 1. The results indicate that these lizards are generalist predators that will consume a wide variety of arthropod prey. This is in general agreement with previous studies on diet composition in teiid lizards of similar size that are found in similar habitats (Pack 1923; Milstead 1957b; 1958; McCoy 1968; Scudday & Dixon 1973; Mitchell 1979; Pianka 1986; Anderson 1993; Paulissen et al. 1993; Eifler & Eifler 1998). Measurement of trophic niche breadth indicated that most prey species are represented by relatively few resource categories (B = 0.44). The results also show that there was a high degree of dietary overlap between C. marmoratus and C. tesselatus (D = 0.78).
At MC, the major prey items (PV) for C. tesselatus and C. marmoratus included insects such as beetles (37.7 and 26.5%, respectively), orthopterans (24.1 and 24.9%), termites (12.1 and 9.9%), lepidopteran larvae (9.2 and 10.5%) and spiders (10.1 and 8.6%). Individuals of both species tongue-flicked regularly and were observed digging in sandy soil and using their forelimbs to explore under surface debris. As a result, they were able to find scorpions and the larvae of beetles and ant lions (although these prey types accounted for only a small proportion of their diets).
One individual of C. marmoratus was observed in the act of capturing a scorpion. This potentially dangerous prey was grasped by the cephalo-thorax, vigorously shaken, released, quickly grasped and shaken again, and then rubbed forcefully against the ground surface until all movements ceased. This behavior was not observed at any time when non-dangerous types of arthropods were captured. O'Connell & Formanowicz (1998) observed a similar behavioral pattern in the handling of scorpions by captive Texas spotted whiptails, Cnemidophorus gularis from Terrell County, Texas. When juveniles or adults of C. gularis encountered a scorpion, it was grasped in the jaws, shaken violently, and often thrown across the cage. This was followed by repeated attacks until the scorpion was immobilized. Non-dangerous prey such as crickets were grasped and sometimes rubbed against the ground, but never shaken or thrown.
Other species of arthropods such as meloid beetles, millipedes and velvet ants (mutillids), which were common at this site and are known to possess effective chemical defenses (Punzo 2000b), were not found in the stomachs of these lizards. This suggests that they learn to avoid arthropods on the basis of unpleasant encounters with them early in life.
The diel periodicity for C. marmoratus and C. tesselatus is shown in Table 2. Both species are strongly diurnal. No individual of either species was observed at the ground surface before 0500 or after 1900 hr CST. There was no significant difference in time of activity between spring and summer months at this site (G = 1.1, P[greater than] 0.5) Most of the individuals of C. marmoratus (36.7 - 36.9%) and C. tesselatus (38 - 40.6%) were observed at the surface between 0700 - 1059 hr. Basking occurred primarily between 0700 - 0830 hr, when soil temperatures reached 28 - 30[degrees]C, followed by foraging activities. Activity decreased markedly after 1300 hr when soil temperatures reached 50[degrees]C. The index for temporal niche overlap (D) yielded a value of 0.71 and 0.78 for spring and summer, respectively, indicating a high degree of overlap in activity patterns for these two species.
Pianka (1970) reported a bimodal pattern of activity for C. tigris from the Sonoran Desert, with peak periods of activity between 0700 - 1200, and 1400 - 1530. Mitchell (1979) also reported a bimodal activity pattern for C. inornatus (0900 - 1159; 1500 - 1730 hr) and C. uniparens (0700 - 100; 1600 - 1800 hr) from southeastern Arizona. Milstead (1957a) found that the peak period of activity for C. exsanguis, C. tesselatus and C. tigris from Trans-Pecos Texas was between 0800 - 1130 hr and ended between 1100 - 1300 hr. It remained low for the rest of the afternoon and then increased again between 1630 - 1730 hr. Bimodal activity patterns have also been reported for representatives of a number of lizard families inhabiting the deserts of North America, Africa, and Australia (Pianka 1986). Interestingly, at the MC site, the activity period was not bimodal, and afternoon activity levels remained low.
The utilization of various microhabitats by C. marmoratus and C. tesselatus is shown in Table 3. The classification of microhabitats follows that described by Pianka (1986). Cnemidophorus marmoratus prefers open areas in the sun (30.8%) or in the shade (36.5%), with little vegetation and rock cover. Cnemidophorus tesselatus, in contrast, was most frequently observed basking or foraging near rocks exposed to direct sunlight (38.8%) or foraging within shaded areas of rock cover (25.6%). There were not many locations on the floor of MC where grasses were found, which may account for the low frequencies observed for grassy areas.
Neither species exhibited any intra- or interspecific aggression whenever they were observed within close proximity ([less than] 0.5 m) to one another. They avoided any physical contact, and once aware of the other's presence they would move away from each other. This is consistent with previous studies which have indicated a reduced level of aggression and general lack of home-range defense in lizards of the genus Cnemidophorus (Stamps 1977; 1994; Eifler & Eifler 1998). It has been argued that this reduced level of aggression may be related in some part to their preference to forage on the ground, close to bushes, grasses, or surface debris which would obstruct their visual field, coupled with their reluctance to climb or use elevated perch sites (Stamps 1977). Both of these factors would reduce visibility thereby making it more difficult to locate potential intruders. In addition, although many teiids utilize burrows as shelter sites, no species has been shown to consistently defend its burrow against intr uders (Leuck 1982). In contrast, Carpenter (1960) found some level of intraspecific aggression and territorial defense in C. sexlineatus from Oklahoma.
In comparison to actively foraging C. tesselatus (mean TFR: 11.4 [+ or -] 1.9 SE), C. marmoratus exhibited a significantly higher TFR (t = 4.8, P [less than] 0.05) when foraging (17.3 [+ or -] 2.3) and when in close proximity to C. tesselatus (12.8 [+ or -] 2.8 vs. 7.1 [+ or -] 1.3, P [less than] 0.05). No significant interspecific differences were found (P [greater than] 0.5) when basking (8.5 [+ or -] 1.1 vs. 6.9 [+ or -] 0.8, for C. marmoratus and C. tesselatus, respectively), or when a conspecific was approached (8.9 [+ or -] 0.8 vs. 7.5 [+ or -] 1.2). Cnemidophorus tesselatus exhibited lower TFR than C. marmoratus under all behavioral states which is interesting in view of the fact that C. tesselatus has been described as being less wary, and less frenetic in its movement patterns when compared to C. marmoratus and C. tigris (cf. Garrett & Barker 1987; Conant & Collins 1998). Since teiids rely heavily on tongue extrusions to sample the environment for olfactory cues associated with potential prey, preda tors or conspecifics (Simon 1983; Cooper 1997), less wary individuals might be expected to exhibit lower TFR.
In summary, C. marmoratus and C. tesselatus in Madera Canyon have similar foraging behaviors, diets, thermal requirements and temporal activity patterns. As a result, the potential for interspecific competition is high. The Trans-Pecos region of Texas can experience annual periods of drought (Medellin-Leal 1982; Punzo 2000b) which have been shown to reduce available arthropod prey (Milstead 1957b; Punzo 1998) and increase intra- and interspecific competition in sympatric desert lizards (Schoener 1977; Dunham 1983; Scheibe 1987; Pianka 1986; Nunez et al. 1989; Punzo 2000a).
This study was supported by a Faculty Development Grant from the University of Tampa. I wish to thank T. Punzo, J. Bottrell, T. Ferraioli, C. Mendez and B. Trivett for assistance in collecting and observing specimens in the field, and C. Bradford, K. Smart, H. Shaw, Jim Dixon and anonymous reviewers for commenting on an earlier draft of the manuscript. B. Garman provided valuable consultation on statistical procedures. This research was conducted with the permission of the Texas Parks & Wildlife Department, Permit # 66-98.
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Table 1. The percentage occurrence by volume of each prey type found in the stomachs of Cnemidophorus marmoratus and C. tesselatus at Madera Canyon. n = 90 adult lizards for each species. NP = number of prey; PV = percent volume; PF = percent frequency; Undet. = undetermined A - adults; N = nymphs; L = larvae. See text for details. Cnemidophorus tesselatus Prey taxon NP PV PF Arachnida (A) Acarina Araneae 80 10.1 68.8 Scorpionida 7 0.8 7.7 Solifugae Undet. 9 1.7 10.0 Chilopoda (N) 3 0.2 3.3 Insecta Blattoidea (A, N) 3 0.2 3.3 Coleoptera Carabidae (A) 33 19.7 35.5 Scarabaeidae (A) 9 5.9 10.0 Tenebrionidae (A) 4 0.4 4.4 Undet. (A,L) 46 11.7 38.8 Diptera (A) 3 0.1 3.3 Hemiptera (A,N) 11 1.9 12.2 Homoptera (A) 6 0.5 6.6 Hymenoptera Formicidae (A) 16 1.1 15.5 Isoptera (A) 604 12.1 97.7 Lepidoptera (L) 27 9.2 23.3 Neuroptera Myrmeleontidae (L) 9 0.3 8.8 Orthoptera Acrididae (A,N) 35 8.7 32.2 Gryllidae (A) 13 3.6 14.4 Tettigoniidae (A,N) 5 0.5 5.5 Undetermined (A,N) 65 11.3 55.5 Total 988 Cnemidophorus marmoratus Prey taxon NP PV PF Arachnida (A) Acarina 11 0.8 8.8 Araneae 67 8.6 60.0 Scorpionida 12 1.1 11.1 Solifugae 9 0.6 10.0 Undet. 6 1.1 5.5 Chilopoda (N) Insecta Blattoidea (A, N) Coleoptera Carabidae (A) 22 11.6 21.1 Scarabaeidae (A) 3 0.4 3.3 Tenebrionidae (A) 2 0.1 2.2 Undet. (A,L) 54 14.4 46.6 Diptera (A) Hemiptera (A,N) 16 2.7 17.7 Homoptera (A) 3 0.2 3.3 Hymenoptera Formicidae (A) 24 1.6 20.0 Isoptera (A) 447 9.9 86.6 Lepidoptera (L) 36 10.5 31.1 Neuroptera Myrmeleontidae (L) 14 1.1 12.2 Orthoptera Acrididae (A,N) 21 6.8 23.3 Gryllidae (A) 23 7.8 20.0 Tettigoniidae (A,N) 7 0.4 7.7 Undetermined (A,N) 56 9.9 51.1 Total 833 Table 2. Daily activity patterns ofCnemidophorus marmoratus C. tesselatus at Madera Canyon (BrewsterCounty, Texas). Data expressed as percent of individualsactive at a particular time interval (CST). Actual number oflizards observed at each time interval given in parentheses.Spring (1 April - 20 June); Summer (21 June - 31 August). Seetest for details. Cnemidophorus marmoratus Time of activity Spring Summer 0500 - 0659 0 0.7 (1) 0700 - 0859 14.3 (19) 18.7 (25) 0900 - 1059 22.6 (30) 18.0 (24) 1100 - 1259 7.5 (10) 3.8 (5) 1300 - 1459 5.3 (7) 1.5 (2) 1500 - 1659 3.0 (4) 1.5 (2) 1700 - 1859 0.7 (1) 2.2 (3) 1900 - 2059 0 0 Cnemidophorus tesselatus Time of activity Spring Summer 0500 - 0659 1.8 (2) 0 0700 - 0859 19.4 (21) 13.9 (15) 0900 - 1059 21.2 (23) 24.1 (26) 1100 - 1259 5.6 (6) 3.7 (4) 1300 - 1459 3.7 (4) 0.9 (1) 1500 - 1659 2.8 (3) 0.9 (1) 1700 - 1859 0 1.8 (2) 1900 - 2059 0 0 Table 3. Percentage utilization of various microhabitats by Cnemidophorus marmoratus (n = 285) and C. tesselatus (n = 188) at Madera Canyon. Values represent the percent of individuals observed in each microhabitat. Numbers in parentheses represent the actual number of lizards observed. See text for details. Microhabitat Cnemidophorus marmoratus Cnemidophorus tesselatus Open sun 30.8 (88) 7.4 (14) Grass sun 0.7 (2) 4.2 (8) Bush sun 9.8 (28) 5.9 (11) Rock sun 2.5 (7) 38.8 (73) Open shade 36.5(104) 5.3 (10) Grass shade 1.8 (5) 3.7 (7) Bush shade 15.7 (45) 9.0 (17) Rock shade 2.1 (6) 25.6 (48)
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|Publication:||The Texas Journal of Science|
|Article Type:||Statistical Data Included|
|Date:||Feb 1, 2001|
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