Diversity and Seasonal Growth Patterns of Lizards at Indio Mountain Research Center
G. Walker Johnson
UTEP Bridges Program
Aug. 9, 1998
Eleven Chihuahuan Desert lizard species from Indio Ranch Research Station, Hudspeth County, Texas were examined from Feb. ë97-August í98 for abundance, movement and growth patterns. Lizard species studied were Cnemidophorus inornatus, C. marmoratus, C. tesselatus, C. exsanguis, Cophosaurus texanus, Coleonyx brevis, Phrynosoma modestum, Uta stansburiana, Eumeces obsoletus, Urosaurus ornatus, and Crotaphytus collaris.
Two lizard species in the area are Parthenogenic. Cnemidophorus marmoratus was the most abundant species with Cophosaurus texanus being second and C. tesselatus being third most abundant. Recaptured lizards showed an increase in growth and usually remained within the same general area throughout the study period.
The University of Texas at El Paso has owned the Indio Mountain Research Station for eleven years and many researchers have had the opportunity to do field studies in this practically pristine desert environment. Indio Ranch is located ca. 26 miles southwest of Van Horn, Hudspeth County, Texas. The Ranch contains about 37,000 acres of rocky mountainous outcrops and surrounding alluvial slopes. The primary vegetation is Chihuahuan Desert Scrub, containing many species if cacti, desert shrubs and grasses, yuccas, and agaves. Elevation ranges from 3200 m to 5000 m and average rainfall is around 10 mm per year. A natural spring, Squaw Spring, contains the only permanent water in the area, however there are many ephemeral earthen tanks. Average high temperatures during the study ranged from a high of 1140F in June and 910F in August. Average low temperatures ranged from 510F in June to 720F in July. Rainfall to date (Aug. í98) is about three and a half mm making it a relatively dry summer as most of the 10 annual inches of rain falls from late June through September.
The major emphasis of this study was to record seasonal growth in lizards from the study area. Even though many studies are available for demographic structure of lizard populations (e.g. Wright and Witt, 1993; Pianka, 1986; Witt and Pianka, 1994; Gaus and Tinkle, 1977), little information on growth rates of Chihuahuan Desert Lizards is available.
My main study area is situated near the Indio Ranch house and in the vicinity of Squaw Springs.
Methods and Materials:
A transect sampling method and random capture by noose or rubber bands were used for collecting lizards. Seven transects, each containing five to nine pitfall traps 20 m apart, were arranged in the vicinity of the ranch house; four on alluvial radiating fingers from the mountain slopes, one in an arroyo directly north of the ranch house, one randomly spaced around the Ranch House, and one additional transect placed by Squaw Spring which is located about two miles north of the ranch house. A total of 49 pitfall traps were used in this study.
A pitfall trap consisted of a five-gallon bucket buried in the ground with the upper lip being flush with ground. Four two-by-four boards are placed in a cross- shaped formation leading toward the bucket, each of which was being held up by two-six inch-spiked nails. Placed on top of the two-by-fours directly over the bucket was a flat board of variable sizes that protected trapped lizards from the elements. Besides holding up the flat board, the two-by-fours also directed lizards toward the bucket. The board on top of the bucket also attracted lizards because they thermoregulate (Huey & Pianka, 1977) by moving from shade to sun thereby keeping their body temperature stable; the area under the flat boards, thus, produced a shaded area that attracted the lizards.
Specimens that fell into buckets were removed by use of hands or forceps and all lizards were then placed into gallon-sized ziplock baggies that were marked with location and time of capture. Lizards were then weighed (grams) and measured (mm) for body and tail lengths.
An identification system for mark-recapture was a toe clipping method similar to that used by Tinkle (1967). I used a number system of one through twenty starting with the specimens left front foot and ending with the right rear foot. Fingernail clippers dipped in alcohol for sterilization were used for the removal of toes. All lizards were later released at the exact position of original capture.
A total of 153 lizards of 11 species were collected through the course of this study. From those, fifteen were recaptured, thus indicating large population numbers and wide home ranges since more new lizards than recaptures were being caught. Four lizards of unknown sex were part of the total number collected. Of the others, seventy-four were males and seventy-five were females. The larger female population is probably due to the fact that 21% of all lizards captured (C. tesselatus and C. exsanguis) belong to a parthenogenic species (Scudday, 1988).
Cnemidophorus marmoratus had the greatest abundance of all lizard species captured totaling 43 individuals, 24 males, and 17 females, and also had the largest recapture rate with five. During the study period, recaptured C. marmoratus gained an average weight of 2.2g, the snout-vent length increased an average of 4.2mm, and tail length increased an average of 5.6mm ( Fig 4). Average distance of movement of recaptures of this species was about 30m.
Thirty-nine Cophosaurus texanusí were documented, 29 being males and 10 being females. Six were recaptures with an average weight increase of 1.3g, an average body length increase of 3.6mm, and an average tail length increase of 6.3mm. Total average distance of movement was 10m.
A total of 31 Cnemidophorus tesselatusí were captured, all of them being parthenogenic females. No recaptures were obtained to date, indicating a large population and wide home range. Cnemidophorus inornatus captured totaled nine, (seven being males and two being females), but none were recaptured.
Coleonyx brevis captured totaled 11, (eight being females and three males), with two recaptures. No weight gain was observed, but an average of 1.5mm increase in snout-vent length and 21mm increase tail length was recorded. The large average increase in tail length was due to the fact that one gecko had a broken tail when first caught (5-30-98) and completely regenerated when recaptured (6-12-98). Total average distance moved was 60m, giving this species the greatest distance traveled for all species studied.
A total of four Phrynosoma modestum were captured, (three being males and one female). One male was recaptured and increased in weight 3g, it grew 1mm in body length, but the tail length remained the same. This recapture was found in the same pitfall trap as it was originally captured in.
Only one Eumeces obsoletus (a male) was captured (6-19-98) during this study and it was recaptured (6-19-98). There was no body length or weight change, but the tail length increased 5mm. This specimen was found in the same pitfall trap as it was originally captured in.
Two other species (Uta stansburiana six total, four males, one female, and one juvenile) and (Urosaurus ornatus, six total, two males, three females, and 1 juvenile) were found, but yielded no recaptures. Only one Crotaphytus collaris and C. exsanguis were found, both females, but none were recaptured.
The lack of recapture data is probably due to the short duration of research time. I will continue this study through October 1998 and begin again in February 1999. More transects and pitfall traps will be constructed to expand the study to cover a broader area to better my chances at recaptures.
All lizards in this study were released at the same spot where they were recaptured. The use of the pitfall traps improved the study greatly due to the fact they are open twenty-four hours a day and require little effort to collect lizards. One lizard species found near the spring, (Eumeces tetragramus), was not included in this study because it was preserved due to the fact that it was an unexpected range extension for the species in Texas. The record will be published in Herpetological Review in the near future.
I would like to thank Dr. Jerry D. Johnson, Al Dominguez, Brett Johnson, Nic Lannutti, Travis LaDuc, and Brian Wooldridge, for helping me throughout this study, and the University of Texas El Paso for allowing me access to the Indio Ranch Research Center.
Gans, C. and D. W. Tinkle, 1977. Biology of the Reptilia, Ecology and Behavior. Academic Press, New York.
Huey, R. B., and E. R. Pianka, 1977. Seasonal Variation in the Thermoregulatory Behavior and Body Temperature of Diurnal Lizards. Ecology 58:1066-1075
Pianka, E.R. 1986. Ecology and Natural History of Desert Lizards. Princeton University Press, Princeton.
Scudday J. F., 1988. All-Female Lizard Populations-An Advanced Form of Sexual Reproduction. Chihuahuan Desert Discovery Summer 1988 4-5
Tinkle D. W., 1967. The Life History and Demography of the Side-blotched lizard, Uta Stansburiana, Misc. Publ. Mus. Zool. University of Michigan 138:1-182
Vitt, L. J. and E. R. Pianka, 1994. Lizard Ecology, Historical and Experimental Perspectives. Princeton University Press, Princeton Wright, J. W., and L. J. Vitt, 1993. Biology of Whiptail Lizards, (Genus Cnemidophorus). Oklahoma museum of Natural History, Norman?