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Life history characteristics of three morphotypes in the parthenogenetic Cnemidophorus dixoni complex (Sauria: Teiidae) in Texas and New Mexico.

ABSTRACT. -- We compared life history characteristics of three morphotypes (or pattern classes A-C) of the parthenogenetic lizard, Cnemidophorus dixoni. A and B were studied in Trans-Pecos Texas from 1989 through 1993; it was not possible to study C in Hidalgo County, New Mexico. The peak activity period for both C. dixoni A and B appeared to be between 1045 and 1245 Central Daylight Time. No significant differences were found in cloacal body temperature between C. dixoni A and B; however, mean air temperatures at sites of collection were higher for B than for A. C. dixoni C grows to a larger adult body size than either A or B. Populations of A and B contain three age classes; C includes either a fourth age class or has a more rapid growth rate. C. dixoni C also has a larger clutch size than A and B. The population of C. dixoni in western New Mexico is not a disjunct population of B located some 500 kilometers west of Presidio County, Texas. It is a distinctive form worthy of efforts to insure its survival. Key words: Cnemidophorus dixoni; parthenogenesis; activity; body temperature; body size; age classes; reproduction.


Zweifel (1965) briefly characterized an unusual population of parthenogenetic whiptail lizards known to occur only in the Antelope Pass area in the Peloncillo Mountains, Hidalgo County, New Mexico. This form was allocated to Cnemidophorus tesselatus and informally designated pattern class F, one of six morphotypes recognized by Zweifel (1965). Subsequently, Scudday (1973) noted the presence of two somewhat similar parthenogens in the Chinati Mountains, Presidio County, Texas. Scudday maintained that his newly discovered populations and pattern class F should be allocated to the new species, Cnemidophorus dixoni. He identified two color pattern classes of C. dixoni; A, a population in Presidio County from which the holotype was designated, and B which included Zweifel's pattern class F in Hidalgo County and the population that differs from A in Presidio County.

Our unpublished analyses of morphological variation identified three morphotypes in C. dixoni which we call A and B, in southwestern Texas, and C, found in western New Mexico. We agree with Scudday (1973) on the identity of C. dixoni A (lizards with irregular dorsal lines in the Chinati Mountains), but we find that his pattern class B actually includes two distinctive forms. Thus, we allocate lizards with fragmented dorsal lines in the Chinati Mountains to morphotype B, and reallocate lizards with fragmented dorsal lines and distinctive meristic features in the Peloncillo Mountains to morphotype C.

Whether these three morphotypes are best treated as populations without formal taxonomic recognition, separate species, subspecies of C. dixoni, or pattern classes of C. tesselatus is subject to divergent opinions (Zweifel, 1965; Scudday, 1973; Cole, 1985; Walker, 1986; Frost and Wright, 1989; Densmore et al., 1989; Wright, 1993). It is certain, however, that the three pattern classes of C. dixoni are diagnosable entities with different patterns of distribution. Because C. dixoni A and B coexist at several sites in Presidio County that were unknown to Scudday (1973), being allopatric at other sites, and both forms are widely disjunctive and morphologically distinct from C. dixoni C in Hidalgo County, we analyzed these pattern classes separately. Here, we present life history data for C. dixoni A and B, with emphasis on specimens acquired in 1991, 1992, and 1993, and compare these forms with a sample of C. dixoni C.


We necropsied 233 specimens of C. dixoni A, 97 specimens of B, and 27 specimens of C. We were unable to study the Hidalgo County form in the field, which today is variously considered as threatened or endangered. Fortunately, we acquired two live adults of C. dixoni C through the New Mexico Department of Game and Fish and we examined preserved specimens in the American Museum of Natural History and University of Colorado Museum. Most specimens of C. dixoni A and B were collected from 1988 through 1993 by JEC during over 100 field days between the Rio Grande and parts of San Antonio Canyon in the Chinati Mountains, Presidio County (see Appendix). Specimens of A and B deposited in Sul Ross State University were also examined. Field data included Central Daylight Time (CDT), cloacal body temperature (CBT) taken with a Schultheis quick-reading thermometer, and air temperature (AT) 5-6 centimeters above the capture site. Prior to injection with 10% formalin, fresh body mass (FBM) was determined with a Pesola balance to 0.1 gram (g). The snout vent length of each preserved lizard (PSVL) was also measured to millimeter (mm) after storage in 70% ethanol. Clutch size was based on counts of vitellogenic follicles ([greater than or equal to]3.0 mm) and/or oviductal eggs, both measured to the nearest 0.5 mm. Means are reported [+ or -]1 SE.


Daily Activity

Based on 1989 and 1990 data, we suggested that the peak activity period for C. dixoni in Presidio County occurred between 1045 and 1245 (Walker et al., 1991). Addition of the May-June 1991, May-July 1992, and June-July 1993 time records to the data base for C. dixoni did not alter our basic conclusion about the peak activity period. Comparison of activity data for C. dixoni A and B with data presented by Schall (1993; Fig. 2) for other species of Cnemidophorus in TransPecos Texas indicates that both forms resemble gonochoristic C. marmoratus (=C. tigris) more closely than parthenogenetic C. tesselatus in the length of the daily activity period (for discussion of evolutionary relationships see Parker and Selander, 1976; Densmore et al., 1989).

Collection times were grouped by 15 minute intervals for University of Arkansas specimens of C. dixoni A (N=204) and B (N=72). For A, 29.4% were observed between 1045 and 1245 and 21.4% between 1245 and 1445; activity decreased between 1445 and 1645 (14.1%) and a second peak occurred between 1645 and 1845 (23.9%). Only 10.1% of all C. dixoni A were found after 1845. For B, collection time data were biased toward the afternoon (19.4% before 1445 and 80.5% after 1445). We predict that a larger sample of B would be found to have a bimodal pattern of daily activity similar to that of A.

Cloacal Body Temperature and Air Temperature

C. dixoni is similar to other species of Cnemidophorus in having an unusually high mean CB temperature compared with many saurians in other families (see summary of papers in Schall, 1976 [Table 15]; Walker et al., 1991). Addition of 1991 and 1992 temperatures (none taken in 1993) to the 1989-1990 data for C. dixoni permitted us to compare CBT and AT data for A and B. We found no significant differences (Duncan's Multiple Range Tests; P of [alpha] = 0.05) in mean CBT either between A (39.71 [+ or -] 0.13, 34.0-43.2, N=157) and B (40.20 [+ or -] 0.22, 35.0-44.3, N=53) or among samples representing each population. However, the mean AT for the pooled sample of A (35.27 [+ or -] 0.31, 25.1-44.2, N=155) is significantly lower than the mean AT for the pooled sample of B (37.81 [+ or -] 0.62, 26.0-46.0, N=53); this AT pattern also holds for individuals of A and B collected after 1445 (when most specimens of B were collected). The higher mean AT observed for C. dixoni B reflects its utilization of more typically desert habitats with creosote where it is more often sympatric with C. marmoratus than is true of C. dixoni A.

Body Length and Body Mass

A total of 184 adult C. dixoni A collected between 1989 and 1993 averaged 80.89 [+ or -] 0.47 (67-96) mm PSVL (Fig. 1); 166 of these averaged 16.21 [+ or -] 0.32 (8.2-30.3) g FBM. A total of 66 adult C. dixoni B averaged 78.74 [+ or -] 0.94 (65-98) mm PSVL (Fig. 1); 63 of these averaged 14.77 [+ or -] 0.51 (7.5-25.5) g FBM. A total of 25 sexually mature C. dixoni C averaged 90.4 (78-103) mm PSVL.

Although the PSVL and FBM means for C. dixoni A are significantly larger than for B, the differences are not great and it may be that additional specimens of B would show that it has the same average body size as A. We base this on the observation that both morphotypes have a maximum PSVL of 96-98 mm and a maximum FBM of 30-31 g (based on the actual FBM of the largest C. dixoni A and log regression estimate of the FBM of the largest B). One gravid female A, collected 1 June 1991, increased the known FBM for this pattern class by 19.8% from 24.3 g (Walker et al., 1991) to 30.3 g. Although individuals of C. dixoni A (8.69%) and B (9.09%) only rarely attain a body size in excess of 89 mm PSVL in Presidio County, lizards over 89 mm PSVL make up 51.8% of the sample of C. dixoni C from Hidalgo County. Eleven of these individuals range between 94 and 103 mm PSVL; however, only two C. dixoni A and three B exceed 93 mm PSVL.


Age Classes

To investigate age class structure we separately analyzed PSVL data for C. dixoni A and B, adding data for lizards obtained in 1991, 1992, and 1993 to the 1989-1990 data base utilized by Walker et al. (1991). Data for each form were separately arrayed in 5 mm intervals between 38 and 98 mm and analyzed by using probability graph paper (Harding, 1949). The analysis of the large sample of C. dixoni A clearly indicated trimodality in the PSVL distributions. Though somewhat less apparent, similar, if not identical, trimodality is also apparent in the analysis of the smaller sample of C. dixoni B. Lizards of the following size classes formed the three modal distributions: 38-55 mm, 56-75 mm, and 76-100 mm. These separate distributional patterns of PSVL data for C. dixoni A and B are in agreement with the length frequencies reported by Walker et al. (1991) for a pooled sample of the two forms. This analysis was not possible in the small sample of C; however, we predict that it differs from A and B in one or two attributes. The large number of individuals between 94 and 103 mm PSVL is either indicative of a fourth age class in the Hidalgo County population or a much more rapid growth rate than in A and B.

Reproductive Characteristics

Average clutch size for 148 C. dixoni A was 2.93 [+ or -] 0.07; average clutch size for 52 B was 2.90 [+ or -] 0.14. Mean clutch sizes for the 1991 (3.11 [+ or -] 0.17), 1992 (2.84 [+ or -] 0.17), and 1993 (2.80 [+ or -] 0.17) samples of C. dixoni A varied little and were similar to data reported for this form by Walker et al. (1991); however, mean clutch size for the 1993 (3.40 [+ or -] 0.30) sample of B was significantly higher than for the 1993 sample of A. These data increase the known clutch size for both A and B to six. Twelve gravid C. dixoni C had an average clutch size of 4.66 [+ or -] 0.72 and maximum clutch size of eight. Clutch size is correlated with PSVL in each of the three pattern classes.

Separate analyses of vitellogenic follicles in left and right ovaries and oviductal eggs in left and right oviducts revealed larger numbers of ova on the right side of the body in C. dixoni A, B, and C. This could reflect an accomodation for the stomach, which may fill the left posterior quadrant of the coelom when engorged; however, there were no significant bilateral differences in egg number in either A or B lizards with four to six eggs and in C lizards with five to eight eggs. Average oviductal egg dimensions in three C. dixoni C are: 8.5 X 15.0 mm (N=7), 8.5 X 16.0 mm (N=8), and 9.5 X 18.5 mm (N=4) (for egg measurements in A and B see Walker et al., 1991).


Subsequent to their formation, whether or not by separate hybrid origins, C. dixoni A and B have diverged little, if at all, in body size, mean body temperature, average and maximum clutch size, egg size, and timing of reproductive cycles. Both have distinctively different color patterns throughout life and all specimens examined by us (see Appendix) could be unambiguously allocated to one pattern class or the other. Moreover, each form has developed a distinctive pattern of distribution, albeit in the geographically restricted Chinati Mountain region. In our study area, C. dixoni A is often found in sympatry with C. gularis septemvittatus (apparent paternal progenitor) in transition areas with montane slopes, whereas B is usually found in sympatry with C. marmoratus (apparent maternal progenitor) in transition areas with desert flats. A few sites are also known where A and B coexist with each other and both of their progenitors.

C. dixoni C in Hidalgo County can no longer be regarded as a disjunct population of C. dixoni B located some 500 kilometers west of Presidio County. This form is not only distinct from A and B in meristic characters and color pattern, it also has a larger maximum and average body size than either of these forms. Correlated with this larger body size is a larger clutch size. At present, it is not clear whether these differences reflect a separate origin for C or whether they are evidence of postformational divergence. Given the small geographic area inhabited by C. dixoni C in New Mexico, it is clear that the profound management considerations arising from this study recommend the taking of appropriate measures to ensure the survival of this distinctive form.


Museum numbers for specimens deposited in collections: AMNH, American Museum of Natural History; SRSU, Sul Ross State University; UADZ, University of Arkansas Department of Zoology; UCM, University of Colorado Museum.

Specimens of C. dixoni A. examined from Presidio County, Texas: UADZ 3553, 3557-3558, 3560-3563, 3566-3567, 3570, 3609-3610, 3612-3617, 3626-3630, 3648-3652, 3660-3669, 3675-3677, 3679-3680, 3688, 3696, 3704, 3756-3758, 3761, 3766, 3767-3768, 3770, 3779, 3782-3790, 3820, 3842-3843, 3852-3853, 3856, 3866-3868, 3870-3872, 3882-3884, 3887-3892, 3897, 3901-3905, 3944-3945, 3954, 3957, 3961-3964, 3972-3973, 3977-3980, 3983-3984, 4001, 4005, 4011, 4014-4015, 4026, 4030, 4032-4033, 4347-4348, 4352, 4354, 4360, 4366-4368, 4374-4377, 4382, 4384, 4387-4392, 4397, 4399, 4400-4401, 4404-4405, 4407-4409, 4411-4412, 4419, 4427, 4429-4430, 4436-4437, 4439, 4572-4574, 4580, 4602, 4614-4616, 4626, 4629, 4632, 4634, 4636, 4642, 4645, 4689, 4691-4692, 4695-4696, 4707, 4714, 4721, 4737, 4739, 4741, 4744, 4753, 4758, 4765, 4772, 4775, 4922, 4930, 4943, 4948-4949, 4952-4953, 4957, 4963, 4965, 4967, 4970, 4972, 4975, 4982, 5052, 5061-5062, 5065, 5106, 5124-5125, 5127, 5134; SRSU 1216, 1443-1444, 4368, 4384-4387, 4393, 4407, 4424-4427, 4446, 4448, 4558, 4563, 4566-4567, 4769, 4821-4822.

Specimens of C. dixoni B examined from Presidio County, Texas: UADZ 3457-3458, 3461-3462, 3463-3465, 3469, 3473-3474, 3568, 3571, 3602, 3678, 3695, 3697-3698, 3700, 3759, 3769, 3819, 3854, 3869, 3885-3886, 3943, 3955-3956, 3958, 3970-3971, 3985, 4016-4017, 4027, 4031, 4034, 4349-4350, 4359, 4383, 4385, 4417-4418, 4426, 4428, 4607, 4641, 4723-4726, 4733-4736, 4740, 4742, 4750, 4754-4755, 4759-4762, 4764, 4767, 4934, 4936, 4946, 4969, 4973-4974, 4976, 5054, 5056-5057, 5063-5064, 5066-5068, 5117; SRSU 883, 885, 892, 911-912, 1000, 1183, 1296, 3029, 4443, 4555, 4803, 5809, 6040.

Specimens of C. dixoni C examined from Hidalgo County, New Mexico: UADZ 3551-3552, AMNH 73739, 80680-80691, 84834-84836, 86994-86996; UCM 14666-14668, 17252, 49439-49440.


The study would not have been possible without the cooperation of Mr. A. Real, Manager of the Mesquite Ranch in Presidio County, Texas. He generously provided JEC with over 100 days of lodging, access to all areas of the ranch that could be reached by vehicle, and invaluable advice on the logistics of conducting field work in Presidio County. We are grateful for the loan of specimens of C. dixoni C by curators at the American Museum of Natural History, J. F. Scudday, Sul Ross University, and S. K. Wu, University of Colorado Museum. A grant to JEC from LSU-Eunice helped to defray the cost of field work in Presidio County in 1993. Field work in Texas was conducted under Texas Parks and Wildlife permit no. 61.


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Harding, J. P. 1949. The use of probability paper for the graphical analysis of polymodal frequency distributions. J. Mar. Biol. Assoc. U. K., 28:141-153.

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______. 1993. Community ecology of Cnemidophorus lizards in southwestern Texas: a test of the weed hypothesis. Pp. 319-343 in J. W. Wright and L. J. Vitt, (eds.), Biology of whiptail lizards (genus Cnemidophorus), Oklahoma Mus. Nat. Hist., Norman, Oklahoma.

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Walker, J. M., J. E. Cordes, J. F. Scudday, R. V. Kilambi, and C. C. Cohn. 1991. Activity, temperature, age, size and reproduction in the parthenogenetic whiptail lizard Cnemidophorus dixoni in the Chinati Mountains in Trans-Pecos Texas. Amer. Midl. Nat., 126:256-268.

Wright, J. W. 1993. Evolution of the lizards of the genus Cnemidophorus. Pp. 27-81, in J. W. Wright and L. J. Vitt, (eds.), Biology of whiptail lizards (genus Cnemidophorus), Oklahoma Mus. Nat. Hist., Norman, Oklahoma.

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Department of Biological Sciences, University of Arkansas, Fayetteville, Arkansas 72701, Division of Sciences, Louisiana State University at Eunice, Eunice, Louisiana 70535 (JEC), and Department of Biology, Regis University, Denver, Colorado 80221 (HLT)
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Author:Walker, James M.; Cordes, James E.; Cohn, C.C.; Taylor, Harry L.; Kilambi, Raj V.; Meyer, Richard L.
Publication:The Texas Journal of Science
Geographic Code:1U8NM
Date:Feb 1, 1994
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