ISSN 1827-9635 (print) © Firenze University Press ISSN 1827-9643 (online) www.fupress.com/ah Acta Herpetologica 8(2): 81-91, 2013 Life history of the Marbled Whiptail Lizard Aspidoscelis marmorata from the central Chihuahuan Desert, Mexico Héctor Gadsden1, Gamaliel Castañeda2 1 Instituto de Ecología, A. C.-Centro Regional Chihuahua, Cubículo 29C, Miguel de Cervantes No. 120, Complejo Industrial Chihuahua, C. P. 31109, Chihuahua, Chihuahua, México. Corresponding author. E-mail: hector.gadsden@inecol.edu.mx 2 Facultad de Ciencias Biológicas. Universidad Juárez del Estado de Durango. Avenida Universidad s/n. Fraccionamiento Filadelfia. Gómez Palacio, 35070, Durango, México Submitted on 2012, 5th December; revised on 2013, 10th August; accepted on 2013, 3rd September. Abstract. The life history of a population of marbled whiptail lizard, Aspidoscelis marmorata, was examined from 1989 to 1994 in the sand dunes of the Biosphere Reserve of Mapimí, in Northern México. Lizards were studied using mark- recapture techniques. Reproduction in females occurred between May and August, with birth hatchlings matching the wet season in August. Reproductive activity was highest in the early wet season (July). Males and females reached adult size class at an average age of 1.7 years and 1.8 years, respectively. Body size of males attained an asymptote around 90 mm snout-vent length and females around 82 mm snout-vent length, at an age of approximately 3.6 years and 3.0 years, respectively. The density varied from 7 to 85 individuals / 1.0 ha. The Mexican population had late maturity, relatively long life expectancy, and fewer offspring. Overall, the observed data for A. marmorata and the expectations of life history theory for a late maturing species (K-rate selection) are in agreement. Keywords. Aspidoscelis marmorata, lizard, Mexico, life history, reproductive cycle, Chihuahuan Desert. INTRODUCTION Four life history traits (age of maturity, clutch size, reproductive effort, and longevity) are used regularly to segregate populations into one of two states generally rec- ognizing but ignoring that a continuum exists between these two. The contrasting states are: short-lived popula- tions characterized by individuals with short lives, early sexual maturity, large broods, and high annual reproduc- tive effort; and long-lived populations characterized by individuals with long lives, delayed sexual maturity, small broods, and low annual reproduction effort. Life history theory provides two opposing models for the evolution of these two population types. The deterministic model postulates that high levels of density-independent mortal- ity result in fluctuating population density, which favors a high intrinsic growth rate (r) and produces short-lived populations. In the stochastic model (optimization), high density-independent mortality produces variable, and usually low, juvenile survivorship, which favors a stable and long-lived adult population. The conflict arises from the generality of the models and their attempt to explain life-histories evolution of diverse organisms (Zug, 1993). Life history studies within discrete taxonomic groups (e.g., Tinkle et al., 1970; Ballinger, 1973; Dunham, 1980, 1981; James, 1991; Chapple, 2003; Sears and Angilletta Jr., 2004, Mata-Silva et al., 2008, 2010) and among conspe- cific populations (e.g., Pianka, 1970; Tinkle and Ballinger, 1972; Parker and Pianka, 1975; Ballinger, 1979; Dunham, 1982; Van Devender, 1982; Niewiarowski and Roosen- burg, 1993; Lemos-Espinal and Ballinger, 1995; Stearns, 2000; Niewiarowsky et al., 2004; Du et al., 2005; Rojas- González et al., 2008) have been somewhat more success- ful in revealing the relationships among life-history traits and different environmental regimes. Even in these cases, difficulties are encountered that make conclusions less 82 Héctor Gadsden, Gamaliel Castañeda robust than desirable. A major difficulty is the discrimi- nation between a trait´s variation arising from selection pressures for individuals to adapt to their present habi- tat / location and variation resulting from a proximate response to current disturbance in local conditions. Awareness of the difficulties in the data and interpreta- tion promotes caution but does not deny the evolution of life-history patterns (Zug, 1993). Life history of a species can be summarized by demographic parameters such as rates of birth and death, migratory movement, and struc- ture of populations through time, as they deal with the interaction between longevity, reproductive age, growth, and age-specific mortality versus the environment (Zug, 1993; Stearns, 2000). Interactions among these param- eters through life make demographics of populations subject to constant variation through time. The demog- raphy of lizard populations can be influenced by various environmental factors, such as temperature (Adolph and Porter, 1993; Parker, 1994), precipitation (Andrews, 1988; Bull, 1994), food availability (Ballinger, 1977; Howland, 1992; Smith, 1996), as well as morphological and phylo- genetic constraints (Ballinger, 1983; Stearns, 1984; Dun- ham and Miles, 1985). Some life history patterns can be singled out relat- ing the dynamics and structure of populations (Tinkle, 1969a, Stearns, 2000). For example, lizards with delayed sexual maturity tend to have longer life cycles, along with major energetic investments in maintenance instead of reproduction (Tinkle, 1969a; Tinkle et al., 1970), and lower adult mortality (Hasegawa, 1990; Bull, 1995). Liz- ards with early sexual maturity have shorter life cycles and higher mortality rates for the young and adults (Ball- inger and Congdon, 1981; Andrews and Nichols, 1990), resulting in a higher rate a of population turnover (Fer- guson et al., 1980; Tinkle et al., 1993). This dichotomy represents the extremes of a life-history gradient, but many intermediate combinations exist (Dunham et al., 1994; Stearns, 2000). The population structure can also be influenced by the mating system. Populations with higher incidence of polygyny tend to have higher male death rates than populations where polygyny is reduced (Schoener and Schoener, 1980; Stamps, 1983), generating a female biased sex-ratio. Life history theory was devel- oped mainly from data of temperate lizard species in the United States (Pianka, 1970; Ballinger, 1983), nevertheless in the vast desert regions of northern Mexico, which has a rich diversity of lizards, long-term studies of life his- tory, population structure and dynamics are limited (e.g., Gadsden et al., 1995; Gadsden et al., 2001b; Gadsden and Estrada-Rodriguez, 2007). Here we study the life-history of a population of Aspidoscelis marmorata from the central Chihuahuan Desert in northern Mexico (Hendricks and Dixon, 1986; Dixon, 2009). Although there is extensive knowledge of several aspects of the ecology and life history of A. tigris from its northern arid distribution (Turner et al., 1969; Pianka, 1970; Parker, 1972; Asplund, 1974; Vitt and Ohmart, 1977; Mitchell, 1979; Hendricks and Dixon, 1984; Anderson, 1994; Sullivan, 2009), little is known about demography and life history of A. marmorata from the desert zones of northern México (Gadsden et al., 1995; Díaz-Gómez, 2009; Dixon, 2009; Mata-Silva et al., 2008, 2010). Herein we identify seasonal patterns of variation in population size, age structure, sex ratio, growth, repro- duction, survivorship, and life history attributes of the lizard A. marmorata in the central Chihuahuan Desert, and make comparisons with other populations of the same species and A. tigris, to evaluate predictions of life history theory. MATERIALS AND METHODS Study Area We conducted field work in a 1 ha study plot (26°52’N, 103°32’W) within the Mapimian subprovince of the Chihuahuan Desert in the Mapimí Biosphere Reserve, Chihuahua, Mexico (1250 m elev.) (Barbault and Halffter, 1981). The study plot was gridded with wooden stakes placed 20 m apart over an area of about 100 × 100 m. Data from this study plot were used in the evaluation of demographic characteristics of this population. The habitat was sandy dune, and the vegetation is dominated by desert thornscrub of Acacia constricta, Acacia gregii, Larrea tri- dentata, Yucca elata, and Lycium berlandieri (Breimer, 1985). The climate of this region is seasonal and receives mon- soon summer rains, with the highest temperature and rainfall occurring in June through September. Temperatures range from an average winter low of 3.9 ºC to an average summer high of 36.1 ºC. Mean annual precipitation is 230 mm, but variation between years is high (Cornet, 1988). Reproductive Analysis During 1990 we collected monthly samples of A. marm- orata from other sand dunes (within several kilometers to the marked population) for analysis of reproductive condition. These were brought into the laboratory and necropsied within 24 h. In total we collected 48 female specimens by noosing or shot with BB rifles, euthanized with Nembutal, preserved in 10% formalin (Gadsden and Palacios-Orona, 1997; Gadsden et al., 2001a), and deposited the samples in the collection of Instituto de Ecología, A. C. (voucher specimens-Inecol-CT-1-48). We used the small- est females that showed vitellogenic follicles or oviductal eggs to estimate the minimum snout-vent length (SVL) at sexual matu- rity (Ramírez-Bautista and Vitt, 1997). We recorded number of non-vitellogenic follicles, vitellogenic follicles and oviductal eggs 83Life history of the Marbled Whiptail Lizard from the central Chihuahuan Desert, Mexico for females. We determined litter size by counting oviductal eggs of adult females during reproductive season. Population Structure and Dynamics In the staked study plot we caught individual lizards of A. marmorata using pitfall cans (100 traps of two gal- lons volume each one) and funnel traps in spring (4-8 May 1989, 6-12 May 1990, 20-27 Apr. 1991, 13-21 May 1992, 8-11 May 1993, and 20-24 Apr. 1994), summer (11-17 Sep. 1989, 7-16 Jul. 1990, 15-23 Jul. 1991, 21-25 Aug. 1992, 18-22 Jul. 1993, and 21-24 Sep. 1994), and fall (4-14 Nov. 1989, 27-30 Oct. and 1-3 Nov. 1990, 3-11 Nov. 1991, 13-17 Nov. 1992, 10-16 Nov. 1993, and 1-5 Dec. 1994). Traps were checked every day in the morning during different periods of study and were removed after each sampling period. Each individual was permanently marked by toe- clipping, and a number was painted on the back for quick identification (Tinkle, 1967). For each capture, the fol- lowing data were recorded: date, time of day, sex, and snout-vent length (SVL), measured to the nearest 1 mm with a ruler. In addition, abdomens of females were care- fully palpated to determine whether oviductal eggs were present. Once data was acquired, lizards were released immediately at the exact point of capture (James, 1991). In order to increase recapture data with individu- als previously marked on the back, the site searches fol- lowed a standardized procedure. On each sampling date, each person (3 people) chose one row of quadrants (100 × 20 m) and walked slowly across the entire area remain- ing at all times between two rows of stakes (Tinkle, 1967; Howland, 1992). Each bush and other hiding places were disturbed when necessary to locate and capture each liz- ard. Field work was conducted between 9:00 and 14:00 h every day. We recorded SVL, weight and sex for marked and recaptured lizards and estimated the growth curves of females and males. The changes in length (dSVL) and time intervals (dT) were used to estimate body growth rates (GR = dSVL/dT). Data were analyzed consider- ing the following recapture time interval > 60 and < 460 days. Additionally, averages of growth and snout-vent length of lizards recaptured more than once were used to estimate rates of growth representative of the population. Average length during the interval for each lizard (mean snout-vent length) was the average of the first and last snout-vent length observed for each. We used the von Bertalanffy model to evaluate growth rate of A. marm- orata (von Bertalanffy, 1951, 1957; Fabens, 1965; Lemos- Espinal et al., 2005). This model predicts that growth rate will be maximal for small body size (juveniles) and will decrease as the size increases (Lemos-Espinal and Ball- inger, 1995) following the linear function: GR = a – bMeanSVL (1) where a is the initial growth rate and b is the decrease coefficient. MeanSVL is used instead of original size because growth is measured over a limited period and may overestimate the GR for initial SVL (Van Devender, 1978). Asy mptot ic size is pre dic te d as Z = -a/b Equation (1) can be expressed as follows: a - bMeanSVL = a [1-SVL/Z] which is the derivation of Fabens (1965) of differential equation model of von Bertalanffy growth. Knowing the size of lizards at birth (SVLo), and using the Z and b val- ues obtained from GR = a - bMeanSVL, the growth curve can be obtained from: SVL = Z (1 - ke-bT) where SVL is length reached by a lizard after a time T (from birth), k is a constant that can be calculated if SVL0 is known, and T is the number of days elapsed (age of lizard). The estimate of k is performed as follows: k = 1 - SVLo/Z We used the following Fabens (1965) equation to estimate SVL of a lizard at time t + d (SVL2) in terms of the SVL at time t (SVL1): SVL2 = Z - (Z - SVL1) e-bd where d is the time interval for growth. To test how well this model fits the real growth of A. marmorata, lizards of known age were compared with sizes predicted by the model. Linear regressions were used to determine the relationship between GR and SVL. The regressions were calculated separately for each sex and were compared using analysis of covariance (ANCO- VA), using SVL as a covariate. For all statistical analysis we used only one observation per individual, which was selected at random. All values are given as means ±1 SE. Densities were estimated from lizards caught in pit- fall cans and from visual sightings. We estimated season- al density (1989-1991) using Jolly-Seber method (Jolly, 1965; Seber, 1982) for open populations; therefore we did not need to assume the absence of recruitment and mortality. The estimation of population size was obtained from the simple relationship: 84 Héctor Gadsden, Gamaliel Castañeda Population size = Size of marked population / Proportion of lizards marked. Random sampling becomes the crucial assumption, and we assume that: 1) Every individual has the same probability (αt) of being caught in the tth sample, where it is marked or unmarked. The critical assumption of equal catchability was tested for Leslie, Chitty, and Chitty Test of Equal Catchability (Leslie et al., 1953). 2) Every marked individual has the same probability of survivor- ship (Øt) from the tth to the (t + 1)st sample. 3) Indi- viduals do not lose their marks, and marks are not over- looked at capture. 4) Sampling time is negligible in rela- tion to intervals between samples. Density estimates using Jolly-Seber method did not include data for period 1992-1994. The population abun- dance of this species declined dramatically during this last period (likely due to a prolonged regional drought) and the data were not suitable for this method. RESULTS Reproductive Cycle Body size (SVL) of adult females averaged 78.2 mm (SE = 0.62, range 70-90 mm, n = 48). The mean SVL of gravid females was 80.0 mm (SE = 0.84, n = 9). Gravid females represented 28% of all sexually mature females caught during the reproductive season (n = 32). Repro- ductive activity in females (Fig. 1) began in May and declined in middle Aug., with oviductal eggs present from May to August. We observed hatchlings from Aug. to Sep., when most of the annual precipitation occurred and food was abundant. In Jul. and Aug., 50% and 38% of females respectively had eggs in utero. The embryon- ic developmental period was estimated from the date at which the first female had freshly ovulated eggs in utero (early-May) to when the first hatchling was found (late- Aug.). These data suggest a gestation of ca. 75 days and incubation was ca. 45 days. Morphometric Data Field Mean SVL in adult males was 80.95 mm (SE = 0.73, range 70-92 mm, n = 70) and in adult females was 76.80 mm (SE = 0.70, range 70-90 mm, n = 36). Average body mass of males was 14.67 g (SE = 0.42, range, 8.0–26.5 g, n = 69) and in females was 12.00 g (SE = 0.53, range 8.0-22.00 g, n = 36). Based on comparisons of males and females, males attained a significantly greater SVL and body mass than females (F1, 105 = 13.29, P < 0.0001 and F1, 104 = 14.49, P < 0.0001, respectively). Population Structure and Dynamics Growth was significantly higher (ANOVA, F4, 82 = 19.6, P < 0.0001) in juveniles and subadults (x = 0.12 ± 0.01 mm × day-1, n = 13 and x = 0.10 ± 0.01 mm × day-1, n = 22, respectively) than hatchlings (x = 0.06 ± 0.01 mm × day-1, n = 5), adults (x = 0.02 ± 0.003 mm × day-1, n = 36), and old adults (x = 0.005 ± 0.003 mm × day-1, n = 7). Females grew faster than males (0.046 ± 0.008 mm/ day and 0.040 ± 0.006 mm/day, respectively), however this difference is not significant (ANCOVA F1, 52 = 0.012, P = 0.91). The growth rate decreased significantly with respect to SVL (F1, 52 = 45.09, P < 0.0001), suggesting a growth curve of von Bertalanffy type. Growth rates var- ied inversely with the average SVL for males and females (Fig. 2). The values used to estimate constants of the growth curves were the same for females and males, correspond- ing to an SVL of 37.9 mm, with an estimated age of 30.0 days. Using these constants in the equation of Fabens (1965), provides longevity for females of approximately 3.0 yr and 3.6 yr for males, at an SVL of 82 mm and 90 mm, respectively (Fig. 3). Longevity for males larger than 90 mm and females larger than 82 mm was very high and may not be representing reality. The age of an individual can be estimated from data on the earliest date of hatchling emergence, the mean growth rate of individuals in different size classes, and the date of capture. Based on the growth curves of both sex- Fig. 1. Percentage of Aspidoscelis marmorata in various reproduc- tive stages in each month of the year. Sample sizes appear above bars. 85Life history of the Marbled Whiptail Lizard from the central Chihuahuan Desert, Mexico es, males reached the adult size class around 76 mm SVL and females around 75 mm SVL, at an average age of 1.7 yr and 1.8 yr, respectively. Comparing this estimate with the data obtained from the reproduction data (i.e., the data obtained from 1990), the smallest sexually mature female with oviductal eggs was 75 mm SVL, whereas the smallest male measured with enlarged testes was 74 mm SVL. Body size of males attained an asymptote around of 90 mm SVL and females around of 82 mm SVL, at an age of approximately 3.6 yr and 3.0 yr, respectively (Fig. 3). The population structure of A. marmorata (Fig. 4) in spring and fall were similar but not in summer (χ2 = 50.44, df = 8, P < 0.005). A notable feature of summer is the presence of hatchlings and juveniles. The relative number of adult females and males (SVL ≥ 70 mm) did not differ significantly among the six years (χ2 = 1.19, df = 5, P > 0.25). Lizards in the 70-82 mm SVL size class were the most numerous in spring and summer. The seasonal sex ratio (1989-1994 pooled) was heav- ily biased toward males in spring, summer and fall (1.7, 1.6 and 4.0, respectively). In fact, the overall sex ratio (3 seasons pooled) was 1.75 (63 males and 36 females), which is significantly different from 1:1 (χ2 = 7.3, df = 1, P = 0.0068). The mean density in summer, fall, and spring (1989- 1991) of A. marmorata (Table 1) was 32.0 animals/1.0 ha (SE = 7.5, range 24.5-39.6, n = 2), 40.6 animals/1.0 ha (SE = 19.3, range 21.3-60.0, n = 2), and 46.0 animals/1.0 ha (SE = 39.1, range 6.9-85.1, n = 2), respectively. The mean total density (summer, fall, and spring combined) was 39.5 animals/1.0 ha (SE = 11.7, range 6.9-85.1, n = 6). Relationship existed between precipitation and den- sity (r = 0.82, P < 0.04, n = 6). Seasonal density estimates did not include data for the period 1992-1994. The abun- dance of this population declined dramatically during this last period probably caused by a prolonged regional drought. Estimates of probability of survival (Ø) from sample time t to sample time t +1 (Table 1) indicated higher sur- vival in 1989 than in other years. We estimated the total Fig. 2. Growth in males and females of the lizard Aspidoscelis marmorata in Mapimí, Durango. Each point represents the body growth rate per day which has a given average SVL. Fig. 3. Growth curves of von Bertalanffy model for females and males of the lizard Aspidoscelis marmorata in Mapimi, Durango. Abscissa shows age (days); ordinate shows SVL (mm). Estimated from the equation of Fabens (1965), with an initial SVL = 37.9 mm vs. 30 days old. The values used were a = - 0.0022 and b = 0.190 for females, and a = - 0.0016 and b = 0.156 for males. Open rectangles and open diamonds represent means. Fig. 4. Number of individuals of Aspidoscelis marmorata in differ- ent size classes in spring, summer, and fall. 34-40 mm = Hatchlings, 41-49 mm = Juveniles, 50-69 mm = Subadults, 70-82 mm = Adults, and 83-94 mm = Old adults. Open bar represents adult males; diag- onal-lined bar represents adult females; and horizontal lines repre- sent females and males of hatchlings, juveniles, and subadults. 86 Héctor Gadsden, Gamaliel Castañeda number of new additions to the marked population that were made at sampling t (called Z´t). Thus, over this peri- od (1989-1991) we observed a total of 47 new individu- als entering the marked population and we predicted (Z´t values) a total of nine new individuals, and a 19% under- estimate. This result is probably biased, and it suggests unequal catchability for the animals within the marked population. This sort of bias could arise if socially domi- nant individuals are easier to trap. DISCUSSION Reproductive Cycle The reproductive season for A. marmorata in sand dunes of the Mapimí Biosphere Reserve, began in May (spring) and declined in Aug. (middle summer) dur- ing the dry season in May.-Jun. and wet season in Jul.- Aug. Most of the annual rainfall occurs during summer (particularly Aug. and Sep.), and we suggest that hatch- ling emergence (between Aug. and Sep.) corresponds to the wet season when food abundance is high in order to promote juvenile growth and survivorship, as occurs in populations of A. tigris in Arizona (Parker, 1972), and Nevada (Tanner and Jorgensen 1963), and A. marmorata in Texas (Milstead, 1957). Gadsden and Palacios-Orona (2000) found in the same dune area in Mexico that A. marmorata eats insects in abundance during the fall sea- son, mainly isopters, lepidopters, and coleopters. This could increase the reproductive potential of this species of lizard in the spring to emerge from its winter dorman- cy, due to an increase of fat stored in their fat bodies. The reproductive cycle peaks in summer and is differ- ent to that previously reported for A. marmorata in spring (Mata-Silva et al. 2010) and similar to that registered for A. tigris and A. marmorata (McCoy and Hoddenbach, 1966; Goldberg and Lowe, 1966; Goldberg, 1976; Vitt and Ohmart, 1977), see Table 2. Nevertheless the known dif- ferences in reproductive traits among populations of A. marmorata and A. tigris may be manifesting proximate environmental effects. The smallest females of A. mar- morata probably reach early sexual maturity at approxi- mately 20 mo of age. Likewise, data available for other female A. tigris from low elevations (530-1176 m, see Bur- kholder and Walker, 1973), females reach sexual maturity after emerging from their second hibernation in 22-24 mo at 70 mm SVL. Burkholder and Walker (1973) estimated size at sexual maturity of A. tigris females to be 67-70 mm SVL. In the northeast of the Sonoran Desert in Arizona, some individuals of A. tigris reached mature size in their first spring at ages of about 8-11 mo, but many others did not do so until their second spring at ages of 20-23 mo. This is similar to ages at first breeding reported for other areas (Turner et al., 1969; Tinkle, 1969a). Population Structure and Dynamics In this study, A. marmorata was observed in the dunes from early spring (Apr.) to early fall (Oct.), as recorded by Hendricks and Dixon (1984) for A. marm- orata (cited as Cnemidophorus tigris) in Texas. According to these authors, juveniles and subadults represent the majority of the population by early fall, probably because the older adults have already begun torpor. Young A. marmorata may emerge during warm weather through- out the winter, as evidence by several specimens observed in early winter. Growth in reptiles is influenced by both phylogenetic and environmental factors (Andrews, 1982). Aspidosce- lis marmorata exhibits differences in growth rates from A. tigris populations possibly related to proximate envi- ronmental influences, such as temperature, photoperiod, rainfall and food availability (Tinkle, 1967, Andrews, 1982). Cuellar (1993) studied one high-latitude A. tigris population in Utah, which had lower growth rates (0.07 mm × d-1 subadults and 0.008 mm × d-1 adults) than this study. However Parker (1972) studied a daily growth rate of A. tigris populations in Arizona, which had higher growth rates for adults (0.04 mm × d-1) than this study. In Mapimí Biosphere Reserve the mean growth rates for A. marmorata was 0.1 mm × d-1 subadults and 0.01 mm × d-1 adults. The discrepancy may represent the inclusion of many immature individuals in the constructed size classes (Cuellar, 1993), whereas all adult A. marmorata Table 1. Population estimates by used of Jolly-Seber model of pop- ulation estimation. Seasonal samples (1989-1991) of Aspidoscelis marmorata from Mapimí Biosphere Reserve, Chihuahua, Mexico. SP=Spring, SU=Summer, FA=Fall. Population sample Proportion marked (α) Total number (N) Probability of survival (Ø) se N se Ø SP-1989 0.00 0.0 0.38 - 0.09 SU-1989 0.57 24.5 0.98 5.4 0.52 FA-1989 0.53 60.0 0.90 24.7 0.62 SP-1990 0.47 85.1 0.27 48.0 0.19 SU-1990 0.36 39.6 0.28 21.2 0.28 FA-1990 0.40 21.3 0.47 20.5 0.47 SP-1991 0.80 6.9 - 4.8 - SU-1991 0.33 - - - - 87Life history of the Marbled Whiptail Lizard from the central Chihuahuan Desert, Mexico mentioned above were reproductive. However, reciprocal transplant experiments could be useful for examining the relative importance of genetic vs. proximate environmen- tal effects as sources of variation among A. marmorata and A. tigris rates to identify whether differences in rates are adaptatives. Furthermore, according to Tinkle (1967) and Lemos-Espinal et al. (2003), like other lizards, growth rates of this species possibly fluctuate with the proximate environment on both seasonal and annual timescale. Numerous population density data exist for A. tigris, Pianka (1970) found lower densities for A. tigris located in high-latitudes (4-5 individuals per hectare) than in our low-latitudes (mean 39 individuals per hectare), see Table 1 and Table 2. Nevertheless, these density values are within the ranges for other areas summarized by Turner et al. (1969) and are also similar to census estimations by Pianka (1970). The density of adult individuals A. mar- morata in Mapimí Biosphere Reserve was similar to that found in a population of A. tigris in Nevada, fluctuating between 1 to 49 individuals per hectare (Turner et al., 1969); suggesting that, if any relationship between density and individual growth does exist, it is not inversely den- sity dependent as is usually assumed and observed (Scott, 1990). Instead lizard density is influenced by a complex variety of factors, including availability of food resourc- es and thermal environment (Pianka, 1970; Rose, 1982; Christian and Tracy, 1985; Sinervo, 1990). The density of A. marmorata was higher in spring 1990 when compared to the others six seasons (Table 1). This large increase may be due to a larger adult recruit- ment in spring 1990 in response to previous above nor- mal precipitation. Pianka (1970) and Whitford and Creusere (1977) suggested that the density of most lizard species varies directly with changes in productivity and relative abundance of arthropods. Likewise the avail- ability of insect prey change according to the distribution and amount of rain throughout the year (Maury, 1995). Although the availability of arthropods was not evaluat- ed in this study, we found a relationship between rainfall and density of lizards. Mean probability of survival (Ø = 0.55) obtained in this study (Table 1) was similar to rates registered in a high-latitude population of A. tigris in southern Nevada (Ø = 0.57), see Turner et al. (1969). During summer 1990 in Mapimí Biosphere Reserve (i.e., Aug. through Sep.) high precipitation increased the availability of insect prey. In sand dunes of Mapimí, lizards showed an abrupt prey shift in fall, eating many more Isoptera and Hemiptera and fewer other insects (Gadsden-Esparza and Palacios-Orona, 1997). Increase in food abundance has been shown to affect food utilization in A. tigris (Ander- son, 1994), and apparently appear to affect reproductive output (Whitford and Creusere, 1977; Dunham, 1981). Therefore, the autoecological and population responses of A. marmorata to environmental variation may be regard- ed as normal. Life-history characteristics of populations in A. mar- morata (Table 2) did not deviate from estimates of Fitch (1985), and Burkholder and Walker (1973). In fact, Bur- kholder and Walker (1973) suggest that southern popu- lations of A. tigris produce a minimum of two clutches per season. According to these authors the difference in Table 2. Comparison of life history data between Aspidoscelis marmorata (this study, Texas1, and Texas 2) and Aspidoscelis tigris (Arizona, California, Colorado and Idaho). M-S = Mata-Silva et al. (2010), MH = McCoy and Hoddenbach (1966), S = Schall (1978), T = Taylor et al. (2001), Pi = Pianka (1970), P = Parker (1972), M = Mitchell (1979), G = Goldberg (1976), VO = Vitt and Ohmarth (1977), BW = Burk- holder and Walker (1973). Chihuahua Texas1 Texas2 New Mexico Arizona California Colorado Idaho Elevation (m) 1129 1215 500-1845 1059 450-500 1585-1830 432 530-1176 Length of growing season (days) 240 - - - 210 130 - 165 Length of reproductive season (days) 120 100 - - 130 75 100 90 Density (individuals/ha) 7-85 - - - 1-36 1-12 - 4-5 Hatchling size (mm SVL) 34-37 48 - - 37-41 - - 32-35 Size range mature males (mm) 70-92 68-94 - - 71-96 - - 63-97 Size range mature females (mm) 70-90 75-87 60-93 70-80 71-93 - - 69-102 Average size adult males (mm) 80.2 83.7 - - 83.5 - - 81.5 Average size adult females (mm) 76.8 80.9 - - 76.7 - - 81.2 Clutch size 2.6 3.3 2.0-2.2 2.2 2.0-2.2 2.3-4.1 2.9-3.4 2.6 Clutch frequency 2 2 2 - 2 2 1 1 Date hatchling appear August June June - 14-June Mid-August July Mid-August Source This study M-S MH, S T Pi, P, M Pi, G VO BW, Pi 88 Héctor Gadsden, Gamaliel Castañeda clutch size of northern and southern populations and number of clutches laid per year pose some unique prob- lem to be considered. The first factor worth considering is the length of activity period. McCoy and Hoddenbach (1966) suggested that in northern populations the activ- ity period is too short to allow the production of two clutches. Therefore, northern populations partially make up for this by producing one large clutch in each season. The second possibility is that northern populations are larger in average body size and greater longevity, which allows them to produce a larger clutch than those in southern areas. This would again be related to latitude and elevation, and possibly to other factors such as food abundance. However the clutch size varies erratically and geographic trends are not consistent. In most instances, at least, clutch size is probably proportional to body size (Fitch, 1985). In other lizard species, Tanner (1972) suggest that with increasing elevation Uta populations live live longer, attain larger size, and have more eggs per clutch. Alter- natively, Nussbaum and Diller (1967) predicted that high latitude and/or high altitude Uta populations show short- er growing season, shorter reproductive season, reduced per season fecundity (clutch frequency is reduced) and greater survivorship than low altitude and/or low lati- tude populations. These two life history patterns may be significant in A. marmorata, and would be another way of adapting to a short or longer activity season. Pianka (1970) propose that reduced predation, reduced time exposed to predation (shorter active season) in northern populations of A. tigris could explain in part the higher average clutch size and lower clutch frequency in these populations. Tinkle (1969b) has reached similar results in his studies on Uta stansburiana. The studied population of A. marmorata had a late maturity, a long life expectancy, and few offspring. Over- all, the observed data and the expectations of current life history theory for a late maturing species (Ballinger, 1983; Dunham et al., 1994; Stearns, 2000) are in agree- ment with K-rate selection. Further studies on other A. marmorata populations in Mexico are needed to define latitudinal and eleva- tional tendencies. Intraspecific variations of A. marm- orata from dissimilar geographic and climatic locations would amplify the understanding of differing environ- mental conditions and their influence on the life history of the Marbled Whiptail Lizard. These studies will need to gather data on body size, longevity, temperature, mois- ture, predator pressure, parasites, and availability of food, which potentially affect life history parameters of lizards. ACKNOWLEDGMENTS This study was supported by a CONACYT grant (1367- N9206). We thank the Herrera family for field assistance. Permit SEMARNAP-SGPA/DGVS/88173 and it is also a contribution to the UNESCO-MAB program. REFERENCES Adolph, S.C., Porter, W.P. (1993): Temperature, activity and lizard life histories. Am. Nat. 142: 273-295. Anderson, R.A. (1994): Functional and population responses of the lizard Cnemidophorus tigris to envi- ronmental fluctuations. Am. Zool. 34: 409-421. Andrews, R.M. (1982): Patterns of growth in reptiles. In: Biology of the Reptilia. Physiological Ecology, pp. 273-320. Gans, C., Pough, F.H., Eds, Academic Press, New York. Andrews, R.M. (1988): Demographic correlates of vari- able egg survival for a tropical lizard. Oecologia (Ber- lin) 76: 376-382. Andrews, R.M., Nichols, J.D. (1990): Temporal and spa- tial variation in survival rates of the tropical lizard Anolis limifrons. Oikos 57: 215-221. Asplund, K.K. (1974): Body size and habitat utilization in whiptail lizards (Cnemidophorus). Copeia 1974: 695- 703. Ballinger, R.E. (1973): Comparative demography of two viviparous iguanid lizards (Sceloporus jarrovi and Sce- loporus poinsetti). Ecology 54: 269-283. Ballinger, R.E. (1977): Reproductive strategies: food avail- ability as a source of proximal variation in a lizard. Ecology 58: 628-635. Ballinger, R.E. (1979): Intraspecific variation in demog- raphy and life history of the lizard, Sceloporus jarrovi, along an altitudinal gradient in southeastern Arizona. Ecology 60: 901-909. Ballinger, R.E. (1983): Life-history variations. In: Lizard ecology: studies of a model organism, pp. 241-260. Huey, R.B., Pianka, R.B., Schoener T.W., Eds, Harvard University Press, Cambridge, Massachusetts. Ballinger, R.E., Congdon, J.D. (1981): Population ecology and life history strategies of a montane lizard (Sce- loporus scalaris) in southeastern Arizona. J. Nat. Hist. 15: 213-222. Barbault, R., Halffter, G. (1981): A comparative and dynamic approach to the vertebrate community organization of the desert of Mapimí (México). In: Ecology of the Chihuahuan Desert: Organization of Some Vertebrate Communities, pp. 11-18. Barbault, R., Halffter, G., Eds, Instituto de Ecología, A. C., México. 89Life history of the Marbled Whiptail Lizard from the central Chihuahuan Desert, Mexico Breimer, R.F. (1985): Soil and landscape survey of the Mapimí Biosphere Reserve, Durango, México. UNE- SCO Regional Office for Science and Technology for Latin America and the Caribean. Montevideo, Uru- guay. Bull, C.M. (1994): Population dynamics and pair fidel- ity in sleepy lizards. In: Lizard Ecology – historical and experimental perspectives, pp. 159-174. Vitt, L.J., Pianka, E.R., Eds, Princeton University Press. Prince- ton NJ. Bull, C.M. (1995): Population ecology of the sleepy lizard, Tiliqua rugosa, at Mt. Mary, South Australia. Aust. J. Ecol. 20: 393-402. Burkholder, G.L., Walker, J.M. (1973): Habitat and repro- duction of the desert whiptail lizard, Cnemidophorus tigris Baird and Girard in southwestern Idaho at the northern part of its range. Herpetologica 29: 76-83. Cornet, A. (1988): Principales caractèristiques clima- tiques. In: Estudio Integrado de los Recursos de Veg- etación, Suelo y Agua en la Reserva de la Biosfera de Mapimí, pp. 45-76. Montaña, C. Ed, Instituto de Ecología, A. C., México. Cuellar, O. (1993): Lizard population ecology: a long term community study. Bull. Ecol. 24 : 109-149. Chapple, D.G. (2003): Ecology, life-history, and behavior in the Australian scincid genus Egernia, with com- ments on the evolution of complex sociality in lizards. Herpetol. Monogr. 17: 145-180. Christian, K.A., Tracy, C.R. (1985): Physical and biotic determinants of space utilization by the Galapagos land iguana (Conolophus pallidus). Oecologia 66: 132- 140. Diaz-Gómez, E. (2009): Estructura poblacional de los saurios Aspidoscelis marmorata y Phrynosoma mod- estum en las dunas semiestabilizadas de Samalayuca, Chihuahua. Bachelor Thesis. Universidad Autónoma de Ciudad Juárez. Ciudad Juárez, Chihuahua. Dixon, J.R. (2009): Marbled Whiptail. In: Lizards of the American Southwest, p. 362-365. Jones L.L.C., Lovich R.E., Eds, Rio Nuevo Publishers, Tucson, AZ, USA. Du, W.G., Ji, X., Zhang Y.P., Xu, X.F., Shine, R. (2005): Identifying sources of variation in reproductive and life-history traits among five populations of a Chinese lizard (Takydromus septentrionalis, Lacertidae). Biol. J. Linn. Soc. 85: 443-453. Dunham, A.E. (1980): An experimental study of interspe- cific competition between the iguanid lizard Scelopo- rus merriami and Urosaurus ornatus. Ecol. Monogr. 50: 304-330. Dunham, A.E. (1981): Populations in a fluctuating envi- ronment: The comparative population ecology of the iguanid lizards Sceloporus merriami and Urosaurus ornatus. Misc. Publ. Mus. Zool. Univ. Mich. 158: 1-62. Dunham, A.E. (1982): Demographic and life histories variation among populations of the iguanid lizard Urosaurus ornatus: implications for the study of life- histories phenomena in lizards. Herpetologica 38: 208-221. Dunham, A.E., Miles, D.B. (1985): Patterns of covariation in life history traits of squamate reptiles: effects of size and phylogeny reconsidered. Am. Nat. 126: 231-257. Dunham, A.E., Miles, D.B., Reznick, D.N. (1994): Life History Patterns in Squamate Reptiles. In: Biology of the Reptilia. Vol. 16. Defense and Life History, pp. 441-522. Gans, C., Huey, R.B., Eds, Branta Books, Ann Arbor, Michigan. Hendricks, F.S., Dixon, J.R. (1986): Systematics and bio- geography of Cnemidophorus marmoratus (Sauria: Teiidae). Texas J. Sci. 38: 327-402. Fabens, A.J. (1965): Properties and fittings of the von Bertalanffy growth curve. Growth 29: 265-289. Ferguson, G.W., Bohlen, C.H., Woolley, H.P. (1980): Sce- loporus undulatus: comparative life history and regu- lation of a Kansas Population. Ecology 61: 313-322. Fitch, H.S. (1985): Variation in clutch and litter size in new world reptiles. Misc. Publ. Univ. Kans. Mus. Nat. Hist. 76: 1-76. Gadsden, H., Palacios-Orona, L.E. (1997): Seasonal die- tary patterns of the Mexican fringe-toed lizard (Uma paraphygas). J. Herpetol. 31: 1-9. Gadsden, H., Palacios-Orona, L.E. (2000): Composición de dieta de Cnemidophorus tigris marmoratus (Sau- ria: Teiidae) en dunas del centro del Desierto Chihua- huense. Acta Zool. Mex. (n.s.) 79: 61-76. Gadsden, H., Estrada-Rodríguez, J.L. (2007): Ecology of the spiny lizard Sceloporus jarrovii in the central Chi- huahuan desert. Southwest. Nat. 52: 600-608. Gadsden, H., Palacios-Orona, L.E., Cruz-Soto, G. (2001a): Diet of the Mexican Fringe-Toed Lizard (Uma exsul). J. Herpetol. 35: 493-496. Gadsden, H., Aguirre-León, G., Guerra-Mayaudón, G., Palacios-Orona, L.P. (1995): Ecología de gremios parapátricos de lagartijas en dunas del Bolsón de Mapimí. Informe Técnico Final CONACYT 1367- N9206, Instituto de Ecología, A. C., México, D. F. Gadsden, H., López-Corrujedo, H., Estrada-Rodríguez, J.L., Romero-Méndez, U. (2001b): Biología pobla- cional de la lagartija de arena de Coahuila Uma exsul (Sauria: Phrynosomatidae): implicaciones para su conservación. Bol. Soc. Herpetol. Mex. 9: 51-66. Goldberg, S.R. (1976): Reproduction in a mountain pop- ulation of the coastal whiptail lizard, Cnemidophorus tigris multiscutatus. Copeia 1976: 260-266. Goldberg, S.R., Lowe, Ch.L. (1966): The reproductive cycle of the western whiptail lizard (Cnemidophorus tigris) in Southern Arizona. J. Morphol. 118: 543-548. 90 Héctor Gadsden, Gamaliel Castañeda Hasegawa, M. (1990): Demography of an island popula- tion of the lizard, Eumeces okadae, on Miyake-Jima, Izu Islands. Res. Popul. Ecol. 32: 119-133. Hendricks, F.S., Dixon, J.R. (1984): Population structure of Cnemidophorus tigris (Reptilia: Teiidae) east of the continental divide. Southwest. Nat. 29: 137-140. Howland, J.M. (1992): Life history of Cophosaurus tex- anus (Sauria: Iguanidae): environmental correlates and interpopulational variation. Copeia 1992: 82-93. James, C.D. (1991): Growth rates and ages at maturity of sympatric scincid lizards (Ctenotus) in central Aus- tralia. J. Herpetol. 25: 284-295. Jolly, G.M. (1965): Explicit estimates from capture-recap- ture data with both death and dilution-stochastic model. Biometrika 52: 225-247. Lemos-Espinal, J.A., Ballinger, R.E. (1995): Ecology of growth of the high altitude lizard Sceloporus grammi- cus on the eastern slope of the Iztaccihuatl volcano, Puebla, México. T. Nebr. Acad. Sci. 22: 77-85. Lemos-Espinal, J.A., Smith, G.R., Ballinger, R.E. (2003): Variation in growth and demography of a Knob- scaled lizard (Xenosaurus newmanorum: Xenosauri- dae) from a seasonal tropical environment in Mexico. Biotropica 35: 240-249. Lemos-Espinal, J.A., Rojas-González, R.I., Zúñiga-Vega, J.J. (2005): Técnicas para el Estudio de Poblaciones de Fauna Silvestre. Universidad Nacional Autónoma de México y Comisión Nacional para el Conocimiento y uso de la Biodiversidad. México, D. F. Leslie, P.H., Chitty, D., Chitty, H. (1953): The estima- tion of population parameters from data obtained by means of the capture-recapture method. III. An exam- ple of the practical applications of the method. Biom- etrika 40: 137-169. Mata-Silva, V., Bursey, C.R., Johnson, J.D. (2008): Gut parasites of two syntopic species of whiptail lizards, Aspidoscelis marmorata and Aspidoscelis tesselata from the northern Chihuahuan Desert. Bol. Soc. Herpetol. Mex. 16: 1-4. Mata-Silva, V., Ramírez-Bautista, A., Johnson, J.D. (2010): Reproductive characteristics of two syntopic whiptail lizards, Aspidoscelis marmorata and Aspidoscelis tes- selata, from the northern Chihuahuan Desert. South- west. Nat. 55: 125-129. Maury, M.E. (1995): Diet composition of the greater ear- less lizard (Cophosaurus texanus) in central Chihua- huan desert. J. Herpetol. 29: 266-272. McCoy, C.J., Hoddenbach, G.A. (1966): Geographic vari- ation in ovarian cycles and clutch size in Cnemidopho- rus tigris (Teiidae). Science 154: 1671-1672. Milstead, W.W. (1957): Some aspects of competition in natural populations of whiptail lizards (genus Cnemi- dophorus). Tex. J. Sci. 9: 410-417. Mitchell, J.C. (1979): Ecology of southeastern Arizona whiptail lizards (Cnemidophorus: Teiidae): population densities, resource partitioning, and niche overlap. Can. J. Zool. 57: 1487-1499. Niewiarowsky, P.H., Roosenburg, W. (1993): Reciprocal transplant reveals sources of variation in growth rates of the lizard Sceloporus undulatus. Ecology 74: 1992- 2002. Niewiarowsky, P.H., Angilletta, Jr., M.J., Leaché, A.D. (2004): Philogenetic comparative analysis of life-histo- ry variation among populations of the lizard Scelopo- rus undulatus: an example and prognosis. Evolution 58: 619-633. Nussbaum, R.A., Diller, L. (1967): The life history of the side-blotched lizard, Uta stansburiana Baird and Girard, in north-central Oregon. Northwest Sci. 50: 243-260. Parker, W.S. (1972): Ecological study of the western whip- tail lizard, Cnemidophorus tigris gracilis, in Arizona. Herpetologica 28: 360-369. Parker, W.S. (1994): Demography of the fence lizard, Sce- loporus undulatus, in northern Mississippi. Copeia 1994: 136-152. Parker, W.S., Pianka, E.R. (1975): Comparative ecology of populations of the lizard Uta stansburiana. Copeia 1975: 615-632. Pianka, E.R. (1970): Comparative autoecology of the liz- ard Cnemidophorus tigris in different parts of its geo- graphic range. Ecology 51: 703-720. Ramírez-Bautista, A., Vitt, L.J. (1997): Reproduction in the lizard Anolis nebulosus (Polychrotidae) from the Pacific coast of México. Herpetologica 53: 423-431. Rojas-González, R.I., Jones, C.P., Zuñiga-Vega, J.J., Lem- os-Espinal, J.A. (2008): Demography of Xenosaurus platyceps (Squamata: Xenosauridae): a comparison between tropical and temperate populations. Amphib- ia-Reptilia 29: 245-256. Rose, B. (1982): Lizard home range: methodology and functions. J. Herpetol. 16: 253-269. Sears, M.W., Angilletta Jr., M.J. (2004): Body size clines in Sceloporus lizards: proximates mechanisms and demo- graphic constraints. Integr. Comp. Biol. 44: 433-442. Seber, G.A.F. (1982): The Estimation of Animal Abun- dance and Related Parameters. 2nd ed. Griffin, Lon- don. Scott, D.E. (1990): Effects of larval density in Ambystoma opacum: an experiment in large-scale field enclosures. Ecology 71: 296-306. Schall, J.J. (1978): Reproductive strategies in sympatric whiptail lizards (Cnemidophorus): two parthenogenet- ic and three bisexual species. Copeia 1978: 108-116. Schoener, T.W., Schoener, A. (1980): Densities, sex ratios and population structure in four species of Bahamian Anolis lizards. J. Anim. Ecol. 49: 19-53. 91Life history of the Marbled Whiptail Lizard from the central Chihuahuan Desert, Mexico Sinervo, B. (1990): Evolution of thermal physiology and growth rate between populations of the western fence lizard (Sceloporus occidentalis). Oecologia 83: 228-237. Smith, G.R. (1996): Annual life-history variation in the striped plateau lizard Sceloporus virgatus. Can. J. Zool. 74: 2025-2030. Stamps, J.A. (1983): Sexual selection, sexual dimor- phism, and territoriality. In: Lizard Ecology: studies of a model organism, pp. 169-204. Huey, R.B., Pianka, E.R., Schoener, T.W., Eds, Harvard University Press. Cambridge, Massachusetts. Stearns, S.C. (1984): The effects of size and phylogeny on patterns of covariation in the life history traits of liz- ards and snakes. Am. Nat. 123: 56-72. Stearns, S.C. (2000): Life History evolution: successes, limitations, and prospects. Naturwissenschaften 87: 476-486. Sullivan, B.K. (2009): Tiger Whiptail. In: Lizards of the American Southwest, p. 394-397. Jones L.L.C., Lovich R.E., Eds, Rio Nuevo Publishers, Tucson, AZ, USA. Tanner, W.W. (1972): Notes on the life history of Uta s. stansburiana Baird and Girard. Brigham Young Univ. Sci. Bull. Biol. Ser. 15: 31-39. Tanner, W.W., Jorgensen, C.D. (1963): Reptiles of the Nevada test site. Brigham Young Univ. Sci. Bull. Biol. Ser. 3: 1-31. Taylor, H.J., Cole, CH.J., Hardy, L.M., Dessauer, H.C., Towsend, C.R., Walker, J.M., Cordes, J.E. (2001): Natural hybridization between the Teiid lizards Cne- midophorus tesselatus (Parthenogenetic) and C. tigris marmoratus (Bisexual): Assessment of evolutionary alternatives. Am. Mus. Nov. 3345: 1-65. Tinkle, D.W. (1967): The life and demography of the side- blotched lizard, Uta stansburiana. Misc. Publ. Mus. Zool. Univ. Mich. 132: 1-182. Tinkle, D.W. (1969a): The concept of reproductive effort and its relation to the evolution of life histories of liz- ards. Am. Nat. 103: 501-516. Tinkle, D.W. (1969b): Evolutionary implications of com- parative population studies in the lizard Uta stansbu- riana. In: Systematic Biology, pp. 133-160. Nat. Acad. Sci. Publ. 1692. Tinkle, D.W., Ballinger, R.E. (1972): Sceloporus undulatus a study of the intraspecific comparative demography of a lizard. Ecology 53: 570-584. Tinkle, D.W., Wilbur, H.M., Tilley, S.G. (1970): Evolu- tionary strategies in lizard reproduction. Evolution 24: 55-74. Tinkle, D.W., Dunham, A.E., Congdon, J.D. (1993): Life history and demography variation in the lizard Sce- loporus graciosus: a long term study. Ecology 74: 2413-2429. Turner, F.B., Medica, P.A., Lannom, Jr., J.R., Hoddenbach, G.A. (1969): A demographic analysis of fenced popu- lations of the whiptail lizard, Cnemidophorus tigris, in southern Nevada. Southwest. Nat. 14: 189-202. Van Devender, R.W. (1978): Growth ecology of a tropical lizard Basiliscus basiliscus. Ecology 59: 1031-1038. Van Devender, R.W. (1982): Comparative demography of the lizard Basiliscus basiliscus. Herpetologica 38: 189- 208. Vitt, L.J., Ohmart, R.D. (1977): Ecology and reproduction of lower Colorado River lizards: II. Cnemidophorus tigris (Teiidae), with comparisons. Herpetologica 33: 223-234. von Bertalanffy, L. (1951): Metabolic types and growth types. Am. Nat. 85: 111-117. von Bertalanffy, L. (1957): Quantitative laws in metabo- lism and growth. Q. Rev. Biol. 32: 217-231. Whitford, W.G., Creusere, F.M. (1977): Seasonal and yearly fluctuations in Chihuahuan desert lizard com- munities. Herpetologica 33: 54-65. Zug, G.R. (1993): Herpetology – an introductory biol- ogy of amphibians and reptiles. San Diego. Academic Press. Acta Herpetologica Vol. 8, n. 1 - June 2013 Firenze University Press Journal of the Societas Herpetologica Italica ACTA HERPETOLOGICA