Ecology of Growth of the High Altitude Lizard Sceloporus grammicus on the Eastern Slope of Iztaccihuatl Volcano, Puebla, México

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1 University of Nebraska - Lincoln DiitalCommons@University of Nebraska - Lincoln Transactions of the Nebraska Academy of Sciences and Affiliated Societies Nebraska Academy of Sciences 1995 coloy of Growth of the Hih Altitude Lizard Sceloporus rammicus on the astern Slope of Iztaccihuatl Volcano, Puebla, México Julio A. Lemos-spinal CNIO/COMF Royce. Balliner University of Nebraska - Lincoln, rballiner1@unl.edu Follow this and additional works at: Part of the Life Sciences Commons Lemos-spinal, Julio A. and Balliner, Royce., "coloy of Growth of the Hih Altitude Lizard Sceloporus rammicus on the astern Slope of Iztaccihuatl Volcano, Puebla, México" (1995). Transactions of the Nebraska Academy of Sciences and Affiliated Societies This Article is brouht to you for free and open access by the Nebraska Academy of Sciences at DiitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Transactions of the Nebraska Academy of Sciences and Affiliated Societies by an authorized administrator of DiitalCommons@University of Nebraska - Lincoln.

2 1995. Transactions of the Nebraska Academy of Sciences, 22: COLOGY OF GROWTH OF TH mgh ALTITUD LIZARD SCLOPORUS GRAMM/CUS ON TH ASTRN SLOP OF IZTACCIHUATL VOLCANO, PUBLA, MXICO Julio A. Lemos-spinal Proyecto Fauna Silvestre, CNIO/COMF, INIFAP Avenida Proreso # 5, Viveros de Coyoacan 04110, Mexico, O. F. Rapid rowth rates are expected to evolve for oranisms inhabitin environments with hih resource supply/demand ratios, whereas low rowth rates are expected to evolve in oranisms inhabitin hihly competitive environments with low resource supply/demand ratios (Pianka, 1970). This relative speed in rowth rates leads to differences in body sizes within populaand Royce. Balliner School of Bioloical Sciences University of Nebraska-Lincoln Lincoln, Nebraska ABSTRACT Growth in the lizard Sceloporus rammicus livin at two altitudes on the Iztaccihuatl Volcano, Puebla, Mexico, was studied from November 1984 to January Despite differences in the environmental conditions at these two elevations, individual lizards rew at similar rates at both study sites. Females in both populations rew slower and attained asymptotic size earlier in life than males ( mm per day and 49.7 mm snout-vent lenth for females vs mm per day and 53.2 mm snout-vent lenth for males). Maximum rowth rates were observed early in life in both sexes and sites. Growth in Sceloporus rammicus fits the von Bertalanffy rowth model from 0 to 1.5 yrs. of ae. Initial rowth rates were between 0.10 and 0.15 mm per day in both sexes and sites. Rates decreased to less than 0.02 mm per day at maturity at both sites. Differential investment in feedin activities, social interactions and thermoreulation, differences in the number of predators and competitors, and absence of a hibernation period at both study sites are possible explanations for similar rowth rates at both sites. t t t Growth is an important component of an oranism's life history. Allocation of rowth enery is compensatory with other life history traits (e. reproduction, maintenance, storae; Fisher 1958). Williams (1966) observed that enery expended for reproduction may result in reduced rowth. Gadil and Bossert (1970) stressed that rowth is one of the three major life history cateories amon which time-enery budets are divided to maximize fitness. 77 tions. A number of studies have shown a positive relationship between female body size and clutch size, such that larer, more rotund females have larer clutches than smaller and thinner females (Fitch, 1970; Tinkle, 1969; Tinkle et ai., 1970; Vitt and Condon, 1978), suestin a selective advantae for lare body size. Growth rates are also related to competitive ability, since contests for food or space are usually won by lare individuals (Fox, 1983; Tokarz, 1985). Lare individuals also usually have an advantae in predator avoidance. Smaller individuals are excluded from territories with optimal food supplies and are forced to forae more often and to spend more time when predators are apt to be huntin (Feruson et ai., 1982). In addition, female choice for larer males has been suested as a causal factor for hih male rowth rates (Halliday and Verrel, 1988; Van Devender, 1978). Growth has been studied extensively in lizards. Lizard rowth rates vary with food availability (Andrews, 1976, 1979, 1982; Balliner, 1977; Balliner and Condon, 1980; Case, 1976; Dunham, 1978; Feruson and Brockman, 1980; Lewis, 1986; Mayhew, 1965; Medica et ai., 1975; Schoener and Schoener, 1978; Stamps, 1977) 1982; ). Differences in rowth rates related to water supply have also been observed (Jenssen and Andrews, 1983; Nay, 1973; Stamps and Tanaka, 1981). Lizard rowth rates are also affected by temperature because temperature affects diestive rate and efficiency (Christian, 1986; Dunham et ai., 1989; Grant and Dunham, 1990; Noeske and Meier, 1983; Sinervo and Adolph, 1989). Some studies have suested a stron relationship between sexual selection and rowth rates (Halliday and Verrel, 1988; Van Devender, 1978). We examined body rowth rates in two populations of the lizard Sceloporus rammicus occurrin at two different altitudes. Growth rates were measured and

3 78 J. A. Lemos-spinal and R.. Balliner related to weather conditions, food availability, and lizard activity patterns, and were compared between sexes and amon seasons throuh several years. MATRIALS AND MTHODS Species studied: Sceloporus rammicus is a small, sexually dimorphic phrynosomatid (Frost and theride, 1989) lizard that occurs from southern Texas, USA, to the state of Oaxaca, Mexico. On Iztaccihuatl Volcano, Puebla, Mexico, Sceloporus rammicus occurs up to 4,600 m elevation. Adult males are larer than females (x = 52 mm snout-vent lenth and 4.6 body mass for males, versus x = 49 mm snout-vent lenth and 4 body mass for females) (Lemos-spinal, 1992). Study sites: Populations were located at two sites on the eastern slope of the Iztaccihuatl Volcano. These sites in Campo xperimental Forestal San Juan Tetla (CFSJT), of the Instituto Nacional de Investiaciones Forestales y Aropecuarias de Mexico (19 10'N, 98 36'W). At this latitude, the tree line is at 4,000 m elevation. The experimental station is 25 km southwest of the town of San Martin Texmelucan, and approximately 50 km west of Puebla City. The low elevation (3700 m) site (hereinafter Launa) of approximately 4 ha is located in a Pinus hartweii forest surroundin a natural ephemeral lake. Lizards occur primarily on los and stumps, and occasionally under tree bark and in cracks of trunks or rocks. Neonates frequently are found on the sandy soil, in leaf litter or under surface debris. Launa was studied monthly from November 1984 to June 1988 and from September 1990 to January The hih elevation site (4400 m) (hereinafter Pared on) of approximately 1 ha is a volcanic rock formation surrounded by rassland of predominately Festuca tolucensis. Lizards at this site live under rocks and in rock crevices. This site was studied monthly from November 1985 to June 1988 and from September 1990 to January Lizards were captured by hand or noose. Rainfall data were obtained from a weather station located at 3,300 m elevation at the field station (Fi. 1). Maximum rainfall occurs in the summer (June-Auust), and rainfall decreases to very low values in the winter (December-February) and remains low until May. Therefore, the year can be divided into a wet season (May-October) and a dry season (N ovember April). Temperature data were obtained from maximum-minimum thermometers located at both study sites from May 1991 to April Prey availability: Prey availability was estimated by settin sticky traps made of 10 wood squares (10 x 10 cm) coated with the adhesive Tanlefoot. Sticky 200.s ro c 100 " <l> 0> ~ <l> > «o _. Jan Feb Mar Apr May Jun Jul Au Sep Oct Nov Dec Fiure 1. Averae rainfall variation at Iztaccihuatl Volcano, Puebla, Mexico. Data taken (January 1984-December 1991) from San Juan Tetla weather station (3,300 m elevation). trap samples were taken monthly in the wet season from May throuh Auust 1991, and in the dry season from December 1991 throuh January 1992 at both sites. Sticky traps were randomly placed on each study site for 12 h from 0600 to 1800 h. At 1800 h traps were removed and number and size of arthropods (lenth x width) were recorded. The volume (V) of individual insects (assumin an ellipsoid shape) was estimated, usin the equation for an ellipsoid. Lizard Activity: Rates oflizard movements, feedin attempts, social displays and the number of 5-min periods in which a lizard was immobile were recorded for active male lizards durin continuous 30-min observation periods (n = 32 periods) usin methods of Grant and Dunham (1988). These rates are useful in estimatin time budets of lizard activity. In eneral, immobile behavior was an indication of thermoreulatory baskin or ambush feedin behavior. These two behaviors could be distinuished by posture and deree of alertness. Also, thermoreulatory baskin involved loner periods of immobility than did feedin behavior. Observations were made before noon in the summer and winter of 1991, 1992, and Sites and seasons (wet and dry) were compared usin analysis of variance (ANOVA). Growth stimation: Lizards were captured by hand or noose (n = 3044). Lizards were permanently marked by clippin a unique combination of up to two toes for each foot. Before the lizards were released at the site of capture the followin data were recorded: snout-vent lenth (SVL) to the nearest 0.1 mm usin a caliper and a clear plastic ruler; body mass (BM) to the nearest 0.01 usin a Pesola sprin scale; sex; tail condition (broken, unbroken or reenerated); and site

4 coloy of rowth of Sceloporus rammicus 79 Table 1. Reression statistics for rowth curves. Initial rowth rate (a); dampin coefficient (b); asymptotic size (-alb). Site: L == Launa; P = Paredon. Sex Site Season N R2 F L All F L Wet F L Dry F P All F P Wet F P Dry M L All M L Wet M L Dry M P All M P Wet M P Dry and date. Chanes in lenth (dsvl) and rowth interval lenth (dt) were used to calculate rowth rates (GR = dsvudt). Only lizards with recapture intervals >30 and <100 days were used in parameter estimation. Averae size durin the interval (ASVL) was the averae of first and last SVL. Two frequently used rowth models are the von Bertalanffy rowth model (Fabens, 1965; von Bertalanffy, 1951, 1957) and the loistic-by-ienth rowth model (Schoener and Schoener, 1978). Andrews (1982) discussed the difference between the von Bertalanffy and the loistic-by-ienth rowth models. The von Bertalanffy model predicts the maximal rate of rowth in lenth for the smallest size class and a decreasin rate of rowth as size increases. In contrast, the loistic model predicts that the maximal rate of rowth in lenth will be observed at 50% ofthe asymptotic lenth (Andrews, 1982). The von Bertalanffy model predicts that rowth rate in SVL is a linear function of body lenth: GR = a - basvl... 1 where a = initial rowth rate, b = dampin coefficient, and ASVL = averae snout-vent lenth. ASVL is used rather than initial size because the GR is measured over a limited period and will underestimate instantaneous GR for the initial SVL (Van Devender 1978). Asymptotic size is predicted as Z = -alb. quation # 1 can be expressed as: GR = a [1- (SYUZ)]... 2 or a - basvl = a{ a [ 1- (SYUZ)])... 3 which is Fabens (1965) derivation of the differential equation of the von Bertalanffy rowth model. Knowin the size of neonate lizards (SVLO), and usin the values of Z and b obtained from GR = a - basvl, the rowth curve can be obtained by: SVL = Z (1- ke-bt)....4 where SVL is the size that the lizard has reached after a b -alb time T (since parturition) k is a constant that can be calculated if SVLO is known, and T is the number of days of rowth (ae). k can be estimated as follows: k = 1 - SVLoIZ... 5 Fabens (1965) proposed the followin equation to estimate SVL of a lizard at time t+d (SVL2) in terms of its SVL at time t (SVL1)' SVL 2 = Z - (Z - SVL 1 ) e-bd... 6 where d is the time interval of rowth. In order to test the oodness of fit for the rowth curves obtained, animals with known aes were compared with sizes predicted by the model.linear reression was used to determine the relationship between GR and SVL. Separate reressions were calculated for each combination of sex, site, and season. The reressions were tested for statistical validity and compared usin analysis of covariance (ANCOVA), usin SVL as a covariate. Since our rowth data followed the von Bertalanffy model, we do not detail the predictions of the loistic model here (see Andrews, 1982; Schoener and Schoener, 1978) RSULTS Growth and body size relationships Growth rates: Growth rates for both populations of Sceloporus rammicus were maximal for the smallest size class and decreased as size increased, indicatin a von Bertalanffy rowth trajectory. Growth rates varied inversely with ASVL for males and females at both sites (Fi. 2). We used these data to calculate a predicted rowth curve (ae vs. SVL) and this areed well with data from lizards of known ae (Fi. 3; Table 1). Females from both populations rew slower than males (P < , F 1479 = 23.42, for Launa, and P < 0.002, F 1379 = 9.36, for Paredon). Growth curves for both sexes and sites indicated that female rowth decreased with size more rapidly than did male rowth

5 80 J. A. Lemos-spinal and R.. Balliner 0.2 Mele body rowth rete. at Peredon 0.2 Femlle body rowth rite. It Plredon y = x R =0.68 Y = x R = >-.. >-.. ~ ~..., Ii lie a: :.. s:. s:.. f... f CI.. CI...- I i ~.... _.-..., ~.. : i. -.-:: Averae Snout-Vent Lenth (rnm) Averae Snout-Vent Lenth (mm) Male body rowth rate. at Launl Femlle body rowth rete. It Laune Y = OO294x R = Y = R = 0.84 >-.. >- c ~ ~!., Ii 0.1 i 0.1 lie a: s:. s:. i f CI i f CI ~--r--~--r--~l:e-""'-'--"" ~---r--~-~-'; Averae Snout-Vent Lenth (mm) Averee Snout-Vent Lenth (mm) Fiure 2. Growth in Sceloporus rammicus for females and males at both study sites. ach point represents rowth rate (GR) and averae snout-vent lenth obtained for lizards with recapture intervals of days. Data are from both seasons (wet and dry). Growth rates for males (both populations) Growth rates for females (both populations) s...J > en LAGUNA PARLOII LAGUNA PARLOII r------~r_------~------~ AG (days) AG (days) Fiure 3. Size and ae in Sceloporus rammicus for females and males at both study sites. Solid points represent the predicted rowth curve from its correspondin reression line in Fi. 2. Open points are lizards marked within their first 3 months oflife (20 mm SVL < lizard < 30 mm SVL) and capture later in the study.

6 coloy of rowth of Sceloporus rammicus 81 Table 2. Variation in available prey of Sceloporus rammicus. Numbers represent volumes (ml) of insects on 10 Tanlefoot traps at each site per time sample. June = early June; June L = late June. Data are means ± one standard error and ANOVA for comparin data by sites. Month Launa Paredon F 1,17 P May ± June ± JuneL ± July ± Auust ± December 6.53 ± January 5.22 ± ± ± *** ± *** ± *** ± ** 2.12 ± ** 2.15 ± * (*P < 0.05, **p < 0.01, ***p < 0.001) (Fi. 4). Growth rates between sites did not differ for either sex (P > 0.05, F 1580 = 0.32 for females, and P > 0.05, F I 272 = 1.94 for males). Althouh no sinificant difference was detected between rowth rates of the same sex at different study sites, rowth rates oflizards from Paredon have a much reater variability than those from Launa (R2 = 0.35 for females and R2 = 0.46 for males at Paredon, VS R2 = 0.71 for females and R2 = 0.76 for males at Launa). Growth rates of adult individual lizards (SVL > 40 mm) were not sinificantly different between wet and dry seasons for both sexes and sites (Launa: F lllo = 0.24, P > 0.05, for females, and F I,43 = 0.88, P > 0.05, for males; Paredon: F I,99 = 1.2, P > 0.05, for females, and F I46 = 1.33, P > 0.05, for m~~). ' Mean asymptotic sizes for females were 49.5 mm (SVL) at Launa and 49.9 mm (SVL) atparedon. Mean asymptotic sizes for males were 52.6 mm (SVL) at Launa and 53.8 mm (SVL) at Paredon (Fi. 2). Comparisons of individuals of known aes with aes predicted by the von Bertalanffy model (Fi. 3), clearly demonstrate that this model is reliable for animals less than or equal to 1.5 years of ae. Beyond this ae, model predictions are very poor. There is considerable overlap in sizes amon lizards between 1.5 and 4 years of ae. Strict applicability ofthe model is limited to individuals 1.5 years old or youner. Ae determination oflizards older than 1.5 years should be verified by mark-recapture techniques. Body-size relationships: Within a iven site, males and females did not differ in the relationship of mass to SVL (P> 0.05, t = 0.257, N = 1932 for Launa, and P > 0.05, t = 0.855, N = 1112 for Paredon, for t test oflo-transformed data). However, the relationship of mass to SVL differed between sites for females (P < 0.05, t = 2.13) and for males (P < 0.005, t = 2.95), for t test of lo transformed data. Individual lizards at Launa were slihtly but not sinificantly heavier than individual lizards at Paredon (averae BM = 3.71 for Launa, and 3.61 for Paredon). Low altitude (Launa) lizards of both sexes are relatively more robust at all SVLs compared to hih altitude (Paredon) lizards. Factors affectin rowth Weather conditions: Averae minimum temperatures 2.03 C ± and 2.24 C ± at Launa and Paredon, respectively (Fi. 5) were not sinificantly different (P > 0.05, F I52 = 0.06). Averae maximum temperatures were hiher at Launa than at Paredon (13.09 C ± vs C ± 0.509, P > , F I52 = 53.53). The averae difference between maximum'and minimum temperatures at Launa (l1.05 C ± 1.162) was sinificantly reater than at Paredon (3.45 C ± 0.574; F I52 = 34.37, P < ). Althouh lizards at Launa face the same minimum temperatures as lizards at Paredon, the former face hiher maximum temperatures. However, individual lizards at Paredon are able to maintain similar body temperatures as individual lizards at Launa by usin microhabitats with reat insolation. Prey availability: The volume of available insects was sinificantly reater at Launa for all the months sampled except May (Table 2). In addition, insect volumes were reater in summer than in winter for both sites. The increase in insect availability at Launa probably results from the hiher temperatures and hiher rainfall in summer. In addition, a reater variability in insects collected in the adhesive Tanlefoot traps was observed at Launa than at Paredon. Arthropod orders at Launa included Diptera, Coleoptera, Hymenoptera, Hemiptera, Dermaptera, and Lepidoptera, alon with some arachnoidea. Arthropod

7 82 J. A. Lemos-spinal and R.. Balliner MALS AT LAGUNA 60 FMALS AT LAGUNA J > > CI) CI)...J AG (days) ~~----~------_, o AG (days) 60 MALS AT PAR DON 60 FMALS AT PARDON J > > CI) CI) J ~ ~------, ~------, o AG (days) AG (days) Fiure 4. Size and ae in Sceloporus rammicus for females and males at both study sites. Curves are the interal form for reression of all seasons in Table 1. Paredon Launa 10 U U Cb Cb :; :; c;; c;; Q; a.! Cb Cb f- f ,.-_.----~-~-,_-~-, , ~ r---., o o DAYS Fiure 5. Variation of minimum and maximum temperatures at low (Launa, 3,700 m) and hih (Paredon, 4,400 m) altitude study sites on Iztaccihuatl Volcano, Puebla, Mexico. X-axis in days from January to December (0 = January 1). DAYS

8 coloy of rowth of Sceloporus rammicus 83 Table 3. Behaviors of Sceloporus rammicus durin 30-min observation periods. Data are means ± one S. All data have units of number per 30-min except total distance which has units ofm per 30-min. The term "5-min immobile" refers to the number of complete 5-min periods per 30-min observations durin which no movement or behavior occurred. Based on 74 observation periods at Launa (3,700 m in elevation) and 42 observation periods at Paredon (4,400 m in elevation), for summer (May, June, July, Auust 1991, 1992, 1993), and 15 observation periods at Launa (3,700 m in elevation) and 17 observations periods at Paredon (4,400 m in elevation), for winter (December 1991, January, December 1992, and February 1993). Launa Paredon Summer Winter Summer Winter Discrete movements 21.0 ± ± ± ± 0.37 Total distance moved (m) 16.9 ± ± ± ± 0.31 Feedin attempts 14.2 ± ± ± ± 0.31 Displays 22.9 ± minutes immobile 0.14 ± ± ± ± ± ± ± 0.2 orders at Paredon were Diptera, Coleoptera and Hymenoptera. Lizard activity: In eneral, lizards at Launa allocated more time to feedin and social interactions than lizards at Paredon (Table 3). Lizards at Paredon spent more time thermoreulatin as reflected in the reater time spent immobile. There are sinificant differences in lizard activity patterns between summer and winter at each site. Summer activity patterns entailed more time spent in feedin and social interactions. These activities declined in winter with an increased amount of time spent thermoreulatin. Increased thermoreulation in winter occurred at both hih and low altitude sites. Thermoreulatory behavior by individual lizards at Paredon allowed similar active body temperature at both study sites (x = C ± O.l1S at Launa, and C ± 0.16S at Paredon, F 1536 = 3.67, P > 0.05), which may account for the similar rowth rates. DISCUSSION Lizard food availability (insect abundance) is hihly correlated with weather conditions, such as rainfall (Balliner, 1977; Dunham, 1978). Grant and Dunham (1988, 1990) demonstrated that lizard rowth rates are the result of interactions between food abundance, thermal environment and lenth of the rowin season. Greater availability of insects at Launa should result in reater rowth if food availability limits rowth, or in increased robustness. Heavier lizards at Launa compared to Paredon miht be the result of reater food availability combined with reater amounts of time spent in feedin activities. The data obtained in this study suest that populations livin in different environments can attain similar rowth patterns reardless of differences in food availability, time spent in feedin activities, social in- teractions and thermoreulation, or different:es in weather conditions as lon as thermoreulatory ~pportunities allow maintenance of similar patterns of body temperatures. Some comparative studies in lizard rowth rates have shown no differences in rowth rates in populations of a species. Tinkle (1967) found similar rowth rates of Uta stansburiana from Colorado and Texas. Grant and Dunham (1990) reported that individual rowth rates for the desert lizard Sceloporus merriami did not differ sinificantly between populations at 560 m and 1609 m elevation at Bi Bend National Park, Texas. However, rowth rates of an intermediate population (1036 m elevation) were sinificantly hiher than those for the two other populations because of loner activity periods (relative to the hihest elevation site) and more insect availability (relative to the lowest elevation site). Walter (1991) demonstrated that rowth rates of the lizard Aama tuberculata, of comparable ae-size cateories, were not sinificantly different between two sites at 690 m and 2215 m in elevation in the western Himalayas. However, all these studies demonstrated differences in ae at maturity between populations livin at different elevations due to a loner rowin season in one of the sites. In contrast, Sceloporus rammicus populations in this study have similar aes at maturity (i.e. between 13 and 14 months; Lemos-spinal 1992). In spite of differences in patterns of food availability, time invested in feedin activities, social interactions, and thermoreulation, and differences in weather conditions, populations of Sceloporus rammicus at Launa and Paredon had similar rowth rates and body sizes. Several hypotheses may explain this similarity in rowth. First, thermoreulation at Paredon may require lower eneretic expenditure than at Launa as there are no shaded areas. Lizards at this site can find a safe baskin spot more easily than lizards at Launa that live in a heavily shaded forest environment.

9 84 J. A. Lemos-spinal and R.. Balliner Moreover, individual lizards at Launa deal with a reater number of possible competitors and predators. At Launa Sceloporus rammicus coexist with another insectivorous lizard (Sceloporus bicanthalis), one fro (Hyla plicata), and 20 insectivorous birds which are potential competitors (Perez and Lemos-spinal, 1988). In addition, possible predators of Sceloporus rammicus at Launa include Crotalus triseritus, Thamnophis scalaris, Falco columbarius, and Falco sparverius. On the other hand, at Paredon, there are no amphibians or other reptiles, and the only possible bird competitors that we observed were Junco phaeonotus and Sialia mexicana. Only one potential predator (Falco sparverius) occurs at Paredon. There is a reater probability of bein eaten by a predator for individual lizards at Launa than at Paredon, based on three indices of predation (tail-break. frequency, flushin distance, and number of missed lizards per 24 hrs) (Lemos spinal, 1992). These combined data suest that individual Sceloporus rammicus at Launa face reater biotic interactions with predators and competitors than individuals at Paredon. Thus, even thouh maximum environmental temperatures are lower and food availability is reduced at Paredon, lizards at this site may suffer less interspecific competition and may spend less time avoidin predators than lizards at Launa. As a result, lizards at both sites may be maintainin equivalent thermal conditions to enable similar rowth rates at the two altitudes. Another possibility is that the minimum amount of food required to attain the observed rowth rate is present at Paredon and the additional food at Launa represents an excess that cannot be put into individual rowth. However, ifthis were the case, Launa miht be expected to support a denser population. This appears not to be the case (Lemos-spinal, 1992). The eoraphical position of our populations on Iztaccihuatl Volcano may be an important factor. Those comparative studies that have found differences in rowth rates resultin from differences in the lenth of the rowin season have been carried out at latitudes hiher than 30 N (Grant and Dunham, 1988; 1990; Tinkle, 1967; Walter, 1991). The present study was conducted at 19 N, with a radient of 700 m between sites, and with no sinificant differences between minimum temperatures. Perhaps populations livin at these elevations do not experience sufficient differences to divere in life history traits such as rowth rates. ACKNOWLDGMNTS For field assistance and use of facilities we thank Gabriel Praxedis-Martinez and workers from Campo xperimental San Juan Tetla. Financial support to JL was provided by INIFAP (Instituto Nacional de Investiaciones Forestales y Aropecuaries) and CONACyT (Consejo Nacional de Ciencia y Tecnoloia). JL is especially rateful to In. Carlos Gonzales Vicente and Susy Sanoja Sarabia for advice and support. LITRATUR CITD Andrews, R. M Growth rate in island and mainland Anole lizards. Copeia 1976: Reproductive effort of female Anolis limifrons (Sauria : Iuanidae). Copeia 1979: Patterns of rowth in reptiles. In: C. Gans, and F.H. Pouh (eds.), Bioloy of the reptilia, Vol. 13., Physioloy D. New York, Academic Press: Balliner, R Reproductive strateies: food availability as a source of proximal variation in a lizard. coloy 58: , and J. D. Condon Food resource limitation of body rowth rates in Sceloporus scalaris (Sauria: Iuanidae). Copeia 1980: Case, T. J Body size difference between populations of the chuckwalla Sauromalus obesus. coloy 57: Christian, K. A Physioloical consequences of nihtime temperature for a. tropical herbivorous lizard (Cyciura nubila). Canadian Journal ofzooloy 64: Dunham, A Food availability as a proximate factor influencin individual rowth rates in the iuanid lizard Sceloporus merriami. coloy 59: , B. W. Grant, and K. L. Overall Interfaces between biophysical and physioloical ecoloy and the population ecoloy of terrestrial vertebrate ectotherms. Physioloical Zooloy 62: Fabens, A. J Properties and fittin of the von Bertalanffy rowth curve. Growth 29: Feruson, G. W. and T. Brockman Georaphic differences of rowth rate of Sceloporus lizards (Sauria:Iuanidae). Copeia 1980(2): , K. L. Brown, and V. G. DeMarco Selective basis for the evolution of variable e and hatchlin size in some iuanid lizards. Herpetoloica 38: Fisher, R. A The enetical theory of natural selection. New York, Dover: 291 pp. Fitch, H. S Reproductive cycles in lizards and snakes. Miscellaneous Publications of the Kansas Museum of Natural History 52: Fox, S. F Fitness, home-rane quality, and aresion in Uta stansburiana.,in: R.B. Huey,. R. Pianka, and T. W. Schoener (eds.), Lizard coloy: Studies of a model oranism. Cambride, Massachusetts, Harvard Univ. Press:

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