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Demography of the Side-Blotched Lizard, Uta stansburiana, in Sand Dunes of theCentral Chihuahuan DesertAuthor(s): Héctor Gadsden and Gamaliel CastañedaSource: The Southwestern Naturalist, 57(3):292-303.Published By: Southwestern Association of NaturalistsDOI: http://dx.doi.org/10.1894/0038-4909-57.3.292URL: http://www.bioone.org/doi/full/10.1894/0038-4909-57.3.292

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THE SOUTHWESTERN NATURALIST 57(3): 292–303 SEPTEMBER 2012

DEMOGRAPHY OF THE SIDE-BLOTCHED LIZARD, UTASTANSBURIANA, IN SAND DUNES OF THE CENTRAL

CHIHUAHUAN DESERT

HECTOR GADSDEN* AND GAMALIEL CASTANEDA

Instituto de Ecologıa, Asociacion Civil-Centro Regional Chihuahua, Cubıculo 30C, Miguel de Cervantes 120,Complejo Industrial Chihuahua, C. P. 31109, Chihuahua, Chihuahua, Mexico (HG)

Facultad de Ciencias Biologicas, Universidad Juarez del Estado de Durango, Avenida Universidad s/n, Fraccionamiento Filadelfia,Gomez Palacio, 35070, Durango, Mexico (GC)

*Correspondent: hgadsden@gmail.com

ABSTRACT—We studied demography of a population of side-blotched lizards, Uta stansburiana, in the sanddunes of Reserva de Biosfera de Mapimı, Durango, Mexico, during 1989–1994. Reproduction in femalesoccurred during January–July, which coincided with late dry and early wet seasons. Reproductive activity washighest in the middle of the dry season (February–March). At an average age of 10 months, 32% of femaleshad reached sexual maturity. A notable feature of summer and autumn was the greater number of hatchlingsand juveniles, coinciding with the wet season. Overall sex ratio did not differ from 1:1. Density of adults was 1–59/ha. This population had early maturity, a relatively short life expectancy, and many offspring.

RESUMEN—Estudiamos la demografıa de una poblacion de la lagartija de manchas laterales, Uta stansburiana,en las dunas de arena de la Reserva de Biosfera de Mapimı, Durango, Mexico, durante 1989–1994. Lareproduccion en las hembras ocurrio durante enero–julio, lo cual coincidio con el fin de la estacion seca yprincipio de la estacion humeda. La actividad reproductora fue mas alta a la mitad de la estacion seca(febrero–marzo). A una edad promedio de 10 meses, el 32% de las hembras habıan alcanzado la madurezsexual. Una caracterıstica notable en el verano y el otono fue el mayor numero de crıas y jovenes, coincidiendocon la estacion humeda. La proporcion global de sexos no fue diferente de 1:1. La densidad de adultos fue de1–59/ha. Esta poblacion tenıa una madurez temprana, una esperanza de vida relativamente corta, y muchascrıas.

The number of studies of ecology of lizards hasincreased greatly since 1960 (Huey et al., 1983),improving our knowledge of life history, populationdynamics, and demography (e.g., Tinkle, 1967a; Dun-ham, 1982; Ballinger, 1983). Some research has comparedvariation in life history among species (e.g., Tinkle et al.,1970; Ballinger, 1973; James, 1991) and others haveevaluated differences among populations of the samespecies (e.g., Tinkle and Ballinger, 1972; Parker andPianka, 1975; Dunham, 1982; Van Devender, 1982;Lemos-Espinal and Ballinger, 1995). Life history of aspecies can be summarized by demographic parameterssuch as rates of birth and death, migratory movements,and structure of populations through time, as they dealwith the interaction between longevity, reproductive age,growth, and age-specific mortality versus the environment(Zug, 1993). Interactions among these parametersthrough life make demographics of populations subjectto constant variation through time. Demography ofpopulations of lizards can be influenced by variousfactors, such as temperature (Adolph and Porter, 1993;

Parker, 1994), precipitation (Andrews, 1988; Bull, 1994),availability of food (Ballinger, 1977; Howland, 1992;Smith, 1996), and morphological and phylogeneticconstraints (Ballinger, 1983; Stearns, 1984; Dunham andMiles, 1985).

Some patterns can be relate dynamics and structure ofpopulations to life history (Tinkle, 1969a). Lizards withdelayed sexual maturity tend to have longer life cycles,along with large energetic investments in maintenanceinstead of reproduction (Tinkle, 1969a; Tinkle et al.,1970), and lower mortality of adults (Hasegawa, 1990;Bull, 1995). Lizards with early sexual maturity haveshorter life cycles and higher rates of mortality for youngand adults (Ballinger and Congdon, 1981; Andrews andNichols, 1990), resulting in a larger turnover in popula-tions (Ferguson et al., 1980; Tinkle et al., 1993). Thisdichotomy represents extremes of a gradient, but manyintermediate combinations exist (Dunham et al., 1988).Structure of populations also can be influenced by matingsystem. Populations with higher incidences of polygynytend to have higher rates of death for males than

populations where polygyny is reduced (Schoener andSchoener, 1980; Stamps, 1983), generating a female-biased sex ratio. Life-history theory, however, was devel-oped mainly from data on temperate species of lizards inthe United States (Ballinger, 1983); nevertheless, in thevast desert region of northern Mexico, which has a richdiversity of lizards, long-term studies of structure anddynamics of populations are scarce (e.g., Gadsden et al.,2001; Gadsden and Estrada-Rodrıguez, 2007; H. Gadsdenet al., in litt.).

One population of Uta stansburiana that inhabits thecentral Chihuahuan Desert in northern Mexico is thefocus of our study. Data regarding several aspects of theecology and life history of U. stansburiana are extensive forpopulations north of the Rio Grande (e.g., Asplund andLowe, 1964; Nussbaum and Diller, 1967; Tinkle, 1967a,1967b, 1969a; Turner et al., 1970; Tanner, 1972; Parker,1974; Parker and Pianka, 1975). However, little is knownabout demography and life history of populations fromarid habitats in northern Mexico (Rivera-Hernandez,2009; H. Gadsden et al., in litt.). We identify seasonalpatterns of variation in size of population, age structure,sex ratio, growth, reproduction, finite rate of survival, andlife-history traits for U. stansburiana in the centralChihuahuan Desert and make comparisons with otherpopulations of the same species to evaluate predictions ofcurrent life-history theory.

MATERIALS AND METHODS—We monitored a population of U.stansburiana on a dune area and we established a 1-ha study plotof (268520N, 1038320W) within the Mapimian subprovince of theChihuahuan Desert in Reserva de Biosfera de Mapimı, Durango,Mexico (1,250 m elevation; Barbault and Halffter, 1981).Vegetation was dominated by Acacia constricta and Acacia gregii(thornscrub) and Larrea tridentata (creosotebush; Breimer,1985). The study plot was gridded with wooden stakes spaced20 m apart over an area of ca. 100 by 100 m. Data from this plotwere used in evaluation of demographic characteristics of thispopulation.

Climate of this region is seasonal, with highest temperaturesand rainfall in summer. Temperatures range from an averagelow in winter of 3.98C to an average high in summer of 36.18C.Mean annual precipitation is 230 mm. There is seasonal andannual variation, but most rain occurs in summer (Cornet,1988).

We captured U. stansburiana by buried pitfall traps (100 7.6-Lcans, each at 20-m intervals), by noosing, or by hand in winter(20–28 February 1989, 13–18 January 1990, 10–13 February1992, 16–20 February 1993, and 19–22 February 1994), spring(4–8 May 1989, 6–12 May 1990, 20–27 April 1991, 13–21 May1992, 8–11 May 1993, and 20–24 April 1994), summer (11–17September 1989, 7–16 July 1990, 15–23 July 1991, 21–25 August1992, 18–22 July 1993, and 21–24 September 1994), and autumn(4–14 November 1989, 27–30 October and 1–3 November 1990,3–11 November 1991, 13–17 November 1992, 10–16 November1993, and 1–3 December 1994). Traps were checked everymorning during our study and were removed after eachsampling period.

We systematically searched sites by following a specific path

that was constant during each censusing interval. On eachsampling date, each person (three people) chose one row ofquadrants (20 by 20 m each) and walked slowly across the entirearea remaining at all times between two rows of stakes followingmethods of Tinkle (1967a) and Howland (1992). Bushes andother potential refugia were shaken when necessary to locate alizard. Sampling occurred during 0900–1400 h each day asdescribed by Ballinger (1973) and Smith and Ballinger (1994a).

Each individual captured was marked permanently by toe-clipping, and a number was painted on the dorsum with apermanent marker for quick identification. For each capture, werecorded date, sex, and snout–vent length (measured to thenearest 1 mm with a ruler). In addition, abdomens of femaleswere palpated carefully to determine if oviductal eggs werepresent. Lizards were released at point of capture as describedby Dunham (1982), Van Devender (1982), and James (1991).Snout–vent length at initial capture was used to determinestructure of the population following Griffiths (1999) andRamirez-Bautista et al. (2002). The number of mature males andfemales in the study area was used to compute sex ratio ofadults.

Data for snout–vent length, weight, and sex of marked andrecaptured lizards were used to estimate growth curves for malesand females. Changes in length (Dsnout–vent length) and timeintervals (DT) were used to estimate rates of growth (rate ofgrowth = Dsnout–vent length/DT). Data were analysed takinginto account the time between recaptures of >60 and <420 days.Additionally, averages of growth and snout–vent length of lizardsrecaptured more than once were used to estimate rates ofgrowth representative of the population. Average length duringthe interval for each lizard (mean snout–vent length) was theaverage of the first and last snout–vent length observed for each.The model used for evaluation of growth of U. stansburiana wasthat of von Bertalanffy (1951, 1957) as modified by Fabens(1965) and Lemos-Espinal et al. (2005). This model predictsmaximum rate of growth in length for small sizes (juveniles) anddecreases as size increases as determined by Lemos-Espinal andBallinger (1995). The von Bertalanffy model predicts that rate ofgrowth in snout–vent length is a linear function of length ofbody: rate of growth = a - bMean snout–vent length (equation1), where a is the initial rate of growth and b is the decreasecoefficient. Mean snout–vent length is used instead of originalsize because growth is measured over a limited period and mayoverestimate rate of growth for the initial snout–vent length asdiscovered by Van Devender (1978). Asymptotic size is predictedas Z = -a/b. Equation (1) can be expressed as follows: rate ofgrowth = a [1 - (snout–vent length/Z)] or a - bMean snout–vent length = a [1 - snout–vent length/Z], which is thederivation of Fabens (1965) of the differential-equation vonBertalanffy model of growth. Knowing size of lizards at birth(snout–vent length0), and using the Z and b values obtainedfrom rate of growth = a - b Mean snout–vent length, the growthcurve can be obtained from: snout–vent length = Z (1 - ke-bT),where snout–vent length is length reached by a lizard after atime T (from birth), k is a constant that can be calculated ifsnout–vent length0 is known, and T is number of days elapsed(age of lizard). The estimate of k is obtained as follows: k = 1 -snout–vent length0/Z. Fabens (1965) proposed the followingequation to estimate snout–vent length of a lizard at time t + d(snout–vent length2) in terms of the snout–vent length at time t

September 2012 Gadsden and Castaneda—Side-blotched lizards in Durango 293

(snout–vent length1): snout–vent length2 = Z - (Z - snout–ventlength1) e-bd, where d is the time interval for growth.

To test how well this model fits the real growth of U.stansburiana, lizards of known age were compared with sizespredicted by the model. Linear regressions were used todetermine the relationship between rate of growth and snout–vent length. Regressions were calculated separately for each sex.For comparison of regressions for each sex, we used analysis ofcovariance (ANCOVA), using snout–vent length as a covariate.For all statistical analyses, only one observation (selected atrandom) per individual was used. All values are given as mean–1 SE.

Densities were estimated using trapping data and visualsightings. In our study, we worked for short periods to mark-recapture lizards (closed population); therefore, we estimateddensity of individual adults and all size classes using the small-sample Lincoln-Petersen and Schumacher-Eschmeyer methods(Krebs, 1998). The Lincoln-Petersen method is based on asingle episode of marking lizards and a second single episode ofrecapturing individuals. The basic procedure is to mark anumber of individuals over a short time (basic premise forimplementing this method), release them, and then recaptureindividuals to check for marks. For this method to be valid,marked and unmarked individuals must have the same chanceof being captured in the second sample. The Schumacher-Eschmeyer method distinguishes only two types of individuals:marked, caught in one or more previous samples; unmarked,never caught previously.

Tinkle (1967a) and Wilson (1991) noted that individual U.stansburiana were regularly observed in a restricted area andcenters of activity were near specific small patches of vegetation.However, we sampled outside the study area to validate thatindividual U. stansburiana remained at the site. Consequently,the possibility of immigration or emigration of individuals inthis population was reduced.

We calculated finite rate of survival (S0) as number ofindividuals alive at end of time period (Nt) divided by number ofindividuals alive at start of time period (N0). If the time intervalis 1 year, this will give a finite annual rate of survival (Krebs,1998). Finite rates of survival can range from 0 to 1, and theyalways apply to some specific time period (Krebs, 1998). Weconsidered the proportion of individuals marked in one seasonthat were recaptured the following year in the same season.Finite rate of survival may be underestimated, however, becausesome lizards may not have been recaptured, despite havingsurvived as noted by Smith and Ballinger (1994b).

During 1990, we collected monthly samples of U. stansburianafrom other sand dunes (within several kilometers of the markedpopulation) for analysis of reproductive cycles. Individual lizardswere brought into the laboratory and autopsied within 24 h. Wekilled specimens with Nembutal and, subsequently, preservedthem in 10% formalin following procedures in Gadsden andPalacios-Orona (1997) and Gadsden et al. (2001). We depositedspecimens in the collection of the Instituto de Ecologıa,Asociacion Civil, Centro Regional Chihuahua (voucher speci-mens-Inecol-US-1-84). We used snout–vent length of thesmallest females with vitellogenic follicles or oviductal eggs asan estimate of size at sexual maturity following Ramırez-Bautistaand Vitt (1997). We recorded number of non-vitellogenic andvitellogenic follicles and oviductal eggs for females. Size of

clutch was determined by number of oviductal eggs in adultfemales during the reproductive season.

A life table was constructed from data on age-specificfecundity (mx) for each age class from size of clutch andfrequency of clutches (data were adjusted to account for theproportion of females of each age that actually were reproduc-tive), age-specific finite rate of survival (S0), and age at firstreproduction. Age classes were chosen to correspond to selectedevents in the life of lizards. The first age class was composed offemale eggs (a 1:1 sex ratio at fertilization was assumed), andsubsequent age classes represented ages at about the midpointof successive reproductive seasons as described by Tinkle andBallinger (1972) and Stearns (1992).

Because of the small number of hatchlings we marked, it wasnot possible to estimate numbers of eggs or survivorship ofhatchlings. However, appropriate data for survival can beobtained from ratios of age classes in the population, if weassume that the population had a stationary age distribution.Life-table analysis used average age composition over the 6 yearsof our study and ignored annual changes in age structure as wasdone by Tinkle and Ballinger (1972) and Stearns (1992).

RESULTS—Average size of adult females was 44.0 – 0.2mm (range = 40–54 mm, n = 175). Mean snout–ventlength of gravid females was 46.9 – 0.8 mm (n = 16).Gravid females represented 29% of all sexually maturefemales caught during the reproductive season (n = 55).Reproductive activity in females (Fig. 1) began in Januaryand declined by mid-July, with oviductal eggs presentduring February–May and July. We observed hatchlingsduring May–October, when most of the annualprecipitation occurred and food was abundant. InFebruary and May, 71 and 50% of females, respectively,had eggs in utero. The first clutch was in early April andthe second was in early July. Period of embryonic

FIG. 1—Percentage of side-blotched lizards Uta stansburianafrom Reserva de Biosfera de Mapimı, Durango, Mexico, invarious reproductive stages at different times of the year. Size ofsamples appear above bars.

294 vol. 57, no. 3The Southwestern Naturalist

developmental was estimated from the date at which thefirst female had freshly ovulated eggs in utero (mid-February) to when the first hatchling was found (lateMay). These data suggest a gestation of ca. 100 days andincubation was ca. 50 days.

Size (snout–vent length) of adult males averaged 49.6– 0.4 mm (range, 45–56 mm, n = 47) and size of adultfemales averaged 44.4 – 0.4 mm (range, 40–51 mm, n =74). Average mass was 4.4 – 0.1 g (range, 3.0–7.0 g, n =47) for males and 2.9 – 0.1 g (range, 1.8–5.0 g, n = 74)for females. Based on comparisons of males and females,males attained a significantly greater snout–vent lengthand mass than females (F1,120 = 84.94, P < 0.001 andF1,120 = 80.08, P < 0.001, respectively). Based oncomparisons of largest males (n = 15) and females (n =15), males attained a significantly greater snout–ventlength and mass than females (Mann-Whitney U-test, Z =-4.4, P < 0.001 and Mann-Whitney U-test, Z = -3.7, P <0.001, respectively).

Snout–vent length averaged 47.6 – 0.9 mm (range,36.5–53.0 mm, n = 36) for males and 41.6 – 0.7 mm(range, 31.5–50.5 mm, n = 27) for females. Based oncomparisons of males and females, males attained asignificantly greater snout–vent length than females (F1,62

= 28.14, P < 0.001). Males grew faster than females (0.03– 0.01 mm/day and 0.02 – < 0.01 mm/day, respectively);however, this difference was not significant (ANCOVAF1,62 = 0.64, P = 0.430). Rate of growth decreasedsignificantly with respect to snout–vent length (F1,62 =6.47, P < 0.001), suggesting a von Bertalanffy growthcurve. Rates of growth varied inversely with averagesnout–vent length for males and females (Fig. 2a).

Values used to estimate constants of growth curveswere the same for males and females, corresponding to asnout–vent length of 24.6 mm, with an estimated age of29.0 days. Using these constants in the equation of Fabens(1965), we determined longevities of ca. 1.3 years formales and 1.2 years for females, at a snout–vent length of48 and 45 mm, respectively (Fig. 2b). Longevity for largerindividuals �51 mm for males and �47 mm for femaleswas high and may not be representing reality.

Age can be estimated from data on earliest date ofemergence of hatchlings, mean rate of growth ofindividuals in different size classes, and date of capture.Based on growth curves, males attained size of adults ofca. 45 mm snout–vent length in an average of 12.0months, while females attained size of adults of ca. 41 mmsnout–vent length at an average age of 10.0 months.Comparing this estimate with reproductive data (i.e., dataobtained in 1990), the smallest male with enlarged testeswas 45 mm snout–vent length and the smallest sexuallymature female with oviductal eggs was 40 mm snout–ventlength. Size of males attained an asymptote at ca. 51 mmsnout–vent length and females at ca. 48 mm snout–ventlength; an age of ca. 2.0 and 1.8 years, respectively (Fig.2b).

One adult male was captured in February 1989 (snout–vent length, 47 mm) and recaptured in November 1991(snout–vent length, 56 mm). Given that males require 1year to attain a snout–vent length of 45 mm, it would beca. 3.8 years old. One adult female was captured in August1989 (snout–vent length, 26 mm) and recaptured in July1991 (snout–vent length, 47 mm). Age of this female wasca. 1.8 years, given it takes females 10 months to reach 40mm in snout–vent length.

Structure of the population (Fig. 3a) was similar inspring and autumn, but not in summer (v2 = 47.52, df =12, P < 0.005). A notable feature of summer is presenceof hatchlings and juveniles. Relative numbers of adultmales and females (snout–vent length ‡40 mm) did notdiffer significantly among the 6 years (v2 = 3.46, df = 5, P= 0.620). Lizards 40–47 mm snout–vent length were themost numerous in spring and autumn.

The sex ratio (males:females) was heavily biased

FIG. 2—a) Growth in males and females of side-blotchedlizards Uta stansburiana from Reserva de Biosfera de Mapimı,Durango, Mexico; each point represents rate of growth per daywhich is given as average snout–vent length. b) Growth curves ofvon Bertalanffy models for males and females; estimated fromthe equation of Fabens (1965), with an initial snout–vent lengthof 24.6 mm versus 30 days old; values used were a = 0.2109 and b= 0.0040 for males and a = 0.2355 and b = 0.0049 for females;rectangles and diamonds represent means.

September 2012 Gadsden and Castaneda—Side-blotched lizards in Durango 295

toward females in winter, spring, and summer (0.58, 0.69,and 0.16, respectively). During autumn, the sex ratio ofadults became biased toward males (1.7). Overall sex ratio(four seasons pooled) was 0.81 (51 males:68 females),which did not differ from 1:1 (v2 = 2.0, df = 5, P > 0.05).

Density of adults (Fig. 3b) in winter, spring, summer,and autumn of 1989 was 59.0, 16.2, 7.0, and 20.3/ha,respectively. Density in winter, spring, summer, andautumn of 1990 was 20.0, 5.6, 7.0, and 15.0/ha,respectively. Density in spring, summer, and autumn of1991 was 12.2, 3.0, and 6.5/ha, respectively. Density inwinter, spring, summer, and autumn in 1992 was 1.0, 7.0,0.0, and 3.0/ha, respectively. Density in winter, spring,summer, and autumn in 1993 was 7.0, 1.0, 0.0, and 23.0/ha, respectively. Density in winter, spring, summer, andautumn in 1994 was 6.6, 11.0, 1.0, and 16.5/ha,respectively. Density of individuals in all age classes (Fig.3b) showed similar fluctuations except in autumn 1993,due to an abundance of hatchlings and juveniles in thisseason. In autumn, we observed recruitment, predomi-

nantly of adults, which significantly increased density ofadults.

Estimates of survivorship, size of population, and age-class structure are in Table 1. For the five annual periods(1989–1990, 1990–1991, 1991–1992, 1992–1993, and1993–1994), mean estimates of survivorship for the entirepopulation was 0.10 – 0.02 (adult, 0.08 – 0.02; subadults,0; juveniles, 0.15 – 0.10; hatchlings, 0.07 – 0.07). Forwinter 1989–1990, juveniles and adults had the highestsurvivorship followed by adults during summer. Insummer and autumn 1990–1991, adults had the highestsurvivorship followed by adults during winter. Duringwinter 1992–1993, juveniles had a higher survivorshipthan during winter 1989–1990.

Numbers and proportions of each age class (Table 2)captured over a 6-year period (1989–1994) permittedcalculation of survivorship and construction of a life table(Table 3). The calculated rate of replacement (0.47) forthis population indicated that the population cannotmaintain a stable size given the schedules of survivorshipand fecundity that we calculated. In this population, ca.40% of the rate of replacement is contributed byindividuals ca. 1.6 years old and 34% is attributable toindividuals ca. 1.0 year old.

DISCUSSION—The reproductive season for U.stansburiana in sand dunes of Reserva de Biosfera deMapimı began in January (winter) and declined in mid-July (early summer) through the driest months andbefore the rains commenced in July–September. Mostannual rainfall occurred during summer (mostly inAugust and September). We suggest that production ofoffspring during May–October is timed to the season withabundant food to promote growth and survivorship ofjuveniles, as occurs in populations of this species inOregon and Arizona (Nussbaum and Diller, 1967).

Reproductive activity of U. stansburiana in winterprobably favors greater survivorship of individuals dueto limited activity of potential predators in this season.Grenot et al. (1978) noted that the greater roadrunnerGeococcyx californianus, one of the main predators of smalllizards in Reserva de Biosfera de Mapimı, displaysreduced activity in winter. Palacios-Orona and Gadsden-Esparza (1995) reported that during winter in the samedune area, U. stansburiana fed on abundant insectsincluding ants, beetles, termites, and true bugs. Thesekinds of prey probably could increase reproductivepotential of this lizard during winter. In contrast, mostother species of lizards in the Chihuahuan Desert reducemetabolic and physiological capacity during winter andbegin hibernation (Gadsden et al., 1993). Risk ofpredation may be lower during winter. For example, weobserved a leopard lizard Gambelia wislizenii hunting a U.stansburiana in Reserva de Biosfera de Mapimı. Thispredatory species was not seen during winter. It is difficult

FIG. 3—Population parameters for side-blotched lizards Utastansburiana at Reserva de Biosfera de Mapimı, Durango,Mexico. a) Number of individuals in size classes (snout–ventlength), seasons, and years: hatchlings, �27 mm; juveniles, 28–33 mm; subadults, 34–39 mm; adults, 40–47 mm; and old adults,‡48 mm. b) Seasonal density (Lincoln-Petersen estimator)during a 6-year period and monthly precipitation for the studyarea: X, adults; m, all age classes; and A, precipitation.

296 vol. 57, no. 3The Southwestern Naturalist

to determine seasonal variation of predation on U.stansburiana from these anecdotal observations.

In our study, activity of U. stansburiana in the dunesoccurred throughout all months of the year, as recordedby Asplund and Lowe (1964) for the same species inArizona. According to these authors, U. stansburiana doesnot have true hibernation and there is considerablevariation in their activity from year to year in winter,depending on severity of weather.

A reproductive cycle that peaks in winter is not similarto that previously reported for this species (Nussbaumand Diller, 1967; Tinkle, 1967a; Table 4). Differences inreproductive traits among populations of U. stansburiana

could be due to phenotypic plasticity, different points of asingle-reaction norm, different reaction norms, or phys-iological restrictions.

Female U. stansburiana in Reserva de Biosfera deMapimi probably attain sexual maturity at ca. 10 monthsof age. Likewise, data available for other female U.stansburiana from a lower elevation (884 m; Tinkle,1967a) indicated that they did not reach sexual maturityuntil an age of ca. 9–10 months, which was during the latewinter and spring following birth. Tinkle (1961) estimat-ed size at sexual maturity of female U. stanburiana to be 40mm. At high latitude in the Sonoran Desert, most U.stansburiana reproduce when 10 months old, but a small

TABLE 1—Survivorship of side-blotched lizards Uta stansburiana from Reserva de Biosfera de Mapimı, Durango, Mexico: survivor-ship = proportion of class surviving from one year to the next; n = size of sample; total = total survivorships are weighted means.

Year and season Parameter All

Adults Subadults Juveniles Hatchlings

?? // Total ?? // Total ?? // Total ?? // Total

1989–1990 Winter Survivorship 0.28 0.33 0.33 0.33 0 0 0 1 0 0.5 — — —n 21 6 9 15 1 3 4 1 1 2 — — —

1989–1990 Spring Survivorship 0.17 0.16 0.22 0.2 0 0 0 — — — 0 0 0n 23 6 9 15 3 2 5 — — — 2 1 3

1989–1990 Summer Survivorship 0.29 0.25 0.25 0.25 0 0 0 — 0 0 0 0 0n 17 4 4 8 0 2 2 — 3 3 1 3 4

1989–1990 Autumn Survivorship 0.08 0.25 0 0.07 0 0 0 0 0 0 — 0 0n 25 4 10 14 5 1 6 1 3 4 — 1 1

1990–1991 Winter Survivorship 0.06 0.2 0 0.08 0 0 0 0 — 0 — — —n 16 5 7 12 1 2 3 1 — 1 — — —

1990–1991 Spring Survivorship 0 0 0 0 — — — — — — — — —n 5 3 2 5 — — — — — — — — —

1990–1991 Summer Survivorship 0.15 0.5 0.2 0.28 0 0 0 0 0 0 — — —n 13 2 5 7 2 1 3 1 2 3 — — —

1990–1991 Autumn Survivorship 0.16 0.2 0.5 0.28 0 0 0 0 0 0 0 0 0n 18 5 2 7 4 1 5 1 3 4 1 1 2

1991–1992 Winter Survivorship 0 0 0 0 — — — — — — — — —n 2 1 1 2 — — — — — — — — —

1991–1992 Spring Survivorship 0 0 0 0 — — — — — — — — —n 9 2 7 9 — — — — — — — — —

1991–1992 Summer Survivorship 0.37 0 0 0 — — — 0 — 0 — 0 0n 8 2 1 3 — — — 2 — 2 — 3 3

1991–1992 Autumn Survivorship 0.25 0 0 0 0 0 0 — 0 0 — 0.5 0.5n 10 3 2 5 1 1 2 — 1 1 — 2 2

1992–1993 Winter Survivorship 0.25 — 0 0 — 0 0 — 1 1 — — —n 4 — 2 2 — 1 1 — 1 1 — — —

1992–1993 Spring Survivorship 0 0 0 0 — 0 0 — — — — — —n 5 1 3 4 — 1 1 — — — — — —

1992–1993 Summer Survivorship 0 — — — — 0 0 — — — — — —n 2 — — — — 2 2 — — — — — —

1992–1993 Autumn Survivorship 0 0 0 0 — — — — — — — — —n 3 2 1 3 — — — — — — — — —

1993–1994 Winter Survivorship 0 0 0 0 — 0 0 — — — — — —n 5 2 2 4 — 1 1 — — — — — —

1993–1994 Spring Survivorship 0 0 — 0 — — — — — — — — —n 1 1 — 1 — — — — — — — — —

1993–1994 Summer Survivorship 0 — — — — — — — — — 0 — 0n 1 — — — — — — — — — 1 — 1

1993–1994 Autumn Survivorship 0 0 0 0 0 0 0 0 0 0 — — —n 16 5 4 9 1 1 2 1 4 5 — — —

September 2012 Gadsden and Castaneda—Side-blotched lizards in Durango 297

percentage of yearlings (late-season yearlings) are toosmall to reproduce in their first spring and breed for thefirst time when 20–22 months old (Nussbaum and Diller,1967). Size of females in Texas (average snout–ventlength, 48.9 mm; Tinkle, 1961) was larger than that offemales in our study. This difference in size is consistentwith Bergmann’s rule (Bergmann, 1847) that describedan increase in size of a species as latitude increases orenvironmental temperature decreases. Although Berg-mann’s rule originally was formulated to describeinterspecific variation in endotherms, today it is usedmore loosely to describe an increase in the size of aspecies as latitude increases or environmental tempera-ture decreases (a phenomenon that also is referred to asJames’s rule; Blackburn et al., 1999). Current evidencesuggests that Bergmann’s rule applies not only toendotherms (Ashton et al., 2000; Meiri and Dayan,2003), but also to certain groups of ectotherms (e.g.,

TABLE 2—Number and proportion of each age class of side-blotched lizards Uta stansburiana captured in Reserva de Biosfera deMapimı, Durango, Mexico, over a 6-year period.

Age class(months) Sex

1989 1990 1991 1992 1993 1994

n Proportion n Proportion n Proportion n Proportion n Proportion n Proportion

11.5 Male 0 0 0 0 0 0 0 0 0 0 0 0Female 17 0.41 4 0.19 4 0.36 1 0.14 4 0.29 4 0.18

19.5 Male 9 0.22 4 0.19 2 0.18 2 0.29 5 0.36 4 0.18Female 11 0.27 9 0.43 3 0.27 3 0.43 1 0.07 8 0.36

27.5 Male 4 0.10 4 0.19 0 0 1 0.14 3 0.21 5 0.23Female 0 0 0 0 2 0.18 0 0 1 0.07 1 0.05

Total 41 21 11 7 14 22

TABLE 3—Life table for side-blotched lizards Uta stansburianafrom Reserva de Biosfera de Mapimı, Durango, Mexico. Rate ofreplacement/generation = 0.47.

Ageclass

Survivorship from firstage class to midpoint of

age class over whichnumber of female eggsproduced by each adult

female in eachreproductive season is

measured

Number offemale eggsproduced byeach adult

female in eachreproductive

season

Age-specificfecundity ·Age specificsurvivorship

0 1.00 0 011.5 0.08 2.0 0.1619.5 0.08 2.5 0.2027.5 0.03 4.0 0.12

TABLE 4—Comparison of life-history data for six populations of side-blotched lizards Uta stansburiana from Reserva de Biosfera deMapimı, Durango, Mexico (modified from Nussbaum and Diller, 1967).

Parameter Durangoa Texasb ColoradobNevada

(Rock Valley)cNevada

(Rainier Mesa)d Oregone

Elevation (m) 1,129 884 1,295 1,295 2,389 732Length of growing season (days) 270 215 175 225 200 110–140Length of reproductive season (days) 180 121–141 120 135 — 70Habitat Sandy Sandy Rocky — — RockyDensity (individuals/ha) 2–129 90–272 44 60 25 177Size of hatchling (snout-vent length, mm) 22 22 22 22 22 22Size range of mature males (mm) 45–56 40–60 42 — 40–56 40–53Size range of mature females (mm) 40–54 40–60 37 — — 41–49Average size of adult males (mm) 49.6 — — — 49 48.4Average size of adult females (mm) 44 48.9 42.8 — — 45.4Size of clutch 3.6 4.0 3.2 3.5–4.5 4.85 3.33Frequency of clutches 2–3 3–5 3 3–5 — 1–2Percentage of males ‡2 years old 30 7 33 28 36–65 57.6Percentage of females ‡2 years old 15 7 33 28 36–65 69.4Date hatchlings appeared 12 May 20 June 25 July 25 June 17 July 25 July

a Durango (this study).b Texas and Colorado (Tinkle, 1961, 1967a, 1967b, 1969b; Tinkle and Woodward, 1967).c Nevada–Rock Valley (Turner et al., 1970).d Nevada–Rainier Mesa (Tanner, 1972).e Oregon (Nussbaum and Diller, 1967).

298 vol. 57, no. 3The Southwestern Naturalist

Ashton, 2002; Heinze et al., 2003; Morrison and Hero,2003). Life-history theory predicts that a higher survivor-ship of juveniles in colder environments can favorevolution of a cline in size. Consistent with this theory,juveniles in colder environments have a higher rate ofsurvival and they delay maturation until reaching a largersize than lizards in warmer environments (Angilletta etal., 2004). Experimental work with the eastern fencelizard Sceloporus undulatus and U. stansburiana suggestedthat a combination of environmental and genetic sourcesof variation is required to explain differences in lifehistories across the range of a species (e.g., Ferguson andTalent, 1993; James and Whitford, 1994; Svensson andSinervo, 2000). Dunham et al. (1989) presented aconceptual model (allocation model) linking physiolog-ical ecology and life history of individuals with populationbiology. The model views life history as a heritable set ofrules that, given the physiological state of the individualand constraints imposed by its environment, determineallocations of time and energy. Life-history characterssuch as rate of growth, age at maturity, size of adults, andfecundity are manifestations of the time-ordered (agedependent) sequence of allocations made over a lifetimeof the individual. Three main sources of variations in life-history phenotypes are indentified by this model:allocation rules, sets of operative environments, andphysiological state of the individual (Dunham, 1993).The first two sources represent genetic and environmen-tal variation, respectively, in life-history phenotypes. Thethird source is an umbrella for a variety of factors thataffect allocation decisions (Niewiarowski, 1994).

Growth in reptiles is influenced by phylogenetic andenvironmental factors (Andrews, 1982). Uta stansburianaexhibits differences in rates of growth among populationspossibly related to proximate environmental influences,such as temperature, photoperiod, rainfall, and availabil-ity of food (Tinkle, 1967a), but still conforms to ageneralized model of reptilian growth. Nussbaum andDiller (1967) studied a high-latitude population of U.stansburiana in Oregon, which had higher rates of growth(hatchlings, 0.22 mm/day; adults, 0.03 mm/day) thanthat in our study, probably due to higher productivity andreduced length of activity season. Worthington and Arvizo(1973) studied daily rate of growth of a population of U.stansburiana in New Mexico that also exhibited higherrates of growth for adults (males, 0.03 mm/day; females,0.02 mm/day) than rates we report. In the Reserva deBiosfera de Mapimı, mean rates of growth for U.stansburiana were 0.18 mm/day for hatchlings and 0.01mm/day for adults. Tinkle (1967a) reported considerablyhigher daily rates of growth for adult U. stansburiana inTexas (males, 0.17 mm/day; females, 0.24 mm/day).According to Worthington and Arvizo (1973), it ispossible that the discrepancy may represent inclusion ofmany immature individuals in the size classes used byTinkle (1967a), but all adult U. stansburiana were

reproductive. However, reciprocal-transplant experimentsor common-garden experiments (i.e., Smith et al., 1994)could be useful for examining relative importance ofgenetic versus proximate environmental effects as sourcesof variation in rates of growth of U. stansburiana. Giventhe complex of behavioral and physiological processesthat determine rates of growth and size, no singlemechanism is likely to prevail as a general explanationfor temperature-size relationships. Rather, mechanismsthat underlie evolution of thermal plasticity will becomplex, and more progress will be made throughcollective study of numerous model systems as suggestedby Angilletta and Sears (2004).

Overall sex ratio did not differ from 1:1. Likewise,individuals in our population were almost always in pairsin the field. When home ranges were plotted, there wasgenerally an indication that each female was associatedwith a specific male as described by Guerra-Mayaudon(1995). According to Tinkle (1967a, 1967b), a sex ratio of1:1 in U. stansburiana indicates presumably facultativemonogamy and is produced by aggressiveness of bothsexes toward individuals of the same sex.

Much data exist on population densities of U.stansburiana. Nussbaum and Diller (1967) noted greaterdensities of populations at high latitudes (177 individu-als/ha) than in populations at low latitudes (maximum,129 individuals/ha). These values are within the range ofdensities reported by Tinkle (1967a) for two populationsin Texas (90–272 individuals/ha). Density of adults inReserva de Biosfera de Mapimi was similar to densitiesreported for Arizona (2–30 individuals/ha; Parker, 1974),suggesting that, if any relationship between density andgrowth exists, it is not inversely density dependent asusually is assumed and observed (Scott, 1990). In fact,density is influenced by a complex of factors, includinglinks between rainfall, availability of food, and thermalenvironment (Whitford and Creusere, 1977; Rose, 1982;Christian and Tracy, 1985; Sinervo, 1990).

Density of adult U. stansburiana was higher in autumn1993 compared to previous years and the following year(Fig. 3b). This large increase may be due to largerrecruitment of adults in autumn 1993 in response toprevious supranormal precipitation. Whitford and Creu-sere (1977) suggested that density of most species oflizards varies directly with changes in productivity andrelative abundance of arthropods. Likewise, availability ofinsect prey changes according to distribution and amountof rain through the year (Maury, 1995). Availability ofarthropods was not evaluated in our study.

Mean estimates of survivorship of adult females in ourstudy (Table 1) was similar to rates reported for apopulation of adult female U. stansburiana in Texas(Tinkle, 1967a) and lower than observed in a populationin high latitude of Nevada (Turner et al., 1970).Abundant precipitation during summer 1990 in Reservade Biosfera de Mapimi (i.e., August–September; Fig. 3b)

September 2012 Gadsden and Castaneda—Side-blotched lizards in Durango 299

increased availability of insect prey to this lizard. In sanddunes of Reserva de Biosfera de Mapimı, lizards showedan abrupt shift in prey during autumn, eating many moretermites and true bugs and fewer other insects (Palacios-Orona and Gadsden-Esparza, 1995). Increase in abun-dance of food affects use of food by U. stansburiana(Waldschmidt, 1983; Palacios-Orona and Gadsden-Espar-za, 1995) and, apparently, appears to affect reproductivepotential by accumulating fat (Waldschmidt, 1983).Increased availability of prey may explain higher survivalof adults during summer and autumn 1990–1991 thanduring summer and autumn 1992–1993.

Life-history characteristics (Table 4) of the populationdid not deviate from estimates of Tinkle (1967b),Nussbaum and Diller (1967), and Tanner (1972). In fact,Tinkle (1967b) predicted that in the northern portion ofits range U. stansburiana would have a shorter reproduc-tive season and greater longevity. Tanner (1972) suggest-ed that, with increasing elevation, U. stansburiana livelonger, attain larger size, and have more eggs per clutch.Nussbaum and Diller (1967) predicted that populationsat high latitude, high elevation, or both, have a shortergrowing season, shorter reproductive season, reducedper-season fecundity (frequency of clutches is reduced),and greater survival than populations at low latitude, lowelevation, or both. The population we studied had anearly maturity, a relative short life expectancy, and manyoffspring. Data we obtained and expectations of currentlife-history theory for an early-maturing species are inagreement.

Limited time and resources are allocated amongcompeting functions to maximize lifetime reproductivesuccess (Stearns, 1992). Allocation to current reproduc-tion involves trade-offs with survival, growth, and futurereproduction. Populations of U. stansburiana have a trade-off between growth and fecundity. If fecundity increasesquickly, then growth is reduced and future fecundity iscompromised. Short-lived populations focus on higherfertility (southern populations) and faster growth. High,extrinsic mortality of adults favors the increase of short-term reproduction at the expense of survival of adults andfuture reproduction. Long-lived populations are morefocused on growth (northern populations) than onimmediate fecundity. Our data (such as high mortalityof adults) indicate that this population appears toconform more to the alternative of reproducing rapidlythan to increasing their fecundity.

According to Parker and Pianka (1975), along an r-K-selection continuum (Pianka, 1972), our population isrelatively r-selected, producing more than one relativelylarge clutch annually. Interactions of U. stansburiana inthe north appear to be primarily climatic, and in thesouth, biotic interactions (e.g., predation and competi-tion) may assume relatively greater importance (Parkerand Pianka, 1975). In our study, ca. 32% of femalesmatured in their first reproductive season, although rates

of replacement under the estimated schedules of survi-vorship did not allow for a stable pattern. Nussbaum andDiller (1967) speculated that reduced predation andreduced time exposed to predation (shorter activeseason) in northern populations could explain in partthe higher average life expectancy in those populations.

An alternative model that could explain geographictrends in life history of U. stansburiana would be thehypothesis of a fast-slow continuum (Stearns, 1983;Promislow and Harvey, 1990), which is more acceptedto explain life-history strategies that operate withinspecies, because it is based solely on mortality of adultclasses and their effect on the population (Franco andSilvertown, 1996). That is, populations that are subject tohigh levels of mortality, evolve a faster rate of growth, havehigh fecundity, and have short life cycles (southernpopulations of U. stansburiana), while those subject tolow mortality, grow slowly, have low fecundity, and havelong life cycles (northern populations of U. stansburiana;Read and Harvey, 1989; Promislow and Harvey, 1990).

Additional studies of U. stansburiana in other parts ofits distribution are needed to define latitudinal andelevational tendencies. Intraspecific comparisons fromdissimilar geographic and climatic locations wouldamplify our understanding of differing environmentalconditions and their influence on life history of thislizard, especially in southern populations. Studies willneed to gather data on temperature, moisture, pressurefrom predators, and availability of food, which potentiallyaffect life history of lizards.

This study was supported by a grant from Consejo Nacionalde Ciencia y Tecnologıa (1367-N9206). We thank the Herrerafamily for assistance in the field. O. Hinojosa de la Garza gavesupport and encouragement throughout analyses of data andpreparation of the manuscript. Permit SEMARNAP-SGPA/DGVS/88173 allowed this research, which is a contribution ofthe United Nations Educational, Scientific and CulturalOrganization-The Man and the Biosphere Program.

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Submitted 7 December 2009. Accepted 10 May 2012.Associate Editor was Fausto Mendez de la Cruz.

September 2012 Gadsden and Castaneda—Side-blotched lizards in Durango 303