Growth effort of Sceloporus scalaris ( Sauria : Phrynosomatidae ) at La Michilía Biosphere Reserve , Mexico

Using Mark-Recapture methods the growth effort of Scelof'orus scalaris was studied at La Michilla Biosphere Reserve, Durango, Mexico. A total of 146 recaptures on 68 individuals were obtained over four years on a 50 000 m' transect. To calculate the growth rate of Sceloporus scalaris individuals, we analyzed the data from two sources. The first using the recapture records of the same individuals over different seasons and the second using the records of cohorts during different períods. The growth effort of S. scalaris drastically diminishes as the organism grows. The growth rate of males and females is about equal for individuals from both clutches. In adults, where it is possible to compare among sea�ons, we measure quicker growth duríng the spríng. The growth paltern of S. scalaris at La MichiIla follows the predíctions proposed by the Bertalanffy model. Maximum growth rates are in the younger age classes and these rates decrease as size increases. The growing períod of S. scalaris is correlated with the seasons at La MichiÜa Biosphere Reserve. Newborn S. scalaris appear when the availability of nouríshment is still the most propítíous for growth at faster rates. Such adaptations to the environment determine many key population attríbutes of this species in this zone. The sexual maturíry age of S. sealaris is very early at La MichiÜa, only 4.5 ro 6 monrhs. Undoubtedly, the growth partern of S. scalaris at La MichiIla Biosphere Reserve can help explain the srructure and dynamics of this populatíon.

The bunch grass lizard, Sceloporus scalaris Wiegmann is an small (62 mm SVL), very common and abundant Mexican lizard which distribution includes 22 states of the Mexican Republic (Smith 1939).In spiteoftheirabundance and wide distribution, there exists relatively few studies concerning the ecological aspects of this animal (Ballinger and Congdon 198 1).Most of the previous studies are devoted to taxonomical (Srnith andpoglayen 1958, Smith and Hall 1974, Thomas and Dixon 1976, Stebbins 1966, Anderson 1972, Van Devender and Lowe 1977), and reproductive aspects (Stebbins 1954, Anderson 1962, Greene 1970, Smith and Hall 1974, Newlin 1976).Thereare noprevious works dealing with the growth patterns of this species.
The growth pattern is a key aspect in the life history of any species (Andrews 1982).Growth rates determine, among other important attributes, the length reached at sexual maturity and the maximum size (Andrews 1976, Barbault 1975, Kaufmann 1981, Van Devender 1978).Body size, in many reptiles.determines crucial reproductivecharacteristics such as reproductive effort arid clutch size (Barbault 1974(Barbault , 1981)).Thus, the study of growth patterns can help us to understand the structure, dynamics, and demography of any lizard population (Barbault 1975, 198 1, Van Devender 1978).

Study area:
The study site, La ~i c h i l l a BiosphereReserve, is in thesoutheastof theStatc of Durango, México, between 104"20 ' and 104 "07 ' W and 23 " 20 ' and 23" 30 ' N. Field work was done in the buffcr zone of the Reserve.
The climatcof thc zone is temperate subhumid with a mean annual tcmpeiature range between 17.4 " C and 20.7 " C and a inean annual precipitation o f 567 mm, conccntrated in the summcr.The vegetation oí' the zone is typically an oak-pine forest but highly diversilicd, including 207 plantspecies thatcontain 18diI'l'erent Quercus species and I O differcnt Pin~ls \pccies ( ~a r t ~n e z and Saldivar 1978).
Methods: A transcct of 50 x 1 000 m marked with stakeseach I Om, wasestablishcd andcensus \vere takcn over 4 ycars during the following months: September and December 1979;March, May, and Septcrnbei 1980and 1982and al1 the months oi' 1981.Each one of thcse 20 stays comprised 15 days.Duringeach day, the transect was traversed by 3 pcrsons lor 4 to 7 hours in the search for liaard individuals.For each lizard obscrvcd, we rccoi-ded the datc, the hour, its sex, its location in relation b thc nearest stake and thcn we capturcd the liaard by hand.
For cach l i ~a r d captured, we rccorded the lollowing data: body temperaturc with a cloaca1 thermomcter (Wescott): snout-vcnt length and tail lcngth to the ncarcst 0. I mm with a metal calipcr (Scala 222); and body mass to the nearest 0. I g with a Pesolii (TM).Captured individuals were marked both by toc clipping and by paint code (Tinkle 1967).
To calculate the growth rate of Sceloporus scalaris individuals, data from two sources were analyzed.The first uses the recapture records of the same individualsduringdifferent seasons and the second uses the records of cohorts followed during different periods.
The data of individual recapture records were pooled by age class and by season.Using the size differences between capture and recapture of a particular individual, the Instantaneous Growth Rate (IGR) was calculated using the Barbault (1 973) formula: Based on the IGR, the Growing Effort index (GE) was calculated using the Barbault (1973) formula: Where Lis the averageindividual lengthduring the time interval T, -T I , and it was calculated using the formula: Thedifferences in individual body data, among captures and recaptures, were adjusted to the differential cquations of the curves and models rnost frequently used to describe animal growth.For this purpose, we calculated the size iricrease per day percentage, also called Relative Growth Ratc(RGR), using the methodof Kaufman (1 981): Where: S , = Body size at the beginning of the time period.S,= Body size at the end of the period.t-Time period.
graph paper, then the logistic or the Bertalanffy curves be can be used (Bertalanffy 1957, 1960, Kaufman 1981).It is possible to calculate the linear regression for the plotted data, and calculating the origin and the slope, to establish the differential equation that defines the pattern observed.

RESULTS
A total of 146 recaptures on 68 individuals RGR value is thenplottedagainstthegeometric weremade during the4 years of study.Individual means (S) ofthe individual body size, S, which is averages were grouped by seasons on Table 1.At calculated in the following way: La ~i c h i l i a , summer was humid and hot and S= (S, winter was cold and dry (Martinez and Saldivar 1978).The age classes considered were (Ortega 1986): juveniles, individuals < 3 months for both clutches; subadults, individuals between 3 and 7 If the plotted data yield a straight line using months for the first clutch individuals and í'rom 3 semilog paper, a Gompertz curve can be used to to 5.5 months for the second clutch individuals; describe the observed data pattern (Kaufman Adults 1, individuals reaching sexual maturity 1981).If a straight line is obtained using from7 to 12months forthefirst clutch individuals logarithmic paper, then a potency curve must be and from 5.5 to 12 months Por the second clutch used and, if a straight line is obtained with linear individuals; Adults 11, individuals older than one year.As we can observe in Tables 1 and 2, GE drastically diminishes as the organisms grow.The maximurn GE is showed by the juveniles during the autumn.Males and females exhibit practically equal GEin al1 ageclassesconsidered.In the case of the adults, where il is possible to compare among seasons, we see a slight quicker growth during spring.
Thereexists goodfit both with thesemilogand linear plots, for both clutches individuals.Testing statistically the correlation coefficients found in each plot (with Student's t-test and f test), we found thebestfit is forthe plot number 1 d) for the first clutch individuals, where y = 0.3205 X -0.0049; r = 0.81; and the plot number 2 d) for those of the second clutc h, where y = 0.2426 x -0.0028; r = 0.65.

Season
Where: The Bertalanffy model is based on the hypothesis that the growth rcpresents the difference between synthetic (anabo1ic)processes and degradation (catabolic) processes and the synthetic processes are directly proportional to the body mass (Bertalanffy 1957, Fabens 1965).Although it is more accurate to use the integrated equation, it iseasier touse thedifferential equation, which establishes Lhe relationship between the Relative Growth Rate and the s i ~e (Kaufmann 1981): Where: a is the axis interccpt and b is the slope of the line (Figs.Id and 2d).
ThegrowthpatternofS. scalaris, a t ~a ~i c h i l i a follows the predictions proposed by the Bertalanffy model, i.e., maximum growth rates are reached in the younger age classe; and these rates decrease as the size increases.
'i'he second data source, independent of the mark-recapture data analyzed, was the measure of the monthly size of individuals of the same cohort during one year.This could be only achieved because S. scalaris, newborn appear in two short well-defined period: 15 days during September for the first clutch individuals and 15 days during October for the second clutch.
The measured growth rates by day and by month, for 1981, are showed in Table 3.Growth rates are relatively high Srom October to March for both clutch individuals.Themaximum growth forfirst clutch individualsoccurs from October to December.During January to March exhibit lower growth values than second clutch individuals, which reach their highervalues during this period.From May to September the growth rates fa11 drastically, for both clutch size individuals, and show littlechange up to theageof 1 year (the next October).
Integrating the data by the mark-recapture methods and by the cohort-record method, it was possible to build the body evolution curves for both males and females (Fig. 3).Juvcnile males grow quickly Srom September to April and relatively slowly from May.Juvcnile females show, Srom September to April, a lower growth rate Lhan juvenile males, but from May they do not exhibit an abrupt decline in rate as do the males.Thus, at the age of 13 months and with a size of 51 mm (he females reaches the average size of the males.From the second April (month 17) females not only are bigger, but grow more quickly than males.The secondclutch individuals follows basically the sarne pattern, but their growth rate is slightly faster, for both males and fernales, which is even more easily observed from the second January (month 16) to their second May (month 20).At the age of 20 months and with a size of 55 mm the second clutch individuals reaches in size to the first clutch individuals and it is not possible posteriorly to differentiate arnong them only by their size.

DISCUSSION
There are rnany interdependent factors that determine the growthrateofany animal: available nourishment and water, phenological phase of theindividual, inter-andintraspecific cornpetition, predation, social environment, and for lizards, even tail breakage.
Individual growth varies with the available energy, thus rnany lizard species exhibit quicker growth rates during the season of higher prey availability (Dunham 1978, Medica et al. 1975) or when they have access to supplementary food (Licht 1974).However, the direct relationship between nourishrnent and growth rate could be rnasked by theeffect on the growth ratesproduced by water availability (Nagy 1973, Smith 1977) or by the existence ofcircadian rhythrns of appetite .and other endogenous factors (Jackson 1970, Licht 1972).
At the La ~i c h i l i a Biosphere Reserve, the rnaxirnum prey-availability period for S. scalaris is frorn June tooctober.During thesemonths the abundance and biomass of their prey reaches their peak (Ortega and Herwndez 1983).Because of this, wemust expect themaximum growthrate for S. scalaris individuals to be from June to October, which does not occur.
From June to Septernber there are only adult individuals of this species in the zone (Ortega 1986).Juvenile individuals reaches their highest numbers from October to January, and the subadults from January toMarch.Forthis reason, in spite thatmaxirnum nourishment availability is foundduring thesurnrnermonths, thefaster growth rates are found from October to March.The younger age classes shows the quickest growth rates in spite than they growthonly during the last optimurn rnonth of the year (October).Tlie reproductive period of S. scalaris is correlated with theseasonality oftheenvironment at La ~i c h i l i a Biosphere Reserve.Adult S. scalaris oviparous females exhibit two periods carrying oviducal eggs, June and August (Ortega 1986).Reproductive activities occurs just when the nourishment availability is themostpropitious for use this energy for reproductive purposes (Ortega 1983).Besides, S. scalariseggsare laying duririg the rainy season, it is to say during the optimum humidity conditions to conclude the embrionary development (Ortega 1986).The new born S. scalaris individuals appears during the last month of the optimum period ofthe year and growth at the fastest rates.
Such adaptations to theenvironmentdetermine many key population attributes of this species in this zone (Ortega 1986).The sexual maturity age ofS.scalarisis very early at ~a ~i c h i l i a , only 4.5 to 6 months (Ortega 1986).Undoubtedly, the growth pattern oí' S. scalaris at La ~i c h i l ~a Biosphere Reservc can help explain the structure and dynümics of this population.
size at time T I ; at initial capture.L,= Individual size at time T,; at recapture.

Fig. l .Fig. 2 .
Figs.I and 2 show the calculated pair values for the RGR and the geometric mean of the individual body size for each individual recaptured, plotted in semilog, log, and linear paper.