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POPULATION DYNAMICS OF SCELOPORUS GRAMMICUS
(SAURW PHRYNOSOMATIDAE) AT DURANGO, MEXICO
ALFREDOORTEGA-RUBIO,
ROBERTBARBAULT,
AND GONZALO
HALFFTER
Instituto de Ecolog'a. Apdo. Postal No. 63, Xalapa, Veracruz 91000, México (AO-R, GH)
Ecob N m a b Superieure, 46 Rue D'Ulm, Pan's Cedex 05, France (RB)
Present Address of AO-R. Centro de Investigaciones Biológtcas del Noroeste, Apdo. Postal No. 128,
La Paz, 23000, Baja California SU^ México, aortega@aInzozmx
kbs~n~c~-Populationdynamics of Scelopmus grammicus were studied at La Michilía Biosphere
Reserve, Durango, Mexico, from 1979 to 1982 using mark-recapture methods. Estimated population density waq 166 individuals (adults and subadults) per hectare, with adults averaging 42
individuals per hectare. Sex ratio wai approximately 1:1, with females slightly dominating at older
ages. Six differerit age classes were established: a) juveniles from O to 3 months of age, b) s u b
adults from 3 to 5 months of age, c) adults 1, individuals in their first reproductive season from
5 to 12 months of age, d) adults 11, individuals in their second reproductive season and older
than 1 year, e) adults 111, individuals in their third reproductive season and older than 2 years,
and f ) adults IV, individuals in their fourth reproductive season and older than 3 years. Average
litter size for females was 6.17 neonates I
1.65 SE and varied widely (three to nine neonates) as
a function of female body size. Estimated number of neonates in 10 hectares was 1,482 and embryo
mortality was 7.2%. Mter hatching, he average mortality was higher than 55% from one age-class
to the next. The mortality rate was lowest for the male adult 1 class (53.8%), and highest for the
male adult 111 class (89.0%), below only the 100% mortality exhibited by adult IV individuals (the
final age class). The population life table indicates a Slobodkiri Type IV survivorship curve, with
high mortality occurring at younger ages and a population replacement rate (net reproductive
rate) close to 1.00. Average generation time for this population was 1.09 years. This is the first
study of the population dynamics of this species.
RESUMEN-La dinámica poblaciorial de Scelopmus grammicus fue estudiada en la Reserva de la
Biósfera de La Michilía, Durango, México, de 1979 a 1982 usando méi:odos d e marca recaptura.
La densidad poblacional estimada fue de 166 individuos (adultos más subadultos) por hectárea,
promediando los adultos 42 individuos por hectárea. La proporción sexual fue d e aproximadamente 1:l dominando las hembras en las mayores clases d e edad. Seis diferentes clases de edad
fueron establecidas: a) juveniles, de O a 3 meses de edad, b) subadultos, de 3 a 5 meses de edad,
c) adultos 1, individuos en su primera estación reproductiva de 5 a 12 meses d e edad, d) adultos
11, individuos en su segunda estación reproductiva y mayores que 1 año, e) adultos 111, individuos
en su tercera estaci6n reproductiva y mayores que 2 años, y f ) adultos 1V. individuos en su cuarta
estación reproductiva y mayores que 3 años. El tamaño promedio de camada para las hembras
1.65 DE, variando ampliamente (de 3 a 9 neonatos) en función del tamaño
fue de 6.17 neonatos I
corporal de la hembra. El número estimado de neonatos en 10 hectáreas fue de 1482 y la mortalidad de embriones fue de 7.2%. Después del nacimiento la mortalidad promedio fue superior
al 55% de una clase de edad a la siguiente. La tasa d e mortalidad fue menor para la clase de
machos adultos 1 (53.8%) y la más alta para la clase de machos adultos 11 (89.0%), por debajo
sólo del 100% de mortalidad que alcanzan los adultos IV (la clase final). La tabla d e vida de la
población indica una curva de sobrevivencia d e tipo IV d e Slobodkin,.con la más alta mortalidad
en las clases d e edad más jóvenes y con un crecimiento poblacional (tasa neta reproductiva)
cercana a 1.OO. El tiempo promedio generacional para esta población fue de 1.O9 años. Este es el
primer estudio sobre la dinámica poblacional de esta especie.
Knowledge of life history traits, such as age
at maturity, age-specific fecundity, and survivorship, is basic to understanding how a species is adapted to its environment (Barbault,
1975; Vinegar, 1975; Barbault, 1981; Andrews
and Wright, 1994; Smith, 1996). Furthermore,
from studies of life history have grown attempts to uncover adaptative patterns of de-
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Ortega-Rubio ei al.-Popiilation
mographic attributes of species (Tinkle, 1969;
Tinkle et al., 1970; Ballinger, 1973; Barbault,
1975; Parker and Pianka, 1975; Barbault,
1981). To see these patterns, it is necessary to
gather more data on population dynamics of
more species. Because of their diurna1 habits
and their usual ease of capture, lizards serve as
admirable subjects for studies of population
dynamics (Van Devender, 1982; Grant and
Dunham, 1990; Anderson, 1994; Smith, 1996).
Although Sceloporus grammicus is a widespread and abundant lizard whose distribution
includes 25 states of México (Smith, 1939),
there exist relatively few studies of this species.
Of those studies that exist, most are of a taxonomic nature (Hall, 1973; White, 1978; Sites,
1980, 1983), or concern its reproduction (Axte11 and Axtell, 1970; Guillette and Casas-Andreu, 1980, 1981; Reed and Sites, 1995), with
only a few dealing with ecological aspects (Lemos-Espina1 and Ballinger, 1992, 1995a,
1995b), and none on its population dynamics.
The purpose of this work is to report the principal population attributes and dynamics of
one population in northwest Mexico over a
four-year period.
METHODSA N D MATERIALS-The study site, La
Michilía Biosphere Reserve, is located in the solitheast portion of Durango, México, between 104"20'
and 104"07'W, and 23"201 and 23"301N.The climate
of the zone is ternperate with a mean annual temperature ranging between 17.4"C and 20.7"C. and a
mean annual precipitation of 567 mm. with most
rain occurring during summer. Vegetation of the
zone is typically a highly diversified oak-pine forest
including 207 plant species of which 18 are (2uercus
species and 10 are Pinus species (Martinez and Saldívar, 1978).
A stiidy plot measuring 500 by 1,000 m was
marked with stakes every 10 m and cerisused over 4
years duririg the following months: September and
December of 1979; March, May, and September of
1980 arid 1982, and every month of 1981. Each one
of these 20 intervals of field work lasted 15 days.
'Three people searched the plot for lizards frorn 4 to
7 hours per day. For each observed lizard we recorded its location in relation to the nearest stake. We
then captured the individual by hand. Captured individual~were marked both by toe clipping arid by
paint code (Tinkle, 1967a), and the following data
were recorded: sex, snout-vent length (SVL), tail
length, and body mass. Body lengths were measured
1.0 the nearest 0.1 mm with metal calipers (Scala
222), and body rnass was measiired to the nearest
dynarnics of
SceZ@ms grammicus
65
0.1 g using a spring scale (Pesola). During the last 3
days of each 15-day period we coriducted eight 50rnin censuses each day to count marked and unmarked individiials found inside the plot. Lizard
captures were not rnade during these censuses, and
we started each census froni a different locatioii inside the plot to try to avoid btases caiised by changing lizard activity over the course of the day.
Population StructureLizards were classified by sex
and age group from morphological and chrornatic
characters. Because most of the S. grammicus neonates are born in a single period of only 15 days
during May, age class can be easily determined ~ising
SVL cohort data. Secondary sexiial characteristics of
males and fernales, such as ventral color patches in
males, were evident frorn a very early age and were
used to determine the sex of individuals.
DensityDensity of adults and subadults was estimated each rnonth f r o ~ nrnark-recapture data usiiig
the Petersen index (Bailey, 1952; Caughley, 1977).
Natali+Natality was computed by applying sizespecific fecundity estimates to resident fernales. We
conducted 120 autopsies of different-sized gravid females frorn outside the study plot to obtain these
size-specific fecundity estimates.
Mortality and SurvivorshipMortality rate was estimated by analyzing recapture data of marked individual~frorn each age class to the next. This estimation was correci.ed by siibtracting the knowii
nurnber of emigrants for each age class. We periodically searched for rnarked individuals in a 500-m
wide zone adjacent to the margins of our study plot.
Marked individiials found in this zone were considered emigrants and were not tallied as deaths. Prenatal or embryoriic mortality was deterrnined by
counting the number of non-viable embryos and
atrophic eggs in ovidiicts of autopsied fernales and
by contrasting the niimber of corpora lutea in their
ovaries with the total number of eggs found in oviducts.
RESUI.TS-Dmsity-Table 1 shows the average estimated density for adults and subadults
per hectare calculated for each field visit. The
estimates for adults, where it is possible to compare among al1 months, were significantly different among months (ANOVA: F,,,,, = 9.1 1 ;
P < 0.001).There exist two well-differentiated
groups of months according to their density
values: October, March, February, and December were the months with statistically highest
density, whereas June, July, August, and November were those with statistically lowest densities (Tukey-Kramer post hoc test: F,,,,, =
7.28; P < 0.01).
Population StructureWe differentiated six
66
Thc South711estmNaturalist
TABL.E
1-Sceloporus p m m i m s average density per
hectare (1 SD in parentheses) calculated for the last
three days of each field visit.
Month
Year
Number
of adult
individuals
January
February
Marrh
April
April
May
May
June
Jilly
August
Septeniber
Septeniber
October
OctobeiNovember
December
1981
1981
1981
1981
1982
1981
1980
1981
1981
1981
1981
1982
1981
1979
1981
1981
42.3
54.2
55.4
47.6
54.8
44.4
42.3
25.6
26.8
24.5
46.1
58.2
58.3
41.1
31.4
52.3
(8.1)
(7.3)
(3.2)
(12.7)
(12.9)
(6.7)
(12.1)
(3.3)
(5.3)
(4.4)
(12.1)
(8.3)
(16.8)
(2.4)
(2.1)
(12.8)
Number of
sub-adult
individuals
0.0
0.0
0.0
0.0
0.0
3.2 (1.1)
51.5 (21.9)
190.7 (37.8)
184.9 (39.9)
72.8 (19.1)
51.3 (18.5)
0.0
0.0
0.0
0.0
0.0
age-groups for Sceloporus grammicus at La Michilía Biosphere Reserve: 1) juveniles, younger
than 3 months, with SVL <38 mm for males
and <34 mm for females; 2) subadults, 3 to 5
months, with SVL from 38.1 to 48 mm for
males, and from 34.1 to 42 mm for females; 3)
vol. 44, n o . i
adult I , 5 to 12 months old and reaching sexual
maturity, with SVL froin 48.1 to 53 mm for
males and from 42.1 to 50 mm for females; and
4) adult 11, individuals in their second reproductive season, reaching a maximum size of 62
min SVL for both inales and females. It was not
possible to determine from size alone whether
the individual was 2 , 3 , or 4 years old. However,
from skeletochronology cohort data (Ortega et
al., 1993) we differentiated 5) adult 111, individual~in their third reproductive season; and
6) adult IV, individuals in their fourth reproductive season.
Table 2 shows estimated numbers (derived
from density estimates) of each age and sex
group for 10 ha throughout the year. Juveniles
appear during May and their nuinbers increase
quickly, reaching a maximum in June. Juveniles are replaced in August by subadults,
which decline during September, being replaced by the adult 1 group. The adult 1 group,
particularly males, can show abrupt oscillations. Nuinbers of adult 1 males reach their
highest from October to December, declining
slowly from February to May. Female adult 1
numbers also are high in October through December, but are lower than those of same-aged
males. Females of this age group reach their
highest numbers during February and March,
or at least more are seen then than earlier.
TABI.E
2-Demographic striicture for the population of S. g-rammicus at La Michilía. Nuinber of individuals
is calciilated for 10 hectares.
Class
Month
January
February
March
April
May
June
July
August
Septeniber
October
November
December
Totals
Male Female
silbsubIiivenile adult adult
Male
adult
1
Female
adult
1
Male
adiilt
11
FeFeFemale Male male Male male
adult adult adult adult adult
11
111
111 IV
IV
Totals
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dynarnics of Sceloporus gramrnicu.,~
Ortega-Rubio et al.-Population
TABLE
3-Percentage
67
o f females i n different age groups o f t h e studied population.
Class
Month
Juveniles
Subadults
Adults 1
Adults 11
Adults 111
Adults N
Average
January
February
M arc h
April
M ~ Y
June
.Ju~Y
Augus~
Septernber
October
November
Decernber
Average
Male and female adult 11 classes reach their
maximum density during September and their
lowest during November and December.
Table 3 shows the percentage of females in
different age groups throughout the year. At
younger ages, the sex ratio is almost 1:1, but
changes in favor of females in older age classes.
Additionally, for older ages there is a tendency
for females to be more abundant during the
first half of the year, but during the second half
this proportion is reversed. This tendency is
even greater in the adult 11 group. The overall
sex ratio is 56.2% females.
NatalityFrom the autopsied females we established that average litter size for S. p m m i cus females was 6.17 (SE = 1.65) neonates per
female. Litter size varied widely from female to
female, ranging from three to nine neonates
depending on female SVL. There exists a
strong significant relationship between litter
size and body size (litter size = 0.361 SVL 12.891; r = 0.88; df = 43; P < 0.05). By multiTABLEG E s t i r n a t e d productivity o f S. grammicus
for t h e nionth o f May
Group
Adult
Adult
Adult
Adult
1
11
111
N
Fernales Average
i n 10
size
hectares
mrn
180
74
15
4
47.03
58.00
58.00
58.00
Average Newborn
clutch
prosize
duced
4.08
8.04
8.04
8.04
734
595
121
32
plying the number of females per age class by
average litter size per age class, we calculated
natality. The estimated number of S. grammicus
born in 10 ha was 1,482 (Table 4).
Mortality and Survival-From the 120 autopsies performed on females during the reproductive season, only 25 atrophic eggs from a
total of 348 eggs were found, thus percent embryo failure was relatively low ('7.2%),suggesting a low prenatal mortality rate (Table 5 ) . Dis
appearance of individuals estimated by markrecapture methods also is shown in Table 5.
The average disappearance from one age class
to the next was higher than 58% for al1 age
groups and both sexes. Minimum disappearance occurred for adult 1 males, and the maxTABLE
5-Mortality, emigratiori, and survival o f S.
grammicus iridividuals.
Class
Prenatal
Juveniles
Male adult I
Female adult 1
Male adult 11
Female adult 11
Male adult 111
Fernale adult 111
Male adult iV
Fernale adult iV
Average
percent Percent
disap- ernigrapearance tion MortalitySurvival
68
vol. 4 4 , no. 1
The Southwestern Naturail~t
TABL.E
6-Life tahle for the S. grarnrnicus populatioii at the Michilía biosphere reserve; X = age in years;
Lx = proportion of original populatiori alive at the start of age iiiterval; dx = proportion of the origirial
populatioii dying in the age interval; qx = age-specific mortality (dx/lx); mx = age-specific fecundity; Lxmx
= age-specific contribution to the net reproductive rate (Ro); Vx = Reproductive Value; Vx* = Residual
reproductive valur; and Ex = Life expectancy of those attaining age x.
Age group
Embryo
.Tuvenile
Adult 1
Adult 11
Adult 111
AdultIV
X
O
0.17
0.63
1.63
2.63
3.63
Lx
dx
qx
mx
Lxmxl
1.00
0.929
0.283
0.087
0.012
0.002
0.071
0.646
0.196
0.075
0.010
0.002
0.071
0.696
0.692
0.862
0.862
1.000
O
O
2.04
4.02
4.02
4.02
O
O
0.577
0.048
0.048
0.008
Xlxmx
O
O
0.364
0.126
0.126
0.029
Vx
Vx*
Ex
0.983
1.058
3.475
4.667
4.690
4.020
0.983
1.058
1.435
0.647
0.670
0.000
2.313
1.413
1.357
1.161
1.067
1.000
' 2 lxmx = Ro == 0.983.
imum value occurred for adult 11 males. Dis
appearance less emigration is mortality. Emigration is highest for juveniles, who also show
high mortality (Table 5). As expected, mortality is highest for the oldest age classes (Table 5).
The life table for Sceloporus gramminrs (Table
6) indicates a Slobodkin (1962) Type IV survivorship pattern as described by Deevey
(1947) in which heavy mortality occurs in the
younger age groups. Net reproductive rate is
0.98 and average generation period is 1.09
years.
ture is length of the hatching period. S. grammicus has a discontinuous, short and welldefined reproductive season (Type 11-Barbault,
1975). This kind of reproduction is similar to
that of most lizards in temperate and desert
habitats (Blair, 1960; Fitch, 1970; Ballinger,
1971; Marion and Sexton, 1971; Schrank and
Ballinger, 1973; Burkholder and Tanner, 1975;
Goldberg, 1975), and several tropical lizard
species (Carpenter, 1966; Fitch, 1973). In the
case of age-specific
survival, the main factor in.
fluencing age structure is average longevity. In
this aspect, S. grammicus is relatively short-lived,
DISGUSSION-BensityLizard density is ex- similar to many tropical and temperate lizard
.
combitremely variable between species, families, and species (Barbault, 1973, 1 9 7 6 ~ )The
regions of the world. Lizard density varies from nation of short birthing periods and reduced
less than 1 individual/ha as in the case of Ure longevity determines the age structure obmastix acanthinurus in Algeria (0.01 individu- served in the S. grammicus population at La
als/ha; Grenot, 1976) and the Komodo dragon Michilía. Age groups were clearly defined,
in Indonesia (0.02 individuals/ha, Dareskivj showing the highest density in the juveniles
and Terentev, 1967), up to densities higher and adult 1 groups.
Our results show that the number of centhan 1,000 individuals/ha, as in the case of Anolis limiji-onsin Panama (1,171 individuals/ha; sused adults oscillates from month to month.
Sexton, 1967). However, the most common This is probably an artifact related to the visual
densities fluctuate from 10 to 100 adults/ha recapture method used. Our censuses depend
(Barbault, 1975), and according to calculations on the number of active individuals observed
of Tumer (1977), average density for lizards is during the last three days of each visit. The
around 51 individuals/ha. The average adult activity of each individual depends on a wide
density for S. grammicus at La Michilía (42 in- range of factors, physiological and behavioral,
dividuals/ha) is close to the general averages. both intrinsic aspects of the individual, and exWe were unable to find any previous study on trinsic factors of the physical and even social
environment. For instante, male adult 1 numS. grammirus density at other locations.
Population StructureThe age structure of a bers reach their peak during December, even
population depends on both age-specific fe- though that is a-relatively cold month. Howcundity and age-specific survivorship schedules ever, December is the month when copulation
(Barbault, 1975). In the case of age-specific fe- takes place (Ortega and Barbault, 1984). So,
cundity, the main factor influencing age struc- that is when male adult 1 individuals become
March 1999
Ortega-Riibio et al.-Population
more active, patrolling and defending territories and looking for females. Thus, a greater
proportion of the adult 1 males alive then are
seen and counted by our method than at later
times.
Likewise, numbers of adult 1 females
reached their peak during February and
March, even though just as many had to be
alive in the months immediately preceding.
From October to December the rainy season
yields maximum availability of prey for S. grammicus (Ortega and Hernández, 1983). During
these months adult 1 females do not need to
move very far in search of food. Also, from October to December, the activity leve1 of S. grammicus predators shows its maximum (Ortega,
1986). So, as a posible adaptation to preclude
predation, adiilt 1 females remain secretive.
Both these aspects may decrease the chance
that an adult female is seen and counted, even
while males are active defending territones
and seeking mates. On the contrary, during
February and March, the drought arrives and
prey grow scarce (Ortega and Hernández,
1983). During these months adult 1 females,
which are pregnant and with elevated nutrition
requirements as compared to post-reproductive males, must move more in search of food.
This increased activity likely translates to a
greater chance that these females are seen and
coun ted.
Sceloporus grammicus sex ratio at birth is very
near 1:1, similar to most other lizard species
(Barbault, 1975) with the exception of parthenogenetic species or subspecies (Grassé, 1970).
With increasing age, it is common to observe
a change in sexual proportion, usually in favor
of females (Hirth, 1963; Barbault, 1974a), but
in some cases favoring males (Alcala, 1966;
Turner et al., 1969). In other cases, numeric
equality between males and females remains
constant (Brooks, 1967; Telford, 1969). In the
case of S. grammicus at La Michilía, the sexual
proportion is biased in favor of females in older classes.
The tendency for a greater female-bias during the first part of the year within an age class
c,ould be an artifact of differential activity of
the sexes. As we explained in the last section,
due to their reproductive behavior, S. grammicus males are much more active during the last
part of the year, and females are more active
during the first part of the year. This differ-
dynamics of Srelopoms (~rammict~s
69
ential activity could affect numbers of males
and females counted, and thus the observed
sex ratio.
Natality-Lizard species show a wide variety
of clutch and litter sizes, varying from the single egg of Anolis up to more than 50 eggs for
large chameleons (Barbault, 1975). The average number of lizard eggs per female varies
from two to six (Fitch, 1970). Comparing the
average litter size of the S. grammicus population of La Michilía with other S. grammicus populations, we found that females of La Michilía
were more prolific (6.2 young) than those
studied at Zoquiapan (Guillette and Casas-Andreu, 1980) and in Texas (Werler, 1951), averaging 5.2 and 5.7 young, respectively. However, S. grammicus females of La Michilía had
smaller litters than females of other Texas populations (11.7 young) studied by Axtell and Axte11 (1970).
Clutch or litter size in lizards is closely related to female body size (Barbault, 1975). The
average body size of lizard females of La Michilía (52.8 i 4.1 mm) was considerably larger
than the body size of S. grammicus females at
Zoquiapan (48.5 2 0.8 mm; Guillette and Casas-Andreu, 1980). This larger size could help
to explain the larger litters of La Michilía females compared to those at Zoquiapan. There
were no data on female body sizes for Texas
populations (Werler, 1951; Axtell and Axtell,
1970).
Clutch frequency per reproductive season
also varies widely in lizards, ranging from only
one (Iverson, 1979) to more than nine (Porter,
1972). Clutch size and frequency in lizards are
higher in tropical species (Wilhoft, 1963; Dunham, 1982) than in species inhabiting temperate climates (Ballinger, 1979). Also, clutch size
and frequency are greater for smaller and earlier-maturing lizards (Barbault, 1976b) than
larger-sized lizards at maturity (Crisp, 1964).
Viviparous lizard species, such as S. grammicus,
have only one litter per year (Ballinger, 1973),
whereas oviparous species frequently have several clutches per year (Parker, 1972).
As a result of variation in clutch and litter
size and frequency, total fecundity varies widely
among females of different species. However,
the average varies between five and 20 eggs for
oviparous species, being more fecund than the
viviparous ones, with an average from three to
70
The Southwtlrtn7i Natu?ahst
10 neonates per female (Barbault, 1975). S.
grammicus total fecundity is within this average.
Mortality and Survival-Prenatal mortality
varies widely among lizard populations, ranging from less than 5% (Ballinger, 1971) to 90%
(Blair, 1960; Barbault, 1973), but the most
common values range from 40% to 60%
(Brooks, 1967; Tinkle, 1969; Barbault, 1974b).
Thus, S. grammicus prenatal values were low
(7.2%) compared to these averages.
Juvenile mortality also varies widely among
lizard populations, ranging from less than 10%
(Blair, 1960) to nearly 50% (Zweifel and Lowe,
1966; Brooks, 1967), up to more than 90%
(Ballinger, 1971). However, the most common
values vary around 50% to 80% (Barbault,
1975). Juvenile mortality for S. grammicus
(69.7%) was close to average values for al1 lizards.
Mortality rates also vary widely among adult
lizards, ranging from less than 20% from year
1 to year 11 (McCoy, 1965; Zweifel and Lowe,
1966; Burrage, 1973) to very high mortality
rates of more than 90% (Tinkle, 19676; Turner
et al., 1969), and u p to 100% for the annual
species (Barbault, 1974b). Adult mortality for
S. grammicus from year 1 to year 11 (61.3%) was
close to that of species with high mortality rates
during this adult period of life.
Adult survival determines the longevity of
the species. For lizards, published data indicate
a wide variation in longevity. In captivity, several lizard species live from 20 to 50 years
(Mertens, 1959; Pfeffer, 1959; Moehn, 1962),
but in nature, lizard longevity averages from 1
to 9 years (Barbault, 1975). Frequently, the
species that are similar in size to S. grammicus
live from 1 to 3 years (Blair, 1960; Tinkle, 1969;
Porter, 1972).ALLa Michilía, the oldest recaptured S. grammicus individuals were 4 years old.
According to general life-history theory
(Tinkle, 1969; Tinkle et al., 1970; Barbault
1976c), S. grammicus as a viviparous and singlebrooded lizard should show lower mortality
and a longer life compared to oviparous, multiple-brooded lizards. However, our data show
a clear contradiction to this expectation. Comparing the demographic data of S. grammicus
with the general rules proposed by r-Kgeneral
theory (Tinkle et al. 1970; Barbault 1976c),we
conclude that the marked seasonality of the
environment, arid the rigorous conditions of
the physical environment and probably the
vol. 44, no 1
high predation pressure on this population,
determine the observed mortality patterns. In
turn, the species' fecundity schedule is adapted to this imposed pattern of mortality. It will
be necessary to conduct more studies, particularly on the impact of predators, to confirm
this idea.
This work was supported jointly by the Consejo
Nacional de Ciencia y Technología (CONACyT), the
Instituto de Ecología, and the Centro de Investigaciones Biológicas del Noroeste, of México; by the
Ecole Normale Superieure and the Council National
de la Recherche Scientifique (CNRS) of France and
by the MAB UNESCO program. We thank three
arionymous reviewers for their kind and helpful suggestions, which highly irnproved an early version of
the manuscript. We also thank R. Bowers for editing
the English and L. Vazquez who typed the niaiiuscript.
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