Growth and sexual size dimorphism in Alberta populations of the ...

Growth and sexual size dimorphism in Alberta populations of the eastern
short-horned lizard, Phrynosoma douglassi brevirostre
G. LAwRENcE PowELL AND ANTHONY P. RussELL
Department of Biology, University of Calgary, 2500 Universi~’ Drive NW., Calgary, Alta., Canada T2N 1N4
Received February 13, 1984
PowELL, G. L., and A. P. RuSSELL. 1985. Growth and sexual size dimorphism in Alberta populations of the eastern
short-horned lizard, Phrvnosoma douglassi brevirostre. Can. J. Zool. 63: 139—154.
Alberta populations of Phrynosoma douglassi brevirostre display marked sexual size dimorphism, adult females being
considerably larger than adult males. Discriminant analyses of whole mensural characters and of scaled mensural characters
indicate that this dimorphism is present from birth, although it is more strongly expressed after sexual maturity. Recapture data
were used to generate modified logistic by weight growth models for snout—vent length (SVL), and allometric models for each
sex were generated for growth in tail length, head length. and head width. The SVL growth model for females indicates delayed
maturity leading to greater adult size, an expected feature of a female viviparine. The SVL growth model for males indicates
that growth ceases sooner than in females, resulting in a smaller adult size. This is possibly a result of male dispersal
competition, an hypothesis further borne out by the results of a preliminary analysis of mobility in the two sexes, and may
also be influenced by intersexual dietary competition. Differences in head dimensions between the sexes are a function of the
differences in SVL at adulthood, but there is a significant sexual difference in the allometric relationship of tail length to SVL.
No difference in the growth patterns and adult size of either sex was found to exist over the range in Alberta.
PowELL, G. L.. etA. P. RuSSELL. 1985. Growth and sexual size dimorphism in Alberta populations of the eastern short-horned
lizard, PhG’nosoma douglassi brevirostre. Can. J. Zool. 63: 139—154.
Les populations de Phrynosoma douglassi brevirostre d’Alberta se caractérisent par un dimorphisme sexuel important quant
a Ia taille et les femelles adultes sont beaucoup plus grosses que les males. Des analyses discriminantes des mesures dans leur
ensemble et des mesures considérées par classes indiquent que le dimorphisme est déjà present a Ia naissance et qu’il se precise
surtout après Ia maturation sexuelle. Des données sur Ia recapture ant servi a construire des modèles de croissance logistiques
modifies, en fonction de Ia masse, de Ia longueur du museau a l’anus (SVL) et des modèles allometriques de croissance de
Ia longueur de Ia queue, de Ia longueur de Ia tête et de Ia largeur de Ia tête, pour chacun des sexes. Le modèle de croissance
SVL des femelles indique que Ia maturité est retardée, cc qui permet aux femelles adultes d’atteindre une plus grande taille,
caractéristique qui n’a rien de surprenant chez une femelle vivipare. Le modèle de croissance SVL des males indique que Ia
croissance des males cesse avant celle des femelles, d’oü leur plus petite taille. Ces phénomènes sont peut-être causes par Ia
competition entre les males au moment de Ia dispersion, une hypothese corroborée par les résultats d’une analyse préliminaire
de Ia mobilité chez les deux sexes; Ia competition alimentaire entre les males et les femelles peut aussi jouer un role. Les
differences entre males et femelles quant aux dimensions de Ia tête sont fonction des differences de longueurs SVL a l’âge
adulte, mais il y a dimorphisme sexuel significatif dans Ia relation allometrique entre Ia longueur de Ia queue et Ia longueur
SVL. II n’y a pas de differences dans les courbes de croissance ou Ia taille des adultes chez l’un ou l’autre des sexes chez les
populations d’Alberta.
[Traduit par Ic journal]
Introduction
The Alberta populations of Phiynosoma douglassi brevi
rostre are range marginal in their distribution and are markedly
sexually dimorphic (Powell and Russell 1984). In the context
of this, growth parameters were examined for each sex to gain
a better understanding of the maintenance of the observed
dimorphism and to investigate whether both sexes grow at the
same or at different rates. Thus, the primary objective of this
study was to accrue mensural data from a sufficient sample of
recaptures from Alberta to be able to generate quantified
growth models for both sexes. These data were also used to test
whether there were significant differences in adult size and
growth pattern between populations over the range in Alberta.
The secondary objective, employing the mensural data base
outlined above, was to attempt to formulate discriminant func
tions that enable itidividuals to be classified to sex, to define the
observed sexual size dimorphism in an ontogenetic context.
The tertiary objective was to utilize the data collected in this
study to examine the hypothesis that the sexual size dimor
phism displayed by these populations is produced and main
tained by differing sexual selection pressures on the two sexes
associated with mating strategy and parental investment.
As a biological process, growth is amenable to mathematical
modelling and a number of widely applicable growth models
have been outlined (Ricklefs 1967; Turner, Bradley et al. 1976;
Fitzhugh 1976; Dunham 1978; Schoener and Schoener 1978;
Kaufmann 1981). In the context of growth in reptiles, these
have been reviewed by Andrews (1982) and it can be demon
strated that they all share a unity of mathematical form, with
the major types being recognizable as special cases of one
“generic growth model.” The basic postulates of this have been
outlined by Turner, Bradley et a!. (1976).
For iguanid lizards a variety of modelling methods have been
applied to the study of growth. Notable among these have been
the approaches of Turner, Medica et a!. (1976), who included
increase in weight as part of a computer simulation of popu
lation energetics of Uta stansburiana; Andrews (1976), who
employed Ricklef’s (1967) method of applying the logistic
growth model in intraspecific comparisons of Anolis growth
rates; and Van Devender (1978), who compared growth rates
between populations of Basiliscus basiliscus by fitting his data
to the Von Bertalanffy model. The most rigorous assessments
of modelling as applied to lizard growth, however, have been
those of Dunham (1978) and Schoener and Schoener (1978).
The latter authors concluded that the logistic by weight model,
as modified for length, best defined growth in Anolis; Dunham
(1978) arrived at the same decision for Urosaurus ornatus and
Sceloporus merriami. It is essentially this model that has been
adopted in this study.
Behind the application of any mathematical modelling
technique to organismal growth lies the fundamental assump
tion that there is a regular, underlying pattern of growth charac
139

140 CAN. J. ZOOL. VOL. 63. 965
112° W III°W
‘30’ N
490 N
1100W
FIG. I. The range of Phn’nosorna douglassi brevirostre in southeast Alberta (hatched area of inset map). Locations marked with stars represent
study sites discussed in this paper (B. Bow Island: R. Rose Ranch; M. Mckinley’s Ranch: N. Nemiskam; C. Comrey).
teristic of the species in question (Ricklefs 1967). However,
this innate, species-specific pattern may be altered in varying
ways by environmental factors (Huxley 1932; Kaufmann 1981;
Andrews 1982). Thus, any growth model generated from field
data, such as that presented in this study, will inevitably be an
approximation of the intrinsic growth pattern of that population
or species. If comparisons are to be made, careful attention
must be given to the extrinsic environmental factors that may
affect each case. In this context, appropriate use of growth
models may cast light on the relative effects of such extrinsic
factors (Dunham 1978).
In the particular case of the genus Phrvnosoma, most consid
erations of growth have been descriptive and feature little or
no statistical analysis. Guyer (1978), however, utilized a
regression of snout—vent length (SVL) on time of year to
differentiate between growth rates of males and females of
P. d. douglassi in southern Idaho. He also considered observed
individual growth rates.
Sexual size dimorphism in a species implies that the two
sexes have somewhat different ecological or reproductive
demands (Howard 1981). The sexual size dimorphism and
I F
0 10 20 30 40 50
km
P50° N
growth patterns characteristic of the Alberta populations of
P. d. brevirostre can initially be attributed to one of two causes:
either intraspecific dietary competition (Schoener 1969) or
intrasexual selection (Ghiselin 1974). Fitch (1981) has con
sidered sexual size dimorphism in reptiles in considerable
detail and has found the emergent pattern to be extremely
variable. Andrews (1982) has discussed this phenomenon in
the context of growth in reptiles.
For Phrvnoso,’na Fitch noted that the female is the larger sex
in the seven species that he examined. Pianka and Parker
(1975) noted this for P. douglassi and P. platyrhinos and indi
cated a clinal trend in the latter, with dimorphism being most
strongly marked in the northernmost extension of the range.
Methods and materials
Field inethochv
Lizards were captured by hand and subjected to measurement in
the field immediately after capture during the spring and summer of
1979 and 1980. and in the spring of 1981. After all of the data had
been recorded, each newly captured lizard was given a unique toe
clip for that study site. The following measurements were recorded:

(I) snout—vent length (SVL). distance from the tip of the snout to the
anterior lip of the vent, measured ventrally. to the nearest 0.5 mm:
(ii) weight, mass of the lizard to the nearest 0.5 g: (iii) tail length
(TL), distance from the posterior lip of the vent to the tip of the tail.
measured ventrally, to the nearest 0.5 mm (phrynosomes are not
autotomic. and so tail length can be used as a consistent and repeatable
statistic); (iv) head width (HW). linear width of the jaws at the corners
of the mouth, to the nearest 0.5 mm: (v) head length (HL). linear
distance from the tip of the snout (rostral scale) to the occipital notch.
to the nearest 0.5 mm.
HW and HL were taken with a pair of vernier eallipers. Weight
was measured with a 5 or 30 g Pesola spring balance, depending on
size. SVL and TL measurements were recorded employing a stainless
steel ruler, the lizard being stretched firmly against it to minimize
measurement error owing to muscular contraction. To minimize the
masking effect of measurement error on growth increment, no mea
surements were taken on lizards recaptured less than 14 days after a
previous set of measurements.
The data used in this study were collected at five separate localities
in southeastern Alberta (Fig. I): Comrey (I lO°4l’ W, 49°05’ N).
Nemiskam Community Pastures (I l0°31’ W, 49°l7’ N). Mckinley’s
Ranch (I l0~25’ W, 49°l7’ N), Rose Ranch (I lO°06’ W, 50°l7’ N),
and Bow Island (II I030~ W, 49°55’ N). Detailed accounts of the five
sites and the distribution of the lizards at each are available elsewhere
(Powell 1982).
Sexual dimorphism
Male lizards of all ages are immediately distinguishable from
females by the presence of enlarged postanal scales. Sexual dimor
phism in SVL, weight. and TL are obvious after sexual maturity, but
not in sexually immature individuals. The morphological data set for
all captures was subjected to diseriminant analysis to derive a discrim
inant function which would describe some aspects of this sexual
dimorphism (McRae et al. 1981). Weight, SVL. TL. HW. and HL
were the variables used. No attempt was made to adjust the variables
for the effect of size in the initial analysis. since weight and SVL were
two of the variables being tested. A two-group stepwise discriminant
analysis (Cooley and Lohnes 1971) was performed by means of the
SPSS (statistical package for the social sciences) subprogramme
DISCRIMINANT (Nice! al. 1975). This yielded a discriminant func
tion which can be used to classify any individual lizard by sex on the
basis of the five measurements used in the analysis.
In addition, for each case, proportional values for several of the
morphological variables were derived (“case” here refers to a com
plete set of measurements taken from one lizard at one capture). TL.
HW, and HL were expressed as percentages of SVL (equivalent to the
“percra” of Werner(197I)) and for each case, the ratio of I-IL to HW
was calculated. A two-group stepwise discriminant analysis. similar to
that described above, was performed on the scaled values of the entire
morphological data set to produce another classificatory diseriminant
function.
Growth
The results of the analysis of sexual size dimorphism indicated that
the growth patterns of each sex should be considered separately.
Longitudinal data (successive measurements of an individual animal
over a period of time (Fitzhugh 1976)) were used to fit the model to
the growth patterns of each sex. Recapture data for each sex were
organized into cases, each case consisting of a measurement taken at
the first capture of an individual (L1). the number of days between
capture and recapture (D). and the equivalent measurement taken at
the recapture of the individual (L2). L1 and L2 are linear measurements
such as SVL or HW. A number of individuals were recaptured more
than once; in these eases, L1 was given the value obtained at the
immediately preceding recapture. Each case was considered sepa
rately in the analyses, regardless of whether other cases of the same
lizard were included in the data set. This may have biased the results,
but was unavoidable in view of the relatively small sample sizes. If
winter intervened between capture and recapture, a value for D was
approximated by subtracting the estimated time spent in brumation
POWELL AND RUSSELL 141
from the actual number of days between capture and recapture. The
estimated brumation period was determined from the dates of the last
capture in the fall and the first capture in the following spring, as
precise dates for onset of brumation and spring emergence could not
be obtained, especially for any particular individual, Cases in which
there was an apparent decrease in size over time were assumed to be
the result of measurement error and not used in the analysis: i.e.. the
data set was “fixed” (Dunham 1978. 1980: Schoener and Schoener
1978).
Dunham (1978) and Schoener and Schoener (1978) found that the
most suitable model for describing lizard growth was the logistic by
weight model, modified for SVL. This model assumes a determinate
asymptotic size, reached by an inflected. S-shaped growth pattern
(Schoener and Schoener 1978). It is derived by substituting the cubed
linear dimension for weight into the simple logistic by weight growth
model (Schoener and Schoener 1978). Andrews (1982), in a review of
the various models applied to reptile growth. recommended the
logistic by weight model for small iguanids. such as Phrvnosoma.
However, weight in free-living lizards can vary with condition. food
availability, and, particularly in females. with reproductive condition,
all of which will obscure growth rate (Dunham 1978. 1980: Schoener
and Schoener 1978). Change in weight is thus not as satisfactory an
index of growth as is a change in a linear measurement such as SVL
for use in a static model such as that employed here.
The differential form of the logistic by weight equation, as modified
by Schoener and Schoener (1978) for length. is
[I] dL/dt = (rL/3)(l — (L3/a~))
where L is length (a linear dimension of the lizard). t is time. r is the
characteristic growth rate, and a1 is the asymptotic or maximum length
attained by the lizard. The instantaneous solution to Eq. I is
[2] L = (a~/(l + he”))~
where e is the base of the natural logarithm and
[3] h = (a~/L~) —
where L1 is the length at birth.
Schoener and Schoener (1978) also derived an interval equation for
this model:
[4] L~ = (a~L~/L~ + (a~ — L~)e “~‘~
where L~ is the length of the lizard at the beginning of the interval. L2
is the length at the end of the interval, and D is the length of the
interval. Equation 4 can be used to predict the size of a lizard of length
L1 after the passage of D units of time.
The parameters a1 and r can he estimated by the use of nonlinear
least squares regression techniques. In this study. the IMSL (Interna
tional Mathematical and Statistical Libraries. Inc.) computer routine
RSMITZ (Anonymous 1979). as modified by the University of
Calgary Academic Computer Services (NONLIN). was used to esti
mate these parameters from the recapture cases. NONLIN converges
on those values of the parameter nearest the hypothesized starting
values which give a minimum sum of squares for a user-supplied
model. The model in this case was Eq. 4. A data matrix of a series of
recapture cases with one dependent variable (L2) and two independent
variables (L1 and D) was used to arrive at the parameter estimates.
NONLIN used this data matrix in conjunction with Eq. 4 to estimate
values for a1 and r that fit cases which resemble those in the original
data matrix. In addition, it yielded asymptotic standard deviations of
each estimate, which permit the calculation of two kinds of confidence
intervals..~~conventional” confidence intervals and support-plane con
fidence intervals. The support-plane confidence interval is the more
conservative of the two estimates (Dunham 1978: Schoener and
Schoener 1978).
Maximum size (a1) and characteristic growth rate (r) were esti
mated for the pooled SVL cases for each sex from all study sites.
These parameters were also estimated for pooled TL. I-lW. and HL
cases for each sex from all study sites. Growth curves for SVL. TL,

142 CAN. J. ZOOL. VOL. 63. 985
TABLE I. Statistics pertaining to Eq. 5, discrimi
nant analysis using whole morphological values,
and classification success of Eq. 5.
Canonical correlation 0.7362
Wilks’ X 0.4580
x2 331.86
Box’sM 234.39. F,.=23.184*
% of all cases
classified correctly 86.01
Predicted group
niembership
Actual group
membership Males Females
Males 83.6% 16.4%
Females 12.5% 87.5%
*SignhIicanl at p 0.001.
HW, and HL, extrapolated to their respective asymptotes, were
modelled by inserting the estimated parameters for each measurement
into Eq. 2; L, was determined by averaging the appropriate mea
surement of lizards born in captivity and used to calculate b. The 95%
confidence intervals for each extrapolated curve were calculated by
inserting the upper and lower 95% support-plane confidence interval
values for the two parameters into the appropriate form of Eq. 2. In
addition, SVL recaptures for each sex were grouped into northern
(Bow Island, Rose Ranch) and southern (Comrey, Nemiskam,
McKinley Ranch) location groups (Fig. I). The a and r values for
SVL were estimated for each location group by sex and used to
construct a growth curve and limits as described above. In all pairwise
comparisons of parameter estimates, they were considered to be dif
ferent if their 95% support-plane confidence intervals did not overlap
(Dunham 1978, 1980).
The difference in goodness of fit of the male and female SVL
growth models was tested by means of an F test (Sokal and Rohlf
1969). Error variance for each model was derived by generating pre
dicted L2 values for each recapture case by means of the appropriate
interval equation (Eq. 4) and summing the differences between
observed and predicted values.
A two-tailed F statistic of 0.05 probability was used as the critical
value. Change in growth rate with change in SVL was approximated
by inserting the appropriate values of a1 and r into Eq. I and calcu
lating dL/dt over the SVL range to a1 for each of the sexes. The same
was done with 95% support-plane confidence limits of a1 and r for
each sex to establish upper and lower 95% confidence limits for
dL/dt. Observed growth rates were calculated for each recapture
series by (L2 — L~ )/D. The corresponding SVL was approximated by
(L2 — L)/2.
As a basis for comparison for the logistic growth models of TL,
HW, and HL, allometric models of growth in each of these mea
surements were generated for each sex. Each allometric model was
derived from the linear regression of the logarithm of the measurement
on the logarithm of SVL, using the entire morphological data base.
Because the least squares regression method assumes no error in the
independent variable. Bartlett’s “best-fit” method or reduced major
axis are normally advocated for deriving the parameters of an allo
metric equation (Grine et a!. 1978: Kermack and Haldane 1950). It
was felt, however, that the large sample size involved here would
adequately offset the potential measurement error factor and would
permit a least squares regression method to be employed for this data
set. In addition, the aptness of a particular method depends upon the
degree of correlation between the two variables: when the correlation
is high, the results given by the different methods are very similar
(Gould 1966; Seim and Saether 1983). The correlations between each
pair of variables in this study were sufficiently high that we felt
justified in using least squares regression for our parameter estimates.
The strength of the relationships between variables was examined
by means of their correlation coefficient (Smith 1980. 1981; Aiello
1981) and by an inspection of residual plots (Smith 1980, 1981).
Comparisons between the regression slopes of the two sexes for each
measurement were performed by means of an analysis of covariance
(ANCOVA). If there was a significant difference between slopes, the
mensural character was held to be sexually dimorphic in allometry. All
statistics for the allometric analysis were obtained by means of SPSS
(Nie et a!. 1975). The allometric models were compared with the
logistic models for each sex by fitting the appropriate extrapolated
lines with confidence limits to scatterplots of the observed distribu
tions of the mensural character against SVL.
Mobilit
Sexual size dimorphism can possibly be promoted by male dispersal
competition. in which males are in a race to find and inseminate as
many females as possible (Ghiselin 1974). This sort of intrasexual
selection will be associated with greater mobility in males than in
females (Ghiselin 1974).
It was hypothesized that there might be a difference in home range
size between males and females if male dispersal competition is active
in maintaining the sexual size dimorphism characteristic of these
lizards. Consequently. an effort was made to examine home range size
in the Alberta populations. A strip of plastic surveyor’s tape with the
date and clip code was tied in place to the closest piece of vegetation
to the point of capture or recapture of each lizard. The distance
between the tags of successive recaptures was measured at the end of
each field season. In cases where an intervening tag was missing, the
distance between the tags for the immediately preceding and immedi
ately succeeding captures was used.
There were not enough sequential recapture series of individuals,
particularly of males, to permit the computation of a sufficiently large
sample of minimum polygon home range estimates for statistical
purposes. This lack precluded the use of Schoener’s (1981) test for
independence of successive recaptures. The distance in metres
between capture and recapture, the number of days between these
events, and the estimated rate of movement in metres per day were
pooled by sex without regard to the individual lizards which yielded
the data. Each pair of recaptures was considered as a “case,” in much
the same manner as the morphological recapture data were treated in
the estimation of the growth parameters. The three variables were
found to be normal in distribution by a Kolmogorov—Smirnov good
ness of fit test. Differences in rate of movement, distance covered, and
number of days between recaptures between the sexes were tested by
a Student’s t-test. Independence of successive recaptures was evalu
ated by the mean number of days between them. Schoener (1981)
allowed at least 1/2 day between observations in an attempt to main
tain independence, and this amount of time was used as a minimum
acceptable interval. All statistical computations were performed by the
SPSS package (Nie et a!. 1975; Hull and Nie 1979).
Results
Capture and recapture success
A total of 446 sets of measurements were taken from 316
individuals in the course of the entire study. Sample size varied
between locations, with Comrey, Nemiskam, and Bow Island
supplying most of the captures. Elimination of cases with
missing values resulted in a sample size of 429 cases which
were used in the analyses.
There was a high overall recapture rate of cases yielding
data useable in growth studies (29.2% of total captures). There
were far more recaptures of females (102) than of males (30),
although the degree of disparity varied from study bite to study
site. Recapture success itself varied between study sites, partly
as a function of the size of the study site itself and partly as a
function of the amount of time spent investigating each. Thus,
Comrey, which is a small site and was intensively studied, is

TABLE 2. Statistics pertaining to Eq. 6, discrimi
nant analysis using percrasr and HL/HW. and
classification success of Eq. 6.
Canonical correlation 0.7119
Wilks’ X 0.4933
x2 300.35
Box’sM 226.32. F,.=22.386t
Predicted group
membership
Actual group
membership Males Females
Males 75.2% 24.8%
Females 6.8% 93.2%
See Materials and methods and Werner (1971).
tSignilicant at p = 0.001.
represented by a higher recapture rate than Bow Island, which
has a much larger area.
Sexual dimorphism
The two-group discriminant analysis using whole mor
phological values yielded the following function:
[5] 5 = —0.2207(SVL) — 0.2549(HW) + 0.l714(HL)
+ 0.4075(TL) + 2.2322
where S1 is the determinant value of the sex. Lizards with a
value of S~ > 0.449 are classed as male, those with an S~ <
0.449 are classed as female. The pertinent statistics for this
analysis are presented in Table I. There is some overlap in the
distributions of the two sexes in the plot of case values along
the discriminant function, but the two groups are distinct, as is
evidenced by the high cannonical correlation and the large
Box’s M with its associated significant F. The Wilks’ X is not
great, but has a significant x2 value and can be considered
adequate in view of the high percentage of cases classified
correctly by the discriminant function. The majority of incor
rectly classified male cases are in the 28.0—39.0 mm SVL
range. The total SVL range within which misclassified male
cases were found extends from 28.0 to 45.5 mm. Misclassified
cases constituted 22.12% of all male cases with an SVL of less
than 46.0 mm. Most incorrectly classified female cases range
from 22.5 to 39.0 mm SVL, with a total SVL range of
22.5—53.0 mm. Within this SVL range, 32.38% of all female
cases were incorrectly classified. The absolute values of the
standardized discriminant scores indicate that TL is the most
important variable, followed in order by SVL, HW, and HL.
Weight was removed from the discriminant function as part of
the stepwise process. HL and HW contribute little to the dim
inution of Wilks’ X. The size of the TL and SVL coefficients
indicate that these two variables account for most of the sexual
dimorphism in the data set.
The two-group discriminant analysis using scaled mor
phological values yielded the following function:
[6] S2 = l9.6663(TL/SVL) — 6l.6984(HW/SVL)
+ 47.6984(HL/SVL) — l9.628l(HL/HW) + 3 1.9734
where S2 is the discriminant value for the sex. Lizards with a
value of S2 < —0.466 are classed as male, those with a value
of 52 > —0.466 are classed as female. The statistics for this
analysis are summarized in Table 2. The Wilks’ K for Eq. 6 is
slightly larger than that for Eq. 5 (Table I), indicating slightly
POWELL AND RUSSELL 143
less acuity in distinguishing between the sexes by Eq. 6. The
Box’s M is large and its associated approximate F is signifi
cant, indicating that there is a significant difference in the
group dispersions. Overlap along the discriminant function is
greater than in the whole value analysis and the spread of the
male cases is greater than that of the female cases. The absolute
value of the standardized discriminant coefficient for HL/SVL
is the greatest, followed in order by those for HL/HW,
HW/SVL, and TL/SVL. Thus, in this analysis scaled head
dimensions are the most useful variables in discriminating
between the sexes, in contrast to the preceding analysis in
which SVL and TL were the most useful. A slightly higher
percentage of the total cases were classified correctly by Eq. 6
than by Eq. 5 (Table I), but there is a marked discrepancy
between the percentages of each sex classified incorrectly.
Very few females (6.8%) were classified as males, whereas
almost one-quarter of the males (24.8%) were classified as
females by Eq. 6. All of the females classified incorrectly are
neonates or yearlings (total range, 23.0—54.0 mm SVL) and
these cases constitute 15.38% of all female cases falling within
this SVL range. Most of the misclassified males are neonates
or early yearlings (25.0—37.0 mm SVL). The total SVL range
of incorrectly identified males is 25.0—46.0 mm and these
constitute 36.11% of all male cases below 46.0 mm SVL.
The discriminant analysis using whole morphological values
shows that the greater part of the obvious sexual dimorphism is
in terms of SVL and TL, the two most conspicuously dimor
phic features. The discriminant analysis using scaled values
shows that head measurements proportional to SVL also differ
between the sexes. The SVL distributions of misclassified
cases of both sexes in both analyses are below the adult size
ranges. By the time that a lizard is large enough to breed for the
first time, it will be classified correctly to sex by both discrim
inant functions.
Growth
Estimates of the values of a1 and r, together with their 95%
conventional and support-plane limits, for SVL of the two
sexes are given in Table 3. There is considerable difference
between the estimates of asymptotic SVL of the two sexes and
the 95% support-plane confidence intervals of these estimates
do not overlap. The extrapolated relationships between TL,
HL, and HW and SVL as modelled by the logistic equation for
each of these measurements did not fit the observed distribu
tions for each sex as well as the extrapolated allometric lines
(Figs. 7, 8, and 9). For this reason the logistic models of TL,
HW, and HL are not discussed further, and subsequent dis
cussions of sexual dimorphism in these measurements will be
based on the allometric models.
The estimates of a1 and r for male and female SVL were
inserted into Eq. 2 to produce models of SVL growth for each
sex. These models were used to generate a series of growth
curves typifying the expected SVL distributions and growth
patterns of each sex over the entire study period. The 95%
confidence intervals of these curves were included to further
define the expected population SVL distributions against the
observed distributions. SVL distributions of all males captured
over the study period generally correspond with the extrapo
lated SVL distributions (Fig. 2). Data points for neonates and
very small males tend to fall outside the 95% confidence inter
vals of the extrapolated curves, probably because the initial
value of the model is the mean of observed neonate SVLs. It
is possible that instances of larger individuals also fall outside
of the 95% confidence limits of the line that they are associated

144 CAN. J. ZOOL. VOL. 63, 985
S
S
-c 0,
a
C
>
0
C
U,
75.
25,
140 160 180 200 220 240 260120 140 160 180 200 220 240 260120 140 160
1979
a1 (asymptotic 95% conventional 95% support-plane
length, mm) interval (mm) interval (mm)
Female SVL 69.03 67.67~a1≤7l.23 67.23~a1~7l.68
Male SVL 52.34 48.39≤a1≤56.29 47.34≤a1≤57.33
(B) Estimates and confidence limits of characteristic growth rates (r)
r (characteristic 95% conventional 95% support-plane
growth rate) interval interval
Female SVL 0.015661 0.0l2620≤r≤0.0l7750 0.01 l968~r≤0.0l8398
Male SVL 0.023847 0.01 6980≤ r≤ 0.030720 0.0! 5162 ≤r≤ 0.032533
with, but the overlap of confidence limits near the asymptote
makes this impossible to ascertain. No observations lie above
the asymptote of the upper 95% confidence limit. Since the
asymptotic SVL is approached rapidly (Fig. 6) no clear differ
entiation can be made between age—size classes above 45 mm
SVL or after a cohort of recruits has finished the summer after
their first brumation (Fig. 2). However, the observed SVLs in
Fig. 2 can be divided into three age—size groups: (I) young of
the year (YoY), individuals of approximately 22—37 mm SVL,
present between the end of July and the onset of brumation in
September; (ii) yearlings, individuals of approximately 3 I —46
mm SVL, born in the previous summer and present throughout
the active season; (iii) adults, individuals of ≥46 mm SVL,
present throughout the active season.
Recapture series of males do not extend over long periods of
time, and so it is not possible to say how long the life-span is
after asymptotic SVL is reached. No histological examinations
1980
Time (days since Jan. 1)
FIG. 2. All observed male SVLs over entire study period (estimated brumation period excluded), with extrapolated male growth patterns (solid
lines) and their upper and lower 95% support-plane confidence limits (dashed lines). Origins of extrapolated growth patterns were estimated from
approximate time of parturition and average SVL of neonate males. Lines with no origin on figure were extrapolated from preceding years.
TABLE 3. Parameter estimates with 95% conventional and support-plane confidence
limits for SVL of females and males
(A) Estimates and confidence limits of asymptotic length (a1)
1981
of yearling males were made to determine the age of onset of
sexual maturity, but enlargement of the hemipenial sacs behind
the vent and production of femoral pore wax were taken as
evidence of the onset of sexual maturity in males. These
features became evident in yearlings between the end of May
and the middle of June (90—120 active days afterbirth), when
the yearlings are 32—38 mm SVL. On this basis, no sexually
immature males were present in any population after the middle
of June in 1979 and 1980, excluding each season’s YoY.
Asymptotic SVL in males is reached in approximately 200—
400 active days after birth, according to the model (Fig. 6), or
(on average) towards the end of the individual’s second full
summer (Fig. 2). From the SVL distributions about the asymp
tote in Fig. 2, a life-span of 3 years, at the least, is implied.
The extrapolated growth rate of males, when plotted against
SVL (Fig. 3), increases from birth to a peak within the yearling
SVL range, then declines sharply as asymptotic SVL is

0.45
0.40
>, 0.35
~0
E
0
S
0
0.30
0.25
0.20
0.15
0.10
0.05
—
.
-‘. .
..
20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0 65.0 ~0.0 75.0 80.0
Snout—vent length (mm)
FIG. 3. Observed male growth rates ((L2 — L1)/D) plotted against
average SVL over growing period ((L1 + L2)/2). The extrapolated
curve of growth rate against SVL (solid line) with upper and lower
95% support-plane confidence limits (dashed lines) was plotted from
a differential form of the male growth model.
approached. The extrapolated 95% confidence interval for the
growth rate includes most of the observed growth rates,
although the model tends to underestimate growth rates within
the yearling SVL range. Observed growth rates within the large
yearling—adult SVL range are almost all within the 95%
confidence interval.
The relationship of all observed female SVLs, over the entire
study period, to the extrapolated SVL growth patterns with
95% confidence intervals (Fig. 4) is generally one of con
formity, although there are a number of observations lying
above the upper 95% confidence limit of the asymptote and a
few observations in the 40—55 mm SVL range which are not
clearly associated with any growth curve covering that size
range. Again, observed SVLs of very young individuals and
neonates tend to lie outside of the 95% confidence limits
of their associated curves because the model uses the average
SVL of female neonates at parturition. Asymptotic SVL is
approached at a slower rate in females than in males and female
growth is more prolonged, although the growth patterns of the
two sexes are similar until approximately 120—160 active days
after birth (Fig. 6). As with males, three age—size groups are
evident: (I) Young of the year (Y0Y), individuals of approxi
mately 22—35 mm SVL, present between the end of July and
the onset of brumation in September; (ii) yearlings, individuals
of approximately 28—60 mm SVL, born the previous summer
and present throughout the active season; (iii) adults, individ
uals of approximately ≥60 mm SVL, present throughout the
active season.
Variable growth rates (Fig. 5) and the overlap of 95% con
fidence intervals near the asymptote (Fig. 4) make further
attempts to subdivide the female age—size groups inadvisable.
Longitudinal recapture series of some individuals, interpreted
in the light of the female SVL growth model, indicate that the
female life-span may extend at least into the fifth summer after
birth. Since growth in SVL may continue after the model’s
asymptote is reached, statements about when asymptotic SVL
is reached are perforce rather vague. Extrapolation from the
model (Fig. 6) indicates that asymptotic SVL is reached be
tween 350 and 600 active days after birth, in the summer after
the second brumation, at the earliest (Fig. 4). There are no
obvious external signs of sexual maturity in female lizards, and
no dissections were performed to determine at what SVL the
POWELL AND RUSSELL 145
gonads mature. Sexual maturity most likely occurs in the year
ling age—size stage. since individuals that were marked at that
stage in 1979 were observed to breed in 1980.
The extrapolated female growth rate curve plotted against
SVL (Fig. 5) is smoother than that for males (Fig. 3), with a
lower maximum amplitude and without a well-marked peak.
The extrapolated growth rate curve tends to underestimate the
growth rates of YoY and yearlings and to over- and under
estimate the growth rates of adults. A similar phenomenon was
noted by Dunham (1978) and Schoener and Schoener (1978).
The variability of observed female growth rates is higher than
that of observed male growth rates (Fig. 3). Nonetheless, the
pattern of observed female growth rates broadly follows the
pattern set in the extrapolated growth rate curve. As in the male
growth rate curve (Fig. 3), there is an increase in the extrapo
lated female growth rate between the YoY and yearling SVL
ranges and a decrease to the SVL asymptote from the yearling
SVL range.
The model SVL growth curves for the two sexes are similar
in shape (Fig. 6). A mixed cohort of males and females would
display similar growth rates until they reached an age of
roughly 270 active days, by which time the slowest growing
female with the lowest asymptotic SVL would have a greater
SVL than that attained by any male and growth would have
virtually ceased in all but the slowest growing males. The
females of the cohort would continue growing until they
reached an asymptotic SVL at least 14 mm greater than the
greatest asymptotic SVL reached by any male. Growth in
females continues as the lizards enter the adult SVL range and
slows down gradually within this range, unlike growth in
males, which levels off at the upper end of the yearling SVL
range and continues, if at all, at a very slow rate within the
adult SVL range. There is no significant difference in goodness
of fit between the male and female SVL growth models (F test,
p > 0.05).
Table 4 contains the statistics relevant to the least squares
regressions used to arrive at the parameters of the allometric
equations. In all cases, the correlation coefficient between the
variables is large. The distribution of the residuals from the
regression of TL on SVL in males indicates that there is some
nonlinearity in the relationship between TL and SVL in the
lower reaches of the SVL range. This is also true of the residual
plot of the HW regression in males, to a lesser extent, and of
the residual plot of HW in females. However, the variations are
not great and the strength of the correlations (Table 4) indicates
that the allometric equations generated from these regressions
are a good approximation of the scaling of the mensural charac
ters with regard to SVL.
There is considerable difference in the allometric relation
ship of TL to SVL between the sexes (Fig. 7, Table 4), with a
significant difference in the slopes of the regressions (Table 4),
indicating sexual dimorphism in both size and allometry. Males
have a longer tail relative to SVL. However, there are no
significant differences between the slopes of the allometric
equations of HW (Fig. 8, Table 4), nor between the slopes of
the allometric equations of HL (Fig. 9, Table 4) between the
sexes. These two features must be considered to be sexually
dimorphic only as a function of SVL, since their growth in
males can be expected to cease with the cessation of SVL
growth and so their maximum sizes will be smaller than in
females, which prolong SVL growth (Fig. 6), and thus growth
in HL and 1-lW. No great difference in head dimensions is to be
expected between a male and a similarly sized female.

146
E
E
-c
C
a)
C
a)
>
0
C
a.’)
CAN. J. ZOOL. VOL. 63. 1985
Time (days since Jan. 1)
FiG. 4. All observed female SVLs over entire study period (estimated brumation period excluded), with extrapolated female growth patterns
(solid lines) and their upper and lower 95% support-plane confidence limits (dashed lines). Origins of extrapolated growth patterns were estimated
from approximate time of parturition and average SVL of neonate females. Lines with no origin on figure were extrapolated from preceding years.
0.45
0.40
5’
(5 0.35
~0
E 0.30
E
CS
0.25
0.20
0.15
0
0 0.10
0.05
.
•.
.
.
. .
• .
FIG. 5. Observed female growth rates ((L2 — L, )/D) plotted
against average SVL over growing period ((C1 + L2)/2). The extrapo
lated curve of growth rate against SVL (solid line) with upper and
lower 95% support-plane confidence limits (dashed lines) was plotted
from a differential form of the female growth model.
The estimates of a1 and r, with their 95% confidence limits,
for the grouped populations from the northern and southern
parts of the range in Alberta are given in Table 6. Insufficient
sample size prevented estimation of a1 and ,. for northern males.
There was no significant difference between the parameter
estimates for the SVLs of the grouped southern males and those
of all males. That the elimination of the northern male sample
did not significantly alter the male parameter estimates implies
that male growth patterns and asymptotic SVL do not vary
significantly over the geographic range examined. There is
some overlap of the 95% support-plane confidence intervals of
.
._~_—~ .
•I
1979 1980 1981
•.
•
.
.
• .
20.0 25.0 3o.o 35.0 40.0 45.0 so.o 55.0 60.0
Snout— vent length (mm)
• .
•
•I
asymptotic SVL between the northern and southern females
and considerable overlap between the intervals of r for these
two groups. Growth patterns and asymptotic SVL of females
must, therefore, also be considered as nonvarying over the
species’ range in Alberta.
Mobility
The statistics resulting from the analysis of the mobility data
are summarized in Table 5. The mean distance between capture
and recapture of males is considerably greater than the mean
distance between capture and recapture of females, but the
difference between the means of the two sexes is only mar
ginally significant (p 0.05). This should be taken as indi
cating that a difference does exist, but, due to the low level of
probability, it does not seem wise to predicate a great deal upon
it. Nonetheless, that there were no significant differences in the
number of days between capture and recapture or in the rate of
movement between the sexes, indicates that the difference in
the average distance between capture and recapture between
the sexes is a real one. Independence of successive recaptures
is suggested by the large mean number of days between them
(Table 5).
Discussion
Possible sources of error
Despite efforts to reduce measuring error, such error is
inevitable in the taking of mensural data from a living animal,
as is evidenced by the necessity of fixing the recapture data.
The morphological data base is sufficiently large, however,
that measurement error of any particular individual should be
offset. Similarly, sample size should compensate for mea
surement error in individual recapture cases.
The recapture sample for females is roughly thrice that of the
males. Few males were recaptured more than once, whereas

80.0
75.0,
70.0
65.0
60.0
E
E 55.0
f 50.0
0)
C
45•Q
C
5) 40.0
>
35.0
C
ci) 30.0
25.0
sequential recaptures of individual females, extending over
several months, were not uncommon. It is possible that male
mortality is much higher, but this cannot be substantiated from
the data available.
The larger number of female recapture cases available for
analysis of growth patterns means that the confidence intervals
(conventional or support plane) calculated for parameter esti
mates used in the model of female growth are narrower than
those for equivalent parameter estimates used in the model of
male growth. The degrees of freedom of these statistics, and
POWELL AND RUSSELL
0 30 60 90 120 150 180 210 240 270 300 330 380390 420 450 480510540 570 600 630 660
Time (days since birth)
50 50- B
A
Females
FIG. 6. Extrapolated male (solid line) and female (dashed and dotted line) SVL growth over time, from birth, with upper and lower 95%
support-plane confidence limits (male, dotted lines; female, dashed lines). Origins of curves were estimated by average SVLs of male and female
neonates.
40 40-
~30~ ~30-
~20- ~20-
I- I-.
10- 10-
FIG. 7. (A) Distribution of tail length against SVL for all male captures, with allometric line (solid) and ± I SD of its exponent (dashed lines).
TL 0.181 ISVL’2979, n 165. (B) Distribution of tail length against SVL for all female captures, with allometric line (solid) and ± I SD
of its exponent (dashed lines). TL 0.3651SVL1°584, n = 264.
Males
0— 0— 1
20 30 40 50 60 70 80 20 30 40 50 60 70 80
Snout— vent length (mm)
Snout — vent length mm
thus the absolute size of the resulting confidence limit, are
partly determined by the sample size (Sokal and Rohlf 1969;
Schoener and Schoener 1978). The smaller number of male
recapture cases results in a larger t or F statistic, as well as a
larger standard deviation for the parameter estimate, and thus
wider confidence intervals will be calculated for estimates of
male parameters than for estimates of female parameters. It is
possible that the degree of sexual size dimorphism displayed by
P. d. brevirostre in the study area is greater than that indicated
by the model presented here, although there is no significant
147

148
C
C
20
6
~ 12-
I
8-
4
A
CAN. J. ZOOL. VOL. 63. 1985
20 30 40 50 60 70 80 20 30 40 50 60 70 80
Snout — vent length (mm) Snout— vent length (mm)
FIG. 9. (A) Distribution of head length against SVL for all male captures, with allometric line (solid) and ± I SD of its exponent (dashed
lines). HL = O.7OIOSVLO70v1, a = 165. (B) Distribution of head length against SVL for all female captures, with allometric line (solid) and
± I SD of its exponent (dashed lines). HL O.7157SVL°7°°2, a = 264.
difference in goodness of fit between the two models. A larger
male recapture sample should result in narrower 95% con
fidence intervals for the male parameter estimates and thus a
reduced possibility of overlap between the support-plane con
fidence limits of the two sexes’ parameter estimates. This is not
likely true, however, of thea1 estimates for SVL of the northern
and southern females. Both recapture samples in this case were
sufficiently large that further diminution of the t and F statistics
with increased sample size would have been negligible.
One assumption made in using the modified logistic by
weight growth model is that growth is determinate in P. d.
brevirostre. This may not be the case. Growth is determinate
in Anolis (Schoener and Schoener 1978), to which this model
was first fitted, but there is some evidence for nondeterminate
growth in many other reptiles (Bellairs 1969). Cessation of
linear growth in lizards is signified by fusion of the epiphyses
C
C
C
I
20
12
4
11111’ I I
20 30 40 50 60 70 80 20 30 40 50 60 70 80
6-
8
Snout — vent length (mm) Snout — vent length (mm)
FIG. 8. (A) Distribution of head width against SVL for all male captures, with allometric line (solid) and ± I SD of its exponent (dashed lines).
HW = O.5295SVLov~e2, a = 165. (B) Distribution of head width against SVL for all female captures, with allometric line (solid) and ± I SD
of its exponent (dashed lines). HW 0.461 lSVL°~’. a = 264.
20- 20-
A B
and diaphysis of each long bone of the limbs (Haines 1969);
this phenomenon does not seem to have been examined in
Phiynosoina. Effectively, however, growth in P. d. brevirostre
appears to be determinate in that it slows down markedly and
in many individuals stops completely within a definite SVL
range, depending upon the sex.
Sexual dimorphism and growth
The degree to which males were correctly distinguished from
females by Eqs. 5 and 6 (Tables 1,2) indicates that there is
significant sexual dimorphism in head, body, and tail dimen
sions and in body proportions. The SVL range, which includes
all of the incorrectly classified cases for both analyses (“zone
of uncertainty”), differs between the sexes, but includes only
YoY- to yearling-sized individuals of either sex. No adults
were misclassified as to sex by either of these discriminant

POWELL AND RUSSELL 149
TABLE 4. Components of allometric equations for TL, HW, and HL of the
two sexes
Exponent
n Constant (.~±SD) R2 F
TL
Males 165 0.1811 1.2979±0.027 0.934
Females 264 0.3651 1.0584±0.013 0.964
HW
Males 165 0.5295 0.8062±0.016 0.939
Females 264 0.461 I 0.8446±0.009 0.972
HL
Males 165 0.7010 0.7081±0.021 0.871
37.03, df=2. 423
(p

150 CAN. 3. ZOOL. VOL. 63. 1985
TABLE 6. Parameter estimates with 95% conventional and support-plane confidence intervals for SVL growth of males and females, grouped
geographically
a (asymptotic 95% conventional 95% support-plane r (characteristic 95% conventional 95% support-plane
length, mm) interval (mm) interval (mm) growth rate) interval interval
Northern female 65.47 63. 17Sa, ≤67.77 62.54~a1 ~68.39 0.019592 0.0l352≤r~0.02566 0.0l202~r~0.027l7
Southern female 70.64 68.50~a1≤72.78 67.90≤a1 ≤73.38 0.014757 0.0l200≤r≤0.0175l 0.01 130≤r≤O.0l82l
Standard male* 52.34 48.39≤a1≤56.29 47.34≤a1~57.33 0.023847 0.01698≤r≤0.03072 O.0l5l6≤r≤0.03253
Southern male 49.74 44.55≤a ~54.93 43. l2≤a~ ≤56.36 0.025106 0.01961 ≤r≤0.03460 0.01300≤r≤0.03721
*Standard male statistics are derived from all male recaptures regardless of geographic location. They are provided br comparison with the southern male statistics, since an inadequate
sample size prevented estimation of northern male parameters.
a difference in HL, accounts for the importance of HL/SVL in
Eq. 6. The zone of uncertainty of each sex for Eq. 6, as for Eq.
5, is limited approximately by the SVL characteristic of lizards
which are thought to be of an age capable of breeding for the
first time. The logistic growth models for both sexes predict a
close similarity in SVL between subadult males and subadult
females, at least until male growth slows down. The SVL
distributions of cases misclassified by both discriminant func
tions reflects this predicted similarity. However, the number of
subadult lizards correctly classified to gender by the two
functions shows that sexual dimorphism in the dimensions
examined is evident throughout the lizard’s life, though more
strongly expressed after sexual maturity is reached. This is not
unequivocally clear from the logistic growth curves (Fig. 6) or
the allometric equations (Table 4) for the two sexes, but could
reflect a difference in acuity between the two analyses.
For the purposes of this discussion, sexual size dimorphism
can be categorized in a tripartite manner: male+, in which the
males of a species are the larger sex at adulthood (male
SVL/female SVL > 1.0); female+, in which the females of
a species are the larger sex at adulthood (male SVL/female
SVL < 1 .0); and monomorphic, in which there is no significant
size difference between the sexes at adulthood (male
SVL/female SVL = 1.0). Fitch (1976) found a correlation
between climatic seasonality and male+ dimorphism in Anolis,
but noted that of the 50 species that he examined, a greater
proportion were nearly monomorphic than were dimorphic. In
a survey of the genus Sceloporus, Fitch (1978) found that more
than one-third of the species examined were female+ dimor
phic. This could be related to selection pressure imposed by
large clutch size and (or) being single clutched. Species from
higher latitudes also tend to display female+ dimorphism
owing to the concentration of reproductive effort associated
with a seasonal climate, which produces a similar result to that
of being single clutched.
Schoener (1977) tabulated the available information on
sexual size dimorphism in lizard species for which data on
social structuring are also available. Of the 59 species exam
ined, 57.75% are male+ dimorphic, 28.88% are female+
dimorphic, and 13.37% are monomorphic. If attention is
restricted to the iguanid species, the breakdown is as follows:
male+ dimorphic, 73.53%; female+ dimorphic, 5.88%;
monomorphic, 20.59%. These percentages indicate that, in
general, lizard species which are monomorphic are in the
minority. Of the majority of species which are dimorphic, a
greater percentage of the species tallied are male+ dimorphic
and this is particularly true of the Iguanidae, to which Phryno
soma belongs. Fitch (1981) performed a comprehensive census
of sexual size dimorphism in the Reptilia and noted that male+
dimorphism is more common in lizards and that monomorphic
reptile species are very uncommon. He likewise pointed out the
predominance of male+ dimorphism in the Iguanidae.
Dietary niche partitioning in association with sexual size
dimorphism has been documented in some lizard species.
Anolis lizards, which generally display male+ dimorphism
or monomorphism, have been discussed in this regard by
Roughgarden (1974) and Schoener (1967, 1969, 1971, 1977).
Female size can be strongly affected by requirements of clutch
size, which may establish a minimum adult size necessary for
reproductive success. Selection for a particular size in one sex
may result in the other sex being forced away from this size by
intraspecific competition (Schoener 1970, 1977).
Phrynosomes can all be classed as model I predators, defined
simply as predators which sit and wait for their prey to come
by, selecting it on the basis of its size and distance (Schoener
1969). Model I predators, from considerations of foraging
theory, are expected to display sexual size dimorphism,
particularly when few competitors are present (Schoener
1969), assuming that food availability is a limiting factor.
Since the two sexes are of different sizes, they would be
expected to have different prey-handling capabilities, and thus
to exploit differing prey size ranges. This would enable the
species as a whole to widen its dietary niche and to reduce
intraspecific competition for food (Roughgarden 1974;
Schoener 1969, 1977).
Prey-handling capabilities will depend upon jaw size in
phrynosomes, and jaw size is determined by head dimensions.
If jaw size is different between the sexes, some difference in
prey-handling capability might be expected as well. The allo
metric models of HL and HW growth (Figs. 8, 9, Table 4)
indicate that there will be a considerable difference in jaw size
between the sexes at adulthood. This lends some weight to the
dietary niche partitioning hypothesis. Sexual size dimorphism
would not be so important in subadult lizards, since its main
importance in dietary niche partitioning would be to reduce
dietary competition between breeding females and adult males.
Females must deal with the metabolic costs of vitellogenesis
(Packard et al. 1977) and, in viviparines, gravidity (Guillette
1982), while the males have no such energetic costs to meet.
Dietary niche partitioning associated with sexual size dimor
phism has been noted in Natrix natrix, another female+ dimor
phic viviparine (Madsen 1983). In this case, size-associated
female fecundity was implicated as the selective force directly
producing the dimorphism, dietary niche partitioning being
regarded as a corollary reinforcing the dimorphism (Madsen
1983).
A proper evaluation of this hypothesis with regard to the
Alberta populations of P. d. brevirostre requires information
on the diet, on whether these populations are food limited, and
also on whether female+ dimorphism is less marked in this
species in areas where there are saurian competitors. Dietary
analysis indicates that the dietary niche is partitioned between

adult females on one hand and all males and subadult females
on the other (Powell and Russell 1984), but it was not
determined if food was limiting for these populations, or if
intersexual dietary competition did occur. information on the
geographical variation in sexual size dimorphism in this
species, and its relationship to diet, are still lacking, and so
intraspecific dietary competition cannot be accepted as the sole
force producing and maintaining the sexual size dimorphism
characteristic of the Alberta populations. Possibly, dietary
niche partitioning is a corollary of a sexual size dimorphism
produced by some other selective force, rather than the selec
tive force itself. It is also possible that it works in conjunction
with other selective forces to produce sexual size dimorphism
(for a more extensive consideration see Powell and Russell
1984).
Ghiselin (1974) discussed the phenomenon of sexual size
dimorphism in the context of sexual selection and set out four
possible types of male— male competition which could promote
it. Three of these would most likely lead to male+ dimorphism
or to monomorphism. The fourth, male dispersal competition,
puts a premium on mobility and early maturation in males,
since they are in a race to find and inseminate as many females
as possible. This last category of male—male competition is
likely to lead to female+ dimorphism. if males are selected for
mobility and early maturation, they are likely to devote more
resources to these functions than to growth in body size, and
thus are likely to be smaller at maturity than females, which are
under no such constraints. Dwarf males are more likely to
occur under certain circumstances: (i) when greater body size
reduces mobility; (ii) when there is a premium on long life in
females, but not in males; (iii) when population densities are
low, intraspecific encounters are rare, and there is little selec
tion for features associated with male—male agonistic encoun
ters but much selection for features facilitating male mobility
and mate-searching abilities. Ghiselin (1974) considers this last
condition to be the most important.
Berry and Shine (1980) employed sexual selection theory to
explain sexual size dimorphism in chelonians, and found that
the mating system and the type of sexual size dimorphism
found in a particular species conformed to the predictions of
Ghiselin’s (1974) theory. They also suggested that the rela
tionship of size to fecundity is important in establishing female
size. A minimum female size could be set by selection pres
sures on clutch size. In addition, viviparity, particularly in
lizards, tends to be correlated with delayed maturity and greater
body size at sexual maturity (Tinkle eta!. 1970). These factors
could serve to hold minimum female size relatively invariable,
while not constraining minimum male size.
Delayed reproduction and fernale+ dimorphism are found in
the viviparous red-sided garter snake (Thainnophis sirtalis
parietatis) near the northern extreme of its range in Manitoba
(Gregory 1977) and viviparity has been suggested as a selective
force contributing to female+ dimorphism in T.sirtalis in the
north-central United States (Benton 1980). Madsen (1983)
found that females of the viviparine Natrix natrix in southern
Sweden exhibited delayed maturity compared with males and
also grow at a faster rate, resulting in female+ dimorphism at
maturity. There is a positive correlation between female size
and fecundity, but no apparent relationship between greater
male size and reproductive success, which probably accounts
for this dimorphism (Madsen 1983).
Delayed maturity in female P. d. brevirostre in Alberta is
evident from Fig. 6, and the viviparity of the species is well
POWELL AND RUSSELL 151
documented. This suggests that female size is under the con
straints outlined above. The predicted SVL growth curves of
both sexes (Fig. 6) show that, while growth patterns are similar
for the first 120—160 active days of growth, male growth
ceases rapidly after this, with males reaching a consequently
smaller SVL at maturity. Female SVL growth continues longer
and tapers off more slowly. A similar pattern was observed in
P. d. douglassi in idaho by Guyer (1978). The similarity of
growth rate until the smaller sex reaches asymptotic size is
characteristic of freshwater turtles, but generally in reptiles the
sex which is larger at adulthood grows faster in the juvenile
stage (Andrews 1982). These observations suggest that matura
tion occurs earlier in male P. douglassi than in females. Pos
sibly more resources are devoted to early maturation than to
growth, with a consequent small size at maturity in males. If
this, indeed, is the case, then male P. d. brevirostre, when
compared with females, exhibit relative paedomorphosis by
means of truncation of somatic growth and acceleration of the
development of reproductive organs (Gould 1977).
Densities in Phrvnosoma populations are usually low
(Pianka and Parker 1975), considerably less than the geometric
mean of 51 individuals/ha estimated for lizards by Turner
(1977). The densities of the populations examined in this study
were not determined with any degree of accuracy, but were
certainly lower than Turner’s (1977) mean. Social behaviours
and structuring are generally held to be weakly developed in
phrynosomes (Carpenter and Ferguson 1977; Stamps 1977) and
associated display organs are not evident. However, Tollestrup
(1981) found that P.~ and P. p!atvrhinos have welldeveloped
display repertoires, implying that social structuring
is more complex in these two species than is generally
imagined. These findings cannot yet be extended to other
species, although Montanucci and Bauer (1982) described a
moderately complex courtship repertoire for P. doug/assi. In
the course of this study, only one male—male interaction, of
short duration and intensity, was observed, in late May of
1979, and lizards were seldom found in close proximity to one
another. The low number of sequential male recaptures sug
gests that males may be relatively mobile. The sedentary habits
of females seem well established by the number of individuals
for which long sequential recapture records exist. if the females
are sedentary, males, by implication, will be mobile, as they
will have to seek the females out to mate with them.
This hypothesis cannot be adopted with a great deal of cer
tainty because there is some question as to the underlying
reason for the paucity of sequential male recaptures. Guyer
(1978) found a preponderance of. female P. doug/assi at his
study site in Idaho and posited heavy winter mortality of year
ling males to explain this. There was no significant difference
in distribution of home range size between adults of the two
sexes (males, ii = 3; females, n 5; Mann—Whitney U-test,
p > 0.05) at this Idaho site (Guyer 1978), which casts some
doubt on the hypothesized greater mobility of males. The
results of the analysis of male and female mobility presented
here (Table 5) are suggestive, however, but cannot be taken as
strong evidence for the male dispersal competition hypothesis.
A sample of male and female home range sizes to which statis
tics could be applied is necessary to be able to establish whether
or not there is a difference in size between them. An investi
gation of sex-specific survivorship in the Alberta populations is
also required to test the possibility that the difference in recap
ture rate between the sexes is due to differential mortality rates
rather than greater mobility in one sex.

152 CAN. 3. ZOOL. VOL. 63. 985
If a smaller size at maturity in males is a result of the
rechannelling of resources into early maturation and vagility,
then the benefits of smallness are evident. There are no grounds
for believing that smaller adult size of itself confers any advan
tage in mobility to males. However, male dispersal competition
as an explanation for fernale+ dimorphism in the Alberta pop
ulations of P. d. brevirostre has considerable plausibility.
Female phrynosomes will be under different selectional con
straints from males with regard to growth. It is evident from
Fig. 4, and particularly from Fig. 6, that females exhibit rela
tively delayed maturity compared with males. Tinkle et at.
(1970) observed that the females of viviparous lizard species
display delayed maturity and a greater body size at maturity.
Delayed reproduction confers a reproductive advantage to the
individual, because the larger a female is at the time of her first
clutch, the larger the clutch mass that she can produce (Tinkle
and Hadley 1975). Tinkle and Ballinger (1972), in their study
of intraspecific demographic variability in the oviparous Scelo
porus undulatus, suggested that growth is faster in southern
populations owing to selection for rapid maturation, but that
life-spans were longer in the northern populations owing to the
selection for delayed reproduction.
The viviparity of P. doug/assi is established (Pianka and
Parker 1975). SVL growth in females continues until 480—600
active days after birth, according to extrapolation from the
model (Fig. 6), a considerable period after the cessation of
male growth as extrapolated from the male model. Placed in a
seasonal context, curves extrapolated from the female SVL
growth model show growth slowing in the summer after the
second brumation and effectively ceasing in the summer after
the third brumation (Fig. 4). No yearling females were found
to be gravid, although this cannot be stated with certainty
owing to problems with the ascertaining of gravidity (see
above). The youngest females that can be said with assurance
to have bred are estimated, from longitudal records, to have
been in the summer after their second brumation, and by the
time of parturition (late July — early August) they would have
achieved near-asymptotic SVL (Fig. 4). Ballinger (1973) found
that females of the viviparous Sceloporus poinsetti likewise did
not breed until the summer after their second brumation,
although they attained sexual maturity a year earlier. However,
females of the lowland populations of the viviparine S. jarrovi
may breed for the first time in the summer after their first
brumation (Ballinger 1973; Tinkle and Hadley 1975). Pilorge
(198 Ia, 198lb) noted similar early maturity in females of the
viviparine Lacerta vivipara in central France, and stated that
viviparity should not be considered as an adaptive strategy
per Se, but as a component of each individual species’ repro
ductive strategy (Pilorge 198 lb). Thus, delayed maturity is not
necessarily a correlate of viviparity in lizards. Nonetheless, in
high-latitude or montane viviparines, the association between
delayed maturity in females and viviparity seems to be valid
(Tinkle ci at. 1970; Ballinger 1973).
Phrynosoina d. douglassi in southernidaho displays similar
female SVL growth patterns tq P. d. brevirostre in Alberta,
growth continuing through the summer after the second
brumation (Guyer 1978), although no data are available on
breeding age. Unfortunately, there are no comparative data on
growth patterns and breeding age for P. douglassi elsewhere in
its range. However, the Alberta populations seem to conform
to other predictions of Tinkle ci at. (1970) with regard to
viviparous lizards. They are long-lived according to the longi
tudinal records, although, as discussed above, this can be said
with more assurance about the females than about the males.
They are single clutched in Alberta, a strategy which is appar
ently characteristic of the species over its range (Pianka and
Parker 1975) and one to be expected in a viviparous lizard
(Tinkle et at. 1970). Slow growth and delayed maturity in
females would also be selected for in lizard populations
dwelling in ecologically marginal situations (Tinkle 1967),
such as montane areas or the northern margins of the species’
range (Ballinger 1979; Ferguson and Brockman 1980). Fitch
(1978) found there to be a correlation between female+ dimor
phism and a large clutch mass and (or) a single clutch in the
genus Scetoporus, and a greater tendency towards female+
dimorphism in temperate zone species of this genus.
Selection for delayed maturity and greater size at maturity
would naturally affect females but not males, since the selec
tion pressure is related to viviparity and (or) clutch size.
Male dispersal competition would put a premium on rapid,
invariate growth in males, as a certain size must be reached to
make mating physically possible; after this point is reached,
resources would be diverted into gonadal maturation and
vagility. Males are engaged in a race to reach a minimum
breeding size and mature sexually as quickly as possible. By
this hypothesis, selection will constrain growth. Females, on
the other hand, are not competing to attain a particular size and
mate as frequently as possible. Selection pressures require
them to delay maturity and maximize size at first breeding, but
after that point is reached there is no reason to believe that these
selection pressures will operate with such force, and so sub
sequent growth may be more variable. Attainment of great size
in a female entails great clutch size and thus reproductive
success, but maximum size attainable will be controlled by
growing season length (Stewart 1979) and the various environ
mental factors enumerated above which affect growth.
The above data and discussion thus indicate that males attain
their asymptotic size more rapidly than females do theirs, by
early cessation of growth, and that females attain a greater
asymptotic SVL by relative prolongation and a more gradual
tapering off of growth. The male growth pattern corresponds to
that expected if selection is for early maturation. This con
forms, at least in part, to the explanation of female+ dimor
phism in these populations as being related to male dispersal
competition.
The female SVL model also reveals positive factors related
to survival in a range-marginal area and can be interpreted as
the result of selection for delayed maturity. Selection pressure
on growth rate and asymptotic size would not be so extreme in
females. Thus, the degree to which maturity is delayed, and the
associated growth rate and asymptotic size, would be expected
to be more variable than is the case with males. The major
criteria being selected for in females, if this line of reasoning
is followed, would be associated with the minimum body size
at which a reasonably sized clutch could be carried. Associated
with this may be the delaying of reproduction until the size of
the female’s cloaca is sufficient to allow the unhindered
passage of neonates of a certain minimum size, this threshold
size being associated with a reasonable chance of surviving the
first brumation. The size of neonates tends to increase towards
the northern limits of the geographical range in viviparines
(Stewart 1979). Such factors would set a premium on female
cloaca (and hence body) size at the time she first breeds.
Together with the selection for adequate clutch size, this factor
will also likely promote delayed maturity.
While dietary competition cannot be ruled out as the selec

tive force producing and maintaining the sexual size dimor
phism found in these populations (Powell and Russell 1984),
the evidence presented here argues strongly for sexual selection
more closely associated with reproductive strategy and
viviparity. At present, neither alternative can be ruled out as
the main selective force, and it is quite possible that the two
work in conjunction to produce and maintain this sexual size
dimorphism.
Acknowledgements
We would like to thank Leonard and Mary Jane Piotrowski,
Phil Flaig, Dick and Janet Rose, Will and Rose McKinley, and
the Laidlaw family for permission to work on their lands.
Dr. E. Swierstra of the Dominion Agricultural Research
Station in Lethbridge is to be thanked for permission to use the
Onefour schoolhouse as field headquarters. We are obliged to
Jan Vrbik, David Schrothe, and Judy Szikora, all of the
University of Calgary Academic Computer Services, for help
with various parts of the computer analysis, and Dr. Gordon
Fick for a discussion of regression methods. We are grateful to
Mr. Larry Linton for assistance in regard to these matters also.
Thanks are also owed to Dr. Thomas Schoener, for his helpful
comments; Dr. Michael Ryan, for his methodological critique;
Craig Guyer, for sharing some unpublished material; and to
Drs. Ronald Davies. Gordon Pritchard, and Robert Weyant for
reading and constructively criticizing an earlier draft of this
manuscript. The comments of two anonymous reviewers
greatly improved the final product. Financial support for this
project was provided partially by the Department of Biology,
University of Calgary, and partially by a Natural Sciences and
Engineering Research Council of Canada grant (A-9745) to
A. P. Russell. The manuscript was typed by Marg Hunik.
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