PII: S0306-4565(98)00018-7
J. therm. Biol. Vol. 23, No. 6, pp. 329±334, 1998
# 1998 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
0306-4565/98 $ - see front matter
GEOGRAPHIC VARIATION IN FIELD BODY
TEMPERATURE OF SCELOPORUS LIZARDS
ROBIN M. ANDREWS{1
1
Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg,
VA 24061-0406, USA
(Received 29 January 1998; accepted in revised form 4 May 1998)
AbstractÐ1. Using data from the literature, I assessed how broad climatic patterns a€ected ®eld body
temperatures (Tb's) of lizards in the genus Sceloporus.
2. Sceloporus at temperate latitudes had mean Tb's of 358C throughout their elevational range. This
pattern is associated with ``tropical'' temperatures that extend into high north latitudes during the summer and the relatively low elevations occupied by the lizards.
3. At tropical latitudes, mean Tb declined from 358C at low elevations to 318C at high elevations.
This pattern is associated with low seasonal variation in temperature at tropical latitudes and the relatively high elevations occupied by the lizards. # 1998 Elsevier Science Ltd. All rights reserved
Key Word Index: Sceloporus; thermoregulation; selected body temperature; lizard
INTRODUCTION
The lizard genus Sceloporus has long been considered
to be thermally conservative. For example, ®eld body
temperatures measured during the normal activity
period (Tb's) typically average 34±368C (Bogert,
1949a,b; Brattstrom, 1965), and the seven species for
which body temperatures have been measured on laboratory thermal gradients exhibit selected temperatures (Tsel) of approximately 358C (McGinnis, 1966;
Greenberg, 1976; Bowker et al., 1986; Crowley, 1987;
Mathies and Andrews, 1997; Andrews et al., submitted, Andrews, unpublished). Moreover, selected
body temperatures do not vary seasonally
(McGinnis, 1966). The maintenance of relatively high
and stable body temperatures during activity is due,
in part, to an association between members of the
genus and open vegetation formations where thermoregulation is relatively easy. Closer scrutiny, however,
indicates that both extrinsic and intrinsic factors
a€ect Tb's of Sceloporus. One extrinsic factor is climate. Some populations exhibit comparatively high
(Grant and Dunham, 1990) or comparatively low
Tb's (Vial, 1984; Lemos-Espinal and Ballinger, 1995;
Andrews et al., 1997), presumably because of thermal
constraints imposed by local environments. Intrinsic
factors also a€ect Tb. Females of some species of
Sceloporus exhibit lower Tb's and Tsel's when gravid
{ Tel.: 540 231-5728; Fax: 540 231-9307; E-mail: [email protected]
329
or pregnant than do males or non-reproductive
females (Beuchat, 1986; Andrews and Rose, 1994;
Mathies and Andrews, 1997), and Sceloporus undulatus individuals that were desiccated had lower Tsel
than normally hydrated individuals (Crowley, 1987).
In this paper, I focus on broad geographic patterns
of Tb. The thermobiology of Sceloporus is conservative and therefore the genus is particularly appropriate for assessing environmental constraints on
thermoregulation. More speci®cally, systematic deviations from ®eld body temperatures of approximately 358C are likely to represent situations where
lizards cannot maintain their preferred body temperatures. The genus is also appropriate because it is
diverse (about 80 species, Sites et al., 1992), and
because it has a wide latitudinal and elevational distribution. Members of the genus range from western
Panama in Central America to the state of
Washington in the United States. Within tropical
latitudes, Sceloporus lizards have an elevational
range from sea level to about 4600 m (Lemos-Espinal
and Ballinger, 1995), and within temperate latitudes,
from sea level to approximately 3300 m in Arizona
(Stebbins, 1966), and to approximately 2700 m in the
Paci®c Northwest (Nussbaum et al., 1983).
MATERIALS AND METHODS
I searched the literature for data on ®eld body
temperatures of active adult Sceloporus and found
330
R. M. Andrews
Table 1. Mean body temperature (Tb) of Sceloporus lizards. Elevation and latitude are given for each site
Sceloporus species
aeneus
bicanthalis (N)
bicanthalis (Z)
graciosus
graciosus
graciosus
grammicus
grammicus
grammicus
grammicus
jarrovi
jarrovi
magister
magister
malachiticus
malachiticus
malachiticus
merriami
merriami
merriami
merriami
occidentalis
occidentalis
orcutti
olivaceus
olivaceus
poinsetti
scalaris
squamosus
undulatus
undulatus
undulatus
undulatus
variabilis
variabilis
variabilis
variabilis
virgatus
woodi
Tb (8C)
Elevation (m)
32.0
28.8
32.3
34.1
34.8
33.9
31.2
31.6
33.0
33.6
34.5
34.2
36.8
34.9
28.6
32.9
33.3
32.2
33.6
36.2
37.0
34.6
35.9
32.6
32.5
36.5
34.2
32.6
35.3
35.3
36.8
34.8
35.1
33.4
34.5
35.4
36.9
34.0
36.2
2800
4000
3200
2580
2230
1500
4400
3700
3400
1100
2250
1650
1900
1200
2850
1850
1600
1610
1500
1040
560
2230
1250
260
190
350
1100
2550
800
2400
1750
1400
1950
1000
45
800
150
1725
60
Tb's for 39 populations and 18 species of Sceloporus
(Table 1). I report data for more than one population per species if these populations represented
sites di€ering in latitude or elevation. Observations
were unevenly distributed geographically; 14 were
from tropical latitudes and the remaining 25 from
temperate latitudes. If latitude and elevation were
not given for particular sites, I determined these
data from other sources. If more than one set of
data was available for the same site, the best data
set was selected judging by sample size, time of
year, etc. If data for more than one time of year
were available for a site, I selected that time (usually
month) with the highest mean Tb. This criterion for
data selection assumes that, for most Sceloporus,
heating is a more critical component of thermoregulation than cooling (but see Grant and Dunham,
1990), and that times of year when maximal Tb's
Latitude (8N)
19
19
19
34
34
43
19
19
19
25
33
32
34
33
10
14
10
29
25
29
29
34
34
34
33
30
25
32
14
37
35
28
37
19
19
14
21
32
27
Source
Andrews et al., submitted
Andrews et al., submitted
Andrews et al., submitted
Adolph, 1990
Adolph, 1990
Guyer and Linder, 1985
Lemos-Espinal and Ballinger, 1995
Lemos-Espinal and Ballinger, 1995
Andrews et al., 1997
Bogert, 1949a
Beuchat, 1986
Andrews, 1984
Vitt et al., 1981
Bogert, 1949a
Vial, 1984
Bogert, 1949a
Fitch, 1973
Grant and Dunham, 1990
Bogert, 1949a
Grant and Dunham, 1990
Grant and Dunham, 1990
Adolph, 1990
Adolph, 1990
Mayhew, 1963
Fitzpatrick et al., 1978
Blair, 1960
Bogert, 1949a
Smith et al., 1993
Bogert, 1949a
Crowley, 1985
Crowley, 1985
Bogert, 1949a
Gillis, 1991
Benabib, unpublished
Benabib, unpublished
Bogert, 1949a
Bogert, 1949a
Smith and Ballinger, 1994b
Bogert, 1949b
are exhibited are also when individuals are most
likely to attain `preferred' Tb's. These criterion will
generally provide the Tb's that most closely
approach Tsel, particularly for environments where
opportunities for thermoregulation vary seasonally.
If mean Tb's were not presented in the original publication, I calculated the mean Tb for the normal activity range as de®ned by Bogert (1949a,b) from
histograms or scatter plots. If Tb's of reproductive
(gravid or pregnant) females di€ered from those of
males or non-reproductive females, I present the
means only for males and non-reproductive females.
To compare climates at tropical and temperate
latitudes, I used monthly means of daily maximum
temperatures collected at meteorological stations as
a rough index of the thermal environment of active
lizards. Climatic records were obtained from
stations within the latitudinal range of Sceloporus. I
Geographic variation in ®eld body temperature of Sceloporus lizards
Table 2. Mean body temperatures (SE, n) of Sceloporus
lizards as a function of latitude and elevation
LOW ELEVATION
(<1500 m)
HIGH ELEVATION
(r1500 m)
TROPICAL
(<248N)
TEMPERATE
(r248N)
35.1 (0.57, 5)
34.9 (0.47, 11)
31.5 (0.58, 9)
34.5 (0.35, 14)
used the month with the highest mean maximum
temperature for both temperate and tropical latitudes. For temperate latitudes, this month was
always July. For tropical latitudes, the highest
monthly means of maximum temperature typically
occur in the spring (March±May). I used two locations in the United States to represent temperate
latitudes. These were Arizona and Oregon east of
121.58N longitude. I selected the ®rst ®ve stations
in alphabetical order within 500 m increments in elevation from data presented by Blanchard (1985).
Because of the paucity of data for high elevations, I
supplemented the data set for Arizona with the
highest station in New Mexico and the highest
station in Colorado and the data set for Oregon
with the highest ®ve stations in Idaho.
I used three sources of weather data for tropical
latitudes. The ®rst was Pearce and Smith (1984)
from which I used all observations for Mexico
(n = 6) and Central America as far south as Costa
Rica (n = 6). Because few high elevation stations
were represented in this data set, I used two further
sources for weather records. One was the Servicio
Meterologico Mexicano, Ciudad de Mexico, for
Milpa Alta (2420 m), Ajusco (2839 m), Rio Frio
(3000 m), and Nevado de Toluca (4140 m). These locations are located within 75 km of Mexico City.
The other was the Annuario Meteorologico for 1965
and 1968 (Servicio Meteorologico y Sismologico
Nacional, San Jose, Costa Rica) for Rancho
Redondo (1780 m), Zarcero (1736 m), Sanatorio
DuraÂn (2337 m), and Villa Mills (3096 m).
I used latitude and elevation as crude estimates
of climate for geographic comparisons of Tb's of
Sceloporus. I conducted three kinds of analyses.
The ®rst was a two factor ANOVA in which the
factors were elevation (low and high) and latitude
(tropical and temperate). I considered observations
for latitudes less than 248N to be tropical (corresponding to the Tropic of Capricorn) and observations for latitudes of 248N or greater to be
temperate. Following the convention of Guillette et
al. (1980), I considered observations from elevations
less than 1500 m to be lowland and observations
from elevations of 1500 m or more to be highland.
331
The second analysis was a regression analysis that
compared the slopes of the regressions of Tb as
function of elevation for tropical and temperate
latitude populations. In the third analysis, I evaluated the joint e€ects of elevation and latitude in a
step-wise multiple regression.
RESULTS
At lowland sites at both tropical and temperate latitudes, and at highland sites at temperate latitudes, Tb's
averaged 35.18C (Table 2). In contrast, the average Tb
at highland sites at tropical latitudes was 31.58C, or
almost 48C lower. The conclusion that Sceloporus at
temperate latitudes exhibit high Tb's even at high elevations while tropical species do not is supported by
the signi®cant interaction between elevation and latitude (F1,35=9.3, P = 0.004, two-factor ANOVA) and
by the signi®cant interaction between elevation and
latitude in the regression analysis in which Tb was the
dependent variable, elevation was the covariate, and
tropical or temperate latitude was the class variable
(Fig. 1, F1,35=5.0, P = 0.032, heterogeneity of slopes
test). Finally, elevation (Elev.) entered on the ®rst step
of the step-wise multiple regression and explained
37% of the variation in Tb. Latitude (Lat.) entered on
the second step and explained a further 11% in the
variation in Tb. The resultant multiple regression
equation was: Tb=33.55 ÿ 0.0010 Elev. + 0.0814
Lat. (F2,36=16.9, P < 0.001, R2=0.48). Field body
temperatures thus decreased with increasing elevation
and increased at higher latitudes.
DISCUSSION
In general, results of this study support the static
view (Hertz et al. 1983) of the thermobiology for
the genus Sceloporus. Field body temperatures of
Sceloporus approximated 358C over a wide range of
elevations and latitudes. Only at high elevations in
the tropics did Tb's average less than 358C. The low
Tb's of tropical Sceloporus at high elevations do not
appear to re¯ect a downward shift in Tsel; Tsel's of
three species of Sceloporus from high elevations
(2800±3400 m) in Mexico ranged from 34±368C
(Andrews et al., submitted, Andrews, unpublished
data for S. grammicus). Low Tb's must therefore
re¯ect an increasing inability to attain preferred
Tb's at increasing elevations in the tropics.
Body temperatures of Sceloporus showed two
major geographic patterns. One pattern was that
Sceloporus at temperate latitudes exhibited relatively
high Tb's (about 358C) throughout their elevational
range. The reason is that essentially tropical temperatures extend far into temperate latitudes during
332
R. M. Andrews
Fig. 1. Body temperatures of Sceloporus lizards as a function of elevation. Species from tropical latitudes are indicated with ®lled circles and from temperate latitudes are
indicated with open circles.
the summer. Standard weather records indicate that
ambient temperatures are actually warmer at some
temperate than tropical latitudes during this time
(Fig. 2). The reason is that during the months of
May, June, and July, solar radiation is greater
between the latitudes of 20±408N than at the equator
(Crowe, 1971, Table 1 and Fig. 7 therein). At this
time, ''the highest values [of solar radiation] are
found over the southern half of the Intermontane
region [of the western United States] where a low incidence of cloud cover complements higher than average elevation. In fact, insolation is greater in this area
than anywhere else in the world at this latitude''
(Bennett, 1965). Moreover, the occurrence of
Sceloporus almost to the Canadian border at 498N in
the Paci®c Northwest is facilitated by an east±west
gradient in solar radiation across the United States
during the summer (Bennett, 1965).
As a consequence of high insolation during the
summer (and perhaps their limited elevational range,
see below), Sceloporus at temperate latitudes are able
to maintain ®eld body temperatures that approximate
Tsel during their activity seasons (McGinnis, 1966;
Adolph, 1990; Smith et al., 1993; Smith and
Ballinger, 1994; Sinervo and Adolph, 1994). For
example, during June, July, August, September, and
October, S. undulatus consobrinus in New Mexico
(1750 m) and in Colorado (2400 m) had mean
monthly Tb's of 35.6, 35.4, 36.8, 36.0, 35.18C and
33.1, 35.3, 34,9, 33.6, 31.78C, respectively (Crowley,
1985). Moreover, lizards are able to maintain high
and stable Tb's over a wide range of elevations as well
(Beuchat, 1986; Sinervo and Adolph, 1994).
The second pattern is a decline in Tb with elevation
at tropical latitudes. While Sceloporus of tropical and
temperate regions have similar Tb's at low elevations,
Tb's of tropical species decline with increasing el-
Fig. 2. Monthly means of daily maximum ambient temperatures from meterological stations located at tropical
and temperate latitudes (see text for details). Note that
temperatures in Arizona are higher than most tropical
values and that Oregon and tropical values are similar at
all elevations.
evation. A partial explanation for this pattern is that
tropical species live at elevations considerably higher
than the elevational limit for temperate zone species.
Because temperature declines with elevation at all
latitudes (Fig. 2), some tropical species live in colder
environments than any temperate zone species (comparisons based on activity seasons only).
What accounts for the di€ering elevational ranges
and Tb's of Sceloporus at high elevations at tropical
and temperate latitudes? An important di€erence
between tropical and temperate zone climates is the
magnitude of seasonal ¯uctuations in temperature
(Janzen, 1967). At temperate latitudes, mean monthly
temperatures vary substantially; the mean for the
coldest month may be below freezing and the mean
for the warmest month may be as warm or warmer
than mean temperatures at tropical latitudes. As a
consequence, lizards at temperate latitudes have a
limited activity season, typically from February or
March through September or October. At temperate
latitudes, Sceloporus lizards emerge from hibernation
in the spring and must complete their reproductive
activities within a short enough period that eggs
hatch by fall. This temporal constraint on reproduction may limit Sceloporus to elevations where individuals can maintain preferred temperatures during
most of their annual activity period, and thus maximize ingestion, digestive eciency, productivity, etc.
(Huey, 1982; Stevenson et al., 1985).
In contrast, at tropical latitudes, mean monthly
temperatures vary by only a few degrees between
the coldest and the warmest month (Janzen, 1967).
Tropical sites usually reach their maximum ambient
temperatures in early spring (end of the dry season)
when cloud cover is low. In summer (wet season),
when solar radiation is the highest, cloud cover
Geographic variation in ®eld body temperature of Sceloporus lizards
reduces ambient temperature (Janzen, 1967, see
Fig. 1 therein). Sceloporus at tropical latitudes are
active year round, even at the upper limits of their
elevational range. Body temperatures of lizards may
or may not vary seasonally, perhaps depending on
microhabitat features. For example, Tb's of S.
aeneus and S. bicanthalis at 2800±3200 m and S.
grammicus at 3700 and 4400 m do not vary seasonally (Lemos-Espinal and Ballinger, 1995; Andrews
et al., submitted). On the other hand, Tb's of S.
grammicus at 3400 m are higher during the dry than
the wet season (Andrews et al., 1997).
Despite ambient temperatures that permit activity
year round, at high elevations at tropical latitudes
Sceloporus cannot maintain preferred body temperatures. For example, at elevations of 3700±
4400 m in Mexico, Sceloporus grammicus had average Tb's of 31±328C in both winter and summer
(Lemos-Espinal and Ballinger, 1995). As a consequence of their relatively low Tb's, Sceloporus at
high elevations in the tropics will experience
impaired performance in locomotion (Crowley,
1985), digestion, and other physiological attributes
(Huey, 1982; Stevenson et al., 1985). Greatly protracted reproductive cycles may be a consequence
of such low Tb's. At elevations of approximately
3000 m and higher, Sceloporus females are reproductive (vitellogenic or pregnant) from fall through
spring (MeÂndez et al., 1988). In contrast, oviparous
species at low elevations at both tropical and temperate latitudes complete their reproductive cycle
during summer (MeÂndez et al., 1998).
The higher elevational range of Sceloporus at tropical than temperate latitudes may thus be related
to the respective lengths of their activity seasons in
these two regions. Year round activity at high elevations in the tropics may allow sucient time for
females to complete their annual reproductive cycle
despite the reduced eciency associated with relatively low body temperatures. In other words, year
round activity may compensate for reduced eciency during the diel cycle. On the other hand, the
short activity season of female Sceloporus temperate
latitudes may limit them to habitats where they can
attain preferred body temperatures. The high eciency associated with preferred body temperatures
may be necessary to rapidly accrue energy for
reproduction and other needs.
The thermal conservancy of Sceloporus is paralleled by some (Van Damme et al., 1989, 1990), but
not all lizard taxa (Hertz et al., 1983). In South
America, the large tropidurine genus Liolaemus parallels Sceloporus in geographic range, habitat, and
thermal biology. Like Sceloporus, Liolaemus exhibit a
genus typical ®eld Tb of about 358C over a wide geo-
333
graphic range (Fuentes and Jaksic, 1979), and Tsel is
about 358C as well (Valencia and Jaksic, 1981). In addition, Liolaemus species that live high elevations (up
to 5000 m) at tropical latitudes have relatively low
Tb's (Marquet et al., 1989). Unlike Sceloporus, however, high latitude species of Liolaemus also have low
Tb's (Jaksic and Schwenk, 1983). This latter observation suggests that a comprehensive review of the
thermobiology of Liolaemus would make an interesting contrast with Sceloporus. Because the northern
and southern hemispheres have such di€erent relative
areas of land and ocean at comparable latitudes their
climates di€er considerably. One speci®c consequence
is less seasonal climates at high southern than high
northern latitudes; such a climatic di€erence would
a€ect the thermobiology of the lizard faunas of these
two parts of the world.
AcknowledgementsÐI thank T. Mathies for his comments
on the manuscript. The research was supported, in part,
by National Science Foundation grant BSR-9022425.
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