Hot rocks and much-too-hot rocks: seasonal patterns of retreat-site selection by a nocturnal ectotherm

Journal of Thermal Biology 27 (2002) 205–218

Hot rocks and much-too-hot rocks: seasonal patternsof retreat-site selection by a nocturnal ectotherm

Michael Kearney*

Department of Biological Sciences, Monash University, Clayton, Vic 3168, Australia

Received 5 May 2001; received in revised form 22 July 2001; accepted 18 August 2001

Abstract

(1) I studied seasonal patterns of diurnal retreat-site selection by a nocturnal, rock-dwelling lizard Christinusmarmoratus, as well as the way physical characteristics of potential retreat-sites affect the thermal conditions withinthem. (2) Lizards were selective of the diameter and degree of shading of rocky retreat-sites, and these two variables

significantly influenced the thermal conditions beneath rocks. Rock thickness also exerted a strong influence on thermalconditions but lizards were not strongly selective of this characteristic. (3) Seasonal changes in the pattern of retreat-siteselection, and in thermal conditions within retreat-sites, strongly suggest that temperature affected quantitative changes

in the types of rocky retreat-sites used by the lizards. A significant seasonal shift in the qualitative nature of retreat-sitesused by lizards, from rocks during spring to deep crevices in the bedrock during summer, also appears to be thermallydriven. r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Christinus marmoratus; Ectotherm; Gekkonidae; Nocturnal; Retreat-site; Reptile; Rock-dwelling; Seasonal patterns

1. Introduction

Animals are potentially vulnerable to predators and

to environmental extremes during periods of inactivity,and they may also become inactive specifically to avoidsuch hazards (e.g. Porter et al., 1973; Whitford andBryant, 1979). Thus an animal’s habitat requirements

will often include a suitable place in which to retreat.For terrestrial ectotherms, the availability of suitableretreat-sites may be a particularly important determi-

nant of their distribution and abundance because theyare often inactive for long periods of time (Huey, 1982;Leather et al., 1993). Since the behavioral and physio-

logical performance of ectotherms (and endotherms) isclosely tied to the microclimates they experience (Huey,

1982; Stevenson et al., 1985; Huey and Kingsolver, 1989;Porter et al., 2000), the environmental conditions withinretreat-sites could have significant implications for their

overall fitness and for the evolution of their thermalsensitivity of performance (Humphreys, 1978; Christianet al., 1984; Buttemer, 1985; Walsberg, 1985; Huey,1991; Schwarzkopf and Alford, 1996).

Despite these considerations, few studies have mea-sured environmental conditions within retreat-sitesused by ectotherms. Those that have indicate that these

conditions can vary widely in space and time, withsignificant physiological and ecological implications(Humphreys, 1978; Christian et al., 1984; Huey et al.,

1989b; Schwarzkopf and Alford, 1996; Webb andShine, 1998; Kearney and Predavec, 2000). For instance,thermal conditions within potential retreat-sites of

the garter snake Thamnophis elegans may have a widerange of potential physiological consequences, includingdeath from heat stress, depending on the thermalproperties of retreat-sites the snakes selected (Huey

et al., 1989b). A similar study of the broad-headed snake

*Corresponding author. Present address: School of Biologi-

cal Sciences, University of Sydney, Heydon Laurence Building

A08, Sydney, NSW 2006, Australia. Tel.: +61-2-9351-7661;

fax: +61-2-9351-5609.

E-mail address: [email protected] (M. Kearney).

0306-4565/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 3 0 6 - 4 5 6 5 ( 0 1 ) 0 0 0 8 5 - 7

Hoplocephalus bungaroides found a seasonal shift in thetype of retreat-site used that appeared to be thermally

driven (Webb and Shine, 1998).In this study, I examined the pattern of retreat-site

selection and its thermal consequences in a nocturnal

ectotherm, the marbled gecko Christinus marmoratus.Nocturnal ectotherms such as geckos may be especiallysensitive to the thermal conditions within retreat-sitesfor two reasons. First, in comparison with diurnal

ectotherms they are more committed to remain in theirchosen retreat-site during the daytime, when thermalconditions are most heterogeneous. Second, the thermal

physiology of nocturnal geckos is optimized at muchhigher body temperatures than they experience at nightwhen active (Huey et al., 1989a; Autumn and De Nardo,

1995), and accordingly they have been shown tothermoregulate during the day from within their re-treat-sites at body temperatures similar to diurnal

lizards (Bustard, 1967; Dial, 1978; Werner andWhitaker, 1978; Autumn et al., 1994; Schlesinger andShine, 1994; Kearney and Predavec, 2000; Kearney,2001). For instance, spot-measurements of the operative

temperatures available within retreat-sites used by themarbled gecko are closer to its preferred range thanwithin randomly selected retreat-sites (Kearney and

Predavec, 2000). Thus nocturnal ectotherms may requireretreat-sites that offer very specific thermal environ-ments.

Marbled geckos use rocks as retreat-sites. Thismakes them excellent candidates for a study of thethermal significance of retreat-site selection becauseanimals sequestered beneath rocks are easily accessed,

and because the thermal environment within theseretreat-sites is easily characterized (Huey et al., 1989b;Webb and Shine, 1998; Kearney and Predavec, 2000).

During spring and summer, I collected field data on theselectivity of marbled geckos to a number of retreat-sitecharacteristics that are known to influence thermal

conditions. I used these data, together with detailedmeasurements of the daily thermal regimes withindifferent types of retreat-site each season, to assess the

thermal consequences of patterns of retreat-site selec-tion exhibited by this lizard. The results indicate thatretreat-site selection can have significant effects on thethermal environment experienced by these lizards, and

that the nature of these effects can change dramaticallybetween seasons.

2. Methods

2.1. Study species and site

Marbled geckos are small (adult snout-vent length

E50mm, adult body mass E2.5 g), arboreal, insectivor-ous lizards with a geographic distribution encompassing

the southern seaboard of Australia. Physiologicallyrelevant body temperatures (Huey, 1982; Hertz et al.,

1993; Christian and Weavers, 1996; Hertz et al., 1999)for this species have already been determined inlaboratory studies; the preferred, or set-point range

(Tset) of this species is 23.6–261C (Kearney andPredavec; 2000), its voluntary thermal maximum(VTmax) is 311C (Kearney and Predavec, 2000), andits upper lethal temperature is 43.51C (Licht et al.,

1966).I studied a population at Mt Korong Scenic Reserve

in central Victoria (3612704200S, 14314501000E, average

altitude 250m). The habitat at the study site consists of a400 ha granitic outcrop that rises to form several smallknolls up to 423m in altitude. Vegetative cover is sparse,

consisting of an open overstorey dominated by Acaciabushes and a low, grassy understorey. Here, marbledgeckos occur at densities of around 150 ha�1 beneath

rocks, which are also plentiful and range in size fromfragments less than 50mm in diameter to boulders over5 m diameter. The climate of the region is temperate,with hot, dry summers and cool, wet winters. Mean

daily minimum and maximum air temperatures rangefrom 3.51C to 12.01C in July and 14.21C to 28.91C inJanuary (Anon., 1993).

2.2. Measurement of selected and available retreat-sites

Three properties of rocky retreat-sites that influencetheir thermal regime are diameter, thickness and degreeof shading (Huey et al., 1989b; Webb and Shine, 1998). I

collected data on these three variables for rocks at thesite with the use of haphazardly positioned 10� 10 m2

quadrats (see Kearney and Predavec, 2000). Within eachquadrat I searched all rocks for geckos and recorded the

diameter (ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffihorizontal length � width

p), maximum ver-

tical thickness and degree of shading for all rocksunder which geckos were found. The degree of shading

is difficult to estimate precisely because it varies with thetime of day, but I assigned rocks to three broadcategories: high (directly underneath vegetation or the

edges of boulders), medium (within a few meters fromsources of shade), or low (positioned in completelyopen areas). These same variables were also recorded

for a subset of ‘‘available’’ rocks in each quadrat thatintercepted a 7� 1 m2 transect, running from the centerof a quadrat to a randomly chosen corner, irrespectiveof whether a gecko was present. I also recorded the

maximum and minimum temperatures beneath eachrock by using an infrared thermometer (Model C-600 M,Linear Laboratories, Los Altos, California, USA, see

Kearney and Predavec, 2000). An advantage of usingthis thermometer was that I could rapidly scan the entirerock underside and substrate within 5 s of lifting the

rock to locate the point of maximum and minimumtemperature, which varied depending on the weather

M. Kearney / Journal of Thermal Biology 27 (2002) 205–218206

conditions, the time of day and the position of the rock(see ‘Lizardhenge’ results below). I rarely encountered

rocks that were too large to lift during the quadratsurveys although such rocks were present on the site.I excluded rocks from the data set that were flush with

the substrate (typically rocks on soil) or embedded:although they were searched, geckos were never foundbeneath these rocks, presumably because they cannotburrow under them. I also recorded the sex/age (male,

female, juvenile) of each gecko found. Marbled geckosare often found in aggregations within retreat-sites(Kearney et al., in press), and in such cases only one

randomly chosen individual was included in theanalyses.

Quadrat surveys were conducted from dawn to

dusk during spring (September and October 1997) andsummer (December 1997 and January 1998). In total, Isampled 112 quadrats in spring and 155 in summer, with

survey durations of 82 and 86.5 h, respectively.To analyze the pattern of retreat-site selection, I

performed a multiple logistic regression with ‘‘geckopresent vs. gecko absent’’ as the dependent variable. The

data was also re-analyzed with ‘‘aggregation present vs.solitary gecko present’’ for the spring data-set (whenmost aggregations were found). The independent vari-

ables were: (i) season; (ii) degree of shading; (iii) rockdiameter; (iv) maximum thickness. I converted the cate-gorical variables (season, degree of shading) to indicator

variables for the analysis, with ‘summer’ as the referencecategory for season and ‘intermediate’ as the referencecategory for degree of shading (see Hosmer andLemeshow, 1989).

To assess the influence of the independent variablesand their interactions, I employed a hierarchical gene-ralized linear model fitting strategy. This involved calcu-

lating the difference in log-likelihood (a measure of fit)between a model including the independent variable orinteraction term of interest and a model excluding it

(Tabachnick and Fidell, 1996). This difference wasmultiplied by two to produce a G statistic (Sokal andRohlf, 1995) which could then be compared to the Chi-

square distribution for the appropriate degrees offreedom and significance level (a ¼ 0:05). I assessed theinfluence of the interaction terms first, beginning withthe term of highest order and proceeding in a hier-

archical fashion through the lower interaction termsuntil the influence of the independent variables them-selves were assessed. All terms of equal or lower order to

the term being tested were included in the full andreduced models.

2.3. Pigment tracking

During the study I regularly conducted nocturnal

surveys for active geckos. To gain a more independentpicture of the retreat-sites used by geckos each season I

pigment tracked 27 active geckos to their retreat-sites(12 during spring and 15 during summer) (Butler and

Graham, 1993). This involved rubbing a small amountof non-toxic fluorescent pigment on the geckos’ bodies(excluding the head) and releasing them at the point of

capture. The trails of powder were followed to theretreat-sites at dusk the following day with the aid ofan UV lamp (Raytech, Middletown, CT).

2.4. Measurement of rock temperature cycles(Lizardhenge)

I performed a similar study to the ‘‘Snakehenge’’ ofHuey et al. (1989b) to examine the nature of daily

temperature cycles beneath rocks at Mt Korong andhow this was influenced by rock diameter, thickness anddegree of shading. Twenty-four rocks of representativediameter and thickness (Fig. 1) were randomly assigned

to a grid on an east-facing unshaded rocky slope (E101)of a small granite hill at the study site. A further eightrocks of different dimensions (Fig. 1) were arrayed

beneath an Acacia bush in deep shade 5m away. I shallrefer to these 32 rocks as ‘‘Lizardhenge’’.

On 11 October and 15 December 1998, I recorded

the maximum (Tmax) and minimum (Tmin) temperaturebeneath each rock at E11

2 h intervals from sunrise tosunset using the infrared thermometer as described

above. Weather conditions during the study were typicalfor the time of year on both days (based on Anon.,1993); cloud cover and wind speed was low, although thesummer day was of longer duration (spring: 14 h 20min;

0

5

10

15

20

25

30

35

40

45

10 20 30 40 50 60 70 80 90

Rock diameter (cm)

Max

imum

thic

knes

s (c

m)

Available

Lizardhenge Unshaded

Lizardhenge Shaded

Fig. 1. Distribution of the diameter and thickness of rocks

available to marbled geckos at Mount Korong, as measured

in the quadrat surveys, and of the 32 rocks used in the

‘Lizardhenge’ study. Twenty-four of the Lizardhenge rocks

were positioned in an unshaded, open area, and eight were

positioned in deep shade beneath an Acacia bush.

M. Kearney / Journal of Thermal Biology 27 (2002) 205–218 207

summer: 15 h 30 min) and had a higher maximumshaded air temperature (spring: 161C; summer: 291C).

I also used portable Tiny Tag temperature dataloggers (Gemini Data Loggers Ltd., West Sussex,England) to simultaneously record the shaded air

temperature at ground level, and the temperature withina 20 cm deep vertical crevice in the bedrock, at 10-minintervals.

For each season and shade category, I used multiple

linear regressions to analyze the effects of rock diameterand thickness on the following dependent variables: (i)time within Tset; (ii) time above VTmax; (iii) highest Tmax;(iv) highest Tmin:

3. Results

3.1. Presence/absence of geckos

The hierarchical model fitting strategy revealed asignificant influence of season, rock diameter and degreeof shading on the presence of marbled geckos (all

P 0so0:0001). Since season was involved in two sig-nificant interactions (season� shading, Po0:0001; sea-son� thickness� shading, Po0:001), I analyzed the

spring and summer data separately.

3.1.1. Spring analysis

During spring, the diameter and the degree ofshading of a rock had a significant influence on thechance of a gecko occurring beneath it (Table 1a). The

effect of rock diameter also appeared to depend on rockthickness but, since this interaction was not detected inthe overall analysis (P ¼ 0:054), I consider it to be aType I error. The odds ratios (Table 1b) indicate the

nature of these influences. First, for a 1 cm increase inrock diameter, the chance of a gecko occurring beneatha rock increased by 2.2%. Second, with respect to a

medium-shade rock (the reference category) the chanceof a gecko occurring beneath a low-shade rockdecreased by 43.9%, and the chance of a gecko

occurring beneath a high-shade rock decreased by64.9% (see also Table 3). That is, during spring thegeckos were more likely to be under large rocks withmedium shade.

3.1.2. Summer analysisSimilarly to spring, the diameter and degree of

shading of a rock had a significant influence on thechance of a gecko occurring beneath it during summer(Table 1a). The odds ratios (Table 1b) indicate that first,

for a 1 cm increase in rock diameter, the chance of agecko occurring beneath it increased by 4.2%. Second,compared to medium-shade rocks, the chance of a gecko

being present beneath low-shade rocks decreased by83.5% whereas the chance of high-shade rocks harbor-

ing a gecko increased by 181.4% (see also Table 3). Thatis geckos were more likely to be under large, highly

shaded rocks during summer. There was a significantinteraction between degree of shading and rock thick-ness (Table 1a) whereby geckos were more likely to be

present under low-shade rocks in summer if the rockswere also thick.

3.1.3. Presence of aggregationsThe presence of aggregations of marbled geckos

(in spring) was significantly related to rock diameter(Table 2a). The odds ratios indicate that aggregationswere more likely to occur beneath large rocks (Table

2b); for a 1 cm increase in rock diameter, the chance ofan aggregation being present increased by 2.3%.

3.2. Intraspecific and seasonal comparisons of selected

retreat-sites

Two-way ANOVAs, with sex/age and season asindependent variables, provided no evidence for sex/

age-specific differences in the diameter or thickness ofrocks used by marbled geckos (diameter: F2;168 ¼ 0:557;P ¼ 0:547; thickness: F2;168 ¼ 1:645; P ¼ 0:196), nor for

any interactions between these variables (all P 0s > 0:05).There was an overall effect of season on the rockdimensions selected (diameter: F2;168 ¼ 8:335; P ¼ 0:004;thickness: F2;168 ¼ 4:499; P ¼ 0:035) whereby geckoswere using larger and thicker rocks during summer thanduring spring (Table 3).

In both seasons, contingency table analyses indicateda significant relationship between gecko sex/age and thedegree that retreat-sites were shaded. In these analyses,small sample sizes for high-shade rocks in spring,

and for low-shade rocks in summer, required meto pool each of these categories with the nextclosest category (i.e. medium-shade rocks) for the

respective comparisons. In spring, more juvenile geckoswere beneath medium/high-shaded rocks than expec-ted, and more adult male geckos were beneath low-

shade rocks than expected (w2 ¼ 6:060; df ¼ 2; P ¼0:048; Table 3). In summer, more adult male geckoswere beneath high-shade rocks than expected, and more

adult female geckos were under low/medium-shaderocks than expected (w2 ¼ 7:050; df ¼ 2; P ¼ 0:029;Table 3).

Pigment tracking of geckos indicated that significantly

fewer geckos used rocks as retreat-sites in summercompared with spring (w2 ¼ 11:402; df ¼ 1; Po0:001).In spring, of 12 marbled geckos tracked, nine used

unshaded rocks and two used crevices in the bedrock. Insummer, of 15 marbled geckos tracked 11 used crevices,and one each were using a shaded rock, an unshaded

rock, a (human-made) rock pile and a cavity inside asmall log.


3.3. Instantaneous temperatures beneath rocks

In spring, instantaneous measurements of maximumtemperature beneath rocks available to geckos wereunrelated to rock diameter or thickness irrespective of

degree of shading (multiple linear regression (MLR)analyses; all P 0s > 0:05). During summer, however, rockdiameter exerted a significant negative influence over

maximum temperature for low-shade rocks (MLR:

ANOVA, F2;167 ¼ 3:769; P ¼ 0:025; regression, t ¼�1:995; P ¼ 0:048) whereas for high-shade rocks,diameter had a significant positive influence andthickness had a significant negative influence (MLR:ANOVA, F2;167¼7:064; P ¼ 0:003; regression (diameter),

t ¼ 3:143; P ¼ 0:003; regression (thickness), t ¼ �2:151;P ¼ 0:038).

Minimum temperature during spring was negatively

related to rock diameter for medium-shade rocks only

Table 1

Results of a multiple logistic regression analysis of the effect of rock diameter, maximum rock thickness and degree of shading (high,

medium, low) on the presence/absence of marbled geckos beneath rocks during spring and summera

Term excluded Log likelihood

Full model Reduced model Chi-square df P

(a) Results of a generalized linear model fitting strategy used to assess the influence of the independent variables and their interactions on

gecko presence/absenceb

Spring

Diameter �217.70 �222.90 10.41 1 0.001

Thickness �217.70 �218.02 0.63 1 0.426

Shading �217.70 �221.73 8.05 2 0.018

Diameter� thickness �215.19 �217.48 4.57 1 0.032

Diameter� shading �215.19 �215.28 0.17 2 0.920

Thickness� shading �215.19 �215.27 0.16 2 0.925

Diameter� thickness� shading �213.98 �215.19 2.43 2 0.297

Summer

Diameter �85.33 �91.01 11.36 1 0.001

Thickness �85.33 �85.54 0.41 1 0.523

Shading �85.33 �104.19 37.71 2 o0.001





Variable Estimate SE Odds ratio 95 CI (upper) 95 CI (lower)

(b) Maximum likelihood estimates of parameters, standard errors of parameter estimates, odds ratios and 95 confidence intervals of the

odds ratios for main effects

Spring

Constant �1.071 0.32 F F FRock diameter 0.022 0.01 1.022 1.036 1.008

Rock thickness 0.016 0.02 1.016 1.056 0.977

Shade (low) �0.578 0.24 0.561 0.902 0.349

Shade (high) �1.046 0.50 0.351 0.929 0.133

Summer

Constant �3.210 0.682 F F FDiameter 0.041 0.012 1.042 1.067 1.017

Thickness �0.019 0.030 0.982 1.042 0.925

Shade (low) �1.803 0.580 0.165 0.514 0.053

Shade (high) 1.035 0.452 2.814 6.821 1.161

aReference coding was used for the categorical variable ‘degree of shading’ (reference category was ‘medium-shade’).bLog likelihoods were generated by fitting a series of logistic regression models to the data (see methods and materials).


(MLR: ANOVA, F2;566:174; P ¼ 0:004; regression, t ¼�3:471; P ¼ 0:001), and was unrelated to rock thick-

ness for any degree of shading (MLR analyses; all

P0s > 0:05). During summer, minimum temperature wasnegatively related to rock diameter for low-shade rocks

(MLR: ANOVA, F2;167 ¼ 7:363; P ¼ 0:001; regression,

Table 3

Summary statistics of the diameter, thickness and degree of shading available to, and selected by, marbled geckos during spring and

summer

Rock type Diameter (cm) Thickness (cm) Degree of shading [frequency, (proportion)]

Mean7SE Mean7SELow Medium High N

Available 35.370.7 8.27 0.3 308 (0.61) 126 (0.25) 67 (0.13) 501

Selected

Spring

Overall 41.271.7 8.570.6 66 (0.50) 60 (0.45) 7 (0.05) 133

Male 45.673.0 8.170.8 32 (0.60) 21 (0.40) 0 (0) 53

Female 39.173.1 9.871.1 18 (0.51) 16 (0.46) 1 (0.03) 35

Juvenile 37.972.3 8.171.0 16 (0.36) 23 (0.51) 6 (0.13) 45

Aggregations 48.772.9 7.671.0 20 (0.49) 19 (0.46) 2 (0.05) 41

Solitary geckos 40.571.6 8.170.5 74 (0.55) 54 (0.40) 7 (0.05) 135

Summer

Overall 49.772.7 10.571.0 5 (0.12) 13 (0.31) 24 (0.57) 42

Male 45.073.3 8.371.7 0 (0) 2 (0.14) 12 (0.86) 14

Female 55.074.9 11.071.5 3 (0.18) 7 (0.41) 7 (0.41) 17

Juvenile 47.375.7 12.671.9 2 (0.18) 4 (0.36) 5 (0.45) 11

Table 2

Results of a multiple logistic regression analysis of the effect of rock diameter, maximum rock thickness and degree of shading (low,

medium, high) on the presence/absence of aggregations of marbled geckos beneath rocks during springa

Term excluded Log likelihood

Full model Reduced model Chi-square df P

(a) Results of a generalized linear model fitting strategy used to assess the influence of the independent variables and their interactions on

gecko presence/absenceb

Diameter �91.852 �94.921 6.138 1 0.013

Thickness �91.852 �92.054 0.404 1 0.525

Shading �91.852 �92.907 2.11 2 0.348





Variable Estimate SE Odds ratio 95 CI (upper) 95 CI (lower)

(b) Maximum likelihood estimates of parameters, standard errors of parameter estimates, odds ratios and 95 confidence intervals of the

odds ratios for main effects

Constant �1.910 0.531 F F FRock diameter 0.023 0.009 1.023 1.042 1.005

Rock thickness �0.020 0.032 0.980 1.043 0.921

Shade (low) �0.238 0.376 0.788 1.648 0.377

Shade (medium) �0.380 0.865 0.684 3.728 0.125

aReference coding was used for the categorical variable ‘degree of shading’ (reference category was ‘medium- shade’).bLog likelihoods were generated by fitting a series of logistic regression models to the data (see methods and materials).


t ¼ �3:351; P ¼ 0:001) and negatively related to rockthickness for high-shade rocks (MLR: ANOVA, F2;37 ¼4:634; P ¼ 0:016; regression, t ¼ �2:360; P ¼ 0:001).

Two-factor ANOVAs indicated that the instanta-neous maximum (Tmax) and minimum (Tmin) tempera-

tures beneath rocks were both significantly influenced bythe degree of shading of the rock and by season, withshaded rocks being cooler than unshaded rocks, androcks being hotter in summer than in spring (Tmax:

(shade) F2;440 ¼ 13:8; Po0:0001; (season) F2;440 ¼ 204:6;Po0:0001; Tmin: (shade) F2;440 ¼ 16:8; Po0:0001;(season) F2;440 ¼ 385:0; Po0:0001). However, there were

significant interactions between the degree of shadingand season (Tmax: F2;440 ¼ 7:7; Po0:001; Tmin: F2;440 ¼10:5; Po0:0001) whereby the effects of shading were

significantly diminished during spring (Fig. 2).

3.4. Daily thermal cycles beneath rocks (Lizardhenge)

3.4.1. General patterns

Measurements taken with the infra-red thermometerprovide a detailed picture of the thermal conditionsbeneath the rocks of Lizardhenge. In both seasons, daily

thermal cycles beneath unshaded rocks displayed asimilar pattern (Figs. 3a and b). At sunrise, a smallthermal gradient was present beneath the rocks with

the minimum temperature (Tmin) located on the rockunderside and the maximum temperature (Tmax) locatedon the rock substrate. As solar radiation heated the

rocks, the location of Tmax shifted to the rock undersideand the location of Tmin shifted to the rock substrate.Furthermore, overall temperature gradients, and gradi-ents across both rock underside and rock substrate,

widened (up to 24.91C), with the position of the highesttemperature tracking the sun throughout the day. Rock

temperatures typically peaked in the mid- to lateafternoon, with the largest, thickest rocks peaking latest;Tmin often peaked a few hours after Tmax: As sunset

approached and solar radiation loads dropped, rocksbegan to cool down and thermal gradients diminished.The outer perimeters of rock undersides began to coolfirst and it was often the case that both Tmax and Tmin

were located on the rock underside at this time.Eventually, the rock substrate became the location ofTmax but, again, this occurred latest with the largest,

thickest rocks.The pattern of thermal cycles beneath shaded rocks

each season was similar to that of unshaded rocks but

thermal gradients and cycle amplitudes were diminished,and rock temperatures closely tracked shaded airtemperature (Fig. 3c; Table 4). Temperatures within

the unshaded 20 cm crevice also showed a diminisheddaily cycle that was slightly out of phase in comparisonto the exposed rocks, and which approximately trackedthe minimum temperature beneath the thicker exposed

rocks (Figs. 3a and b).

3.4.2. Seasonal patterns

Temperatures beneath unshaded rocks in summer farexceeded those in spring (Figs. 3a and b; Table 4); themean highest Tmax in summer was 18.11C greater than in

spring (paired t-test; n ¼ 24; t ¼ 23:402; Po0:0001;Table 4) and the mean highest Tmin in summer was14.21C greater than in spring (paired t-test; n ¼ 24;t ¼ 51:845; Po0:0001; Table 4). The magnitude ofthermal gradients beneath rocks also differed betweenseason, with the mean largest gradient in summer 4.51Cgreater than in spring (paired t-test; n ¼ 24; t ¼ 6:549;Po0:0001; Table 4).

Seasonal differences were also present beneath shadedrocks; the mean highest Tmax in summer was 10.11C

greater than in spring (paired t-test; n ¼ 8; t ¼ 10:091;Po0:0001; Table 4) and the mean highest Tmin insummer was 8.31C greater than in spring (paired t-test;

n ¼ 7; t ¼ 12:735; Po0:0001; Table 4). The magnitudeof thermal gradients beneath rocks also differed betweenseason (Table 4), with the mean highest gradient insummer 4.51C greater than in spring (paired t-test;

n ¼ 24; t ¼ 6:549; Po0:0001). Mean temperature in theunshaded 20 cm crevice was 7.61C hotter during summerthan in spring (Figs. 3a and b).

3.4.3. Influence of rock diameter and thicknessMultiple linear regression analyses, with rock dia-

meter and rock thickness as independent variables,explained a significant amount of the variation in thehighest Tmax; highest Tmin; and time spent at or above

gecko thermal preferences beneath unshaded rocks eachseason (Table 5; Fig. 4). Specifically, in spring, highest

0

10

20

30

40

50

Low Med High

Shade

Tem

per

atu

re (

deg

. C)

Fig. 2. Interaction plots of the effects of degree of shading on

maximum (closed symbols) and minimum (open symbols)

temperatures beneath rocks available to marbled geckos during

spring (dashed lines) and summer (solid lines).


0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18 20 22 0

Hour of day

Tem

per

atu

re (

deg

. C)

Lethal

VTmax

T set

T max 4 cm thickT min 4 cm thick


Crevice 20 cm deep

0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18 20 22 0

Hour of day

Tem

per

atu

re (

deg

. C)

Lethal

VTmax

T set



Crevice 20 cm deep

0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18 20 22 0

Hour of day

Tem

per

atu

re (

deg

. C)

T max springT min springAir 1 cm spring

T max summerT min summerAir 1 cm summer

Lethal

VT max

T set

(a)

(b)

(c)

Fig. 3. Representative daily rock temperature cycles beneath a thick and a thin rock, and within a 20 cm crevice, during (a) spring and

(b) summer, and (c) beneath a rock in deep shade during spring and summer. Concurrently measured air temperatures at 1 cm are also

indicated for shaded rocks in spring and summer (c). Superimposed on the figures are the bounds of the set-point range (Tset; thin,

dashed horizontal lines), the voluntary thermal maximum (VTmax; thin solid horizontal line) and the lethal limit (thick, dashed

horizontal line) of the marbled gecko. Vertical lines represent the times of local sunrise and sunset.


Tmax; highest Tmin; and the amount time spent withinTset were all negatively related to rock thickness

(Table 5b, Figs. 4a–c). During summer, highest Tmax;and highest Tmin were negatively related to rockthickness (Table 5b, Figs. 4a–b), while highest Tmax

was positively related to rock diameter (Table 5b,

Fig. 4e). In addition, during summer the amount oftime spent above VTmax was also relevant because atsome stage Tmin of all rocks rose above this threshold

and Tmin of one rock approached the lethal limit(Table 4). Only rock thickness had a significant(negative) influence on the time spent above VTmax

(Table 5b, Fig. 4d).The same analyses for the shaded rocks indicate that

rock diameter and rock thickness had no influence onthe highest Tmax; highest Tmin; and time spent at or

above gecko thermal preferences (multiple linearregression, all P0s > 0:05). Moreover, during spring onlyone shaded rock spent time within Tset (circa 30 min)

and, during summer, no shaded rocks spent time aboveVTmax:

5. Discussion

The results of this study clearly demonstrate that boththe pattern of retreat-site selection exhibited by marbledgeckos, and the thermal environment within potential

retreat-sites, were related to the physical attributes ofretreat-sites. In particular, geckos were selective of thediameter and the degree of shading of the rocks they

used as retreat-sites (Table 1), and the spatial andtemporal variation in the thermal environment beneathrocks was significantly affected by these rock attributes

(Figs. 2–4). This further supports the notion that thelocation of an animal in its habitat during periods

of inactivity can have important physiological conse-quences (Christian et al., 1984, Huey et al., 1989b,

Huey, 1991). The existence of significant variationin temperature within and between retreat-sites alsomeans that nocturnal ectotherms could potentiallythermoregulate during their ‘inactive’ period by

retreat-site selection and by adjusting their positionand/or posture within a retreat-site, and marbled geckoshave been demonstrated to use both mechanisms

(Kearney and Predavec, 2000, Kearney, 2001). To whatextent, however, was the marbled gecko’s pattern ofretreat-site selection driven by thermoregulatory

requirements?The seasonal patterns of retreat-site selection provide

strong evidence for temperature as a causal factor. Thethermal environment within rocky retreat-sites at Mt

Korong changes seasonally in important ways from theperspective of a marbled gecko. Kearney and Predavec(2000) found that, during spring, a gecko positioning

itself randomly within randomly selected retreat-siteswould deviate on average around 71C below its set-pointrange (Tset) and would almost never exceed its volun-

tary thermal maximum (VTmax). In contrast, a geckobehaving similarly in summer would deviate around 71Cabove its Tset; 3.41C above its VTmax; and may even

exceed its lethal limit (see also Fig. 3). Since bodytemperatures above Tset are more deleterious than bodytemperatures of equal magnitude below Tset (DeWittand Friedman, 1979; Huey, 1982; Huey and Kingsolver,

1993), one would expect geckos in spring to chooseretreat-sites that peak at high temperatures and thatspend maximal time within Tset; and in summer to

choose retreat-sites that peak at low temperatures andthat spend minimal time above VTmax:

This is consistent with the observed selective behavior

of marbled geckos with respect to the degree of shading

Table 4

Summary statistics of maximum (Tmax) and minimum (Tmin) temperatures as well as the maximum temperature gradients (Gmax)

beneath shaded and unshaded rocks during spring and summer

Shading Season Variable Mean 7 SE (1C) Range (1C)

Spring Shaded Tmax 20.670.770 17.0–24.5

Tmin 15.870.340 14.5–17.0

Gmax 5.070.639 2.6–8.1

Unshaded Tmax 32.070.383 27.0–34.0

Tmin 23.670.323 21.5–27.5

Gmax 10.470.502 6.6–17.8

Summer Shaded Tmax 30.771.264 26.5–36.0

Tmin 24.170.538 22.5–27.0

Gmax 7.471.040 2.6–10.9

Unshaded Tmax 50.170.921 42.0–59.0

Tmin 37.870.365 35.5–42.0

Gmax 14.970.765 10.5–24.9


of their retreat-sites. During spring the geckos avoidedhigh-shade rocks, which were cooler than low- andmedium-shade rocks, and which spent minimal time

within Tset (Tables 1, 3 and 4, Figs. 2 and 3). Conversely,during summer high-shade rocks were frequently used,despite their low availability (Table 3), and these rocks

spent minimal time above VTmax (Fig. 3, Table 4).Additionally, in comparison with spring the number ofgeckos found beneath rocks per quadrat during summer

was reduced by 74%, corresponding with an 82%reduction in number of pigment-tracked geckos foundto use rocks in the summer, when most geckos weretracked to crevices in the bedrock. Seasonal shifts in the

qualitative nature of selected microhabitats by reptiles

have previously been related to temperature effects(Christian et al., 1983; Christian et al., 1984; Paulissen,1988; Webb and Shine, 1998) and this may also be the

case for the marbled gecko since crevices are likely to beconsiderably cooler than surface rocks (Figs. 3a and b,see also Webb and Shine, 1998), and high-shade rocks

were relatively rare at the study site (Table 3).These seasonal patterns are supportive of tempera-

ture as a causal factor in determining retreat-site

selection by marbled geckos, whereby the thermalenvironment within spring retreat-sites (low- andmedium-shade rocks) moves out of the climate space(Porter and Gates, 1969) of marbled geckos in summer,

whereas the thermal environment within summer

Table 5

Summary statistics of multiple linear regression analysis of the effect of rock diameter and thickness on the highest maximum (Tmax)

and highest minimum (Tmin) temperatures recorded beneath rocks as well as the time spent above the set-point range (Tset) and

voluntary thermal maximum (VTmax) of the marbled gecko during spring and summer for the rocks of Lizardhengea

Season DV Source df MS F-ratio P R2

(a) Results of analyses of variance

Spring Highest Tmax Regression 2 0.019 8.91 0.002 0.677

Residual 21 0.002

Highest Tmin Regression 2 0.024 10.319 0.001 0.704

Residual 21 0.002

Time Tset Regression 2 0.209 18.469 o0.001 0.638

Residual 21 0.011

Summer Highest Tmax Regression 2 0.069 30.129 o0.001 0.742

Residual 21 0.002

Highest Tmin Regression 2 0.012 9.726 o0.001 0.481

Residual 21 0.001

Time Tset Regression 2 0.062 4.668 0.021 0.308

Residual 21 0.013

Time above VTmax Residual 2 0.142 37.216 o0.001 0.805

Regression 18 0.004

Season DV IV Coeff. SE t P

(b) Results of analyses of regression slopes

Spring Highest Tmax Diameter 0.001 0.024 0.047 0.963

Thickness �0.065 0.016 �4.125 o0.001

Highest Tmin Diameter �0.032 0.025 �1.294 0.210

Thickness �0.065 0.017 �3.958 0.001

Time Tset Diameter 0.000 0.000 1.632 0.118

Thickness �0.002 0.000 �6.021 o0.001

Summer Highest Tmax Diameter 0.086 0.025 3.458 0.002

Thickness �0.123 0.016 �7.544 o0.001

Highest Tmin Diameter 0.010 0.018 0.555 0.585

Thickness �0.052 0.012 �4.389 o0.001

Time Tset Diameter 0.004 0.002 2.359 0.028

Thickness 0.007 0.004 1.604 0.124

Time above VTmax Diameter �0.002 0.001 �1.932 0.069

Thickness �0.022 0.003 �7.717 o0.001

aAll variables were natural log-transformed.


retreat-sites (high-shade rocks and deep crevices) movestoo far below Tset during the cooler months for them toremain there (Fig. 3). Although this conclusion is yet tobe tested experimentally, it is especially convincing if one

considers that the absence of such behaviors wouldlikely result in death from overheating during summer.However, it was notable that marbled geckos were

largely impartial to rock thickness (Table 1), despite thestrong influence this variable had on rock temperature inthis study (Table 5b, Figs. 3 and 4) and in other

theoretical (Huey et al., 1989b) and empirical studies(Huey et al., 1989b; Webb and Shine, 1998). In contrast,marbled geckos were highly responsive to rock diameter,

favoring larger rocks in both seasons (Table 1), despitethe relatively limited effects this variable had on rocktemperature (Table 5b, Fig. 4).

The patterns of selection with respect to rock diameter

and thickness may be explained in part by constraints

on the effectiveness of the thermoregulatory behaviorsavailable to nocturnal ectotherms. First, in this study Ihave assumed that all rocks have sufficient spacebeneath them to allow a gecko to behaviorally regulate

body temperature anywhere within the gradient oftemperature beneath the rock at any one instant,whereas in reality rocks may vary in the amount of free

space beneath them. Larger rocks tend to provide largerand more variable crevices beneath them (personalobservation) and thus may enhance the potential for

geckos exploit thermal gradients by adjustments of theirposition and posture (Dial, 1978; Kearney andPredavec, 2000; Kearney, 2001).

Second, although the thermal effects of rock thicknessin spring were statistically significant, the resultantthermal variation between rocky retreat-sites may havebeen too small, compared to that within retreat-sites, to

make retreat-site selection an effective thermoregulatory

(a)

20

25

30

35

40

45

50

55

60

65

0 5 10 15 20 25

Maximum thickness (cm)

Hig

hest

Tm

ax (

deg.

C)

15

20

25

30

35

40

45

0 5 10 15 20 25


Hig

hest

Tm

in (

deg.

C)

(b)

2

3

4

5

6

7

8

9

0 10


Tim

e ab

ove

VT

max

(h)

(d)

20


2

3

4

5

6

7

8

9

Tim

e w

ithin

Tse

t (h)

0 5 10 15 20 25

(c)

Fig. 4. Relationships of rock thickness and rock diameter with the highest maximum (Tmax; a and e) and highest minimum (Tmin; b and

f) temperatures recorded beneath the unshaded rocks of ‘Lizard Henge’, as well as with the time within the set-point range (Tset; c and

g) and time above the voluntary thermal maximum (VTmax; d and h) of the marbled gecko during spring (crosses) and summer (open

circles). Slopes were fitted by OLS. (No rocks spent time above the voluntary thermal maximum during spring.)


behavior. Indeed, during spring marbled geckos arerelatively ineffective thermoregulators, and there isevidence that positional/postural adjustments within

retreat-sites are more significant thermoregulatorybehaviors in this season than retreat-site selection(Kearney and Predavec, 2000). In addition, althoughthe thermal effects of rock thickness were stronger

during summer, the rocks available to geckos may nothave been thick enough to provide protection from hightemperatures during this season in the absence of shade.

For instance, even 22 cm thick rocks can spendsignificant amounts of time above the marbled gecko’sVTmax during summer when unshaded (Fig. 3b), and

only 4.1% of the rocks at the study site are this thicknessor greater (Fig. 1).

Of course, the rock variables I measured may have

other implications for geckos besides thermal ones.First, marbled geckos often occur in aggregations, whichmay be of social significance (Kearney et al., in press),and large rocks were indeed more likely to harbor

aggregations (Table 2). Thus, by virtue of the increased

area beneath them, large rocks may provide socialadvantages. Alternatively, such aggregations may haveformed beneath large rocks because of the thermal or

other benefits that such microhabitats confer. Alter-native hypotheses for the significance of rock diameterthat are unrelated to temperature must also account forwhy geckos used larger rocks in summer than in spring

(Table 3).Second, the proximity of retreat-sites to foraging sites

may be correlated with the degree that a rock is shaded;

marbled geckos predominantly forage in trees andbushes at the study site (personal observation) and, sincevegetation was the main source of shade, this could

explain the tendency for geckos to occur beneathmedium-shade rocks in spring (Tables 1 and 3). In thisrespect it is notable that temperatures beneath low- and

medium-shade rocks available to the geckos were verysimilar during this season (Fig. 2). Intraspecific differ-ences in degree of shading selected by marbled geckoswere also apparent; female and juvenile geckos were

more likely to be under medium- and high-shade rocks

2

3

4

5

6

7

8

9

10 20 30 40 50 60 70 80

Rock Diameter (cm)

Tim

e ab

ove

VT

max

(h)

(h)

20

25

30

35

40

45

50

55

60

65

10 20 30 40 50 60 70 80

Rock Diameter (cm)

Hig

hest

Tm

ax (

deg

C)

15

20

25

30

35

40

45

10 20 30 40 50 60 70 80

Rock Diameter (cm)

Hig

hest

Tm

in (

deg.

C)

(e) (f)

2

3

4

5

6

7

8

9

10 20 30 40 50 60 70 80

Rock Diameter (cm)

Tim

e w

ithin

Tse

t (h)

(g)

Fig. 4. (continued)


than males in spring, while the reverse was true insummer (Table 3). This pattern is unlikely to be directly

related to temperature because there is no evidence forintraspecific differences in the thermal preferences of thisspecies (Kearney and Predavec, 2000), but may relate to

nesting behavior since egg-laying occurs in spring andnests are frequently beneath shaded rocks (personalobservation).

Third, geckos beneath rocks may be vulnerable to

predators such as snakes (personal observation,Downes and Shine, 1998b) and centipedes (Kearneyand Downes, 1998), and rock characteristics such

as diameter may influence the relative safety of a geckobeneath it. If factors such as these conflict withthermoregulatory requirements, the degree to which

retreat-site selection is used as a thermoregulatory beha-vior may be restricted (Downes and Shine, 1998a).

For seasonal shifts in retreat-site selection to occur in

marbled geckos, they must use certain cues for both thetime to shift and the types of retreat-sites that aresuitable (Levins, 1968), and this deserves further study.Cues for the seasonal shift would need to be highly

reliable and perhaps also conservative, because thetiming of the onset of hot weather can change markedlyfrom year to year (Webb and Shine, 1998), and because

an incorrect choice of retreat-site by a gecko at nightmay not be easily corrected during the day. Cues as tothe thermal suitability of different retreat-sites may more

likely relate to structural features rather than instanta-neous temperatures, which are often poor indicatorsof general thermal conditions (Heatwole, 1977) andwhich have largely converged by the latter stages of the

night, when geckos presumably select their diurnalretreat-sites (personal observation, see also Fig. 3). Thenature of such cues is currently unknown but may be

revealed by simple laboratory or field experiments(e.g. Schlesinger and Shine, 1994; Downes and Shine,1998a).

In summary, results of this study highlight thepotential consequences of retreat-site selection forthermal physiology (Huey, 1991). They also strongly

suggest that environmental conditions played an im-portant role in affecting patterns of retreat-site selectionof the marbled gecko. The most striking result in thisrespect was the seasonal shift in the qualitative nature of

the retreat-sites from rocks in spring to crevices insummer, which was apparently the result of extremelyhigh (>401C) temperatures beneath rocks in summer.

However, if geckos remained in summer retreat-sitesduring the cooler months they would spend minimaltime within Tset; which could have significant conse-

quences for their physiological capacity (Huey et al.,1989b; Autumn and De Nardo, 1995). A wide variety ofdiurnal and nocturnal ectotherms living in temperate

environments use rocks as retreat-sites (e.g. Bayly,1999). Such seasonal shifts in microhabitat selection

may be widespread in these organisms (e.g., Webb andShine, 1998), especially considering that few ectotherms

can tolerate body temperatures above 401C for very long(Licht et al., 1966; Spellerberg, 1972) and that suchtemperatures commonly occur beneath rocks in summer

(Huey et al., 1989b; Webb and Shine, 1998). Clearly, thephysiological and ecological consequences of retreat-siteselection for ectotherms are potentially profound anddeserve more attention that they have received (Huey,

1991).

Acknowledgements

I thank Dan Harley, Michael MacLean, Nicole

Kearney, Murray Logan, Pete Luckock and JamesSmith for help with field work. Thanks also to SharonDownes, Allen Greer, Martin Predavec, Ian Stewart,

Mike Thompson, Pat Whitaker and Warren Porterfor discussion and practical advice. Ralph MacNallyand Gerry Quinn provided statistical advice and RayHuey, Dave O’Connor and Sohan Shetty made helpful

criticisms of the manuscript. Financial support wasgenerously provided by Australian Geographic P/L, anEthel Mary Read research grant from the Royal

Zoological Society of New South Wales, and a researchgrant from the Peter Rankin Trust Fund for Herpe-tology. The Department of Mechanical Engineering

(Monash University, Clayton) allowed me the use of aninfra-red thermometer. This project was approved bythe Animal Care and Ethics Committee at MonashUniversity, and conducted under Research Permit RP-

97-097 issued by the Department of Environment andNatural Resources, Victoria, Australia.

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Documents

Hot rocks and much-too-hot rocks: seasonal patterns of retreat-site selection by a nocturnal ectotherm