Asiatic Herpetological Research

2001

Vol. 9, pp. 25-29

Influence of Temperature on Burrow Use by the Monitor Lizard Varanus

panoptes o\ the Coastal Dunes at Fog Bay, Northern Australia
Sean J. Blamires
Morning Bell Language School, 671-3 Jukdo 2 Dong, Pohang City, Kyongsangbuk Do 791-052, South Korea.
email: s_blamires@hotmail.com

Abstract.- An increase in the number of Varanus panoptes burrows appearing among the sand dunes at Fog Bayinnorthern Australia was noticed during the dry season (June to August). Entrances of marked burrows weresmoothed, and their interiors investigated using a burrowscope, to determine the monthly number of foraging andretreat burrows appearing over a 12 month period. In the dry season, increased numbers of both types of burrows

were found. A temperature data logger was used to record temperatures of the sand surface, and at a depth of 50
cm. Burrow depths were measured as how far the burrowscope could be lowered into each burrow. There was apositive correlation between the number of retreat burrows and burrow depths. There was a negative correlation

between the number and depth of retreat burrows and the minimum sand-surface temperature and the maximum
sand temperature at 50 cm. The number of foraging burrows was independent of sand temperatures. The results
indicate a likely thermoregulatory use of retreat burrows with more and deeper burrows prevalent when overnightsurface and subterranean sand temperatures decrease.

Key words.- Reptilia, Varanidae, Varanus panoptes, Australia, Northern Territory, fossorial, thermoregulation

Introduction
Burrows serve many ecologically important purposes
to many animals (Hansell, 1993). Lizards expend lessenergy throughout the day if a large portion of time isspent in burrows (Bennett and Nagy, 1977). Monitorlizards (Varanus spp.) utilise burrows for a variety ofpurposes
Bogert,suchasthermoregulation(Cowles1944), reducing water loss (Green,and1972),

finding prey (Pianka, 1969), and for oviposition andretreat (Cowles and Bogert, 1944; Auffenberg, 1983).

In a recent study of themonitor lizard Varanus

panoptes (Fig. ) inhabiting the coastal dunes at Fog
Northern
Australia
Bay,(12°42'S;
Territory,1130°20'E), I detected a seasonal change in the number of burrows present. A greater number of burrows

appeared among the dunes in June-August (dry season) compared to the rest of the year. These burrows

Figure 1. Varanus panoptes'm the coastal dunes at

Fog Bay, Northern Australia.

also seemed quite deep compared to those seen in theature, as has

wet season.

Use of these burrows for oviposition is unlikely
since Varanus panoptes produces eggs during the wetseason. Free access to other water sources might ruleout conservation of water loss. Overnight temperatures in the area often drop below 15°C in the dry sea-son.

A likely explanation may be that the monitors

retreat intoburrows at a depth where sand tempera-

tures remain high to conserve overnight body temper-

been found for some other monitors(Cowles and Bogert, 1944; Auffenberg, 1983). The
monitors at Fog Bay inhabit the dunes and forage on
the beach and dunes (Blamires, 1999). Increased for-aging activities may be another explanation for thenumber of burrows seen in the dry season. The aim ofwas to determine whether changes
number of burrows between the wet and dry

this investigationin theAsiatic Herpetological ResearchVol. 9, p. 26

seasonsis2001a result of foraging, thermoregulation,

retreat or a combination of these, or other, influences.

y-(Material and Methods'The area of the investigation is approximately 5 km of~/'gpj

beach along the northern-most mainland beach at Fog

I

Bay (12°42'S, 130°20'E), approximately 80 km from
Darwin, Northern Territory, Australia. The area is privately owned and closed to the public. The sandy*%dunes are immediately backed by grassland dominated by spinifex vegetation and dispersed Pandanustrees.

Black soil plains, mangroves and salt flats backthe grassland. The dunes of the southern-moststretch of beach are

1kmbacked by monsoon forest. The

entire 5 km of beach, and the grassland and monsoon
forest backing the dunes, was walked twice

monthlyover a 12 month period.
Allburrows encountered were marked with adepth of surveyor's tape tied

tonearby vegetation

Figure 2. Burrow of Varanus panoptes.

below the surface. A digital temperature reading wastaken by the logger every 30 minutes. The data weredownloaded and compiled at the end of every month.
Correlation analysis was done between the number of foraging and retreat burrows counted eachmonth and the mean burrow depth. Correlation analy-

was also done between the number of foraging
retreat burrows and burrow depths and(Fig. 2). To estimate visitation rates the entranceswere smoothed and checked, on revisiting, for varanid

sistracks leading into it.

monthly maximum and minimum sand temperature at
the surface and at 50 cm depth.

A small video-camera device, called a "burrowscope" (Dyer and Hill, 1991; Dyer and Aldworth,
1998) was used to investigate the inside of burrows.

burrows,Results

The model I used was a modified version of that ofOverall 93 burrows were identified as either retreat orDyer and Aldworth, (1998). A small black and white
CCD, 38mm x 38mm, camera (Samsung MOD-BW

foraging burrows. Fifty-one were identified as foraging burrows and 42 as retreat burrows, three of the204), now popular for home security systems (Capel,1993) encased in a 375 ml jar was used. Infrared lighting was used to illuminate the burrow for the camera,provided by 10 high intensity light emitting diodes fitA small black and white

ted on a piece of Vero board.

video monitor (260 mm; 5.8 kg) was used for viewing, operated directly from a

12V battery. The wiringfrom the camera to the monitor was approximately 2

m long and encased in a garden hose. The depth ofeach burrow was determined by measuring the depthof hose that fed into the burrow to reach its end.

Burrows were assumed to be for foraging when
were noted to intersect the burrow of a potential prey
item (crab, skink or bird). The number of foraging
burrows was totalled each month. Occupied burrows,

and those with tracks appearing from the entrance
after

weresmoothing, not identified as foraging burrows,
classified as retreat burrows. The number ofretreat

burrows were identified as such because they

were occupied by a monitor lizard. The numbers ofretreat and foraging burrows observed each month are

shown in Figure 3. There was a peak in the number ofburrows in July with a steep drop in Augustand September. The number of foraging burrows
observed peaked in April, although the number stays
retreathigh until July when the number drops. The numberof retreat burrows exceeded the number of foraging

burrows between May and September.

There was a positive correlation between the numberof retreat burrows and the burrow depths recorded

each month (r = 0.67; P = 0.017; Table 1).

Table 1 Correlation coefficients between the number.

of retreat and foraging burrows and burrow depths.
*denotes a significant correlation

retreat burrows observed was totalled monthly.

A temperature data logger (model 6003 A, Unidata
Australia, Perth)

was placed on top of one of thedunes with two probes extruding from the logger'scase. One of the probes was placed 1 cm below thesand surface, while the other was buried

at50 cm

Retreat Burrows

0.67

0.017*

Foraging Burrows

0.097

0.763

2001

Asiatic Herpetological ResearchVol. 9, p. 27

1210O
3
SQ-o6zO

RETREAT BURROWS -Q FORAGING BURROWS-2

JANMAR

MAYJULSEPNOVMONTHFigure 3. Monthly number of foraging and retreat burrows ofHowever,number of foraging burrows is nottheV.

panoptes at Fog Bay.

a non-corrosive sealant (glass/window sealant) wasused. From then on, the system provided clear images,

retreat burrows are negatively correlated with the min-with effective viewing of monitor lizard presence inburrows and the depth, width and curvature of all bur-= 0.463; P =rows. The narrower, rounder burrows of other animals

correlated to burrow depths (Tableimum

sand-surface temperatures1).

(rThe number of0.024) and the maximum sand temperature at 50 cm (rwere easily identifiable at the bottom of foraging bur-= -0.616; P = 0.033; Table 2). Burrowrows.

also negatively correlated to thedepths wereminimum sand sur-face temperature (r = -0.789; P = 0.002) and the maxi-

mum sand temperature at 50 cm (r = -0.64; P = 0.033;
Table 2). The number of foraging burrows was unaffected by sand surface temperature and sand tempera-

50 cm (Table 2). This equates to more andretreatburrows being dug, the cooler the sanddeeperture attemperature at the surface and at 50 cm.

with this modified design of burrow viewing equipment and an assessment of its effectiveness is warjustify

theresults.

The only problem

encountered was when the original silicon glue (ametal sealant), used to hold the camera to the jar casing, was suspected of having a corrosive effect on thecamera. The camera stopped working within a shorttime and,when

was also negatively correlated to minimum sand-surface temperature each month. This is indicative of anincreased need for the lizards to burrow to an increasing depth as the overnight temperatures cooled.

increase in the number and depth of retreat burrows.

The field experiments herein were the first attempt

totheir depth. The number and depth of retreat burrows

Cooler sand temperatures at 50 cm also caused anDiscussion

rantedThe number of foraging burrows was independentof depth or temperature influences. However, thegreater the number of retreat burrows dug, the greater

inspected, small white spots wereseen on the solder of the circuit board. The camera

worked properly when the spots were cleaned off andSince thermal diffusion in sand is slow, sand tempera-

50 cm heats and cools slowly (Packard andPackard, 1988). The sand temperature at 50 cm at Fogture at

Bay was the highest overnight when the sand surface
islowest (Guinea, 1994). Thus, monitors may be bur-rowing more in the dry season to exploit the warmer
overnight internal sand temperatures. Maximum internal sand temperatures fall during the dry season

andthe monitors therefore dig further to find even warmer

temperatures. Foraging burrows were also responsiblefor increasing the number of burrows seen in the dry

Asiatic Herpetological ResearchVol. 9, p. 28

2001

Table 2. Correlation coefficients between the number of retreat and foraging burrows and burrow depth and sandsurface (T so ) minimum and maximum temperatures and sand at 50 cm depth (T S50 ) minimum and maximum tem*peratures.

denotes a significant correlation.

Asiatic Herpetological Research

2001

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Packard, G. C, and M. J. Packard. 1988. The physiological ecology of reptilian eggs and embryos. Pp.
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Vol. 9, p. 29