Optimising the design of artificial refuges for the Australianskink, Egernia stokesii

Evy Ayu Arida1, C. Michael Bull2

School of Biological Sciences, Flinders University, GPO Box 2100, Adelaide South Australia,Australia1 Present address: Museum Zoologicum Bogoriense, Widyasatwaloka Building, Jalan Raya Bogorkm 46, Cibinong 16911, Indonesia2 Corresponding author; e-mail: michael.bull@flinders.edu.au

Abstract. Western subspecies of the Australian skink Egernia stokesii are considered endangered andtranslocation to unoccupied areas of suitable habitat has been proposed as a conservation strategy.We investigated the internal structure of artificial refuges that might induce translocated lizards toremain at the site of release. In a laboratory environment, individual lizards were offered choices ofalternative structures as refuges. They preferred deeper and narrower refuge structures, with a singleentrance rather than two entrances. They showed a slight tendency to avoid PVC structures whenplywood or brick paving alternatives were available. Soft sand or hard brick substrate were equallyaccepted. The results suggest that the use of brick pavers may be a practical management strategy toprovide extra refuges for the lizards, but further trials are needed with a greater range of temperaturesthat are representative of field conditions.

Key words: Artificial refuge; Egernia; lizard; shelter choice; translocation.

Introduction

Habitat loss and fragmentation are major threats to the conservation of endangeredspecies. An increasing threat will be climate-driven changes in the remaining habitatisolates that they occupy (Shoo et al., 2006). In a changed climatic regime, somespecies may be able to disperse and locate newly suitable habitat (Parmesan andYohe, 2003; Root et al., 2003), but less mobile species such as most reptiles,may become trapped in decreasingly suitable patches of habitat. Some ecologistsnow suggest that carefully planned translocations are the best way of conservingthreatened species if their dispersal rate is too low, or the available dispersalcorridors too tenuous to allow for natural colonization processes (Hulme, 2005).Even that option is fraught with difficulties (Westoby and Burgman, 2006).

© Koninklijke Brill NV, Leiden, 2008 APPLIED HERPETOLOGY 5: 161-172Also available online - www.brill.nl/ah

162 E.A. Arida, C.M. Bull

Reptiles have been translocated to remove “nuisance” animals from urban ar-eas (Shine and Koenig, 2001; Sullivan et al., 2004), to rescue animals from de-velopment sites (Platenberg and Griffiths, 1999), or to establish new populationsof threatened species (Dickinson and Fa, 2000; Towns and Ferreira, 2001; Nelsonet al., 2002; Pernetta et al., 2005). Reptile populations sometimes persist follow-ing translocations to small islands where there are suitable microhabitats and lowpredator densities (Dickinson and Fa, 2000; Knapp, 2001; Nelson et al., 2002). Per-sistence on islands may result from a lack of opportunity to disperse, or becausethese are the only remaining predator-free habitats. However, reptile translocations,particularly to mainland locations, are generally either unsuccessful or their suc-cess cannot easily be determined (Dodd and Siegel, 1991). Initial stress during thetranslocation may impair the health and cognitive ability of translocated individuals(Teixera et al., 2007). Some translocated reptiles will remain near the release site(e.g. Lettink, 2007) but they often have poorer body condition than residents (Plat-tenberg and Griffiths, 1999). In addition, translocated individuals have been shownto disperse more and have lower survival compared to individuals in their sourcepopulation or residents at the translocation site (Hein and Whitaker, 1997; Plum-mer and Mills, 2000; Sullivan et al., 2004; Butler et al., 2005; Rittenhouse et al.,2007). This effect can persist for up to two years after the translocation (Reinert andRupert, 1999).

A successful translocation program normally requires the majority of individualsto remain near the release site. For some species, one strategy to achieve thismight be to choose release sites with optimal microhabitat to encourage persistence(Dickinson et al., 2001; Pernetta et al., 2005). Alternatively, penning gophertortoises at the release site for 9-12 months improved the chance of them remainingthere when finally released (Tuberville et al., 2005). Whether penned or immediatelyreleased, translocated animals may be less stressed, disperse less and survive moreif there was an abundance of suitable, unoccupied refuges. Natural refuges could beaugmented with artificial shelters.

Artificial refuges such as brick pavers (Webb and Shine, 2000), or burrows(Souter et al., 2004) have been used to augment natural refuges for existing reptilepopulations, or to assist in reptile surveys (Reading, 1997). For each reptile species,some refuge structures are preferred to others (Milne and Bull, 2000; Goldsbroughet al., 2006), and experimental studies have been conducted to clarify optimalstructures (e.g. Lettink and Cree, 2007). In each of these studies the researchershave suggested three characters that could influence reptile preference, thermalproperties, substrate and security from predation. If artificial refuges are to be usedto encourage persistence of translocated animals, their structure should reflect thesepreferences.

In this study we explored preferences for different structures of artificial refugesby the gidgee skink Egernia stokesii, an Australian scincid lizard. In its naturalhabitat this species often occupies crevices on rocky outcrops (Bull et al., 2000) orhollows in trees (Wilson and Swan, 2003). At least three currently named subspecies

Artificial refuges for gidgee skinks 163

from Western Australia (E. s. stokesii, E. s. aethiops and E. s. badia) have restricteddistributions or are considered endangered (Cogger, 1992; Storr et al., 1999; Wilsonand Swan, 2003). How et al. (2003) advocated relocation of populations of thethreatened E. s. badia from sites that are currently threatened by agricultural activity,and suggested providing suitable, but unspecified refuge habitat at the translocationsite. The focus of current study was on refuges that will protect against predationrather than thermal extremes.

We used a captive colony of the more common eastern form of the species(E. s. zellingi) to investigate aspects of refuge habitat structure preferred byEgernia stokesii. The study was part of a broader program to explore biology andconservation strategies (Gardner et al., 2007; Fenner and Bull, 2008; Mensforth andBull, 2008), including possible release in existing or new population sites, using acommon form of the species with low cost of failure. At the same time results fromthe common form may allow deductions about the taxonomically related but rarersubspecies.

For these lizards, refuges provide shelter from thermal extremes and frompredators. We performed a series of experiments to determine lizard preferencefor internal structures and dimensions of refuges in a thermally benign laboratoryenvironment.

Methods

We used E. stokesii from a laboratory colony of over 100 lizards that had originatedfrom near Hawker in South Australia (31◦54′S; 138◦25′E) in 1998, but that includedmany laboratory reared individuals. The colony was housed in 3 m×3 m pens insiderooms held at 25◦C, with heat lamps turned on to provide basking opportunitiesin the light phase of a 12:12 light cycle. Lizards in the colony were providedwith paving brick refuges in their pens. Each trial within an experiment involveda different individual, chosen randomly from the colony, so the selection includedindividuals of various ages and of each sex. Lizards used in the trials had snout tovent lengths (SVL) in the range 116-191 mm, and body mass 65-295 g. MatureE. stokesii have a SVL of over 170 mm (Duffield and Bull, 2002; Chapple,2003).

Trials were conducted in a room with a 12:12 light cycle and a constanttemperature of 25◦C. Each lizard was individually housed in a glass tank (30 ×35×75 cm) and visually isolated from other lizards with newspaper around the tanksides. The substrate was a 5 cm thick layer of fine sand, and water was available at alltimes. At one end there was a basking brick (19 × 9 × 3 cm) where the temperaturereached 32◦C underneath a heat lamp that was turned on for 8 h each day. Heatlamps were turned off 20 min before the onset of the dark period.

For each trial, two artificial refuges were made available for the lizard away fromthe basking brick. The aim of the experiment was to determine if one or otherof those refuges was preferred by the lizard. A trial ran for two days, and lizard

164 E.A. Arida, C.M. Bull

behaviour was video recorded for 4 h on each day with a Sony Handy Cam (CCD-TR512E PAL) mounted above each tank. Lizards were left in their tank for at least3 h on the first day before filming commenced. The time of filming was the samefor all trials within an experiment, and included three different phases of the heatingand lighting cycle. The Heat Phase was when both heat lamps and room lights wereon, the Light Phase was when heat lamps were off but room lights were on, and theDark Phase was when both heat lamps and room lights were off. To film in the dark,we used the night vision facility of the cameras. Lizards entered the refuges headfirst and during video playback we defined a lizard as “in a refuge” when its headwas completely inside one of the refuges provided. A lizard was defined as “out ofa refuge” when all of its head was emerged from the refuge.

In each temperature-light phase on each day, we recorded: (1) the number of timesa lizard entered either refuge, and (2) the amount of time spent by a lizard in eachrefuge. We standardised these to values per 20 min, and analysed the data usingrepeated measures ANOVA to determine if there were any differences in use of thetwo refuges. Day of filming (first or second day), temperature-light phase (Heat,Light or Dark), and burrow type (first or second alternate refuge) were all repeatedvalues within individual lizards.

Five individual experiments described below, and summarised in table 1, wereperformed sequentially. In each experiment there were three 2-day filming sessions,and in each session 6 to 8 trials were conducted simultaneously. Thus we used 18to 24 different lizards in each experiment. In some cases camera malfunction meant

Table 1. A summary of the five experiments.

Experiment Refuge Basic Refuge 1 Refuge 2 Number ofnumber character refuge trials used

tested structure in analysis

1 Entrance PVC 5 cm wide 8.5 cm wide 18width circular entrance entrance

tube25 cm long

2 Number of Same as 1 One Two 17entrances but both entrance entrances

5 cm diam3 Refuge Same as 2 25 cm long 50 cm long 28

length but both oneentrance

4a Refuge 5 × 5 cm PVC Brick 204b structure square tubes Plywood Brick 174c 28 cm long Plywood PVC 22

One entrance5 Refuge Same as 4 Brick Sand 21

substrate except 3 × 9 cm substrate substratetubes

Artificial refuges for gidgee skinks 165

that there were fewer lizards in the analysis than in the trials. Tanks and refugeswere thoroughly cleaned with 70% ethanol and then distilled water, and fresh sandwas added between trials.

Experiment 1: burrow width

Each aquarium was provided with two initially circular, 25 cm long PVC tubes.One tube remained circular with a 5 cm diameter opening and was called a narrowentrance refuge. The second tube started as a 7.5 cm diameter circular tube, butwas laterally compressed to form a 5 cm high and 8.5 cm wide opening, and thisshape was retained along its length. This was called a wide entrance refuge. The tworefuge types had the same length and the same vertical height. We placed the twotubes, side by side, lengthwise in the middle of the aquarium, randomly assigningthe wide burrow to the left or right side of each tank. Between successive trials inthe same tank we altered their relative positions. We used 18 different lizards forthis experiment, but (in this experiment) took no recordings in the Dark Phase whenboth heat and lighting were off.

Experiment 2: number of entrances

Two 5 cm diameter, 25 cm long circular tubes (narrow entrance refuge as above)were placed side by side in the middle of the tank. One of them had the end furthestfrom the basking brick blocked with a polystyrene plug (single entrance), the otherwas open at both ends (double entrance). As before, the positions of the two refugetypes were alternated. In this and all subsequent experiments filming was conductedover all three temperature and light phases. Films were available for only 17 out of18 lizards in this experiment.

Experiment 3: refuge length

Two 5 cm diameter circular tubes with one end blocked with a polystyrene plug(single entrance refuge as above) were placed as before. One (shallow refuge) was25 cm long. This was just long enough for an adult lizard body and tail. The second(deep refuge) was 50 cm long. Trials were conducted as before with data from18 lizards. Because the tendency to prefer deeper burrows may be more pronouncedin larger lizards, we looked for a correlation between time spent in the deep burrowand lizard SVL.

Experiment 4: refuge structure

For the next three experiments we tested for any lizard preference in the materialused to construct the refuges. We chose three materials, paving brick, plywood andPVC, that may allow simple construction of artificial refuges in field conditions.In each experimental trial two adjacent, parallel refuge structures with a square5 × 5 cm cross-section were constructed. The outer walls and roof were made of

166 E.A. Arida, C.M. Bull

paving bricks and the floor was sand. They were each 28 cm long and the furtherend was blocked with a polystyrene plug. PVC and plywood refuges were made byslipping a 5 × 5 cm square × 28 cm long sleeve of the appropriate material into oneof the brick refuges.

We had data from 20 lizards for a comparison of PVC and sand-floored brickrefuges, 17 lizards for a comparison of plywood and sand-floored brick, and22 lizards for a comparison of plywood and PVC refuges.

Experiment 5: refuge substrate

In this experiment we tested whether lizards preferred soft sand or a hard bricksubstrate for their refuges. In this case the two refuges were separated to oppositesides of the filming tank. The entrance dimensions of each were 3 cm wide × 9 cmhigh, and they differed in that one had a paving brick floor while the other was sandas before. The entrances of each were at the same level and the refuges were each28 cm long and blocked at the far end. There were data from 21 lizards.

Results

Experiment 1: burrow width

Lizards spent significantly longer in the narrow refuges than the wider ones (F1,17 =11.85; p = 0.003), although there were no differences in the number of timeslizards entered either refuge type (table 2). There were no significant effects offilming day or phase, nor any interaction effects, indicating that the preference,indicated by time spent in the narrow refuge, was consistent over the trial period.

Experiment 2: number of refuge entrances

For time spent in refuges there was a significant interaction effect Phase × RefugeType (table 3). Lizards increased the mean time spent in refuges from the Heat

Table 2. Width of burrows: ANOVAs showing effects of refuge type (wide or narrow burrows), day(day 1 or day 2) and temperature-light phase (heat and light or light only) on the time spent by lizardsin a refuge, and the number of times they entered a refuge.

Effect Time in refuge Number of entries

d.f. F p d.f. F p

Refuge type (R) 1, 17 11.85 0.003 1, 17 0.01 0.91Day (D) 1, 17 0.35 0.56 1, 17 2.98 0.10Phase (P) 1, 17 0.16 0.69 1, 17 2.87 0.11R × D 1, 17 1.05 0.32 1, 17 2.53 0.13R × P 1, 17 0.34 0.57 1, 17 0.33 0.57D × P 1, 17 0.08 0.78 1, 17 1.81 0.20R × D × P 1, 17 0.04 0.84 1, 17 0.56 0.46

Artificial refuges for gidgee skinks 167

Table 3. Number of burrow entrances: ANOVAs showing effects of refuge type (one or two burrowentrances), day (day 1 or day 2) and temperature-light phase (heat and light, light only or dark) on thetime spent by lizards in a refuge, and the number of times they entered a refuge.

Effect Time in refuge Number of entries

d.f. F p d.f. F p

Refuge type (R) 1, 16 37.72 <0.001 1, 16 7.97 0.01Day (D) 1, 16 0.16 0.69 1, 16 10.64 <0.001Phase (P) 2, 15 37.84 <0.001 2, 15 8.15 <0.001R × D 1, 16 2.68 0.13 1, 16 0.00 0.95R × P 2, 15 6.22 0.01 2, 15 5.02 0.02D × P 2, 15 0.24 0.79 2, 15 2.20 0.14R × D × P 2, 15 0.37 0.70 2, 15 0.29 0.75

Phase to the Light Phase to the Dark Phase. In the Dark Phase lizards spentalmost all of their time in refuges. However the increase in time spent in thesingle entrance refuges was greater than in the double entrance refuges, and ateach phase lizards had greater mean time in the single entrance refuges. Thus therewas a highly significant main effect of refuge type. Note that in all subsequentexperiments there was a similar effect of the time phase, in that lizards spent moretime in the refuges when it was dark. This will not be reported for the subsequentexperiments.

For the number of times that lizards entered the refuges there was also asignificant interaction effect Phase×Refuge Type (table 3). This was because lizardsmoved in and out of refuges most often in the Light Phase (LP) after the heat lampswere turned off but the lights were still on, but that difference was greater for singleentrance than for double entrance refuges. There was a significant main effect ofday, with lizards moving into refuges more on day 1 than day 2, but the trend formore movement into single entrance refuges was consistent across the days (nointeraction Day × Refuge Type). Note also that the movement patterns in and outof refuges were consistent across experiments and reflected the trends shown bythe time spent in refuges. For the subsequent experiments we only report time inrefuge, although the data and analyses for the number of times entering refuges areavailable.

Experiment 3: refuge length

There was a marginally significant main effect of refuge type (p = 0.051) withlizards spending more time in the deep than the shallow refuges (table 4). That trendwas more apparent on day 1 than day 2, as indicated by the marginally significantinteraction effect Day × Refuge Type (p = 0.07). The 18 lizards used in thisexperiment varied in SVL from 145-185 cm, but there was no correlation betweenSVL and time in the deeper refuge during the dark phase (r = 0.37; n = 18;p = 0.13). That is, lizards of all sizes preferred deeper refuges.

168 E.A. Arida, C.M. Bull

Table 4. Burrow length: ANOVA showing effects of refuge type (short or long burrows), day (day 1or day 2) and temperature-light phase (heat and light, light only or dark) on the time spent by lizardsin a refuge.

Effect Time in refuge

d.f. F p

Refuge type (R) 1, 17 4.40 0.051Day (D) 1, 17 0.80 0.38Phase (P) 2, 16 20.62 <0.001R × D 1, 17 3.78 0.07R × P 2, 16 0.96 0.40D × P 2, 16 0.60 0.56R × D × P 2, 16 0.18 0.84

Table 5. Burrow structure: Main effects of refuge type (PVC, plywood or sand-floored brick) inANOVAs comparing responses to pairs of burrow refuges of different structure (SFB = sand-flooredbrick).

d.f. Time in refuge Entries into refuge

F p F p

PVC vs SFB 1, 19 0.17 0.69 1.35 0.26Plywood vs SFB 1, 17 2.03 0.17 4.99 0.04Plywood vs PVC 1, 21 2.59 0.12 4.97 0.04

Experiment 4: refuge structure

In the comparison of PVC and sand-floored brick refuges there were no significanteffects, other than the time phase, and specifically there was no significant effect ofrefuge type on the time spent in a refuge (table 5).

In the comparison of plywood and sand-floored brick there was a significantmain effect of day, with lizards spending more time overall in refuges on day 2(F1,17 = 6.30; p = 0.02). There was also a significant Refuge Type × Day × Phaseinteraction (F2,16 = 4.14; p = 0.03), indicating that lizards spent more time duringthe Dark Phase in the plywood refuge than the brick refuge, on day 2 but not onday 1.

In the comparison of plywood and PVC refuges there was a similar significantthree way interactions reflecting a preference for plywood over PVC in the DarkPhase that varied in strength between the two days.

Experiment 5: refuge substrate

There were no significant main or interaction effects of refuge type on the time thatlizards spent in the refuges (table 6). Thus lizards showed no preference for softsand or a hard brick substrate.

Artificial refuges for gidgee skinks 169

Table 6. Burrow substrate: ANOVA showing effects of refuge type (sand or brick substrate), day(day 1 or day 2) and temperature-light phase (heat and light, light only or dark) on the time spent bylizards in a refuge.

Effect Time in refuge

d.f. F p

Refuge type (R) 1, 20 3.11 0.09Day (D) 1, 20 1.51 0.23Phase (P) 2, 19 43.35 <0.001R × D 1, 20 3.17 0.09R × P 2, 19 1.93 0.17D × P 2, 19 2.19 0.82R × D × P 2, 19 5.05 0.14

Discussion

The experiments showed some clear trends. First, the temporal patterning wasconsistent across experiments. Lizards spent more time emerged from their refugeswhen the heat lamps and lights were both on. When the heat lamps were switchedoff, lizards started to move in and out of refuges more frequently, but also spent moretime in refuges. We interpreted this to be an exploratory phase as lizards sampledthe refuges available. Finally in the dark phase there was very little movement, andfor most of the recorded time, lizards were in refuges. We assumed that lizards hadnow selected the refuge they were going to spend the night in and stayed there. Insome experiments there was a stronger discrimination between refuge types on thesecond night than the first. It may be that lizards used the experience of the firstnight to make more informed choices on the second night.

The second trend was for lizards to discriminate between alternative refuge typesoffered. They clearly preferred refuges with narrow entrances over ones with widerentrances, refuges with one entrance over refuges with two entrances, and deeperrefuges than shallow ones. This result coincides with findings for other lizards(Cooper et al., 1999; Webb and Shine, 2000). The preference for deep refuges wasnot related to lizard size, as might be expected from results from other studies (Milneand Bull, 2000). An explanation for these preferences is that the tighter and deepera lizard can retreat into a refuge the safer it should be from potential predators.Cooper et al. (1999) reached a similar conclusion in their study of rock crevice useby a cordylid lizard. Egernia stokesii have keeled scales on their tails and, in the fieldthey jam themselves tightly into rock crevices by inflating their body and pushingthe tail spines up against the crevice roof. A predator would have difficulty eithergrasping such a lizard or pulling it out of the refuge. Likely predators that couldenter crevices include pythons, elapid snakes, and varanid lizards. Additionally,conspecifics may prey on juvenile E. stokesii (Lanham and Bull, 2000). A singleentrance crevice may also reduce predation risk, by reducing the openings for apredator to explore, as also suggested by Cooper et al. (1999). Although a reduced

170 E.A. Arida, C.M. Bull

number of entrances also reduces the number of escape routes, this would be lessimportant with a strategy of trying to be difficult to dislodge rather than runningaway.

Although the trials were conducted in thermally benign conditions, refuge selec-tion might still be based on potential thermal conditions. Deeper crevices in therock may have less temperature variation (Duffield and Bull, 1996), and lizardsmay favour sites where there is less thermal stress in extreme temperature condi-tions. Further trials are needed in more realistic outdoor conditions to confirm thatlizard choice reflects tolerable thermal environments.

Lizards also discriminated between the material used in refuge structure, althoughnot as strongly. They tended to avoid the PVC piping when alternative plywoodrefuges were available. The smooth surface of the piping may have preventedthe lizards gaining a firm grip on the refuge substrate. Lizards consistently usedrefuges made from brick pavers. It is unlikely that different thermal properties ofthe materials influenced lizard choice because laboratory temperature conditionswere never excessive. Lizards showed no discrimination between soft sand or ahard paving brick as refuge substrate.

This study provided information that could be used for both captive maintenanceand translocation of gidgee skinks, both valuable techniques in conservation man-agement. In particular brick pavers appear to be suitable for making refuges forthe common sub-species of Egernia stokesii, that could be easily constructed in thefield and would provide suitable habitat that could encourage translocated lizardsto remain near their site of release. The results suggest that these artificial refugesshould be among the designs first trialled in any conservation management programfor the more endangered sub-species E. s. stokesii, E. s. aethiops and E. s. badia.This study is one stage of a program of research investigating refuge structure forall sub-species of gidgee skinks. Further research (Mensforth and Bull, 2008) hasexplored alternative spatial arrangements of refuges within piles of brick pavers inan outside environment.

Acknowledgements. This research was funded by grants from the AustralianResearch Council and the School of Biological Sciences, Flinders University. EvyArida was supported by an AusAID scholarship. We thank Dale Burzacott andLeslie Morrison for technical assistance. The study was conducted according tothe guidelines of the Flinders University Animal Welfare Committee in compliancewith the Australian Code of Practice for the use of animals for scientific purposes.

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Accepted: February 21, 2008.