Sunny side up: lethally high, not low, nest temperatures may prevent oviparous reptiles from...

Biological Journal of the Linnean Society

, 2003,

78

, 325–334. With 3 figures

© 2003 The Linnean Society of London,

Biological Journal of the Linnean Society,

2003,

78

, 325–334

325

Blackwell Science, LtdOxford, UKBIJBiological Journal of the Linnean Society0024-4066The Linnean Society of London, 200378

Original Article

R. SHINE ET AL.THERMAL CONSTRAINTS ON REPTILE OVIPARITY

*Corresponding author. E-mail: [email protected]

Sunny side up: lethally high, not low, nest temperatures may prevent oviparous reptiles from reproducing at high elevations

RICHARD SHINE*, MELANIE J. ELPHICK and ELIZABETH G. BARROTT

Biological Sciences A08, University of Sydney, NSW 2006, Australia

Received 5 May 2002; accepted for publication 6 September 2002

Oviparous (egg-laying) lizards and snakes generally inhabit warmer climates than do related viviparous (live-bearing) taxa. This pattern is widely attributed to the failure of oviparous reproduction in cold climates, but the ther-mal regimes of potential nest-sites above and below the elevational cut-off for oviparous reproduction have neverbeen quantified. We studied oviparous (

Bassiana duperreyi

) and viviparous (

Eulamprus heatwolei

) scincid lizards atsuch a site in the Brindabella Range of south-eastern Australia. Miniature data-loggers monitored temperatures ofnest-sites and lizards in midsummer, partway through the incubation period of eggs in natural nests. Our resultscontradict the simplistic notion that mean nest temperatures determine this elevational limit for oviparity. Instead,potential nest-sites with average temperatures suitable for embryogenesis in

Bassiana

are available well above thethreshold elevation. However, thermal minima decrease consistently with elevation and thus the maximum tem-perature needed for any given mean incubation temperature increases rapidly with elevation. Potential nest-sitesabove the elevational threshold can only attain mean temperatures high enough to sustain embryogenesis by havinglethally high thermal maxima. Such nest-sites are available close to the soil surface, but cannot support develop-ment. In contrast, behavioural thermoregulation allows viviparous lizards to maintain high mean body tempera-tures concurrently with relatively low maximum temperatures, regardless of elevation. Paradoxically, oviparousreptiles may be restricted to low elevations not because nests that provide appropriate mean incubation tempera-tures are unavailable further up the mountain, but because eggs laid in such shallow nests would overheat. © 2003The Linnean Society of London,

Biological Journal of the Linnean Society

, 2003,

78,

325–334.

ADDITIONAL KEYWORDS:

geographical distribution – lizard – oviparity – thermal – viviparity.

INTRODUCTION

Consistent associations between environmental fac-tors and life-history traits, especially if present inmany distantly related lineages of organisms and inmany different parts of the world, provide some of thestrongest evidence for Darwinian interpretations oflife-history diversity (e.g. Harvey & Pagel, 1991).Many such patterns have been identified, and in ear-lier years were often accorded the status of ‘laws’ or‘rules’. For example, colder climates are associatedwith larger body size in endotherms (Eisenberg, 1981),and a higher incidence of brooding in marine inverte-brates (Gallardo & Penchaszadeh, 2001). Similarly,

habitat productivity has been identified as a correlateof the incidence of polygyny (Emlen & Oring, 1977),communal breeding (Brown, 1987) and clutch sizes(Geffen & Yom-tov, 2000) in birds. Although some ofthese patterns are striking, they are rarely so clear-cutas to permit direct identification of threshold condi-tions: that is, where a specific life-history strategy isviable in one area but fails in an adjacent site.

The association between reptilian reproductivemodes (oviparity vs. viviparity) and climate (warm vs.cold) offers an exceptional opportunity in this respect.Oviparous (egg-laying) lizards and snakes arerestricted to relatively warm areas, with the propor-tion of viviparous (live-bearing taxa) increasing dra-matically with increasing elevation or latitude (e.g.Mell, 1929; Weekes, 1933; Shine & Berry, 1978). Thisassociation is extraordinarily consistent across conti-

326

R. SHINE

ET AL

.



2003,

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, 325–334

nents (e.g. it is seen in Africa, Asia, Australia, Northand South America: Tinkle & Gibbons, 1977) andacross diverse reptilian lineages representing

>

100separate evolutionary transitions from oviparity toviviparity (Blackburn, 1982, 1985; Shine, 1985). Thereason for this association presumably involves ther-mal biology. In severely cold areas, viviparity allowsdeveloping eggs (retained

in utero

) to be kept warm bytheir mother’s thermoregulatory behaviour, and thusto avoid detrimentally low nest temperatures (Mell,1929; Sergeev, 1940; Shine, 1983, 1985). Oviparity isprecluded in such areas because soil temperatures aretoo low to permit successful embryonic developmentand hatching.

Despite the virtually universal acceptance of thismodel to explain the absence of oviparous squamatesfrom severely cold climates, there are remarkably fewempirical data on the temperatures of potential nest-sites in such regions (Andrews, 2000). Such data areessential if we are to evaluate reasons for the absenceof egg-layers in cold environments. To understand howthe distribution of oviparous reptiles is constrained bycold, we need to compare potential nest-sites immedi-ately above and below the thermal threshold of a spe-cies’ distribution. This is difficult in practice, becausedistributional limits are rarely so clear-cut. The taskcan be facilitated by focusing on elevation rather thanlatitude as the determining factor, because of thesteeper thermal clines in the former situation. Ideally,then, we need to find the upper distributional limit inan area where similar habitat features (and thus,potential nest-sites) extend into higher, colder regionsthan those occupied by the study species.

We have found such an area in the BrindabellaRange of south-eastern Australia. An oviparous scin-cid lizard,

Bassiana duperreyi

, is abundant at lowerelevations but absent from otherwise similar areas

>

1630 m (Pengilley, 1972). At the highest locality wehave recorded for this taxon (Mount Ginini), the liz-ards nest in large numbers at the lower end of a skirun, but are never seen further up the run (pers. obs.).Extensive searching over four years has revealed

>

1000 eggs (corresponding to the clutches of

>

200females) at elevations

<

1630 m on the site, but no eggsabove this point. The ski-run extends directly up thenorth-east side of the mountain to 1762 m, with noobvious habitat discontinuity between the sites occu-pied by

Bassiana

and those further up. This locationthus provides an ideal opportunity to compare thethermal regimes of nest-sites used by

Bassiana

tothose potentially available above the upper eleva-tional limit of this taxon. Another distantly relatedoviparous lizard species also breeds at the bottom ofMount Ginini but no higher (the agamid

Tympanoc-ryptis diemensis

) whereas at least five live-bearers arefound all the way up to the summit (

Egernia whitii

;

Eulamprus heatwolei

;

E. tympanum

;

Pseudemoiaentrecasteauxii

;

Tiliqua nigrolutea

: pers. obs.). Thisdistributional pattern suggests that reproductivemode is causally involved in whatever mechanism pre-vents

Bassiana

from reproducing successfully athigher elevations. In support of this inference,

Bassi-ana

eggs that were translocated to artificial nest-sitesat higher elevations on Mount Ginini exhibited lowerhatching success than eggs maintained at lower ele-vations (Shine, 2002a). Why are these higher-eleva-tion nests unsuitable for oviparous reproduction?

We conducted a study to quantify thermal regimesabove and below the upper elevational limit for

Bassi-ana

. In particular, we ask:

(1) Does the elevational difference simply involve ashift in mean incubation temperatures, such thatpotential nest-sites above the cut-off are too cool topermit embryonic development? This hypothesis pre-dicts that the threshold elevation should correspond tothe mean nest temperature below which hatching willnot occur (Sergeev, 1940) or that hatching will occurbut produce suboptimal hatchling phenotypes (Shine,1995).(2) Does thermal variance in potential nest-sites shiftwith elevation also? Embryogenesis in

Bassiana

isaffected by thermal variance as well as mean tem-perature, and females select nest-sites that displayhigh thermal variance as well as high mean tem-peratures (Shine & Harlow, 1996; Shine, Elphick &Harlow, 1997). On the other hand, embryos would bekilled if temperatures fell above or below their toler-ance levels (Muth, 1980; Packard & Packard, 1988). Itis thus of interest to examine elevation-related shiftsin maximum and minimum as well as mean nesttemperatures, and the relationship between thesevariables.(3) How do thermal regimes experienced by embryosinside viviparous reptiles shift with elevation?Because viviparous reptiles extend all the way to thetop of Mount Ginini, we can compare the thermalregimes experienced by uterine embryos in such taxato those experienced by eggs of an oviparous form atthe same location. Again, both the mean and varianceof incubation temperatures are of interest.

To answer these questions, we gathered data onthermal regimes at a range of elevations for:

(1) An array of potential nest-sites, including exposedas well as heavily sheltered locations. These sitesallowed us to quantify conditions in all available nest-ing locations, including ones that were less well-insulated (and thus, with higher and more variabletemperatures) than the sites actually used; also, sitesthat occurred at higher elevations than any actualnests.

THERMAL CONSTRAINTS ON REPTILE OVIPARITY

327



2003,

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, 325–334

(2) Actual nest-sites of

Bassiana

over a range of ele-vations.(3) Gravid females of the viviparous lizard

Eulamprusheatwolei

, over a range of elevations. As for potentialnest-sites, these locations included areas above theupper elevational limit for oviparous reproduction, butwithin the usual range of viviparous species.

MATERIAL AND METHODS

S

TUDY

SPECIES

AND

AREA

The three-lined skink,

Bassiana duperreyi

, is amedium-sized (to 80 mm snout

-

vent length) insecti-vore that is abundant in montane areas of southernand eastern Australia (Cogger, 1992). Because of therelatively short warm season in montane areas, ovipo-sition and hatching are highly synchronized in ovipa-rous species. Female

Bassiana

lay a single clutch of3–9 eggs in early summer (December), usually underlogs or rocks (Pengilley, 1972). The eggs hatch 2

-

3months later, shortly prior to the onset of severely coldweather in autumn (Pengilley, 1972; Shine, 1983).Many clutches are communal, containing the eggs ofup to 20 females (Pengilley, 1972; Shine & Harlow,1996). The slightly larger viviparous lizard

Eulam-prus heatwolei

(highland water skink; to 100 mmSVL) is sympatric with

Bassiana

over a large area, butextends into higher colder regions as well (to

>

1800 m:Pengilley, 1972). We have studied both species exten-sively in the Brindabella Range 40 km west ofCanberra, Australian Capital Territory (Shine, 1983,1995; Shine & Harlow, 1993, 1996). The climate is hotin summer (January mean temperature 25.9

∞

C) butcold in winter (July mean 10.4

∞

C) with intermittentsnow cover (Green & Osborne, 1994; AustralianBureau of Meteorology). Nesting by

Bassiana

is con-centrated in open areas among eucalypt forests. Manyof these open areas are anthropogenically derived(especially, clearways cut for powerlines, and ski-runsat higher elevations: Shine

et al

., 1997; Shine, Barrott& Elphick, 2002a).

P

OTENTIAL

NEST

-

SITES

In spring (November) 2000, we laid out sets of fourgrey concrete pavers (Boral Masonry: 30

¥

30 cm,10 cm thick) on flat ground at each of four locations:Picadilly Circus (1240 m asl; 148

∞

50

¢

E, 35

∞

21

¢

S) andon the lower (1615 m), middle (1660 m) and upper(1720 m) slopes of Mount Ginini (148

∞

46

¢

E, 35

∞

32

¢

S).The Picadilly site was in a 60 m wide anthropogenicclearing under powerlines, whereas the Ginini sitewas a 50-m-wide ski-run. The replicate pavers withineach site were spaced 7–29 m apart (depending onproximity of trees), and each was partially dug intothe underlying soil so that it was firmly embedded.

Thermal regimes in natural nests of

Bassiana

areaffected not only by their elevation, but also by theirdegree of exposure to solar radiation, and by the sizeand type of cover items (logs, rocks: Shine

et al

., 1997;Shine, Barrott & Elphick, 2002). To reduce the effectsof this latter confounding factor, we used standardizedcover items (above). These pavers have similar ther-mal characteristics to natural cover items, and arereadily used as nest-sites by

Bassiana

(unpubl. data).To standardize exposure to sunlight, all pavers werein open areas (as is typical for all natural nests of

Bassiana

: Shine

et al

., 1997, 2002a, b). Pavers at thetwo lower localities (Picadilly and the base of MountGinini) were

<

1 m from natural nests of

Bassiana

,whereas those at higher elevations were placed inhabitats and orientations that closely resembled nat-ural nesting areas. To verify that sun exposure wassimilar along the elevational gradient, we took hemi-spherical (180

∞

) photographs with a 35-mm cameraand fisheye lens (Canon F1 7.5 mm) placed on a paverand pointing directly upwards. The resultant photo-graphic records of the size, shape and location of gapsin the forest canopy were scanned and the digitalimages analysed using GLA software (Gap LightAnalyser v.2.0, Frazer, Canham & Lertzman, 1999).This program calculates attributes such as canopystructure and gap light transmission exposure. Wealso took similar photographs at natural nest-sites, tocompare our pavers with natural nests in theserespects.

The GLA calculations confirmed that our paverswere exposed to similar radiation intensity as werenatural nests. The four pavers at the base of MountGinini did not differ significantly from eight nearbynatural nests in the percentage of open sky (vs. can-opy) above them (means 65 vs. 61%,

F

1,9

=

1.26,

P

=

0.29), in the duration of direct sunlight falling onthe paver each day (means 577 vs. 612 min d

-

1

;

F

1,9

=

1.14,

P

=

0.31) or in total incident radiation(means 39.0 vs. 38.3 mol m

-

2

d

-

1

;

F

1,9

=

0.23,

P

=

0.64).At Picadilly, the pavers were in slightly more openareas than the nests (because some nests at this sitewere partially shaded), but overlapped considerably(% open canopy, means 76 vs. 56%;

F

1,18

=

19.60,

P

<

0.001; duration of direct sunlight, means 711 vs.577 min d

-

1

,

F

1,18

=

7.18,

P

<

0.02; total incident radia-tion, means 43.0 vs. 37.2 mol m

–

2 d

-

1

;

F

1,18

=

7.39,

P

<

0.02). Comparing among pavers only, canopy open-ness was greatest for the highest-elevation site (nearthe top of Mount Ginini, 89%), next greatest for thelowest site (Picadilly, 76%) and lowest for the interme-diate sites (63 and 65%;

F

3,11

=

10.54,

P

<

0.002). Theduration of direct sunlight and total incident radiationshowed the same pattern (mean values for duration

=

GT 759, PC 718, GB 586, GM 551 min d

-

1

,

F

3,11

=

76.98,

P

< 0.0001; mean values for total radia-

328 R. SHINE ET AL.

© 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 78, 325–334

tion = GT 43.5, PC 43.0, GM 39.3, GB 39.0 mol m-2 d-1:F3,11 = 20.36, P < 0.0001). Thus, canopy openness,duration of direct sun and total incident radiation var-ied among sites, but did not change consistently withelevation. The most important of these variablesfor thermal regimes (total incident radiation) variedonly slightly among elevations (mean values 39.0–43.5 mol m-2 d-1).

Thermal data-loggers (Thermochron ibutton, DallasSemiconductor, Dallas, Texas, USA; diameter 15 mm,height 6 mm, mass 3.3 g; and Hobo-temp H8 and XTmodels with external leads, Onset Computer Co.,Massachusetts) were used to record temperaturesevery 15 min at three locations associated with eachpaver: on the paver’s upper surface (and thus exposedto direct sunlight for most of the day), directly underthe paver (where eggs would usually be laid) and30 cm deep underground directly beneath the paver(simulating a nest much deeper than any we haverecorded in the Brindabella Range: Shine et al., 1997).These locations were chosen to encompass the range ofpotential nest-sites available to lizards at each site, interms of the thickness of cover items. The exposedthermochron measured regimes that would be experi-enced by extremely superficial nests whereas theunderground probe measured conditions for eggs bur-ied deep under the soil surface. Physical attributesof an exposed thermochron (size, colour, etc.) haveonly a minor influence on its thermal regimes (Vitt &Sartorius, 1999; Shine & Kearney, 2001).

Data on temperatures at these locations weregathered during the period 11–17 January 2001; theweather was fine and warm throughout this period(mean daily maxima 31.4∞C, minima 14.1∞C). Becauseof high temporal correlation of temperatures withinany given nest, even a brief sample such as this shouldprovide a robust indication of thermal regimes expe-rienced during the incubation period (Shine & Harlow,1996; Shine et al., 1997, 2002a). Even if this assump-tion is violated, comparisons between thermal condi-tions measured simultaneously at different elevationsshould be reliable.

BODY TEMPERATURES OF LIZARDS

To monitor the body temperatures of viviparousfemale lizards at the same sites over the same timeperiod, we erected two circular open-topped nylonarenas (‘Space Pop’, Smash Enterprises, Melbourne;48 cm diameter, 56 cm deep) at each elevation. Damppotting mix was provided as a substrate, with logs forshelter and to provide an elevated basking platform.Crickets were provided as food. We placed two recentlycaptured gravid female water skinks (Eulamprusheatwolei) in each arena after attaching thermochrondata-loggers to the mid-dorsal surface of each animal

using super-glue (Loctite 406, Loctite Australia,Caringbah, NSW, Australia). To reduce mass of thedata-loggers, they were removed from their metal can-isters and plastic-dipped before being attached to theanimal (Robert & Thompson, 2002). The final packageweighed <1.5 g, whereas the lizards averaged 13.9 g.The animals showed no overt effect of the thermo-chrons, and moved about freely within the arenas. Thedata-loggers were set to record temperatures every10 min. After 6 days, the thermochrons were removedand the lizards released at their initial sites of cap-ture. Calibration trials show that the temperaturesrecorded by external thermochrons are very similar tointernal body temperatures of the lizards carryingthem (Robert & Thompson, 2002).

NATURAL NESTS

Over a 7-year period (1994-95 to 2000-01), we placedthermal data-loggers in a total of 170 natural Bassi-ana nests, spread among three elevations. These com-prised 75 nests at Coree Flats (1050 m asl; 148∞48¢E,35∞17¢S), 82 nests at Picadilly Circus (1240 m) and 13nests on the lower slopes of Mount Ginini (1615 m).Temperatures were monitored from early December(<1 week after laying) to hatching (generally in March/April). We calculated mean, minimum and maximumtemperatures for each nest over the entire incubationperiod.

DATA ANALYSIS

Data were analysed using Statview 5.0 andSuperANOVA 1.1 on an Apple Macintosh G4 computer.Assumptions of statistical tests were checked priorto analysis. The assumption of equal variances wasviolated for temperature data collected at potentialnest-sites (at every elevation, thermal regimes deepunderground were less variable than for exposedprobes, and no transformation could remedy the prob-lem). We thus used a non-parametric test (Kruskal–Wallis) to examine these data. To quantify therelationship between mean and maximum tempera-tures for potential nests at each site, we used the com-bined data for environmental temperatures (i.e. groundsurface plus under paver plus 30 cm underground) ateach elevation. This procedure was designed to assessthe range of thermal conditions potentially accessibleto a nesting female lizard at any given elevation.

RESULTS

Sample sizes for some thermal measurements werereduced by data-loggers failing to record, being dis-placed by itinerant wombats, or becoming detachedfrom lizards. Thus, we obtained thermal data on three

THERMAL CONSTRAINTS ON REPTILE OVIPARITY 329


Figure 1. Diel variation in temperatures on a fine warmday (15 January 2001) at 1660 m asl (middle slopes ofMount Ginini) in the Brindabella Range. The graph showsoutput from miniature data-loggers that were (a) attachedto the dorsal surface of a gravid female of a viviparouslizard species (Eulamprus heatwolei) in an outdoor arena,and thus free to thermoregulate (‘lizard’); (b) glued to theupper surface of a grey concrete paver (‘ground surface’);(c) placed under the paver, where eggs would typically belaid (‘nest’); or (d) buried 30 cm deep under the paver(‘underground’).

10

20

30

40

50

0000 0300 0600 0900 1200 1500 1800 2100

Time (h)

underground

nest

ground surface

lizard

Tem

per

atu

re o

C

lizards from three sites but only one lizard from thebottom of Mount Ginini. We obtained either three orfour data-sets for all environmental variables at alllocations except for 30 cm underground on the middleslope of Ginini (one data-set only). Nonetheless, vari-ation among replicates within locations and sites wassmall (see below), so these losses will have little effecton overall patterns.

POTENTIAL NEST-SITES

Thermal profiles showed considerable diel variation.Soil temperatures on the ground surface reached>45∞C during the day and fell below 15 ∞C at night(Fig. 1). Deep-soil temperatures showed the least fluc-tuation, and ‘nest’ temperatures (i.e. those under pav-ers) were intermediate in this respect. Becauseheating rates were slow, temperatures under paversdid not peak until late afternoon (Fig. 1). At everyelevation, deep-soil probes recorded lower meantemperatures, lower maximum temperatures, higherminimum temperatures, and less thermal variationthan did probes in more exposed situations (Fig. 2;Kruskal–Wallis tests show P < 0.05 for every suchtest). Temperatures under pavers (potential ‘nest’sites) were intermediate between deep-soil probes andexposed data-loggers in all of these respects (Fig. 2).

The effects of elevation were weaker than theselocation-within-elevation effects. Mean temperatures

decreased at higher elevations (for deep-soil tempera-tures, F3,7 = 4.91, P < 0.04; for ‘nest’, F3,11 = 22.04,P < 0.0001; for exposed, F3,12 = 12.33, P < 0.001). Stan-dard deviation in temperature was not affected by ele-vation (all P > 0.34), nor was maximum temperature(all P > 0.05). Minimum temperatures were muchlower at higher elevations for deep-soil probes(F3,7 = 8.79, P < 0.01) but this effect was weaker for‘nest’ temperatures (F3,11 = 3.46, P = 0.055) and notevident for exposed (soil-surface) temperatures (Fig. 2;F3,12 = 0.80, P = 0.52). To summarize, mean and mini-mum environmental temperatures were generallylower at higher elevations, whereas maximum tem-peratures were unaffected (Fig. 2).

Data on potential nest-sites (i.e. combining readingsfrom all data-loggers, including ground surface as wellas under pavers and well-buried) revealed strong asso-ciations between mean, minimum and maximum tem-peratures. At each elevation, potential nest-sites withhigher mean temperatures also had higher maxima(Picadilly, N = 11, r = 0.91, P < 0.001; Ginini bottom,N = 10, r = 0.95, P < 0.001; Ginini middle, N = 9,r = 0.97, P < 0.001; Ginini top, N = 12, r = 0.99,P < 0.0001). These relationships between mean andmaximum temperatures differed among elevations,with a given mean value corresponding to a highermaximum value at higher elevations than at lowerelevations (ANCOVA slopes F3,34 = 0.35, P = 0.79; inter-cepts F3,37 = 20.78, P < 0.0001).

The opposite pattern was evident for the relation-ship between mean and minimum temperatureswithin each elevation. Potential nest-sites with highermean temperatures had lower not higher minima (Pic-adilly, N = 11, r = 0.88, P < 0.001; Ginini bottom,N = 10, r = 0.83, P < 0.004; Ginini middle, N = 9,r = 0.77, P < 0.02; Ginini top, N = 12, r = 0.93,P < 0.0001; comparing these relationships among ele-vations, ANCOVA slopes F3,34 = 1.54, P = 0.22; inter-cepts F3,37 = 10.45, P < 0.0001). This counter-intuitiveresult reflects differences among sites in exposure:well-insulated sites (such as those 30 cm under-ground) had high minima and low maxima, whereasmore exposed sites (such as those on the ground sur-face) had low minima and high maxima. The negativecorrelations between mean and minimum tempera-tures indicate that average values were determinedprimarily by maxima rather than minima, in turn dueto the much greater range of maximum than mini-mum temperatures within each site (typically aboutfourfold greater, with ranges of 40 vs. 10∞C).

This analysis shows that minimum soil tempera-tures decline with increasing elevation (Fig. 2), andthat potential nest-sites within any given elevationdisplay only a modest range of variation in minimumtemperatures. In contrast, variation in factors such ascover-item thickness and the degree of exposure to

330 R. SHINE ET AL.


solar radiation engender much greater variation inmaximum temperatures. An inevitable consequence ofthese two results is that the only way that a high-elevation nest-site can exhibit a high mean tempera-ture is to have a very high maximum temperature. Inpractice, this will characterize a nest-site close to thesoil surface, with high exposure to solar radiation. Incontrast, the higher minimum temperatures of lower-elevation sites allow the same mean value with alower maximum, and hence may permit successfulembryonic development under a thicker or moreshaded cover-object. The central result is that all elsebeing equal, any given mean temperature will be asso-ciated with a higher maximum temperature in a high-elevation than in a lower-elevation nest-site.

BODY TEMPERATURES OF LIZARDS

Unlike environmental regimes, the body temperaturesof female Eulamprus were largely unaffected by

elevation (Fig. 2; for mean temperature, elevation ef-fect F3,6 = 2.05, P = 0.21; for minimum temperature,F3,6 = 0.32, P = 0.81; for maximum temperature,F3,6 = 1.60, P = 0.29). The sole exception was standarddeviation, which was slightly lower at the bottom ofGinini than elsewhere (Fig. 2; F3,6 = 6.21, P = 0.03; butno posthoc tests were significant, and the aberrantlocation was the one that was represented by only asingle individual). Thus, the overall pattern was thatlizard body temperatures were similar across the dif-ferent elevations, whereas environmental tempera-tures were not.

NATURAL NESTS

Despite the 565 m range in elevation across our threestudy sites (1050–1615 m), our 7-year data set showedthat natural nests at Coree Flats, Picadilly Circus andat the base of Mount Ginini exhibited virtually iden-tical mean temperatures (19.4, 19.7, 19.8∞C, respec-

Figure 2. Thermal regimes at four elevations in the Brindabella Range. The Figures show means and associated standarderrors calculated over a period of six days (11–16 January 2001) for (a) mean temperature (b) the standard deviation ofmean temperature (c) minimum temperature, and (d) maximum temperature, as measured by miniature data-loggers infour places. These units were either attached to dorsal surfaces of highland water skinks in outdoor arenas (‘lizard’); gluedto the upper surface of a grey concrete paver (‘ground surface’); placed under the paver, where eggs would be laid (‘nest’);or buried 30 cm deep under the paver (‘underground’). See text for statistical analysis of these data.

15

17.5

20

22.5

25

27.5 aground surface

0

5

10

15

1240 1615 1660 1720

Elevation (m)

b

10

20

30

40

50

60

1240 1615 1660 1720

Elevation (m)

d

5

10

15

20 c

nestunderground

lizard

Mea

n t

emp

erat

ure

(oC

)

Max

ium

m t

emp

erat

ure

(oC

)S

tan

dar

d d

evia

tio

n (

oC

)

Min

imu

m t

emp

erat

ure

(oC

)



tively; one-factor ANOVA on ln-transformed data toremove variance heterogeneity, F2,167 = 0.99, P = 0.38).However, mean maxima were higher at the higher-elevation sites (32.2, 34.5, 37.2∞C; F2,167 = 8.39,P < 0.0003), and mean minima were lower (13.1, 12.1,10.3∞C; F2,167 = 22.49, P < 0.0001). That is, nests at thehighest elevation (Mount Ginini) exhibited similarmean temperatures to lower-elevation nests, despitesignificantly lower night-time minima, by achievinghigher daytime maximum temperatures.

Closer inspection reveals complex relationshipsbetween mean, minimum and maximum tempera-tures. As was the case for potential nest-sites (seeabove), a higher mean temperature was associatedwith a higher maximum temperature (Fig. 3; 1050 m,N = 75 nests, r = 0.71, P < 0.0001; 1240 m, N = 82nests, r = 0.82, P < 0.0001; 1615 m, N = 13 nests,r = 0.58, P < 0.04), and the relationship between meanand maximum temperatures differed among eleva-tions. ANCOVA with elevation as the factor, mean nesttemperature as the covariate and maximum tempera-ture as the dependent variable, confirmed that max-ima were higher, relative to mean temperature, fornests at higher rather than lower elevations (Fig. 3;

slopes F2,164 = 0.35, P = 0.70; intercepts F2,166 = 9.57,P < 0.0001; posthoc tests have all P < 0.05). However,the relationship between mean nest temperature andminimum nest temperature was much weaker(1050 m, N = 75 nests, r = 0.11, P = 0.34; 1240 m,N = 82 nests, r = 0.13, P = 0.24; 1615 m, N = 13 nests,r = 0.29, P = 0.33). As for the potential nest-sites, thisresult reflected the relative constancy of minimumtemperatures across nests within each elevation. Min-imum temperatures for nests were about 3∞C cooler atthe highest than the lowest elevation site (see above),so that in order for nests to exhibit approximatelyequal mean temperatures over this elevational range(as they did: see above), maximum nest temperaturesaveraged about 4 or 5∞C higher at the highest eleva-tion than at the lowest one (Fig. 3).

Analysis of data (elevations combined) from naturalnests for which we recorded hatching success showedthat a higher proportion of eggs hatched successfullyfrom nests with higher mean incubation temperatures(N = 73 nests, r = 0.30, P < 0.015). The four coolestnests (<18∞C) did not produce any viable hatchlings,with the proportion of successful eggs increasing athigher mean temperatures (50% for 18∞C, 66% for19∞C, 72% for 20∞C, 81% for 21∞C). Hatching successwas low for the hottest nests, however (31% from fournests with a maximum temperature of >40∞C, vs. 80%from 25 nests with maxima 35–40∞C).

DISCUSSION

Our study provides the first quantitative informationon thermal regimes available above and below theupper elevational threshold for oviparous reproduc-tion in reptiles. Thermal regimes did indeed shift withelevation in ways that would affect the viability ofembryos laid in a nest (as also shown experimentally,by the lower viability of eggs translocated to siteshigher on Mount Ginini: Shine, 2002a). However, thenature of these thermal shifts was more complex thanwe expected. Most published discussions on therestriction of oviparous reptiles to warmer climateshave simply taken as self-evident the fact that poten-tial nest-sites at high elevations are too cool to permitembryonic development through to hatching (e.g.Tinkle & Gibbons, 1977; Shine, 1985). Our data chal-lenge this assumption, and suggest instead that therelationship between mean, minimum and maximumtemperatures plays a crucial role in limiting oviparousreproduction to low-elevation nests. Thus, the adap-tive significance of viviparity lies not in a simpleincrease in mean incubation temperatures for theembryos, but in the (mobile) female’s ability to breakthe mathematical link between mean and maximumtemperatures that applies to any fixed point (such as anest). This link between mean and maximum is espe-

Figure 3. The relationship between mean temperatureand maximum temperature for natural nests in the Brind-abella Range (numbers show elevations in m asl). The hor-izontal line at a maximum temperature of 40∞C representsthe critical thermal maximum (CTmax), the level likely tobe lethal to eggs. The vertical line at 16.5∞C (‘developmen-tal zero’) shows the minimum temperature below whichembryogenesis ceases in this species. Thus, developmentcan occur successfully only in the thermal region to thelower right of this Figure, bounded by these two lines (seetext for further explanation). The graph shows data fornatural nests (separately for three sites, at 1050, 1240 and1615 m) monitored over a seven-year period. Higher meanvalues were associated with higher maximum values, andthe relationship between mean and maximum nest temper-atures varied with elevation (see text).

20

30

40

50

Max

imu

m t

emp

erat

ure

(°C

)

15 16 17 18 19 20 21 22 23

1615m

1240m

1050m

CTmax

deve

lopm

enta

lze

ro

Mean temperature (°C)

332 R. SHINE ET AL.


cially strong when (as was the case for our data), min-imum values were relatively invariant within anygiven elevation.

We looked only at thermal factors, and do not doubtthat other variables also modify nest-site availability.For example, the fact that oviparous reptiles at highelevations need to lay their eggs in relatively superfi-cial nests (e.g. under relatively thin cover items) inorder to experience sufficiently high mean tempera-tures (Sexton & Claypool, 1978; see Fig. 2) means thateggs in such nests not only are exposed to high ther-mal variance, but also may be more prone to desicca-tion or predation than those under thicker shelter(Andrews, 2000). Biotic factors may also play a role.For example, Bassiana in the Brindabella Range arebroadly sympatric with large and ferocious ‘bulldog’ants of the genus Myrmecia. At low elevations, manyshelter-items (logs, rocks) partially or fully exposed tosunlight serve as suitable nest-sites for both lizardsand ant colonies, and thus are not a limited resource.At the base of Mount Ginini, however, these thermalconditions can only be obtained under large thin rockswith full sun exposure. Because such rocks are scarce,most shelter thousands of large aggressive ants,severely curtailing their availability as lizard nests(pers. obs.). Thus, the availability of potential nest-sites may decrease at higher elevations not becausethere are no thermally suitable sites, but becausemany such sites are occupied by competing taxa andthus are not available for lizard nesting.

How do elevation-induced changes in nest tempera-tures impact on embryonic development in Bassiana?Soil temperatures in summer are unlikely to fall belowthe critical thermal minima for Bassiana eggs, regard-less of elevation. Mean minima remained above 10∞Cin natural nests and above 5∞C even in fully exposedpositions for ‘potential nest-sites’ (see above), whereaseggs of Bassiana in the laboratory readily toleratetemperatures around 0∞C (Shine, 1983, 2002b). Thus,the elevational limit for successful reproduction inthis species is unlikely to be set by minimum soil tem-peratures. However, our data suggest that both meanand maximum nest temperatures may influence thesuitability of nest-sites. Many natural nests exhibiteither mean values too low for embryogenesis, or max-imum temperatures that are perilously close to lethalvalues (Fig. 3). Hatching success, developmental rateand hatchling phenotypes are all enhanced by incuba-tion at relatively high mean temperatures (Shine,1983, 1995), and clearly, maximum nest temperaturesmust remain below lethal levels also. Extensive labo-ratory studies reveal that embryogenesis in Bassianaceases below a mean incubation temperature of 16.5∞C(Shine & Harlow, 1996); thus, temperatures below thislevel will not support development. In natural nests inthe field, the minimum temperature for successful

hatching was 18∞C and the maximum about 38∞C (seeabove). Laboratory data confirm that eggs cannot tol-erate incubation temperatures above about 40∞C(unpubl. data). Although we do not know how longeggs need to be exposed to such temperatures to expe-rience reduced viability, even a relatively brief expo-sure to this temperature may be deleterious. Thus,female Bassiana need to lay their eggs in sites withminimum nest temperatures above about 0∞C, meantemperatures above 16.5∞C (and probably >18∞C) butwith maximum nest temperatures remaining below40∞C. The first of these criteria (minimum tempera-ture) is satisfied over the entire range of elevationsthat we studied, but the two latter criteria are morechallenging because the strong positive correlationbetween the mean and the maximum makes it difficultto find sites (especially at high elevations) where a suf-ficiently high mean does not entail a lethally highmaximum.

Figure 3 compares these thermal requirements ofBassiana embryos to the incubation regimes recordedin natural nests monitored over a 7-year period at dif-ferent elevations. In this diagram, the range of condi-tions that allow successful development are boundedby the vertical line for mean temperature (belowwhich development ceases) and the horizontal line formaximum temperature (above which the eggs wouldoverheat). Unlike the temperatures under pavers(designed to mimic ‘standardized’ natural nests) thatare shown in Figure 2, mean temperatures in realnests were virtually identical across a wide range ofelevations (see above for mean values). This differencebetween real nests under rocks and potential nest-sites under pavers reflects the highly non-randomattributes of the real nests. Both the pavers and thereal nests experienced lower thermal minima athigher elevations, so that the maintenance of highmean temperatures in real nests at high elevationswas due to the female lizards’ selection of thermallyfavourable locations for egg-laying.

Female Bassiana are highly selective of thermalregimes in potential oviposition sites (Shine et al.,1997) and thus presumably seek out thermallyoptimal sites (high mean, low maximum) for eggdeposition. Even at relatively low elevations, femaleBassiana select nest-sites based on thermal varianceas well as mean temperature, probably because hot,variable regimes accelerate embryogenesis and opti-mize hatchling phenotypes (Shine et al., 1997). Thereis significant spatial variation in thermal regimesunder cover items at any given elevation, as a functionof factors such as slope, aspect and cover-item thick-ness and heat-retention (e.g. logs vs. rocks). Thus,even very high elevations may have occasional siteswhere eggs can experience high mean temperatureswithout lethally high maxima. The wide scatter



around the trend lines in Figure 3 provides evidence ofsuch heterogeneity, although much of this scatterreflects year-to-year variation in weather conditionsand hence overestimates the potential for maternalnest-site selection in any one year to decouple themean from the maximum. Regardless, the significantrelationships between mean and maximum tempera-tures within natural nests (Fig. 3) as well as withinpotential nest-sites, show that females cannot com-pletely escape this constraint. At all elevations that weexamined, maximum temperatures inside some ofthe natural nests approached lethal levels (40∞C),whereas mean temperatures inside other nestsapproached the minimum for viable incubation(Fig. 3). Nests at higher elevations had higher maximarelative to the mean (Fig. 3), suggesting that the nec-essary combination of a high mean temperature andlow maximum temperature becomes increasinglyscarce at higher elevations. Indeed, this situation is amathematical inevitability given the fact that thermalminima decrease with elevation, and show relativelylittle variation within any single elevation.

Thus, the thermal shift with elevation involvesmore than a simple decrease in mean incubation tem-perature. If we restrict attention to replicate nest-siteswith similar orientations and thicknesses (i.e. underour pavers), then mean temperatures do indeeddecline with increasing elevations (Fig. 2, nests). How-ever, much warmer sites are available, under moresuperficial cover items (Fig. 2, ground surface).Female Bassiana take advantage of this diversity andare able to find nest-sites with high mean tempera-tures even at high elevations; but in order to do so inthe face of decreasing minima, they have to use siteswhere daily temperatures reach high maximumvalues (Fig. 3).

Behavioural thermoregulation (heliothermy) allowsgravid females of viviparous species to maintain thesame advantageous combination of thermal meansand variances as are available in (some) low-elevationnest-sites (Fig. 2). Although sample sizes for vivipa-rous females were small in the present study, ourresults are supported by an extensive published liter-ature (including studies on the Brindabella skinks)showing that mean and maximum body temperaturesof heliothermic squamates are relatively unaffected bylocal climatic conditions (Avery, 1982; Shine, 1983;Greer, 1989). Although the combination of a highmean temperature and a relatively low maximumtemperature is difficult or impossible to attain inpotential nest-sites at higher elevations (Fig. 3),gravid female lizards can maintain such regimes evenat elevations far above the upper limit for oviparousreproduction (Fig. 2).

We thus conclude that the factor restricting ovipa-rous reptiles to relatively low-elevation nesting sites is

not a simple absence of potential nests with suffi-ciently high mean temperatures for embryogenesis.Although mean temperatures of the ground surface(the upper limit of available nest temperatures)declined with elevation, they always remained wellabove the mean temperatures of successful nests.Thus, female Bassiana prepared to use superficialnests could find many sites warm enough for incuba-tion, but maximum temperatures in such a nest wouldexceed the eggs’ thermal tolerance. This constraint is adirect consequence of lower minimum (overnight) tem-peratures in high-elevation sites, such that any givenmean incubation temperature is necessarily accompa-nied by a higher maximum temperature than wouldbe the case for a low-elevation nest.

These results suggest that the fundamental reasonwhy viviparous reptiles are able to reproduce inseverely cold climates is their ability – via behaviouralthermoregulation – to break the link between meanand maximum incubation temperatures for their off-spring. Simple mathematics means that any fixedpoint must be subject to that link: if the mean is toremain constant, a lower minimum must entail ahigher maximum. Eggs are immobile and thus cannotescape that relationship. However, there is consider-able spatial heterogeneity in temperatures at anygiven time of day (Fig. 1), such that a gravid femaleviviparous reptile can move around to select appropri-ate thermal environments. The jagged trace of mater-nal body temperatures during daylight hours (Fig. 1)reflects shuttling heliothermy (Huey & Slatkin, 1976),with consequent regulation of a mean temperaturemuch closer to the maximum than can be attained byany immobile egg.

Our interpretation thus differs from previous workon high-elevation reptiles, which has emphasized thefact that maternal body temperatures are on averagehigher than nest temperatures (Shine, 1983; Qualls,1997; Andrews, 2000). Although this was true at highbut not low elevations in our study (Fig. 2), our datasuggest that this difference reflects an elevationalshift in the relationship between mean and maximumincubation temperatures. Contrary to intuition andprevious speculation, higher elevations do not lackpotential nest-sites with mean incubation tempera-tures high enough to sustain embryogenesis. Paradox-ically, the reason that oviparous reptiles cannotreproduce in cold climates may be that although nestswith high mean temperatures are available, any eggslaid in those sites would overheat.

ACKNOWLEDGEMENTS

We thank K. Robert for introducing us to thermo-chrons, D. Hochuli for creative suggestions, and help-ful families (R. and D. Barrott, D. Elphick) for

334 R. SHINE ET AL.


accommodation during fieldwork. This study wasfunded by the Australian Research Council.

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