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Bioscience Horizons Advance Access originally published online on April 17, 2008
Bioscience Horizons 2008 1(2):136-142; doi:10.1093/biohorizons/hzn014
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© 2008 The Author(s)
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Body surface temperature and length in relation to the thermal biology of lizards

Daniel Garrick*

Canterbury Christ Church University, Canterbury, UK

* Corresponding author: Flat 4, 89 Henver Road, Newquay, Cornwall TR7 3DJ, UK. Email: daniel_garrick{at}hotmail.co.uk

Supervisor: Georges Dussart, Canterbury Christ Church University, Canterbury, UK.


   Abstract
Top
 Abstract
Materials and methods
 Results
 Discussion
 Funding
 References
 
This study investigated body surface temperature (Tbs) in 22 lizards of 18 species. The difference between Tbs and ambient temperature (Ta) was correlated with size. The greater Tbs – Ta differentials, which were recorded in larger lizards, may occur as a result of heat transfer from the core to surface in prevention of overheating. The structure of the integument may contribute to heat dissipation. Heliothermy and thigmothermy as forms of thermoregulation were also incorporated into the data set. Heliothermic lizards showed a positive correlation between length and Tbs – Ta. Thigmothermic lizards, however, exhibited a negative correlation. Differences in size and rate of conductive heat transfer are put forward as possible reasons for the negative correlation.

Key words: thigmothermy, heliothermy, lizard, length

Lizards, like most reptiles, are ectothermic. Ectothermy is the reliance on external heat sources to increase body temperature.1, 2 Thus lizards gain heat, principally, from their environment. To achieve optimum body temperature lizards perform different thermoregulatory behaviours. Thermoregulation is a well-documented area of reptile biology.3–5

The use of thermoregulation to achieve and maintain optimum body temperature has contributed to the great diversity of lizards and even allowed colonization of temperate regions. For example, through simple body posturing, some species within the South American genus Liolaemus can attain body temperatures 30°C above the ambient temperature.6 Such a high differential, though uncommon, emphasizes the diversity of thermal relations among lizards.

Heliothermy and thigmothermy are further means of ectothermal thermoregulation.7 Heliothermy is heat gain by short wavelength solar radiation. Thigmothermy involves heat conduction to the reptile body by direct contact with a warm substratum.8 In practise, heliothermy and thigmothermy enable lizards to compensate for the thermal conditions of the habitat.9 Heliothermy and thigmothermy in lizards are therefore related to habitats and life-history strategies.

Heliothermy is a well-documented area in lizard biology.10 A principal reason for this bias is that diurnal basking lizard species are often the dominant vertebrates in hot arid environments and data on these animals are relatively easy to obtain. In contrast, Belliure and Carrascal8 suggest thigmothermy is a less well-studied aspect of animal thermal relations.

In general, thigmothermy is practised by species that have limited access to solar radiation.11 Most studies of lizard thigmothermy have considered diurnal species.12, 13 Among diurnal lizards, thigmothermy is normally practised by species in forest environments where incoming solar radiation is impeded by the tree canopy. Morgan14 suggests that in such environments, thermally discrete microclimates are important. In contrast thigmothermic behaviour is relatively more important for nocturnal lizards, which do not receive any solar radiation input.

Previous studies of thermal interactions between lizard and environment have predominantly used the internal body temperature (Tb) (i.e. cloacal) and ambient temperature (Ta) differential.12, 15–17 Auffenberg15 and Stevenson16 related body temperature to size. These authors concluded that larger size conferred a greater Tb – Ta differential due to thermal inertia. Thermal inertia is based upon the physical properties of heat dissipation whereby larger objects retain heat for longer than smaller objects due to reduced surface-to-volume scaling. Such a phenomenon has been demonstrated in extant crocodilian species and has been proposed for elevated temperatures in large dinosaur species.2, 18, 19

In contrast with previous work, this paper investigates radiant heat transmission using body surface temperature (Tbs). Cowles20 suggested that the dermal surface of reptiles could act as a heat collector and dispenser. Therefore the question arises as to whether the previous identified relationships between core and ambient temperatures are similar to the differentials between Tbs and Ta? The following experiment aims to investigate the correlation between length and Tb – Ta differential. A further aim is to gain a greater understanding of heliothermy and thigmothermy in a range of lizard species.


   Materials and methods
Top
 Abstract
 Materials and methods
Results
 Discussion
 Funding
 References
 
Prior to the main experiment, a preliminary study was carried out. Data were collected from three individual lizards of three species of differing size. The preliminary study was used to determine first, how readings would be recorded and secondly, the general protocol for the main experiment. The study also determined whereabouts on the body it was possible to collect temperatures accurately without the need for physical contact.

Data collected in the study were obtained from captive lizards maintained indoors under artificial conditions. Environments of captive lizards are mostly constant since heat sources and photoperiods are regulated. For the investigation undertaken here, this is an advantage since it allows a fair comparison of inter-specific effects.

Body surface temperature (Tbs) – ambient temperature (Ta) differential
Data were obtained from four private and zoological institutions in the UK and Germany (Table 1). Overall data for the main experiment were collected from 22 individual lizards representing 18 species in 6 families (Table 2). Data acquisition took place in September 2006 between 13:00–16:00 hours. All measurements were taken indoors within enclosures.


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  		Table 1. Abbreviations used in text

 


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  		Table 2. Key to list of collections used in study

 
Temperature was measured using a DT-8812 infrared non-contact thermometer. The thermometer is laser pointed and provides measurement collection at a distance to spot ratio of 8:1. The resolution is –20°C to 200°C in 0.1°C intervals, with a display accuracy of ±2.0%. Temperature readings were recorded in degree Celsius (°C). Measurements were taken within 200 mm of study animal. Each measurement recorded the Tbs at a predetermined anatomical site. The four sites chosen for data collection were mid-body lateral (MBL), lower fore leg (LF), lower hind leg (LH) and mid-caudal (T) (Figure 1). To get a temperature reading the thermometer laser was aimed at a point ~20 mm above the chosen anatomical site (as specified by manufacturers). Five consecutive readings were taken at each site. Ta was also measured by five consecutive readings. For the purpose of the experiment Ta was, as defined by Pough and Gans,7 the mean air temperature outside the boundary layer in the immediate vicinity of the lizard (i.e. within 300 mm radius of study animal). There was a 20-s delay between each reading. Between sites and Ta readings there was a delay of 3 min. This delay allowed the infrared sensor to equilibrate. Most of the study animals (90%) were exposed within enclosure when readings were taken. However, readings from two individuals were taken while sheltered (see Table 3). There was no physical contact with the study animal during data collection.


Figure 1
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  		Figure 1. The four anatomical sites used for data collection in experiment.

 


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  		Table 3. Individuals measured in study with Tbs – Ta differentials and heliothermic/thigmothermic categorization. Note both sheltered and non-sheltered individuals are included in this table. Heliothermic thigmothermic classification is prior to output of discriminant analysis (see Materials and Methods).Abbrieviations for collection are given in Table 2

 
Heliothermy and thigmothermy
The protocols described above were used for the measurement of heliothermic and thigmothermic species. Species were categorized as heliothermic or thigmothermic in accordance to whether a species was diurnal (heliotherm) or nocturnal (thigmotherm). No further thermoregulatory categories were accounted for.

Statistical methods
Statistical analyses were performed in accordance to the protocols described in Zar.21 The correlation method used throughout the experiment was non-parametric Spearman's Rank. Multiple regression analysis (performed through Microsoft Excel) was used in conjunction with Spearman's Rank correlation. One-way ANOVA (as provided by MINITAB) was used to analyse site Tbs – Ta differential versus length. Data input for the one-way ANOVA was preceded by an Fmax test to check homogeneity of variance. The Fmax test is used to screen samples for normal distribution of data. Discriminant analysis was provided by MINITAB. This method is used to separate sampling units into their true groupings. Data input involves pre-classification of sampling units. In the case of my study pre-classification comprised two units: heliothermic or thigmothermic. Each individual's unit was plotted with Tbs – Ta differential.


   Results
Top
 Abstract
 Materials and methods
 Results
Discussion
 Funding
 References
 
Overall data collection: site body surface temperature (Tbs) versus ambient temperature (Ta)
First, the temperature relations of the four anatomical sites on the reptile body were compared with each other for the 22 lizards (Figure 2). Figure 2 shows close correlations between Tbs and Ta for all four sites. All anatomical sites have similar gradients. Coefficients (r2) for the sites range from 0.946 to 0.974. Such results suggest a positive correlation between Tbs and Ta at each of the sites.


Figure 2
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  		Figure 2. Mean site temperature for 22 individual lizards against the mean ambient temperature. r2 values are given as LF: 0.974, LH: 0.966, MBL: 0.946, T: 0.946 (see Table 1 for key to abbreviations).

 
Overall, LH exhibited the lowest temperature (Figure 1). Proportionately, 27% LH readings were lower than Ta. In contrast, 22% of T and 9% of LF readings were lower than Ta. MBL showed no readings lower than Ta (0%). Therefore, attention was focused on MBL as it more closely represents the animal's internal temperature.

Inter-family comparison
The 18 different species measured represent 6 different families (Figure 3, Table 2). Data for each anatomical site for each individual were averaged and categorized by family. Figure 3 shows that the Gekkonidae exhibited the lowest mean Tbs (25.5°C), whereas the Agamidae exhibited the highest mean Tbs (36.9°C).


Figure 3
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  		Figure 3. Comparison of mean Tbs (for all four anatomical sites) between the six lizard families used in study. Standard deviation is given as 4.57.

 
Body surface temperature (Tbs) – ambient temperature (Ta) differential versus length
One-way ANOVA analysis showed MBL to be the anatomical site with the greatest Tbs – Ta differential (Figure 4). Other anatomical sites displayed more variable readings and Tbs – Ta differential between all four sites fluctuated considerably. One-way ANOVA showed significant difference (P < 0.001). The MBL data were used to give Tbs – Ta differential as the independent variable, to be compared with log10 length. Figure 4 indicates a positive correlation between Tbs – Ta differential and log10 length (r2 = 0.666). Multiple regression analysis of the data set gave F as 39.9, thus indicating a strong signal-to-noise ratio (regression MS/residual MS). The given P-value from the regression analysis was <0.001.


Figure 4
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  		Figure 4. Tbs – Ta differentials, from 22 individual lizards, against length (log10).

 
Heliothermy and thigmothermy
Eighteen heliothermic lizards and four thigmothermic lizards were studied in accordance to the prior classification described in Materials and Methods (see Table 2 for individuals; Figure 5). Figure 5 shows a positive correlation between Tbs – Ta differential and length for the heliothermic species data (r2 = 0.623). In contrast, the thigmothermic species data set shows a negative slope (r2 = –0.830).


Figure 5
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  		Figure 5. Comparison of the 18 heliothermic and 4 thigmothermic species measured in study showing Tbs – Ta differential against length (log10).

 
To assess level of confidence in determining heliothermy and thigmothermy in the study species, a discriminant analysis was performed using Tbs – Ta differential from MBL readings. Discriminant analysis showed a confidence level of 77% correct classifications. Five misclassifications resulted in species pre-classified as heliothermic being classified as thigmothermic. The sheltered individuals (Tiliqua gigas, Gallotia stehlini) measured in the study were both incorrectly classified. The further misclassifications were Lacerta lepida, Crotaphytus collaris and Laudakia stellio brachydactylus. No thigmothermic species were misclassified.


   Discussion
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 Abstract
 Materials and methods
 Results
 Discussion
Funding
 References
 
Overall, the data set suggests that anatomical Tbs is associated with Ta (Figure 2), which is congruent with the established concept that lizards are ectothermic.2

Body surface temperature (Tbs) – ambient temperature (Ta) differential versus length
Central to the study was the comparison of Tbs – Ta differential (from MBL reading) with length. The relationship between length and Tbs – Ta differential might be explained by the role of integument. Licht and Bennett22 compared a scaleless snake and a normal snake and suggested that reptile integument was not a significant heat insulator. Scales are undoubtedly important to radiative and conductive heat exchange. However, if convective heat is lost through an integument it could suggest endogenous heat production. Such heat production has been demonstrated for some reptile species. For example, digestion is known to induce both a faster metabolic rate and increased body temperature in some species.23, 24 Brattstrom (cited in Auffenberg15) notes that the integument of the varanid Varanus varius was only poorly effective at retarding heat loss and Stevenson16 includes thermal conductance between core and surface layer in a mathematical model of thermal balance in ectotherms.

Readings from the MBL anatomical site exhibited a mean Tbs – Ta differential of 2.16°C, which represented a thermal difference of 36.8%, 64.9% and 83.2% greater than the other three sites. One-way ANOVA of site Tbs – Ta differentials showed significant variation (P  <  0.001) between all four sites. The MBL site is located on the trunk of the lizard. The proximity of MBL to body organs and thus potential endogenous heat sources may be the reason for the greater Tbs – Ta differentials recorded at this anatomical site.

Bartholomew25 suggested that thermal conductance is so great in lizards that endogenous heat is lost at a rate that is proportional to its production. In small lizards, endogenous heat production is insignificant due to the rapid dynamics of heat gain and loss as a result of greater surface-to-volume ratios. However, in large lizards endogenous heat can be significant.15, 25, 26 In this present study, Varanus gouldii was one of the largest species and it exhibited a Tbs – Ta differential of 4.3ºC. The greatest Tbs – Ta differential, of 5.1°C, was recorded from the iguanid Cyclura cornuta. Although the latter species is shorter than V. gouldii, it is of greater bulk and thus may suggest mass to be a factor in heat exchange. Mass was not taken into account in this study, but could be a consideration for future lines of investigation.

Greater Tb – Ta differential in large ectotherm species is often attributed to thermal inertia.18, 27 The Komodo Monitor (Varanus komodoensis) is the world's largest extant lizard species. Auffenberg15 accredits Tb – Ta differentials in V. komodoensis to physiology. The large differentials recorded in V. gouldii and C. cornuta could therefore indicate the existence of some form of physiological heat production in these species. With thermal inertia, greater differentials in larger species might reflect transfer of internally generated heat from body core to surface. Heat dissipation through the integument may help prevent larger species from overheating.

If Tb – Ta differential is generally correlated with size it could counteract short-term changes in Ta. Small species need to actively bask for a fraction of the time required for large species28. Therefore small lizards are able to resume activity within much shorter time frames than larger species. To counteract an inevitable greater rate of cooling, smaller lizards perform short-term intermittent patterns of thermoregulation and activity throughout the day.

Bergmann's rule states that animals at higher latitudes are generally larger than those in the tropics.29 In contrast, Ashton and Feldman30 observe the opposite for snakes and lizards. Smaller size and therefore greater rate of heat gain would be of greater benefit at higher latitudes due to increased seasonal variation in temperatures. Thus there may be a larger versus smaller trade-off in lizards.

One of the two largest lizards (1800 mm) in my study, Iguana iguana, displayed a Tbs – Ta differential of 2.5°C. In accordance with the other data collected here, a lizard of this size should have a bigger Tbs – Ta differential. To put the result into context, a smaller (750 mm) sub-adult individual of I. iguana demonstrated a Tbs – Ta differential of 3.7ºC. The reason for the disparity may lie in the conditions in which the large individual was housed. The large I. iguana was housed in a large, open display as part of a multi-species exhibit in a zoo. No heat sources were provided and Ta fluctuated with Ta in the building. Avery10 states that iguanids, in general, are active at high constant temperatures. Wild adult I. iguana bask prominently and thus raise their Tb. Therefore without access to any direct radiative or conductive heat source in the zoo, the study individual would not be able to raise its Tb. In addition, the surrounding Ta of the large I. iguana was one of the lowest (23.5ºC) in the data set. Thus it is possible that at certain temperatures Tbs conforms more to Ta. If such a phenomenon does occur, a smaller Tbs and Ta differential would result in reduced heat loss from core. Reduced heat loss at a lower Ta supports the suggestion that integument dissipates heat in large lizards when under periods of high Ta.

Heliothermy and thigmothermy
An objective of the study was to compare heliothermy and thigmothermy in lizards. The results showed that Tbs – Ta differential in heliothermic species was positively correlated with length. Thigmothermic species, on the other hand seemed to show a negative correlation, though there were relatively few data.

Bennett31 correlated maximal locomotor capacity in lizards with Tb. Bennett concluded that species performed at their peak at higher Ta (higher than that would be experienced in the wild for some). Bennett's results suggest heliothermy could enhance lizard performance. Rummery et al.13 view heliothermy as a high cost method of raising body temperature. For example, diurnal species which often bask prominently in open areas to maximize radiative heat gain are exposed to increased risk of predation. Large heliothermic species are at less risk of predation. Also, because of their large size, their higher Tb means that relatively less time is spent basking. Most lizard species, however, are of small to moderate size and therefore heliothermy is risky.

Nocturnal lizards face different constraints since they cannot thermoregulate in the classical sense. Some nocturnal species use protected basking where the protection is afforded by a crevice or thick vegetation that does not impede solar radiation.10 Protected basking therefore, is used by some lizard species that remain inactive during the day but become active at night using the heat gained while at rest. Equally, some species use camouflage to enable discreet basking. For the purpose of these analyses, nocturnal lizards were categorized as thigmothermic. All the thigmothermic lizards in this study were geckos. Geckos control Tb actively through selection of substrata and passively through activity times (according to location or season).10 Inter-family comparison shows that the Gekkonidae exhibit the lowest mean overall Tbs of all six families in the study. Also, all geckos recorded low Tbs – Ta differentials and were of relatively small size (<250 mm). The negative correlation between Tbs – Ta differential and length seen in the study could suggest that geckos might get a metabolic benefit from smaller size.

Thermal conductance is greater during heating than in cooling.32 However, larger lizards require a longer period of contact with the substratum to reach their optimum body temperature. Therefore smaller size enables optimum body temperature to be reached in a shorter time frame. The same principle applies to heliothermic species and radiative heat gain.

The discriminant analysis showed five misclassifications. Sheltered individuals of T. gigas and G. stehlini were among the incorrectly classified taxa. In contrast, non-sheltered individuals of these species were classified correctly as heliothermic. Both sheltered and non-sheltered individuals of the same species were within the same enclosures. However, temperature measurements from one of each species were taken from individuals hiding in an enclosure refuge.

Sheltered individuals of T. gigas and G. stehlini showed markedly low Tbs – Ta differentials compared with non-sheltered individuals. Regal33 stated that lizards seek low temperatures when not active. Data obtained from the present study suggest that low temperature is more important than shelter, which would then be considered coincidental with low Ta. Shelters are often less influenced by short-term environmental temperature fluctuations and thus can provide stable microclimates.17 Therefore, it is probable that shelters are used to maintain selected body temperature through intermittent use.

Adolph4 notes that some terrestrial sheltered microhabitats are warmer than exposed arboreal perches. In relation to the present study the statement is not true, as recorded Ta was much greater outside of the shelter (2.4–3.9ºC). However, such a situation would be expected under captive conditions where oscillations in Ta are almost non-existent. Natural habitats, however, offer a more varied spatial distribution of differing thermal microclimates. Therefore wild lizards are presented with greater opportunities to exert control, which would lead them to establish their optimum body temperature.

In the discussion of heliothermy and thigmothermy, it should be remembered that neither are definitive means of thermoregulation. Pough et al.26 states that heliothermy and thigmothermy are ends of a spectrum in thermoregulation. Within heliothermy, further categorizations include whether a species performs shuttling or posturing thermoregulatory behaviours. Such behaviours were not accounted for the present study.

In summary, the results suggest that radiant heat transmission from the integument is greater in larger individuals. Exactly why this occurs is unknown and thus requires further investigation.

One possibility is that the integument of larger lizards shields them more efficiently than smaller bodied species from radiative heat gain, thus preventing the overheating of bodies that already possess greater Tb–Ta differentials, because of thermal inertia. Therefore, the results obtained could simply be attributed to heat reflecting from the integument rather than transfer from the core. However, this hypothesis does not encompass the younger stages of growth when maximizing radiative heat gain would be imperative.

This study could be improved by examining more individuals and species. A starting point for further study could use measurements of species larger than those in this study. As mentioned earlier, mass could also be measured to compliment length in order to give a better picture of individuals' volume.

A second line of investigation could be to compare dorsal Tbs and ventral Tbs. Tercafs34 notes differences in radiative transmission between dorsal and ventral integuments. Such a comparison could improve our knowledge of the role of conduction in lizard thermal relations. Also substratum temperature could be included in a future study and integuments of a selection of species could be examined.

Most of the species studied here (82%) were heliotherms. The limited number of thigmothermic species for this group means that no strong conclusions can be drawn regarding the relationship between length and Tbs–Ta differential. However, the results raise issues for future study. For example, does Tbs–Ta differential decrease further with greater length in thigmothermic lizards? Also how does the thermal performance of diurnal thigmothermic species, such as those present in deep forest habitats, compare with nocturnal species? The dearth of information on lizard thigmothermy is an incentive to continue the line of investigation.


   Funding
Top
 Abstract
 Materials and methods
 Results
 Discussion
 Funding
References
 
This project was self funded, although Canterbury Christ Church University did provide some funding for equipment.


   Acknowledgements
 
The author would like to thank the invaluable assistance of Stefan Terlinden who provided opportunity to collect data in Germany. Equal gratitude is bestowed to staff at Zoo Duisburg, Zoom Erlebniswelt at Gelsenkirchen and Terrazoo Rheinberg who permitted use of their lizards in the study. In addition the author thanks Chris Davies for allowing his private collection to be used for data collection. Finally, I would like to thank Professor Georges Dussart for his helpful guidance throughout the project.


   References
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 Abstract
 Materials and methods
 Results
 Discussion
 Funding
 References
 

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Submitted on 30 September 2007; accepted on 28 January 2008


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