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Reptiles, thermoregulation and the environment

  • Institute for Development, Ecology, Conservation and Cooperation,
711 Leeds Road, Huddersfield, Yorkshire, UK
Since the early pioneering studies of Strelnikov (1934) Sergeyev (1939) and Cowles and Bogart
(1944) thermoregulation in reptiles has been recognised as a key element in their biology and
extensively studied. Numerous reviews of these works have been published, the most recent and
comprehensive (Avery, 1982) discussed the thermal relations of over 500 species. Much of the work
on reptile thermoregulation has concerned lizards but other groups including chelonians have been
investigated and several reviews published (eg Hutchison 1979; Sturbaum 1982; Meek & Avery
This paper looks at ways thermoregulating reptiles interact with their environments. It also
examines how environmental constraints set fundamental limits on, for example, behavioural
repertoires, thermal set points and distribution patterns. The main purpose is not to attempt a
comprehensive review of the subject since space does not permit what would be a major undertaking,
but to provide an insight into the subject using selective examples of research (citing chelonians
when appropriate) to highlight adaptations and problem areas.
It is hoped that an insight into the thermal ecology of free-living reptiles will enable
herpetologists involved in captive breeding projects to enhance captive environments. For example
Avery (1985) and Avery & Bond (1987) demonstrated that in thermoregulatory arenas, lizard
foraging activity increased with increasing spatial heterogeneity, and Hailey (1982) observed that
common lizards (Lacerta vivipara) switched from basking on grass to wood during cloudy weather to
enhance heating rates. Therefore although other factors such as space, humidity, and nutrition are
involved in captive breeding, thermal distribution patterns and substrate materials, particularly in
outdoor enclosures, are important factors to be taken into account in reptile husbandry.
Thermoregulation; the key to reptile biology
Thermoregulatory behaviour is not an end process in itself but a critical mechanism that enables
reptiles to enhance physiological performance i.e. running speeds, growth, reproduction and
digestion, by exploiting thermal distribution patterns in the environment to attain physiological
optimum body temperatures. The concept of a physiological optimum temperature is related to the
TESTUDO (1995) Volume four, number two, 56 - 78
body temperature which a reptile will often select, given a range of environmental thermal zones
from hot to cold. These are known as the preferred or eccritic body temperatures and are the
temperatures that the physiological processes are said to work best. The progression of efficiency
between the increments in temperatures toward the physiological optimum temperature can be most
clearly expressed in terms of a temperature coefficient, the Q10 value, derived from the equation,
Q10 = (K1/K2)10/( t
where K1 and K2 are the velocity constants proportional to the rates of reaction observed at
temperatures t1 and t2. Very simply this equation gives the predicted increase in physiological
processes for every increase in body temperature of 10°C. Most biological reactions have Q10"s of 2-
3; for example a Q10 of 2 for muscular energy between 20 - 30°C indicates an increase of twice the
amount of energy available in the muscles over that particular increase in temperature range.
There is, as well as thermal set points where physiological processes work optimally, an
optimum range of body temperatures that reptiles may employ. Optimum ranges fluctuate between
lower and upper thermal thresholds and not around a central point in temperature. In addition to
ecological factors, optimum temperature ranges can also be influenced by the physiological state of
the animal at any given time, for example digestion (Gatten, 1974) or reproductive condition (Obbard
& Brooks, 1979): this has led to the hypothesis that there are multiple physiological optimum
temperatures, although Huey (1982) has drawn attention to the inherent difficulties in this theory.
Seasonal differences in preferred body temperatures and ranges have been recorded (Turner et al.,
Reptiles are critically constrained by a series of body temperature thresholds, the ecological
critical maximum or minimum body temperatures. These are the temperatures that because they bring
about an absence of effective muscular coordination, restrict the reptile's ability to escape from
conditions which will ultimately lead to its death. Ecological critical (maximum) temperatures may
occur at temperatures close to operative field body temperatures. Hutchison (1979) gives a list (albeit
a now somewhat outdated list) of chelonian body temperatures. In chelonians for example ecological
maximum body temperatures occur around 41-43°C (Hutchison, Vinegar & Kosh, 1966). Death
occurs at the physiological critical (lethal) maximum or minimum body temperatures. Critical
thermal zones in body temperature ultimately set absolute limits on where or when reptiles can
Beer cans, thermoconformers and thermoregulators
One of the early definitions of evidence of thermoregulatory behaviour in reptiles was if there was an
elevation of body temperature above environmental temperatures. However in a famous " beer can "
experiment Heath (I964) showed that filled beer cans when placed out in the open also had
temperatures higher than environmental temperatures and argued that such a definition of
thermoregulation leads to the conclusion that either beer cans are able to thermoregulate or the
method of observation can be misleading.
Eventually evidence of thermoregulation was defined as a regression of body temperature with
air temperature by use of the regression equation,
y = mx + b
where the slope m relating the variables y (body temperature) and x (an environmental temperature)
is expected to vary between 1 and 0. The value of b in the equation defines the y intercept, the
starting point on the axis of the slope. Figure I shows this relationship. A slope with a value of 0
indicates a perfect thermoregulator since body temperature is completely independent of
environmental temperature. The slope with a value of 1 indicates a thermoconformer since y is in
agreement with x at all times; that is the animal is not attempting to thermoregulate but allowing its
body temperature to closely track environmental temperatures.
Regression equations can however be fitted to data with almost any amount of scatter and a
degree of intellectual judgment should be applied to their use. For example confidence intervals
around factor m can be calculated defining the agreement between m and the distribution of its data
base, and taken into account in any analysis. More recently a new method has been proposed (Hertz
1992). Here the view is that thermoconformity will not necessarily indicate body temperatures equal
to environmental temperatures and that the true measure should be between the body temperature the
reptile has and that which it would have if it did not thermoregulate. To test this hypothesis, models
are placed at random in the environment and their temperatures are compared with those of the
animals. Huey (1982) discusses the various methods of estimating extent of thermoregulation and
their limitations.
Thermoregulatory strategy
The evolution of thermoregulatory behaviour in reptiles almost certainly arose because environments
are not usually thermally stable but fluctuate both on a daily or seasonal basis or because of uneven
heat distributions within environments. Seasonal or daily shifts in reptilian thermoregulatory effort to
compensate would therefore have had selective advantages and although such adaptations have not
been extensively studied, where they have been sought they have been found. The fact that
physiological performance is optimal and profoundly influenced by temperature suggests that the
precision with which a reptile thermoregulates should indicate the cost that the habitat and its climate
imposes on precise thermoregulation; precise body temperatures should only be expected in
environments that are thermally stable and the heat sources easily accessible (low cost habitat). In
habitats where heat flow patterns are intermittent and/or difficult to access (high cost habitats),
reptiles should become imprecise thermoregulators and operate through the use of optimum ranges.
Therefore although a species may have a range of body temperatures for optimum physiological
performance, they can only be considered optimal in an ecological context if the costs incurred in
attaining them are low (Huey 1982). The optimum activity temperature(s) can be defined as the "
ecological optimum", that which enables the reptile to maximise its energy intake with the greatest
efficiency in any particular habitat (Huey & Slatkin 1976).
A theoretical model
Consider a hypothetical reptile, living in an ideal thermal environment where every day a broad range
of heat sources are readily available for it to select from. In addition to a thermo-friendly
environment the hypothetical reptile is free from predators or inter-specific competition and has a
regular and easily accessible food source. Under such conditions physiological optimum body
temperatures should be readily attained and hence the reptile could, through long daily spells at
optimum temperatures, operate at maximum physiological capacity for prolonged periods.
Figure 2 is a theoretical cost-benefit model for a reptile faced with a shift towards a cooler
climate. It shows the costs and benefits of retaining thermal set points or shifting to new lower
thermal set points. The retention of thermal set points requires an increase in the amount of time
spent basking with less time available for other ecological activities like feeding. This may not be
required if adjustments are made to lower set points but physiological costs resulting in (for example)
reduced running speeds are likely. This model is based on the "static" theory (Hertz et al., 1983);
Crowley, 1985) where thermal benefits are believed fixed and must be retained if physiological
performance is to be maintained. An alternative "labile" theory (Berkum, 1986; Huey & Bennet,
1987) argues that thermal optima are dynamic and will evolve with changing environments.
Reptiles in laboratory heat gradients.
Conditions approaching the optimum hypothetical situation actually exist. The experimental
conditions designed to examine reptile thermal biology in laboratories where in thermal gradient
chambers (usually a long enclosure with high temperatures at one end, graduating to cooler
temperatures at the other end) reptiles may experience conditions approaching optimum thermal
Figure 3 is a record of the body temperatures of two species of monitor lizard (Varanus) studied
in a laboratory thermal gradient (Meek 1978). The graph shows that these large lizards (480g, 1.7kg)
display the classical paradigm of heliothermy, that is, early morning basking produces an increase in
body temperatures to around 38°C followed by fluctuations of around 2°C as a result of alternate
periods of basking and movement behaviour. There are no predators, cloudy weather or the necessity
to spend considerable time searching for food to interfere with thermoregulation. The only real
variable (in this experiment) is between the two animals, the larger lizard because of its greater mass,
and hence lower surface area to body mass geometry has slower heating and cooling rates. Altering
the strength of the heating lamps may change the shape of the heating curves but has no significant
influence on the temperatures the lizards maintain at the upper end of the curve-this has important
implications for reptiles operating under natural conditions.
For a reptile to achieve optimum physiological body temperatures in natural environments an
adequate supply of heat to the habitat is vital. The attainment of operative body temperatures is
therefore primarily dependent on a number of important factors that include 1) the distribution of
heat within the habitat 2) the animals' ability to gain access to the heat source (varying degrees of
plant or cloud cover may restrict the heat rays' penetration through the habitat) 3) lifestyle, some
species may be adapted to certain types of microhabitat which receive less amounts of heat (e.g.
fossorial species) 4) the qualitative nature of the thermal energy available, for example the way a
reptile may be able to harness the sun's energy at high altitudes where there may be abundant
sunshine, but very low ambient air temperatures, may require a quite different strategy to a habitat
with a low number of sunshine hours but high ambient air temperatures.
Figure 4 shows a series of reptile habitats of different structural complexity from woodland with
only limited sunlight penetration and large amounts of shade (A) to Mediterranean scrub with almost
equal shade and sunlit areas (B) and hot North African desert with limited plant cover, dried up river
bed and hot sand dunes.
Thermoregulation at high altitudes
The most severe environments in which reptiles live are in high latitudes and altitudes. Thermal
studies have taken place in both types of habitat, but perhaps the most remarkable were in the
Peruvian Andes on the lguanid lizard Liolaemus multiformis above 4500m (Pearson, 1954). In this
region even summer climates are characterised by near freezing air temperatures often with snow on
the ground. However there are usually thick clumps of vegetation which enabled the lizards to
insulate themselves from cold soil temperatures and clear sunny skies - albeit with frequent snow
storms. By basking on the thick vegetation mats the lizards, even when emerging from their burrows
when air temperatures were as low as - 5°C, in two hours or so, elevated their body temperatures to
around 34°C. Depending on the frequency of the snow storms, Liolaemus could maintain high body
temperatures for a good part of the afternoon (Pearson, 1954).
At an altitude of 4100m in the Caucasus in similar harsh climatic conditions the Lacertid lizard
Lacerta agilis also displayed similar thermoregulatory adaptations attaining body temperatures of
almost 30°C above ambient air temperatures (Strelnikov 1944). On the Mediterranean island of
Corsica Bauwens et al., (1990) recorded body temperatures of 35°C in the Lacertid Lacerta
bedriagae at altitudes of 1750-1800m. Precise thermoregulation in a high cost habitat was apparently
achieved by subtle use of the environment such as using the abundant basking sites that were in full
sun and quickly retreating to shelter when the sun was obscured by cloud. It was suggested (Bauwens
et al., 1990) that the high and constant body temperatures were also achieved by restricting activity
only to those periods when environmental conditions made it possible to do so.
However there are high costs in respect of time budgets for reptiles in cold climates. In an
analysis of the time devoted each day to the various activities of Liolaemus, Pearson & Bradford,
(1976) observed that 16% of the lizard's time was spent thermoregulating (3.5 hours per day) only
0.3% (less than 5 minutes) feeding, a further 26 minutes to social behaviour and locomotory activity
and for more than 80% of their time they were inactive in their burrows (however time spent in the
burrowing part at least, is really thermoregulatory behaviour, since one of the primary functions here
is avoiding critically low body temperatures). Long and frequent basking periods are also part of the
costs that high altitude L.bedriagae incur for precise thermoregulation in Corsica. However extensive
basking in L.bedriagae results in similar body temperature levels to the closely related Podarcis
tiliguerta and P.sicula at sea level where there are much higher environmental temperatures
(Bauwens et al., 1990).
The costs for living in harsh environments were demonstrated by research on the viviparous
lizard (Lacerta vivipara). Lacerta vivipara is found over a range of altitudes and its distribution
stretches from north of the Arctic circle to the south to Spain- a distance of almost 3000 km, and east
to the Pacific coast, almost 12,000 km. Within this area L. vivipara populations inhabit a whole series
of thermal environments and provide a useful tool to make direct intra-specific comparisons. Van
Damme,. (1990) compared the thermal physiology of a population at an altitude of 2000-2200m
in the Austrian Alps with one at 25m in Belgium and found that body temperatures of the montane
populations were consistently 3-5° below those of the lowland population. The montane lizards were
often observed active at body temperatures that seriously affected locomotory capacity. There was
however no parallel shift in optimal temperatures for running speeds in the high altitude lizards and
complementary laboratory thermal gradient studies on individuals from both populations reinforced
the findings. These observations support the static view of thermal physiology, that thermal set points
are resistant to directional selection.
Van Damme et al., (1989) investigated altitude variation of the thermal biology of Podarcis
tiliquerta on the island of Corsica. At high altitudes lizard body temperatures were lower and more
variable than low altitude populations. The main response to changing thermal conditions was a shift
to lower thermal set points but results in impaired running speeds, particularly in the early morning.
P. tiliquerta has a thermal physiology that is apparently evolutionary conservative.
Thermoregulation at high latitudes
Even to the casual observer the increasing number of reptile species and their increasing
abundance in terms of sheer numbers of individuals as one moves north or south towards the equator
is apparent. Although the numerical gradient is not continuous -irregularities arise with local climatic
and ecological variables, the general pattern of distribution appears to be consistent (Dobzhansky,
Research into the thermal biology of (for example) both cool and warm temperate European
reptiles indicate that the majority are at least partially heliothermic often living in relatively open
habitats (Fig 4b) where sunshine is easily accessible and, particularly in the case of those lizard
species living in the Mediterranean region, they often maintain high body temperatures of >38°C (see
Spellerberg, 1976; Avery, 1982, for general reviews and Meek and Avery, 1988 for chelonians). Two
species at least are believed to be thigmothermic- utilizing heat from the substrate, although one of
the two, the slow-worm Anguis fragilis- has a rather complex thermal lifestyle often operating at low
body temperatures but on occasion can be seen basking (Spellerberg, 1976 and pers. observation).
Generally those species that live at the highest latitudes maintain the lowest activity temperatures (eg
L. vivipara 30-33°C; A. fragilis <30°C) but there is at present insufficient information to generalise
usefully about latitudinal variations in body temperatures and thermoregulatory patterns. In the slow-
worm it may be that much of its activity patterns are generated by the activity patterns of its prey
species (slugs and earthworms). Thermoregulation may only be of secondary importance in Anguis
and for much of its time it may operate as a thermoconformer.
Behavioural complexity has been shown to be inversely related to climate and latitude in certain
European lacertid lizards (Avery, 1976). Thermoregulatory behaviour in the cool temperate Lacerta
vivipara occupies a greater fraction of the daily time budget than in the warm temperate Podarcis (=
Lacerta) sicula. The northern species consequently has less time available to evolve complex social
Thermoregulation in Chelonians
The thermal relations of Chelonians have received less attention than those of lizards. The work
carried out to date has indicated that most species regulate their body temperature by basking and
retreating into shade or water. Body temperatures of those studied either in the laboratory (Hutchison,
1979) or in the field (Avery, 1982; Meek & Avery, 1982), rarely exceed 34°C, the exceptions are
certain semiaquatic Emydids (eg Graptemys & Trachemys (=Pseudemys) although the desert living
Gopherus maintain relatively high body temperatures (>37°C, Brattstrom, 1965).
Semi-aquatic terrapins. The evolution of the chelonian shell appeared early in the history of the
group (Romer, 1968) but the shells of present day forms vary noticeably between species and such
differences can significantly influence their thermal biology. Many species of semi-aquatic terrapins
often spend long periods basking on river banks or logs to elevate their body temperature (Boyer,
1965). Experiments with water-filled models of aquatic terrapins painted black showed that flatter
bodied species had faster rates of heating but the difference could be modified by the reflectivity of
the integument; therefore although the flat shaped soft shelled terrapins Trionyx had the highest rates
of heat gain as far as the models were concerned, in real life darker shelled emydids of the same mass
had similar rates of heat gain. Apparently the darker colour (and possibly the different texture of the
emydid shell) has selective advantages for their lifestyle and cancels out any differences (Boyer
The more spherical shape retained by the emydids could also have selective advantages since it
would enhance heat retention, particularly useful in the temperate zones where one of the major
problems encountered by a terrapin basking on a log on a sunny Spring day could be in experiencing
very cold temperatures when re-entering the water. In habitats where environmental temperatures are
high, basking even in emydids may be limited (eg Meek, 1983), confined to surface water basking or
be abandoned altogether (Avery, 1982 for review). Basking and precise thermoregulation may be
only of minor importance (if at all) in non-emydid forms, the animals in effect operating as
thermoconformers, for example the Kinosternidae (Cagle, 1944; Edgren & Edgren, 1955; reviews in
Brattstrom, 1965; Avery, 1982)). However basking in terrapins is a complex behaviour and its
duration in (for example) Trachemys ( = Pseudemys) scripta has been shown to vary between season,
sex, temperature and digestive state (Hammond et al., 1988). Female Trachemys basked longer in
spring and summer than males and fed animals of both sexes basked for longer than unfed animals.
In autumn and winter such differences apparently did not exist. At high environmental temperatures
both sexes, whether fed or unfed, spent significantly less time basking.
Ecological influences other than thermoregulation are clearly involved in chelonian
morphometry, for example predation, sexual strategy and movement mechanics have had important
influences in their evolutionary biology. In aquatic forms a low profile shell design may reduce
resistance to the water, but be less useful against predators and for enhancing egg production in
females (Iverson, 1984; Meek,1987). Indeed the primary function of basking although generally
regarded as thermoregulatory may have secondary uses, the drying out of the shell could assist the
animals in controlling parasites or reducing algal growth which can cause severe deterioration of the
shell (Cagle, 1950; Neill & Allen, 1954; Meek, 1987) and may be involved in the production of
vitamin D (eg Holick, 1989).
Terrestrial chelonians. The terrestrial tortoises are slow moving (Jayes & Alexander, 1980) and
often grow to very large sizes which may give rise to acute problems of overheating in areas with
limited shade (Swingland & Frazier, 1979) or in the smaller species restrict movement through open
areas (Meek, 1984; Branch, 1984). Although the larger species, as a consequence of their low surface
to body mass geometry, reduce rapid heat exchange with the environment, this can present particular
problems of overheating and even heat death through thermal inertia (Swingland & Frazier, 1979).
Body geometry and locomotion are clearly important constraints on chelonian thermal ecology,
however predation, a major constraint on thermoregulation (Huey, 1982) is reduced as a result of the
heavy armour provided by the shell.
Environmental constraints on thermoregulatory behaviour has been usefully examined in
Mediterranean tortoises particularly Testudo graeca and T. hermanni. The earliest works concerned
T. graeca populations in North Africa (Lambert, 1981, 1983; Meek & Jayes, 1982) where cloud
cover (Lambert, 1981) or sea mists (Meek & Jayes, 1982) were observed to prolong basking into the
latter part of the day, delaying the attainment of maximum body temperatures of 35°C to later in the
day than on days when the skies were clear and sunny. In Europe T. graeca were observed by Wright
et al, (1988) during the summer months in Greece where they occupied relatively open habitat and
attained body temperatures of 33°C or higher. These body temperatures were slightly higher than
those recorded in N. Africa by Meek & Jayes, 1982 (=29.8°C) under sea mists (Greece, = 30.9, 29.7
& 31.1°C).
In Greece, the closely related T. hermanni was often found with T. graeca in habitats of similar
structural complexity and with similar activity periods and body temperatures (Wright et al,. 1988).
However T. hermanni also occupied less open habitat such as pine and broad leafed woodland, where
activity shifted from the bimodal pattern observed in the open heathland areas to a unimodal pattern
with significant midday activity. Body temperatures of T. hermanni were lower in enclosed habitats.
This is in general agreement with body temperature levels of T. hermanni in woodland areas in
southern France where there is also only limited sunlight penetration and limited basking activity
(Pulford et al, 1984). Seasonal shifts in activity patterns and body temperatures have been observed
in several populations of T. hermanni. Hot summer weather induced bimodal activity and high body
temperatures (34-35°C) in Croatia, Montenegro and Greece ; spring and autumn produced unimodal
activity and generally lower body temperatures (Meek, 1984; 1988a; Panagiota & Valakos, 1992).
Panagiota & Valakos, (1992) observed winter activity at low body temperatures in T. marginate in
Attica although in the same area T. hermanni entered hibernation.
The work of Wright et al, (1988) made an important contribution to the thermal biology of
Mediterranean tortoises since it identified habitat use and separation and the results were discussed in
the context of the group's evolutionary history. For such purposes analysis was restricted to the
thermal loads of the population as a collective unit (ie the data sets from both sexes were pooled).
However significant size differences exist between the sexes of T. hermanni (females may be as
much as twice as large as males in wild populations (eg Meek, 1985; 1989, Meek & Inskeep, 1981)
and in the former Yugoslavia this was found to significantly influence thermoregulatory behaviour in
different ways through different seasons. In spring T. hermanni displays the classical paradigm of
reptilian heliothermy with basking mainly occurring in the earlier part of the day and other activities
such as feeding and locomotory activity initiated once body temperatures of 34°C or so are reached
(Meek, 1984). In summer the total time spent basking is reduced allowing increased time available
for feeding, locomotory and sexual activity, but females due to their larger size experience a thermal
lag not attaining operative body temperatures until later in the day than males (Meek, 1988a).
How does thermal lag affect the daily lives of tortoises and indeed is it significant enough to
have fundamental consequences for their evolutionary biology? In the region where these tortoises
were studied, they are close to their northernmost limits in Europe and the environmental conditions
found here and the thermal lag they produce on the larger species of reptile may ultimately impose
maximum size constraints, since the amount of available basking time sets absolute limits on what
gross body temperatures may be achieved daily and annually. Interestingly large females in this
region do not increase basking intensity during cooler spring and summer weather to compensate for
their larger size but compromise by accepting lower body temperatures whilst expanding feeding
periods. In autumn females increase basking intensity but are still unable to achieve normal operative
body temperatures - although the smaller sized males manage this (Meek, 1988a).
It would appear that T. hermanni females have evolved large size to maximise egg production
and operate a compromise between physiological optimum and ecologically attainable temperatures
(Meek, 1988a). Male T. hermanni size limitations may involve greater mobility, for example to make
contact with greater numbers of females, which would be enhanced by the Q10 effects of higher body
temperatures. Although thermal lag is not as significant in juvenile T. hermanni thermal ecology,
other factors such as predation or habitat familiarity may have an important role in restricting
juvenile body temperatures to slightly lower levels than those of adult males (Meek, 1988a).
Predation has been suggested as a constraint on basking in hatchling terrapins (Jansen eL al., 1992)
and habitat familiarity on the body temperatures of introduced tortoises (Chelazzi & Calzolia, 1986).
The thermal environment does not rigidly constrain all reptiles to a series of homogenous
responses. In former Yugoslavia where T. hermanni were studied, the sympatric glass lizard
(Ophisaurus apodus) is active on cool cloudy days when tortoises remain in their overnight retreats.
The lizards are active at significantly lower body temperatures than is normal and when they are
much more at risk to attacks from predators (Meek, 1984). Hailey (I984) has drawn attention to
similarities in the physiological ecology of Ophisaurus with Testudo, in particular its reliance on
dermal armour as a defence mechanism (Ophisaurus is 70% as armoured as a similar sized Testudo)
and similar low metabolic scope.
Concluding remarks
The study of the thermal ecology in reptiles now encompasses such a wide range of disciplines that
in this relatively short selective review it has not been possible to even briefly mention works from
other major lines of investigation. References to research on, for example, environmental sex
determination (review; Janzen & Paukistis, 1991), thermal constraints on nesting female reptiles
(Spotila & Standora, 1985; Meek, 1988b), partial endothermy in sea turtles, monitor lizards and
female pythons (review in Bartholomew 1982), the role of the parietal eye in reptile thermoregulation
(Firth & Turner, 1982), thermal involvement in reproductive cycles (eg Licht, 1972 and review in
Duval et al., 1982), winter dormancy (Gregory, 1982) and strategies to overcome conflicting thermal
requirements of female lizards and their developing embryos (Beuchat, 1986) have been omitted
although such lines of enquiry are of critical importance to our understanding of the subject. Even the
medical aspects of reptilian infections, again not reviewed here, can tell us much about the
evolutionary background of thermal biology. For example the way certain lizards Dipsosaurus,
Agama and Sceloporus apparently respond to pathogens by inducing an "ectothermic fever" to
enhance their survivorship capability is an intriguing aspect of thermal biology that may ultimately
throw new light on the origins of host defence mechanisms to bacterial infections in vertebrates
(Kluger, 1979; Ortega et al., 1991).
Previously reptile ectothermic physiology was regarded as an inferior physiological system in
comparison to that of the endothermic mammals and birds, as if reptiles were in some way failed
endotherms. Research has now shown the fallacy of this view and has indicated reptilian ectothermy
as an evolutionary route in the direction of a low budget energy cost physiology which drains
environmental resources at a less intense rate than mammals and birds enabling ectothermic
vertebrates to colonise large areas of the world, often in high population densities.
Dr Roger Avery critically reviewed the manuscript.
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... For reptiles, thermoregulatory behavior is a crucial mechanism for maintaining body temperature (Meek 1995;Bansal 2020). Black heat-radiating surfaces help maintain temperature during nighttime hours but may lead to road mortality (Bernardino and Dalrymple 1992;Bambaradeniya et al. 2001;Selvan et al. 2011;Karunarathna et al. 2013;Bansal 2020;Das and Vishnu 2021). ...
... Although reptiles rely on external temperatures, optimal values fluctuate between thermal maximum and minimum thresholds according to their varied physiological needs (Gatten 1974;Obbard and Brooks 1979). Suboptimal thermal values reaching either maximum or minimum thresholds could attain or exceed physiological critical points leading to animal death and possibly to regional extirpation (Meek 1995;Gilman et al. 2010). ...
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Mortality of freshwater turtles on a railway track in Puducherry, India
... Reptiles are ectotherms and therefore depend on the ambient temperature to regulate their body temperature, which must be maintained within a narrow range to carry out necessary physiological and behavioral processes (Adolph & Porter, 1993;Avery, 1982;Huey, 1982;Meek, 1995). According to the costbenefit model proposed by Huey and Slatkin (1976), the cost of thermoregulation is determined by the thermal quality of the environment (d e = the degree of discrepancy between the environmental temperatures available and the body temperatures that animals prefer, Hertz et al., 1993). ...
Aggressive behavior is performed in the context of intraspecific competition for gaining access to mates, food, or suitable territories. However, aggressive confrontations may divert time and energy from other important activities and increase the likelihood of suffering physical injury or predation. Aggressive behavior is particularly costly for ectotherms because it may reduce the time available for thermoregulation, which is a time‐consuming activity but indispensable for adequate maintenance of metabolic processes. In this study, we analyzed the long‐term effect of the thermal quality (i.e. the degree of discrepancy between the temperatures available in a given environment and temperatures that animals prefer) on the aggressive behavior of the mesquite lizard Sceloporus grammicus. Our hypothesis was that time allocated to aggressive behavior in low thermal quality environments is diverted from time spent on the acquisition and maintenance of an adequate body temperature. Accordingly, we found that lizards from the low thermal quality location (i.e. low environmental temperature) exhibited less aggressive behavior than those captured in middle and high thermal quality locations. Our results show that in the low thermal quality location aggressive behavior was almost absent, suggesting that this behavior may interfere with the acquisition and maintenance of an adequate body temperature. Therefore, it is likely that the benefits of thermoregulation outweigh those of aggressive behavior at low thermal quality locations. Aggressive behavior is performed in the context of intraspecific competition, however, aggressive confrontations may divert time and energy from other important activities and increase the likelihood of suffering physical injury or predation. Aggressive behavior is particularly costly for ectotherms because it may reduce time available for thermoregulation. In this study, we analyzed the long‐term effect of the thermal quality (i.e. the degree of discrepancy between the temperatures available in a given environment and temperatures that animals prefer) on the aggressive behavior of the mesquite lizard Sceloporus grammicus. Our results show that in the low thermal quality location aggressive behavior was almost absent, suggesting that this behavior may interfere with the acquisition and maintenance of an adequate body temperature.
... For ectotherms like reptiles thermoregulatory behavior is a critical mechanism (Bansal 2020) for mainting proper body temperature (Meek 1995). Black tar roads help raise body temperatures during the daytime and slow the release of heat at night, attracting reptiles to the roads for thermoregulation (Bernardino and Dalrymple 1992;Bamabaradeniya et al. 2001;Selvan et al. 2011;Karunarathna et al. 2013;Bansal 2020). ...
... For ectotherms like reptiles thermoregulatory behavior is a critical mechanism (Bansal 2020) for mainting proper body temperature (Meek 1995). Black tar roads help raise body temperatures during the daytime and slow the release of heat at night, attracting reptiles to the roads for thermoregulation (Bernardino and Dalrymple 1992;Bamabaradeniya et al. 2001;Selvan et al. 2011;Karunarathna et al. 2013;Bansal 2020). ...
... Age and consequent body size increase may alter an animal's resource use, as this influences the behavioural and physiological strategies the animals may employ, such as predator avoidance or thermoregulation [24]. The physiology of an ectothermic animal influences the time spent within resource areas: larger individuals of a lizard species have an increased capacity for retaining heat through thermal inertia and thus may perform less shuttle heliothermy [25], tending to maintain their preferred body temperature through more subtle behavioural strategies such as mouth gaping and microhabitat selection [26][27][28]. Recent work indicates that captive S. c. crocodilurus thermoregulate by altering habitat use, though overall the species has a weak propensity for accurate thermoregulation [29]. ...
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This study compared the resource use of juvenile zoo-living Chinese crocodile lizards, Shinisaurus crocodilurus crocodilurus across three observation windows, spanning nine months, accounting for time of day and lizard age, and under consistent environmental conditions. Lizards showed a significant difference in proportionate resource use, quantified using a modified spread of participation indices between the second and final sampling period, such that with increasing age, resources were more equally utilised. The time of day did not have a significant effect on resource use. Lizards in this study significantly increased their use of water bodies and branches outside the bask zone and decreased their use of the land areas within the bask zones over time. Resource use data suggests the importance of providing enclosures which cater to ontogenetic shifts in captive individuals or within mixed age groupings.
... Non-avian reptiles need to maintain their body temperature within a specific temperature range in order to adequately accomplish necessary physiological processes and ecological activities (e.g. mating, feeding; Huey, 1982;Adolph and Porter, 1993;Meek, 1995). For lizards, however, thermoregulation is a time-consuming activity that is energetically costly because the energy and time needed for it may compromise the time or energy that otherwise can be devoted to foraging, acquiring and maintaining a territory, mating and to avoiding predators (McConnachie and Alexander, 2004). ...
A 20-month recapture analysis of 1001 individually marked mesquite lizards (Sceloporus grammicus) suggests that variation in thermal quality across three altitudes influences survival probability. Each additional unit of deviation from the temperature selected by these lizards in previous laboratory experiments (i.e. decreased thermal quality) meant an increase of roughly 1.01% in survival probability. Survival probabilities ranged from 0.80 to 0.90 at the lowest elevation site (2600 m), from 0.76 to 0.87 at the middle elevation site (3100 m) and from 0.90 to 0.94 at the highest elevation site (4150 m). These results suggest that in poor thermal quality environments mesquite lizards may employ thermoregulatory strategies (behavioral, physiological and/or morphological) to decrease their metabolic expenditure and their exposure to predators, maximizing survival. These findings highlight the relevance of thermal quality of the habitat in determining survival probability of ectotherms.
... Appropriate environmental temperatures also increase metabolic rate, digestion rate and digestibility, all factors that indirectly contribute to the maintenance of Ca homeostasis. For example, aquatic turtles need an environmental temperature of between 25 and 30˚C to obtain an average body temperature of 34˚C (Meek, 1995; Manning & Grigg, 1997; Bury et al., 2000; Steyermark & Spotila, 2000). Metabolic rate affects energy needs and expenditure, and will influence both interspecies and intraspecies comparisons for dietary formulations. ...
This review reports on a decade of nutrition research into calcium (Ca) homeostasis in freshwater turtles, including research on non-nutritive factors that are essential for Ca homeostasis [i.e. ontogeny, environmental temperature and humidity, and ultraviolet (UV) light and photoreception]. Recommendations for future research include long-term research programmes in three specific areas: (1) photoreception, UV light and biosynthesis, (2) Ca homeostasis and vitamin and mineral supplementation, and (3) developmental indices, gut transit time (GTT) and energy requirements.
Temperature influences physiology and behavior of all organisms. For ectotherms, which lack central temperature regulation, temperature adaptation requires sheltering from or moving to a heat source. As temperature constrains the rate of metabolic reactions, it can directly affect ectotherm physiology and thus behavioral performance. This direct effect is particularly relevant for insects whose small body readily equilibrates with ambient temperature. In fact, models of enzyme kinetics applied to insect behavior predict performance at different temperatures, suggesting that thermal physiology governs behavior. However, insects also possess thermosensory neurons critical for locating preferred temperatures, showing cognitive control. This suggests that temperature-related behavior can emerge directly from a physiological effect, indirectly as consequence of thermosensory processing, or through both. To separate the roles of thermal physiology and cognitive control, we developed an arena that allows fast temperature changes in time and space, and in which animals' movements are automatically quantified. We exposed wild-type and thermosensory receptor mutants Drosophila melanogaster to a dynamic temperature environment and tracked their movements. The locomotor speed of wild-type flies closely matched models of enzyme kinetics, but the behavior of thermosensory mutants did not. Mutations in thermosensory receptor dTrpA1 (Transient receptor potential) expressed in the brain resulted in a complete lack of response to temperature changes, while mutation in peripheral thermosensory receptor Gr28b(D) resulted in diminished response. We conclude that flies react to temperature through cognitive control, informed by interactions between various thermosensory neurons, whose behavioral output resembles that of enzyme kinetics.
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Some giant tortoises (Geochelone gigantea} migrate to the relatively shadeless coast during the rainy season to obtain the seasonal flush of food which results in migrant females laying larger clutches than those tortoises that stay inland. The migrants face the conflict of maximizing food intake and minimizing the risk of death from overheating. Internal body temperature was measured using radio pills: other observations were made of ambient temperature, critical thermal maximum,sward height, tortoise density and body size distribution of grazing tortoises in relation to distance from the nearest shade center. The results are: (i) on cool days tortoises are active all day but on hot days they shade at midday, (ii) CTMax is 36-38°C, (iii) larger tortoises spend longer in the shade, (iv) the need to shade imposes limitations on feeding range, (V) different size tortoises do not have different size feeding ranges. the third, which rainy season (Swi than their conspecifics who have remained inland. However there are hottest time of yi Indeed, death from overexposure is the major apparent cause of 611 often single trees, to which the tortoises must retire during the heat of the day Risk of overheating not only limits the maximum distance from each center at which on feeding range, (v) different size tortoises do not have different size feeding ranges
The view that behavior and physiological performance are tightly coadapted is a central principle of physiological ecology. Here, we test this principle using a comparative study of evolutionary patterns in thermal preferences and the thermal dependence of sprinting in some Australian skinks (Lygosominae). Thermal preferences (Tp ) differ strikingly among genera (range 24° to 35°C), but critical thermal maxima (CTMax) (range 38° to 45°C) and optimal temperatures for sprinting (To , 32° to 35°C) vary less. Diurnal genera have relatively high Tp , To , and CTMax. In contrast, nocturnal genera have low Tp but have moderate to high To and CTMax. Both nonphylogenetic and phylogenetic (minimum-evolution) approaches suggest that coadaptation is tight only for genera with high Tp . Phylogenetic analyses suggest that low Tp and, thus, partial coadaptation are evolutionarily derived, indicating that low thermal preferences can evolve, even if this results in reduced performance. In one instance, thermal preferences and the thermal dependence of sprinting may have evolved in opposite directions, a phenomenon we call "antagonistic coadaptation." We speculate on factors driving partial coadaptation and antagonistic coadaptation in these skinks.
We measured basking duration of fed and nonfed yellow slider turtles (Pseudemys scripta) in a constant-temperature environmental chamber. We acclimated and tested turtles at four temperatures (10, 20, 30, and 35 C), once in spring/summer and once in fall/winter. In the spring/summer there was a statistically significant difference (P < .01) between basking durations of fed and nonfed turtles and between basking times of males and females at all acclimation temperatures except 35 C, when basking time was minimal for both fed and nonfed animals. Fed females basked much longer than fed males (P < .05), and both groups of fed turtles basked much longer than nonfed animals. These differences were greatest at 10 and 20 C but were also apparent at 30 C. In the fall/winter there were no statistically significant differences between the basking times of fed and nonfed turtles or between the basking times of males or females. There were, however, statistically significant differences (P < .01) between basking times at all four acclimation temperatures. We suggest that basking in turtles is a complex behavior that is affected by the nutritional status, acclimation, season, and sex of the individual.