BACKGROUND
geographical variation in environmental conditions is a major ecological factor involved in evolutionary diversification  <cit> .
since thermal regimes are particularly sensitive to latitude and altitude, geographical location imposes profound selection on organisms' metabolism, morphology and behaviour  <cit> , leading to covariation between phenotypic traits and geographical gradients  <cit> .
body size is known to exhibit substantial variation in relation to thermal differences among habitats  <cit> .
however, factors other than environmental temperature  may also impose selection on body size  <cit> .
as a result, models predicting patterns of evolutionary change in body mass in response to thermal variation are controversial  <cit> .
one such example is bergmann's rule  <cit> , which suggests that species body size increases with increasing latitude and elevation, and hence, with decreasing environmental temperatures  <cit> .
different hypotheses have attempted to elucidate the causal factors promoting the pattern predicted by bergmann's rule  <cit> .
potential explanations have focused on heat-conservation strategies  <cit> , later maturation to larger body size  <cit> , phylogenetic constraints  <cit> , interspecific variation in migration  <cit> , differential resistance to starvation  <cit> , and effects on somatic cell sizes  <cit> .
so far, the heat-conservation hypothesis, based on the fact that increases in body volume lead to decreases in relative surface area appears to be the most likely explanation  <cit> .
since the formulation of bergmann's rule more than  <dig> years ago  <cit> , several studies have explored the universality of its predictions across different lineages.
in general, the validity of this rule appears to depend on the thermoregulatory physiology of the studied model systems.
for endotherms, most evidence has supported bergmann's rule  <cit> .
this is presumably because the benefits of reducing heat-loss rates through larger body size are advantageous for cold climate organisms that maintain optimal body temperatures by metabolic generation of heat.
however, the relevance of bergmann's rule for ectotherms is less obvious, and supportive evidence is elusive  <cit> .
bergmann's rule in ectotherms
more than 99% of species are ectotherms  <cit> .
consequently, no biological prediction can be considered universal if it is not supported by these organisms.
the first tests to evaluate bergmann's rule in ectotherms  <cit>  claimed that up to 75% of studied species support its predictions.
however, these studies were based on simplistic statistical approaches, when the importance of conducting phylogenetic comparative studies was not yet appreciated  <cit> .
unsurprisingly, the development of phylogenetic comparative analyses to test bergmann's rule over the last decade has provided divergent lines of evidence.
while a series of studies confirmed previous findings  <cit> , others were more equivocal  <cit> .
for example, ray  <cit>  gathered data from previously published studies to evaluate bergmann's rule.
regarding fishes, this author concluded that it is "obeyed by a great number of fishes as shown by numerous reports in the literature".
curiously, no citations were provided to support this claim  <cit> .
more recently however, in a rigorous study conducted on more than  <dig> fish populations of different freshwater species from north america, belk and houston  <cit>  observed that these ectotherms tend to reverse bergmann's rule.
with regard to tetrapod ectotherms , evidence is similarly controversial  <cit> .
a number of phylogenetic studies have shown that groups of primarily aquatic and tropical-subtropical species follow bergmann's rule.
for example, some anurans, and most urodeles  and turtles exhibit a negative relationship between body mass and environmental temperatures  <cit> .
on the other hand, debate has progressively intensified in relation to squamate reptiles .
non-phylogenetic  <cit>  and phylogenetic  <cit>  studies have revealed that squamates exhibit only a weak tendency to conform to this model.
nevertheless, research conducted on some widespread groups has concluded that these reptiles can exhibit body size trends predicted by bergmann's rule  <cit> .
for example, a recent phylogenetic study conducted on liolaemus lizards supported this temperature-size model  <cit> .
cruz et al.
<cit>  observed that species of the clade liolaemus boulengeri  <cit>  show larger body size at higher latitudes and elevations.
an additional integrative analysis including species belonging to congeneric clades led cruz et al.
<cit>  to conclude that "the strong, positive, size-latitude relationship in the l. boulengeri clade apparently accounted for the pattern observed for the entire dataset".
however, after enlarging the sample of taxa belonging to one of the studied clades , pincheira-donoso et al.
<cit>  observed that these species do not support bergmann's rule.
among factors that might explain the disparity observed across studies testing bergmann's rule in ectotherms, the limited availability of phylogenetic studies is perhaps the most obvious  <cit> .
only scarce or generalized tests have been conducted on ectotherms, and most of this research focuses on the analyses of few species per clade   <cit> .
moreover, studies focused on analysing explicit patterns of body size variation in the species of the same clade in response to continuous geographical gradients are still rare  <cit> .
recent criticism suggests that a rigorous test of bergmann's rule should focus on species belonging to a monophyletic clade exhibiting substantial variation in patterns of body size and occurring in a broad geographical area encompassing a wide range of environmental conditions  <cit> .
lizards of the south american genus liolaemus offer unique opportunities to test predictions concerning the impact of selection on traits recognized as labile to variation in environmental temperatures, such as body size  <cit> .
this clade represents one of the most diverse known amniote lineages.
consisting of more than  <dig> named species, liolaemus occurs in the widest range of environmental conditions recorded for any lizard genus  <cit> .
these iguanians range from tropical-subtropical areas in brazil and peru, and the atacama desert in chile, to austral patagonia in tierra del fuego, the southernmost place where reptiles have been recorded  <cit> .
the altitudinal distribution of liolaemus is also one of the broadest known among squamate reptiles, occurring from sea level to over  <dig> m in the andes range  <cit> .
these biological features satisfy all of the requirements recognized as essential for a model group employed to test predictions concerning evolutionary radiations  <cit> .
here, we investigate the effect that continuous variation in environmental temperatures imposes on the evolution of body size among species of the liolaemus genus, using a comparative approach.
we studied a set of more than  <dig> species ,  <dig> of which are included in an explicit phylogenetic hypothesis .
this species sample represents almost the entire biogeographical, ecological and morphological diversity known for these lizards.
we aim to test the hypothesis that increasing latitude and elevation  are associated with larger body size  <cit> .
we suggest as an alternative hypothesis that large body size is disadvantageous for ectotherms in cold-climates, because it demands longer time basking to achieve optimal metabolic temperatures.
therefore, we expect to find weak or no evidence in support of bergmann's rule.
RESULTS
body size patterns under environmental gradients
we found non significant relationships between environmental gradients and body size in most of the tests performed.
in none of the studied clades was bergmann's rule supported .
for both methods of data analysis, significantly high or low standardized residual  values revealed the presence of outliers which might affect the accuracy of the models.
for independent clade analyses, extreme z-scores  were found in the clades montanus  and donosolaemus-magellanicus .
the relationship between independent contrasts for body size and adjusted latitudinal midpoint  revealed the presence of five outliers affecting the model .
the observed differences in the correlation and regression analyses including and excluding outliers are detailed below.
linear bivariate regression analyses showed that body size does not vary predictably with adjusted latitudinal midpoint  in the genus liolaemus.
for analyses conducted on clades separately, low proportions of variance were explained in the groups chiliensis, fitzingerii, wiegmannii and lineomaculatus .
regression analyses including outliers showed non-significant relationships between svl and alm in the clades montanus and donosolaemus-magellanicus.
when excluding outliers, we found a significant negative relationship between svl and alm for the clade montanus and a non-significant relationship in the clade donosolaemus-magellanicus .
linear regression analyses conducted on phylogenetic independent contrasts  revealed that alm cannot predict body size when including outliers , and when excluding outliers .
effects of sample size reduction
non-phylogenetic linear regressions conducted on the  <dig> species for which phylogenetic information was available , revealed that reduction of sample size for phylogenetic analyses  does not produce qualitative differences in the results.
as observed in the whole dataset , body size does not vary predictably with adjusted latitudinal midpoint  in any of the studied groups.
for these  <dig> species, non-significant proportions of variance were explained in the clades chiliensis , donosolaemus-magellanicus , fitzingerii , lineomaculatus , montanus  and wiegmannii .
the analysis on standardized residuals confirmed that none of the regression models suffered from oultiers.
discussion
this analysis of bergmann's rule is unique in including a representative proportion of the geographical, ecological, morphological and phylogenetic diversity of one of the most species rich terrestrial ectotherm vertebrate genera, liolaemus lizards .
our results fail to support bergmann's rule.
non-phylogenetic and phylogenetic analyses showed that increasing latitude and elevation do not predict increasing body size in liolaemus species.
these results further contradict the only known phylogenetic evidence in favour of bergmann's rule reported in squamate reptiles , and a recent large-scale analysis conducted on assemblages of european lizards supporting this macroecological model  <cit> .
thermal gradients, thermoregulatory physiology and body size
research on bergmann's rule has inspired debate about why increasing body size should be advantageous for cold climate species, and why its predictions are observed in some groups but not in others  <cit> .
for example, it has been suggested that the anatomical characteristics of the skin  in groups as different as birds, mammals and turtles make large body size advantageous in colder environments and that the absence of these skin characteristics in squamate reptiles explains why they generally fail to follow bergmann's rule  <cit> .
in contrast, much less reference has been made to the role that thermoregulatory physiology of organisms  can play during the process of body size evolution  <cit> .
indeed, an important part of the available discussions focuses on the benefits of increasing body size for heat retention in cold climate species.
however, since conservation of body heat in ectotherms is only possible once they have optimally achieved it , it appears to be simplistic to exclude the fact that increasing body mass also reduces the rates of heat gain  <cit> , which may be as critical as the need for retaining heat  <cit> .
this is illustrated, for example, by the observation that gravid females in several lizard species tend to bask more often or for longer than non-gravid females  <cit> .
it might be argued that lizard body size and heating rates do not covary in a linear fashion, as these organisms may adjust  some physiological and behavioural traits to increase heating rates in cold environments  <cit> .
for example, it has been observed that physiological adaptations may increase heating rates up to 17% in cold climate lizards  <cit> .
likewise, behavioural adjustments such as selection of basking sites less exposed to wind  <cit>  or modifications in the postures and body orientation to the sun may contribute to gain heat more rapidly  <cit> .
however, even though these adaptations contribute to increase heating rates in lizards inhabiting cold environments, their efficiency has limits.
indeed, a number of studies have shown that annual and daily activity, and thermoregulatory processes in cold climate lizards may vary substantially in comparison to warm climate lizards.
for example, it has been observed that high elevation  lizards tend to exhibit considerably shorter daily activities, often no more than four or five hours a day  <cit> , than species distributed in lower latitudes and elevations, which may remain active for over twelve hours per day  <cit> .
one of the most plausible factors to explain this pattern is that mean daily temperatures are lower, hot hours per day are fewer, and warm seasons are shorter in cold climates  <cit> .
therefore, even in presence of the above mentioned physiological and behavioural adjustments, cold environments restrict severely the patterns of activity in lizards, as a consequence of their failure to overcome the selective pressures imposed by low temperatures.
this suggests that the thermoregulatory limitations determined by the ectothermal condition of lizards might be one of the main  factors constraining the maximum attainable limits of body size in cold climate species.
large body mass in cold climate lizards would have dramatic consequences for ecological performance  and reproductive success , because in conditions of suboptimal metabolic temperatures most physiological functions  occur at suboptimal rates  <cit> .
for example, performance at prey capture, predator evasion, endurance, digestion, mate courtship, sperm production and conversion of lipids is substantially reduced at suboptimal body temperatures  <cit> .
also, basking for longer periods may increase the risk of predation by diurnal hunters  <cit> .
consequently, large body mass in cold climate lizards would be disadvantageous.
hence, the hypothesis of heat conservation cannot be accepted , as it predicts increasing body size in low-temperature environments.
these claims are supported by a number of previous studies conducted in lizards  <cit> .
finally, it might also be argued that large ectotherms can maintain higher constant minimum body temperatures than do smaller species.
however, it is unlikely that even relatively large cold climate lizards are able to retain heat overnight.
indeed, it has been observed in several species  <cit>  that during the initial period of basking, the body temperature of lizards is very similar to the environmental temperature.
additionally, field observations conducted at high latitudes and elevations in south america  reveal that different sized liolaemus lizards exhibit similar temperatures before initiating basking.
trophic niche evolution and body size
the evolution of trophic niches in lizards may provide additional evidence to support the idea that larger body size is disadvantageous for species living in cold environments.
although most lizards feed on other animals, omnivory and herbivory have evolved independently many times among these squamates  <cit> .
plant matter is a low energy food for lizards  <cit> .
therefore, omnivory and herbivory are advantageous for these reptiles only when certain morphological and thermoregulatory conditions are met.
in general, it has been observed that plant consumption has evolved primarily in species occurring in warm habitats, where vegetation is extremely abundant , or where abundance of animal prey is extremely low   <cit> .
in these environments, lizards can attain large body size and high body temperatures, both traits considered essential requirements for efficient digestion of plant matter  <cit> .
a large body allows for a more voluminous gut, and high body temperature provides optimal conditions for endosymbionts  specialized in breaking down otherwise indigestible plant matter  <cit> .
consequently, according to these rules of herbivory  <cit> , most lizards in which plant consumption has evolved are expected to reverse bergmann's rule, because larger species tend to occur in warm environments.
contrary to predictions, in a recent study espinoza et al.
<cit>  found that omnivory and herbivory have not only evolved in warm-climate and large lizards, but also in small species occurring in cold environments  <cit> .
these authors observed that a large number of small liolaemidae species  restricted to high latitudes or elevations, have substantial proportions of plant matter in their diets .
although the body size pattern observed in omnivorous and herbivorous liolaemidae species differs significantly from the pattern known for plant-consumer lizards occurring in warm climates, the explanation may be exactly the same.
since digestion of plant matter requires the highest possible body temperatures  <cit> , evolution of large body size in cold-climate liolaemidae species would decrease the thermoregulatory efficiency needed to achieve optimal metabolic temperatures.
this in turn would affect negatively the digestion of plant matter.
therefore, in the case of liolaemidae species occurring at high latitudes and elevations, evolution of large size would be disadvantageous, challenging patterns predicted by bergmann's rule.
why do some ectotherms appear to follow bergmann's rule?
thermal environment differs from some other sources of selection in that it is a dominant selective force at low temperatures  <cit> , but at higher temperatures, ectotherms are essentially freed from thermal constraints and hence other factors may become dominant in determining body size.
these include fecundity selection  <cit> , reduction of mass-specific energy requirements  <cit> , greater physiological homeostasis  <cit> , and reduction of mortality rates by predation  <cit> .
therefore, ectotherm clades whose geographical distributions are primarily restricted to tropical or subtropical ecosystems, such as continental turtles  <cit> , may exhibit increasing body size with increasing latitude and moderate elevations.
this pattern probably reflects a response to selective factors other than the thermal environment which is essentially benign throughout the geographic range of the group.
alternatively, observations of bergmann's clines in some ectotherm vertebrates  may be explained by the large taxonomic scale at which studies have been conducted  <cit>  which could mean that different patterns of body size variation at an interspecific scale might be obscured by patterns observed at inter-clade scales  <cit> .
consequently, for example, although ashton  <cit>  observed that salamanders tend to follow bergmann's rule, we could expect that a test incorporating a substantially larger number of species from a wider diversity of areas find that these amphibians do not follow bergmann's clines .
CONCLUSIONS
physiological processes in ectotherms are strongly dependent upon their body temperature  <cit>  with numerous consequences for survival and reproduction.
since heating rates in ectotherms are critically determined by body mass, large body size in species occurring at high latitudes or elevations is likely to be disadvantageous.
we found no evidence to support bergmann's rule in liolaemus lizards: increasing latitude and elevation are not associated with larger body size in these reptiles .
we suggest that bergmann's rule should be recognized as a macroecological prediction employed to investigate patterns of body size evolution only in endotherms, as it was originally proposed by bergmann  <cit> .

 geographical variation in environmental conditions is a major ecological factor involved in evolutionary diversification. since thermal regimes are particularly sensitive to latitude and altitude, geographical location imposes profound selection on organisms' metabolism, morphology and behaviour, leading to covariation between phenotypic traits and geographical gradients. body size is known to exhibit substantial variation in relation to thermal differences among habitats. however, factors other than environmental temperature may also impose selection on body size. as a result, models predicting patterns of evolutionary change in body mass in response to thermal variation are controversial. one such example is bergmann's rule, which suggests that species body size increases with increasing latitude and elevation, and hence, with decreasing environmental temperatures. different hypotheses have attempted to elucidate the causal factors promoting the pattern predicted by bergmann's rule. potential explanations have focused on heat-conservation strategies, later maturation to larger body size, phylogenetic constraints, interspecific variation in migration, differential resistance to starvation, and effects on somatic cell sizes. so far, the heat-conservation hypothesis, based on the fact that increases in body volume lead to decreases in relative surface area appears to be the most likely explanation. since the formulation of bergmann's rule more than years ago, several studies have explored the universality of its predictions across different lineages. in general, the validity of this rule appears to depend on the thermoregulatory physiology of the studied model systems. for endotherms, most evidence has supported bergmann's rule. this is presumably because the benefits of reducing heat-loss rates through larger body size are advantageous for cold climate organisms that maintain optimal body temperatures by metabolic generation of heat. however, the relevance of bergmann's rule for ectotherms is less obvious, and supportive evidence is elusive. bergmann's rule in ectotherms more than 99% of species are ectotherms. consequently, no biological prediction can be considered universal if it is not supported by these organisms. the first tests to evaluate bergmann's rule in ectotherms claimed that up to 75% of studied species support its predictions. however, these studies were based on simplistic statistical approaches, when the importance of conducting phylogenetic comparative studies was not yet appreciated. unsurprisingly, the development of phylogenetic comparative analyses to test bergmann's rule over the last decade has provided divergent lines of evidence. while a series of studies confirmed previous findings, others were more equivocal. for example, ray gathered data from previously published studies to evaluate bergmann's rule. regarding fishes, this author concluded that it is "obeyed by a great number of fishes as shown by numerous reports in the literature". curiously, no citations were provided to support this claim. more recently however, in a rigorous study conducted on more than fish populations of different freshwater species from north america, belk and houston observed that these ectotherms tend to reverse bergmann's rule. with regard to tetrapod ectotherms, evidence is similarly controversial. a number of phylogenetic studies have shown that groups of primarily aquatic and tropical-subtropical species follow bergmann's rule. for example, some anurans, and most urodeles and turtles exhibit a negative relationship between body mass and environmental temperatures. on the other hand, debate has progressively intensified in relation to squamate reptiles. non-phylogenetic and phylogenetic studies have revealed that squamates exhibit only a weak tendency to conform to this model. nevertheless, research conducted on some widespread groups has concluded that these reptiles can exhibit body size trends predicted by bergmann's rule. for example, a recent phylogenetic study conducted on liolaemus lizards supported this temperature-size model. cruz et al. observed that species of the clade liolaemus boulengeri show larger body size at higher latitudes and elevations. an additional integrative analysis including species belonging to congeneric clades led cruz et al. to conclude that "the strong, positive, size-latitude relationship in the l. boulengeri clade apparently accounted for the pattern observed for the entire dataset". however, after enlarging the sample of taxa belonging to one of the studied clades, pincheira-donoso et al. observed that these species do not support bergmann's rule. among factors that might explain the disparity observed across studies testing bergmann's rule in ectotherms, the limited availability of phylogenetic studies is perhaps the most obvious. only scarce or generalized tests have been conducted on ectotherms, and most of this research focuses on the analyses of few species per clade. moreover, studies focused on analysing explicit patterns of body size variation in the species of the same clade in response to continuous geographical gradients are still rare. recent criticism suggests that a rigorous test of bergmann's rule should focus on species belonging to a monophyletic clade exhibiting substantial variation in patterns of body size and occurring in a broad geographical area encompassing a wide range of environmental conditions. lizards of the south american genus liolaemus offer unique opportunities to test predictions concerning the impact of selection on traits recognized as labile to variation in environmental temperatures, such as body size. this clade represents one of the most diverse known amniote lineages. consisting of more than named species, liolaemus occurs in the widest range of environmental conditions recorded for any lizard genus. these iguanians range from tropical-subtropical areas in brazil and peru, and the atacama desert in chile, to austral patagonia in tierra del fuego, the southernmost place where reptiles have been recorded. the altitudinal distribution of liolaemus is also one of the broadest known among squamate reptiles, occurring from sea level to over m in the andes range. these biological features satisfy all of the requirements recognized as essential for a model group employed to test predictions concerning evolutionary radiations. here, we investigate the effect that continuous variation in environmental temperatures imposes on the evolution of body size among species of the liolaemus genus, using a comparative approach. we studied a set of more than species, of which are included in an explicit phylogenetic hypothesis. this species sample represents almost the entire biogeographical, ecological and morphological diversity known for these lizards. we aim to test the hypothesis that increasing latitude and elevation are associated with larger body size. we suggest as an alternative hypothesis that large body size is disadvantageous for ectotherms in cold-climates, because it demands longer time basking to achieve optimal metabolic temperatures. therefore, we expect to find weak or no evidence in support of bergmann's rule.  body size patterns under environmental gradients we found non significant relationships between environmental gradients and body size in most of the tests performed. in none of the studied clades was bergmann's rule supported. for both methods of data analysis, significantly high or low standardized residual values revealed the presence of outliers which might affect the accuracy of the models. for independent clade analyses, extreme z-scores were found in the clades montanus and donosolaemus-magellanicus. the relationship between independent contrasts for body size and adjusted latitudinal midpoint revealed the presence of five outliers affecting the model. the observed differences in the correlation and regression analyses including and excluding outliers are detailed below. linear bivariate regression analyses showed that body size does not vary predictably with adjusted latitudinal midpoint in the genus liolaemus. for analyses conducted on clades separately, low proportions of variance were explained in the groups chiliensis, fitzingerii, wiegmannii and lineomaculatus. regression analyses including outliers showed non-significant relationships between svl and alm in the clades montanus and donosolaemus-magellanicus. when excluding outliers, we found a significant negative relationship between svl and alm for the clade montanus and a non-significant relationship in the clade donosolaemus-magellanicus. linear regression analyses conducted on phylogenetic independent contrasts revealed that alm cannot predict body size when including outliers, and when excluding outliers. effects of sample size reduction non-phylogenetic linear regressions conducted on the species for which phylogenetic information was available, revealed that reduction of sample size for phylogenetic analyses does not produce qualitative differences in the results. as observed in the whole dataset, body size does not vary predictably with adjusted latitudinal midpoint in any of the studied groups. for these species, non-significant proportions of variance were explained in the clades chiliensis, donosolaemus-magellanicus, fitzingerii, lineomaculatus, montanus and wiegmannii. the analysis on standardized residuals confirmed that none of the regression models suffered from oultiers. discussion this analysis of bergmann's rule is unique in including a representative proportion of the geographical, ecological, morphological and phylogenetic diversity of one of the most species rich terrestrial ectotherm vertebrate genera, liolaemus lizards. our results fail to support bergmann's rule. non-phylogenetic and phylogenetic analyses showed that increasing latitude and elevation do not predict increasing body size in liolaemus species. these results further contradict the only known phylogenetic evidence in favour of bergmann's rule reported in squamate reptiles, and a recent large-scale analysis conducted on assemblages of european lizards supporting this macroecological model. thermal gradients, thermoregulatory physiology and body size research on bergmann's rule has inspired debate about why increasing body size should be advantageous for cold climate species, and why its predictions are observed in some groups but not in others. for example, it has been suggested that the anatomical characteristics of the skin in groups as different as birds, mammals and turtles make large body size advantageous in colder environments and that the absence of these skin characteristics in squamate reptiles explains why they generally fail to follow bergmann's rule. in contrast, much less reference has been made to the role that thermoregulatory physiology of organisms can play during the process of body size evolution. indeed, an important part of the available discussions focuses on the benefits of increasing body size for heat retention in cold climate species. however, since conservation of body heat in ectotherms is only possible once they have optimally achieved it, it appears to be simplistic to exclude the fact that increasing body mass also reduces the rates of heat gain, which may be as critical as the need for retaining heat. this is illustrated, for example, by the observation that gravid females in several lizard species tend to bask more often or for longer than non-gravid females. it might be argued that lizard body size and heating rates do not covary in a linear fashion, as these organisms may adjust some physiological and behavioural traits to increase heating rates in cold environments. for example, it has been observed that physiological adaptations may increase heating rates up to 17% in cold climate lizards. likewise, behavioural adjustments such as selection of basking sites less exposed to wind or modifications in the postures and body orientation to the sun may contribute to gain heat more rapidly. however, even though these adaptations contribute to increase heating rates in lizards inhabiting cold environments, their efficiency has limits. indeed, a number of studies have shown that annual and daily activity, and thermoregulatory processes in cold climate lizards may vary substantially in comparison to warm climate lizards. for example, it has been observed that high elevation lizards tend to exhibit considerably shorter daily activities, often no more than four or five hours a day, than species distributed in lower latitudes and elevations, which may remain active for over twelve hours per day. one of the most plausible factors to explain this pattern is that mean daily temperatures are lower, hot hours per day are fewer, and warm seasons are shorter in cold climates. therefore, even in presence of the above mentioned physiological and behavioural adjustments, cold environments restrict severely the patterns of activity in lizards, as a consequence of their failure to overcome the selective pressures imposed by low temperatures. this suggests that the thermoregulatory limitations determined by the ectothermal condition of lizards might be one of the main factors constraining the maximum attainable limits of body size in cold climate species. large body mass in cold climate lizards would have dramatic consequences for ecological performance and reproductive success, because in conditions of suboptimal metabolic temperatures most physiological functions occur at suboptimal rates. for example, performance at prey capture, predator evasion, endurance, digestion, mate courtship, sperm production and conversion of lipids is substantially reduced at suboptimal body temperatures. also, basking for longer periods may increase the risk of predation by diurnal hunters. consequently, large body mass in cold climate lizards would be disadvantageous. hence, the hypothesis of heat conservation cannot be accepted, as it predicts increasing body size in low-temperature environments. these claims are supported by a number of previous studies conducted in lizards. finally, it might also be argued that large ectotherms can maintain higher constant minimum body temperatures than do smaller species. however, it is unlikely that even relatively large cold climate lizards are able to retain heat overnight. indeed, it has been observed in several species that during the initial period of basking, the body temperature of lizards is very similar to the environmental temperature. additionally, field observations conducted at high latitudes and elevations in south america reveal that different sized liolaemus lizards exhibit similar temperatures before initiating basking. trophic niche evolution and body size the evolution of trophic niches in lizards may provide additional evidence to support the idea that larger body size is disadvantageous for species living in cold environments. although most lizards feed on other animals, omnivory and herbivory have evolved independently many times among these squamates. plant matter is a low energy food for lizards. therefore, omnivory and herbivory are advantageous for these reptiles only when certain morphological and thermoregulatory conditions are met. in general, it has been observed that plant consumption has evolved primarily in species occurring in warm habitats, where vegetation is extremely abundant, or where abundance of animal prey is extremely low. in these environments, lizards can attain large body size and high body temperatures, both traits considered essential requirements for efficient digestion of plant matter. a large body allows for a more voluminous gut, and high body temperature provides optimal conditions for endosymbionts specialized in breaking down otherwise indigestible plant matter. consequently, according to these rules of herbivory, most lizards in which plant consumption has evolved are expected to reverse bergmann's rule, because larger species tend to occur in warm environments. contrary to predictions, in a recent study espinoza et al. found that omnivory and herbivory have not only evolved in warm-climate and large lizards, but also in small species occurring in cold environments. these authors observed that a large number of small liolaemidae species restricted to high latitudes or elevations, have substantial proportions of plant matter in their diets. although the body size pattern observed in omnivorous and herbivorous liolaemidae species differs significantly from the pattern known for plant-consumer lizards occurring in warm climates, the explanation may be exactly the same. since digestion of plant matter requires the highest possible body temperatures, evolution of large body size in cold-climate liolaemidae species would decrease the thermoregulatory efficiency needed to achieve optimal metabolic temperatures. this in turn would affect negatively the digestion of plant matter. therefore, in the case of liolaemidae species occurring at high latitudes and elevations, evolution of large size would be disadvantageous, challenging patterns predicted by bergmann's rule. why do some ectotherms appear to follow bergmann's rule? thermal environment differs from some other sources of selection in that it is a dominant selective force at low temperatures, but at higher temperatures, ectotherms are essentially freed from thermal constraints and hence other factors may become dominant in determining body size. these include fecundity selection, reduction of mass-specific energy requirements, greater physiological homeostasis, and reduction of mortality rates by predation. therefore, ectotherm clades whose geographical distributions are primarily restricted to tropical or subtropical ecosystems, such as continental turtles, may exhibit increasing body size with increasing latitude and moderate elevations. this pattern probably reflects a response to selective factors other than the thermal environment which is essentially benign throughout the geographic range of the group. alternatively, observations of bergmann's clines in some ectotherm vertebrates may be explained by the large taxonomic scale at which studies have been conducted which could mean that different patterns of body size variation at an interspecific scale might be obscured by patterns observed at inter-clade scales. consequently, for example, although ashton observed that salamanders tend to follow bergmann's rule, we could expect that a test incorporating a substantially larger number of species from a wider diversity of areas find that these amphibians do not follow bergmann's clines.  physiological processes in ectotherms are strongly dependent upon their body temperature with numerous consequences for survival and reproduction. since heating rates in ectotherms are critically determined by body mass, large body size in species occurring at high latitudes or elevations is likely to be disadvantageous. we found no evidence to support bergmann's rule in liolaemus lizards: increasing latitude and elevation are not associated with larger body size in these reptiles. we suggest that bergmann's rule should be recognized as a macroecological prediction employed to investigate patterns of body size evolution only in endotherms, as it was originally proposed by bergmann. 
the impact of environmental gradients on the evolution of life history traits is a central issue in macroecology and evolutionary biology. our results fail to support bergmann's rule in liolaemus lizards. one such example is bergmann's rule, which predicts that body size will be positively correlated with latitude and elevation, and hence, with decreasing environmental temperatures. non-phylogenetic and phylogenetic analyses showed that none of the studied clades experience increasing body size with increasing latitude and elevation. therefore, evolution of larger body size in colder environments appears to be a disadvantageous thermoregulatory strategy. however, a later reassessment of this evidence, performed on one of the four analysed clades, produced contrasting conclusions. a recent study conducted on some species belonging to four of these six clades concluded that liolaemus species follow bergmann's rule, representing the only known phylogenetic support for this model in lizards. here, we investigate bergmann's rule in the six main clades forming the liolaemus genus, one of the largest and most environmentally diverse genera of terrestrial vertebrates. in cold environments, adaptations to gain heat rapidly are under strong positive selection to allow optimal feeding, mating and predator avoidance. most physiological and behavioural processes in ectotherms depend directly upon their body temperature.
 the impact of environmental gradients on the evolution of life history traits is a central issue in macroecology and evolutionary biology. a number of hypotheses have been formulated to explain factors shaping patterns of variation in animal mass. one such example is bergmann's rule, which predicts that body size will be positively correlated with latitude and elevation, and hence, with decreasing environmental temperatures. a generally accepted explanation for this phenotypic response is that as body mass increases, body surface area gets proportionally smaller, which contributes to reduced rates of heat-loss. phylogenetic and non-phylogenetic evidence reveals that endotherms follow bergmann's rule. in contrast, while previous non-phylogenetic studies supported this prediction in up to 75% of ectotherms, recent phylogenetic comparative analyses suggest that its validity for these organisms is controversial and less understood. moreover, little attention has been paid to why some ectotherms conform to this rule, while others do not. here, we investigate bergmann's rule in the six main clades forming the liolaemus genus, one of the largest and most environmentally diverse genera of terrestrial vertebrates. a recent study conducted on some species belonging to four of these six clades concluded that liolaemus species follow bergmann's rule, representing the only known phylogenetic support for this model in lizards. however, a later reassessment of this evidence, performed on one of the four analysed clades, produced contrasting conclusions.  our results fail to support bergmann's rule in liolaemus lizards. non-phylogenetic and phylogenetic analyses showed that none of the studied clades experience increasing body size with increasing latitude and elevation.  most physiological and behavioural processes in ectotherms depend directly upon their body temperature. in cold environments, adaptations to gain heat rapidly are under strong positive selection to allow optimal feeding, mating and predator avoidance. therefore, evolution of larger body size in colder environments appears to be a disadvantageous thermoregulatory strategy. the repeated lack of support for bergmann's rule in ectotherms suggests that this model should be recognized as a valid rule exclusively for endotherms. 