Testing Bergmann’s rule and the Rosenzweig hypothesis with craniometric studies of the South American sea lion

We tested the validity of Bergmann’s rule and Rosenzweig’s hypothesis through an analysis of the geographical variation of the skull size of Otaria flavescens along the entire distribution range of the species (except Brazil). We quantified the sizes of 606 adult South American sea lion skulls measured in seven localities of Peru, Chile, Uruguay, Argentina, and the Falkland/Malvinas Islands. Geographical and environmental variables included latitude, longitude, and monthly minimum, maximum, and mean air and ocean temperatures. We also included information on fish landings as a proxy for productivity. Males showed a positive relationship between condylobasal length (CBL) and latitude, and between CBL and the six temperature variables. By contrast, females showed a negative relationship between CBL and the same variables. Finally, female skull size showed a significant and positive correlation with fish landings, while males did not show any relationship with this variable. The body size of males conformed to Bergmann’s rule, with larger individuals found in southern localities of South America. Females followed the converse of Bergmann’s rule at the intraspecific level, but showed a positive relationship with the proxy for productivity, thus supporting Rosenzweig’s hypothesis. Differences in the factors that drive body size in females and males may be explained by their different life-history strategies. Our analyses demonstrate that latitude and temperature are not the only factors that explain spatial variation in body size: others such as food availability are also important for explaining the ecogeographical patterns found in O. flavescens.     

Bergmann's rule in shrews: geographical variation of body size in Palearctic Sorex species

According to Bergmann's rule, individuals of a given species tend to be larger in colder (northern) climates. Traditional explanation points to the relatively lower surface‐to‐volume ratio in larger animals and, consequently, relatively lower costs of thermoregulation. We examined intraspecific covariation of body size with geographical location and climate in five species of Sorex shrews, animals that are among the smallest extant mammals. The condylobasal length of skull (CBL), compiled from literature data and measured on museum specimens, was used as an indicator of the overall body size of shrews. Surprisingly, in three out of five shrew species, the CBL was negatively correlated with latitude, and the same trend, although not statistically significant, was found in the fourth species. In general, shrews were smaller in colder areas, as evidenced by the positive correlations between the CBL and temperature. In two species, these positive correlations appeared when the effect of longitude was held constant in the partial correlation analysis. Characteristically, the strongest negative correlation with latitude and positive with temperatures was found in S. minutus, the smallest species under study. Shrews were in general larger in environments with high actual evapotranspiration. Body mass reviewed in S. araneus paralleled the pattern found in the CBL variation in this species, i.e. it decreased northward, both in summer‐ and winter‐caught animals. In addition, contrary to the widely accepted ? but not rigorously tested ? belief, body mass recession from summer to winter (the Dehnel Effect) did not correlate with latitude. We concluded that shrews followed the converse to Bergmann's rule, and hypothesize that part of their body size variation along the west‐east axis may be explained by character displacement. We also hypothesize that scarcity of food, especially in winter, is a major factor selecting for small body size in shrews in northern areas, as smaller individuals should require less food. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 78 , 365–381.     

Ambient temperature correlates with geographic variation in body size of least horseshoe bats

Geographic variation in body size is common within many animal species. The causes of this pattern, however, remain largely unexplored in most vertebrate groups. Bats are widely distributed globally owing to their ability of powered flight. Most bat species encounter a variety of climatic conditions across their distribution range, making them an ideal taxon for the study of ecogeographic patterns in body size. Here, we used adult least horseshoe bats, Rhinolophus pusillus, to test whether geographic variation in body size was determined by heat conservation, heat dissipation, climatic seasonality, or primary productivity. We measured body mass and head-body length for 246 adult bats from 12 allopatric colonies in China. We quantified the ecological conditions inhabited by each colony, including mean maximum temperature of the warmest month, mean minimum temperature of the coldest month, temperature seasonality, precipitation seasonality, and annual net primary productivity (ANPP). Body mass and head-body length, 2 of the most reliable indicators of body size, exhibited marked differences between colonies. After controlling for spatial autocorrelation, the mean minimum temperature of the coldest month explained most of the variation in body size among colonies, regardless of sex. The mean maximum temperature, climatic seasonality, and ANPP had limited power in predicting body size of males or females in comparison with mean minimum temperature. These results support the heat conservation hypothesis and suggest adaptive responses of body size to cold climates in cave-dwelling bats.     

Latitudinal trends in digestive flexibility: testing the climatic variability hypothesis with data on the intestinal length of rodents

Flexibility of digestive features can be understood considering the benefits of digestion, which links animal foraging to metabolizable energy and nutrient gain, and its costs, which are partly indexed by digestive tract tissue mass, one of the most expensive to maintain in terms of energy and protein metabolism. In this article, we applied a meta-analytical approach to current data on rodents' small intestine length flexibility to evaluate the climatic variability hypothesis (CVH). This hypothesis states the following: (1) as the range of climatic fluctuation experienced by terrestrial animals increases with latitude, individuals at higher latitudes should be more flexible to persist at a site; (2) the greater phenotypic flexibility allows species to occupy more habitats and to become more widely distributed. We compiled data from 25 articles, which provided a total of 86 estimations of flexibility involving 20 rodent species. Consistent with CVH predictions, we found a positive correlation between small intestine length flexibility and latitude and between small intestine length flexibility and the number of habitats occupied by different species. When seen from the perspective of digestive physiology, our analysis is an important piece of evidence on the adaptive value of digestive flexibility in small mammals.     

Geographic variation in body size and sexual size dimorphism of a seed-feeding beetle

Body size of many animals varies with latitude: body size is either larger at higher latitudes (Bergmann's rule) or smaller at higher latitudes (converse Bergmann's rule). However, the causes underlying these patterns are poorly understood. Also, studies rarely explore how sexual size dimorphism varies with latitude. Here we investigate geographic variation in body size and sexual size dimorphism of the seed-feeding beetle Stator limbatus, collected from 95 locations along a 38 degrees range in latitude. We examine 14 variables to test whether clines in environmental factors are adequate to explain geographic patterns of body size. We found that body size and sexual size dimorphism of S. limbatus varied considerably with latitude; beetles were smaller but more dimorphic at lower latitudes. Body size was not correlated with a gradient in mean temperature, contrary to the commonly accepted hypothesis that clines are produced by latitudinal gradients in temperature. Instead, we found that three factors were adequate to explain the cline in body size: clinal variation in host plant seed size, moisture (humidity), and seasonality (variance in humidity, precipitation, and temperature). We also found that the cline in sexual size dimorphism was partially explainable by a gradient in moisture, though moisture alone was not sufficient to explain the cline. Other ecological or environmental variables must necessarily contribute to differences in selection on male versus female body size. The main implications of our study are that the sexes differ in the magnitude of clinal variation in body size, creating latitudinal variation in sexual size dimorphism, and that clines in body size of seed beetles are likely influenced by variation in host seed size, water availability, and seasonality.     

Seasonality rather than average temperature explains body size variation in a Neotropical fruit-eating bat

We investigated variation in body size of the widely distributed Neotropical bat Chiroderma villosum across its entire range. Our objective was to verify if the size-related geographic variation in the species is related to environmental variables. We took 13 measurements of 410 specimens from 198 localities in Mesoamerica and South America, and collected information on latitude, longitude, altitude, precipitation, and temperature, totalling 22 variables. We detected clinal variation in size related to latitude and longitude, with a pattern that conforms to the Bergmann's rule. Clinal variation of size along longitude was influenced by the taxonomic component, with subspecies C. v. jesupi being smaller than C. v. villosum. In contrast the latitudinal cline was explained by temperature seasonality and precipitation, with a 14% increase in size between the north and south extremes of the range. In other words, size of individuals is larger in areas with more seasonal oscillations in temperature and with lower precipitation. Our results support the notion that low temperatures alone do not explain large size of mammals in high latitudes. One hypothesis is that large size is favoured in more seasonal climates because somatic growth is faster when resources are abundant, and also larger animals can endure food scarcity better than small ones. We also postulate that pressures related to interspecific competition and resource use may be more intense in more areas marked by seasonal climatic variations. Specifically, a larger size in seasonal areas may allow individuals to explore a wider niche. We suggest that future approaches, refining regional variation in the diet of C. villosum may serve as a further test to this hypothesis.     

Latitudinal shifts in body size of Enallagma cyathigerum (Odonata)

Aim Survey of the latitudinal body size pattern for populations of Enallagma cyathigerum (Odonata) across a south‐north transect. Location A transect covering the whole distribution range from south to north across Europe was sampled. Methods Newly emerged adults were collected from five major sites across Europe and one to four localities were sampled within each site. In total 253 adults were collected from fourteen localities. Body size was measured using thorax length, length of right front wing and length of right hind tibia. These body size estimates were thereafter related to latitude and mean temperature in January and July. Results Body size showed a U‐shaped pattern with latitude, being large at low and high latitudes and small at intermediate latitudes. The same U‐shaped pattern was found for mean January and July temperature, with large animals at low and high temperatures. Conclusion The U‐shaped relationship between body size and latitude is suggested to be a combination of two effects: (1) the length of the season favourable for growth and development, and (2) variation in life cycle length with latitude.     

Determinants of local abundance in a major radiation of Australian passerines (Aves: Meliphagoidea)

Aim  To identify the factors that contribute to variation in abundance (population density), and to investigate whether habitat breadth and diet breadth predict macroecological patterns in a suborder of passerine birds (Meliphagoidea).
Location  Australia (including Tasmania).
Methods  Mean abundance data were collated from site surveys of bird abundance (the Australian Bird Count); range size and latitudinal position data from published distribution maps; and body mass and diet breadth information from published accounts. A diversity index of habitats used (habitat breadth) was calculated from the bird census data. We used bivariate correlation and multiple regression techniques, employing two phylogenetic comparative methods: phylogenetic generalized least squares and independent contrasts.
Results  Body mass and latitude were the only strong predictors of abundance, with larger-bodied and lower-latitude species existing at lower densities. Together, however, body mass and latitude explained only 11.1% of the variation in mean abundance. Range size and habitat breadth were positively correlated, as were diet breadth and body mass. However, neither range size, nor habitat breadth and diet breadth, explained patterns in abundance either directly or indirectly.
Main conclusions  Levels of abundance (population density) in meliphagoid birds are most closely linked to body mass and latitudinal position, but not range size. As with many other macroecological analyses, we find little evidence for aspects of niche breadth having an effect on patterns of abundance. We hypothesize that evolutionary age may also have a determining effect on why species tend to be rarer (less abundant) in the tropics.     

Dense habitats selecting for small body size: a comparative study on bovids

As a crucial adaptation impacting on most aspects of a species' biology, body size is likely to be under multiple selective pressures. This study aims to determine whether the physical structure of dense habitats selects directly for small size in order to reduce resistance to movement from the vegetation and facilitate concealment (Manoeuvrability hypothesis). This is unclear because other selective forces could indirectly cause habitat to covary with body size. Focusing on bovids, I tested the explanatory power of the Manoeuvrability hypothesis in an interspecific comparative analysis controlling for phylogenetic relatedness. Bovids in dense habitats were indeed found to be reduced in size. Confirming the Food choice hypothesis, grazers, who predominantly feed on abundant food in open habitats, were found to be larger than browsers in a multivariate analysis. However, after controlling for diet there remained a residual variation in body size which correlated negatively with habitat density. No support was found for Bergmann's hypothesis as the explanation for this remaining correlation since body size did not correlate with latitude of the species' range. Likewise, sexual selection on male body size in open habitats due to increased polygyny was an unlikely cause since body size did not correlate with sexual body size dimorphism in multivariate analysis. Rather, the finding that tail length was disproportionately shorter in closed habitats points to dense habitat structure selecting directly for compact body conformation and small size, as predicted by the Manoeuvrability hypothesis.     

Disentangling the effects of spring anomalies in climate and net primary production on body size of temperate songbirds

Body size is implicated in individual fitness and population dynamics. Mounting interest is being given to the effects of environmental change on body size, but the underlying mechanisms are poorly understood. We tested whether body size and body condition are related to ambient temperature (heat maintenance hypothesis), or/and explained by variations in primary production (food availability hypothesis) during the period of body growth in songbirds. We also explored whether annual population‐level variations of mean body size are due to changes of juvenile growth and/or size‐dependent mortality during the first year. For 41 species, from 257 sites across France, we tested for relationships between wing length (n = 107 193) or body condition (n = 82 022) and local anomalies in temperature, precipitation and net primary production (NDVI) during the breeding period, for juveniles and adults separately. Juvenile body size was best explained by primary production: wings were longer in years with locally high NDVI, but not shorter in years with low NDVI. Temperature showed a slightly positive effect. Body condition and adult wing length did not covary with any of the other tested variables. We found no evidence of climate‐driven size‐dependent mortality for the breeding season. In our temperate system, local climatic anomalies explained little of the body size variation. A large part of wing length variance was site‐specific, suggesting that avian size was more dependent on local drivers than global ones. Net primary production influenced juvenile size the most through effects on body growth. We suggest that, during the breeding season in temperate systems, thermoregulatory mechanisms are less involved in juvenile growth than food assimilation.     

A test of Allen's rule in ectotherms: the case of two south American Melanopline Grasshoppers (Orthoptera: Acrididae) with partially overlapping geographic ranges

We studied the geographic variation of three morphometric characters in relation to body size in two South American grasshoppers (Acrididae), Dichroplus vittatus Bruner and D. pratensis Bruner to test Allen's rule in these ectotherms. Since both species follow the converse to Bergmann's rule owing to latitudinal and/or altitudinal variation in time available for growth and reproduction, geographic variation in body size proportions of protruding parts may obey to differential allometric growth in different geographic areas. Alternatively, it could reflect true Allenian variation related to thermoregulation. Body proportions were studied by correlation/regression analyses with geographic and climatic variables. In D. pratensis, body proportions increased with latitude and decreased with altitude. These results probably obey to the effects of water balance and seasonality on final body size, and on the allometric growth of the three studied characters not being related to thermoregulation. In D. vittatus, a generally non-significant trend towards the decrease of the mean proportions of all three characters with increasing latitude was observed. Nevertheless, also in this species, it is probable that the environmental gradient responds to seasonality factors (although not to water balance) that affect the length of growing season and, in consequence, body size and its allometric relationships. We conclude that the regularities in the geographic distribution of body proportions of D. pratensis and D. vittatus do not follow Allen's rule in the sense of thermoregulation, and result from variables that determine growing season length and the allometric growth of different body parts.     

Habitat primary production and the evolution of body size within the hartebeest clade

Local adaptation is a key process in the evolution of biological diversity but relatively few studies have identified the selective forces that drive trait divergence at low taxonomic levels, particularly amongst mammals. Variation in body size across taxa is fundamental as shown by allometric relationships with numerous physiological, morphological and life-history traits. Differences in adult size across cohorts within populations of temperate ungulates are determined by variation in trophic resource availability during growth, suggesting that natural selection might promote the evolution of size divergence across sister taxa through local adaptation to variation in habitat productivity. We tested this hypothesis in the hartebeest ( Alcelaphu s sp.), an antelope lineage including eight extant (or recently extinct) allopatric subspecies that evolved within the last million years and colonized all the African savannahs. We predicted that body size across the subspecies should correlate positively with habitat productivity across taxon ranges. Mean body size of all the hartebeest taxa was quantified using skull length from museum specimens, and climatic variables were used as surrogates of habitat productivity. Body size across subspecies was positively correlated with rainfall, suggesting that variation in habitat primary production may drive morphological evolution between taxa. Focusing at a low taxonomic level has allowed us to identify a critical selective force that may shape divergence in body size, without the confounding effect of variation in trophic niche. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 431–440.     

The effect of island area on body size in a primate species from the Sunda Shelf Islands

Aim  We examine the effect of island area on body dimensions in a single species of primate endemic to Southeast Asia, the long-tailed macaque ( Macaca fascicularis ). In addition, we test Allen's rule and a within-species or intraspecific equivalent of Bergmann's rule (i.e. Rensch's rule) to evaluate body size and shape evolution in this sample of insular macaques.
Location  The Sunda Shelf islands of Southeast Asia.
Methods  Body size measurements of insular macaques gathered from the literature were analysed relative to island area, latitude, maximum altitude, isolation from the mainland and other islands, and various climatic variables using linear regression.
Results  We found no statistically significant relationship between island area and body length or head length in our sample of insular long-tailed macaques. Tail length correlated negatively with island area. Head length and body length exhibited increases corresponding to increasing latitude, a finding seemingly consistent with the expression of Bergmann's rule within a single species. These variables, however, were not correlated with temperature, indicating that Bergmann's rule is not in effect. Tail length was not correlated with either temperature or increasing latitude, contrary to that predicted by Allen's rule.
Main conclusions  The island rule dictating that body size will covary with island area does not apply to this particular species of primate. Our study is consistent with results presented in the literature by demonstrating that skull and body length in insular long-tailed macaques do not, strictly speaking, conform to Rensch's rule. Unlike previous studies, however, our findings suggest that tail-length variation in insular macaques does not support Allen's rule.     

Patterns in body size and melanism along a latitudinal cline in the wingless grasshopper,Phaulacridium vittatum

Aim We explore geographic variation in body size within the wingless grasshopper, Phaulacridium vittatum, along a latitudinal gradient, and ask whether melanism can help explain the existence of clinal variation. We test the hypotheses that both male and female grasshoppers will be larger and lighter in colour at lower latitudes, and that reflectance and size will be positively correlated, as predicted by biophysical theory. We then test the hypothesis that variability in size and reflectance is thermally driven, by assessing correlations with temperature and other climatic variables. Location Sixty‐one populations were sampled along the east coast of Australia between latitudes 27.63° S and 43.10° S, at elevations ranging from 10 to 2000 m a.s.l. Methods Average reflectance was used as a measure of melanism and femur length as an index of body size for 198 adult grasshoppers. Climate variables were generated by BIOCLIM for each collection locality. Hierarchical partitioning was used to identify those variables with the most independent influence on grasshopper size and reflectance. Results Overall, there was no simple relationship between size and latitude in P. vittatum. Female body size decreased significantly with latitude, while male body size was largest at intermediate latitudes. Rainfall was the most important climatic variable associated with body size of both males and females. Female body size was also associated with radiation seasonality and male body size with reflectance. The reflectance of females was not correlated with latitude or body size, while male reflectance was significantly higher at intermediate latitudes and positively correlated with body size. Analyses of climate variables showed no significant association with male reflectance, while female reflectance was significantly related to the mean temperature of the driest quarter. Main conclusions Geographic variation in the body size of the wingless grasshopper is best explained in terms of rainfall and radiation seasonality, rather than temperature. However, melanism is also a significant influence on body size in male grasshoppers, suggesting that thermal fitness does play a role in determining adaptive responses to local conditions in this sex.     

Latitudinal pattern in body size in a cockroach,Eupolyphaga sinensis

Body size of insects with flexible life cycles is expected to conform to the saw‐tooth model, a model in which the relationship between size and developmental time depends on length of the growing season. In species with high variability in terms of voltinism, however, more complex patterns can be expected. Few empirical studies have demonstrated the existence of such relationships, or whether climatic factors contribute to these relationships. In this study, we investigated the geographic variation in body size of the Chinese cockroach, Eupolyphaga sinensis Walker (Blattaria: Polyphagidae), which has a variable life cycle length. The sizes of adults – collected from 14 localities ranging from temperate to subtropical zones in China – were measured, using body length, body width, and pronotum width as parameters. The relationship between size, latitude, and climate factors (encompassing 10 variables) was then investigated. We found that the body size of E. sinensis varied considerably with latitude: cockroaches were larger at low and high latitudes, but smaller at intermediate latitudes. Thus, the relationship between climate and body size conformed to a saw‐tooth pattern. Results indicate that two factors were significantly associated with body size clines: season length and variability in life cycle length. Our results also demonstrated that climatic factors contribute to latitudinal clines in body size, which has important ecological and evolutionary implications. It can be expected that global climate change may alter latitudinal clines in body size of E. sinensis.     

Body size variation and sexual size dimorphism across climatic gradients in the widespread treefrog Scinax fuscovarius (Anura,Hylidae)

Variation in body size represents one of the crucial raw materials for evolution. However, at present, it is still being debated what is the main factor affecting body size or if the final body size is the consequence of several factors acting synergistically. To evaluate this, widespread species seem to be suitable models because the different populations occur along a geographical gradient and under contrasted climatic and environmental conditions. Here we describe the spatial pattern of variation in body size and sexual size dimorphism in the snouted treefrog Scinax fuscovarius (Anura, Hylidae) along a 10° range in latitude, 25° longitude, and 2000 m in altitude from Argentina, Brazil and Paraguay using an information‐theoretic approach to evaluate the support of the data for eight a priori hypotheses proposed in the literature to account for geographical body size, and three hypotheses for sexual size dimorphism variation. Body size of S. fuscovarius varied most dramatically with longitude and less so with latitude; frogs were largest in the northwestern populations. Body size was positively related with precipitation seasonality, and negatively with annual precipitation. Furthermore, the degree of sexual size dimorphism was greatest in the western populations with less annual precipitation, as the increase in body size was stronger for females. Our results on body size variation are consistent with two ecogeographical hypotheses, the starvation resistance and the water availability hypotheses, while our results on sexual size dimorphism in S. fuscovarius supports the differential‐plasticity hypothesis but the inverse to Rensch's rule and the parental investment hypothesis. Due to the weak association between environmental variables and body size and sexual size dimorphism variation, we stress that there are other factors, mainly those related to the life history, driving the geographical variation of S. fuscovarius.     

Urbanization disrupts latitude‐size rule in 17‐year cicadas

Many ectotherms show a decrease in body size with increasing latitude due to changes in climate, a pattern termed converse Bergmann's rule. Urban conditions—particularly warmer temperatures and fragmented landscapes—may impose stresses on development that could disrupt these body size patterns. To test the impact of urbanization on development and latitudinal trends in body size, we launched a citizen science project to collect periodical cicadas (Magicicada septendecim) from across their latitudinal range during the 2013 emergence of Brood II. Periodical cicadas are long‐lived insects whose distribution spans a broad latitudinal range covering both urban and rural habitats. We used a geometric morphometric approach to assess body size and developmental stress based on fluctuating asymmetry in wing shape. Body size of rural cicadas followed converse Bergmann's rule, but this pattern was disrupted in urban habitats. In the north, urban cicadas were larger than their rural counterparts, while southern populations showed little variation in body size between habitats. We detected no evidence of differences in developmental stress due to urbanization. To our knowledge, this is the first evidence that urbanization disrupts biogeographical trends in body size, and this pattern highlights how the effects of urbanization may differ over a species’ range.     

Density-range size relationships in French riverine fishes

We examined the relation between the local density of species and the size of the geographic range for French riverine fishes. As for most other taxonomic groups, a positive interspecific relationship is found for this group. This relationship is robust to the confounding effects of phylogeny and is not a priori a product of other potential mechanistic artefacts. We formally tested two of the principal biological mechanisms already proposed (i.e. the niche breadth hypothesis and the resource availability hypothesis). We found no support for the niche breadth hypothesis. In contrast, we found consistent support for the closely related resource availability hypothesis. Species utilising resources (habitats) that are marginal tend to appear at low density and to have narrow distribution whereas species utilising widespread habitats tend to be more abundant and more widely distributed. Using data on body size and reproductive traits we explored the potential influence of these variables in explaining significant variation around the density-range size relationship. Only body size explains significant variation around the relationship, being negatively correlated with local density and positively correlated with range size.     

Variation at high latitudes: the geography of body size and cranial morphology of the muskox, Ovibos moschatus

Abstract Aim Bergmann's rule, one of the most studied and controversial ecogeographical generalizations, has rarely been tested with observations from high latitudes. We tested the rule using cranial measurements of the muskox [Ovibos moschatus (Zimmerman)], a homeotherm with an extremely northern distribution. We also used these data to describe geographical patterns in the species' dental architecture, an extension of the framework developed from interspecific comparisons. Location Specimens were compiled from arctic Canada, Alaska and Greenland, a latitudinal range of 60° N?83° N. Methods Body size was estimated from principal components analysis (PCA) of five cranial characters from 128 specimens. Mean scores on the first principal component from each locality were regressed against latitude and mean temperature to identify geographical variation in body size; scores on the second principal component were regressed against latitude to assess patterns in dental architecture. Regression analyses of the individual characters were performed as a complement to PCA. Results No latitudinal or climatic trend in body size was observed in either sex. On the other hand, for males, significant latitudinal variation was found for the second PCA axis (r = ?0.434), and the feature which loaded most heavily on it, maxillary tooth row length (r = 0.429). For females, this dental structure also tended to increase with latitude (r = 0.423), but the trend was only marginally significant (P=0.12), perhaps owing to a smaller sample size. Main conclusions The geographically invariant body size of muskoxen failed to support current hypotheses of size variation. Behavioural and physiological adaptations may exempt the muskox from selective pressures underlying these hypotheses. We interpret latitudinal variation in dental architecture as a reflection of a cline in diet, dominated by graminoids at the expense of willows at higher latitudes. This intraspecific geographical trend is a recapitulation of the interspecific framework for large mammalian herbivores.     

Climatic adaptation and the evolution of basal and maximum rates of metabolism in rodents

Metabolic rate is a key aspect of organismal biology and the identification of selective factors that have led to species differences is a major goal of evolutionary physiology. We tested whether environmental characteristics and/or diet were significant predictors of interspecific variation in rodent metabolic rates. Mass-specific basal metabolic rates (BMR) and maximum metabolic rates (MMR, measured during cold exposure in a He-O2 atmosphere) were compiled from the literature. Maximum (Tmax) and minimum (Tmin) annual mean temperatures, latitude, altitude, and precipitation were obtained from field stations close to the capture sites reported for each population (N = 57). Diet and all continuous-valued traits showed statistically significant phylogenetic signal, with the exception of mass-corrected MMR and altitude. Therefore, results of phylogenetic analyses are emphasized. Body mass was not correlated with absolute latitude, but was positively correlated with precipitation in analyses with phylogenetically independent contrasts. Conventional multiple regressions that included body mass indicated that Tmax (best), Tmin, latitude, and diet were significant additional predictors of BMR. However, phylogenetic analyses indicated that latitude was the only significant predictor of mass-adjusted BMR (positive partial regression coefficient, one-tailed P = 0.0465). Conventional analyses indicated that Tmax, Tmin (best), and altitude explained significant amounts of the variation in mass-adjusted MMR. With body mass and Tmin in the model, no additional variables were significant predictors. Phylogenetic contrasts yielded similar results. Both conventional and phylogenetic analyses indicated a highly significant positive correlation between residual BMR and MMR (as has also been reported for birds), which is consistent with a key assumption of the aerobic capacity model for the evolution of vertebrate energetics (assuming that MMR and exercise-induced maximal oxygen consumption are positively functionally related). Our results support the hypothesis that variation in environmental factors leads to variation in the selective regime for metabolic rates of rodents. However, the causes of a positive association between BMR and latitude remain obscure. Moreover, an important area for future research will be experiments in all taxa are raised under common conditions to allow definitive tests of climatic adaptation in endotherm metabolic rates and to elucidate the extent of adaptive phenotypic plasticity.