Slaying dragons: limited evidence for unusual body size evolution on islands

Aim Island taxa often attain forms outside the range achieved by mainland relatives. Body size evolution of vertebrates on islands has therefore received much attention, with two seemingly conflicting patterns thought to prevail: (1) islands harbour animals of extreme size, and (2) islands promote evolution towards medium body size (‘the island rule’). We test both hypotheses using body size distributions of mammal, lizard and bird species. Location World‐wide. Methods We assembled body size and insularity datasets for the world’s lizards, birds and mammals. We compared the frequencies with which the largest or smallest member of a group is insular with the frequencies expected if insularity is randomly assigned within groups. We tested whether size extremes on islands considered across mammalian phylogeny depart from a null expectation under a Brownian motion model. We tested the island rule by comparing insular and mainland members of (1) a taxonomic level and (2) mammalian sister species, to determine if large insular animals tend to evolve smaller body sizes while small ones evolve larger sizes. Results The smallest species in a taxon (order, family or genus) are insular no more often than would be expected by chance in all groups. The largest species within lizard families and bird genera (but no other taxonomic levels) are insular more often than expected. The incidence of extreme sizes in insular mammals never departs from the null, except among extant genera, where gigantism is marginally less common than expected under a Brownian motion null. Mammals follow the island rule at the genus level and when comparing sister species and clades. This appears to be driven mainly by insular dwarfing in large‐bodied lineages. A similar pattern in birds is apparent for species within orders. However, lizards follow the converse pattern. Main conclusions The popular misconception that islands have more than their fair share of size extremes may stem from a greater tendency to notice gigantism and dwarfism when they occur on islands. There is compelling evidence for insular dwarfing in large mammals, but not in other taxa, and little evidence for the second component of the island rule – gigantism in small‐bodied taxa.     

Body size evolution in insular vertebrates: generality of the island rule

Aim My goals here are to (1) assess the generality of the island rule – the graded trend from gigantism in small species to dwarfism in larger species – for mammals and other terrestrial vertebrates on islands and island‐like ecosystems; (2) explore some related patterns of body size variation in insular vertebrates, in particular variation in body size as a function of island area and isolation; (3) offer causal explanations for these patterns; and (4) identify promising areas for future studies on body size evolution in insular vertebrates. Location Oceanic and near‐shore archipelagos, and island‐like ecosystems world‐wide. Methods Body size measurements of insular vertebrates (non‐volant mammals, bats, birds, snakes and turtles) were obtained from the literature, and then regression analyses were conducted to test whether body size of insular populations varies as a function of body size of the species on the mainland (the island rule) and with characteristics of the islands (i.e. island isolation and area). Results The island rule appears to be a general phenomenon both with mammalian orders (and to some degree within families and particular subfamilies) as well as across the species groups studied, including non‐volant mammals, bats, passerine birds, snakes and turtles. In addition, body size of numerous species in these classes of vertebrates varies significantly with island isolation and island area. Main conclusions The patterns observed here – the island rule and the tendency for body size among populations of particular species to vary with characteristics of the islands – are actually distinct and scale‐dependent phenomena. Patterns within archipelagos reflect the influence of island isolation and area on selective pressures (immigration filters, resource limitation, and intra‐ and interspecific interactions) within particular species. These patterns contribute to variation about the general trend referred to as the island rule, not the signal for that more general, large‐scale pattern. The island rule itself is an emergent pattern resulting from a combination of selective forces whose importance and influence on insular populations vary in a predictable manner along a gradient from relatively small to large species. As a result, body size of insular species tends to converge on a size that is optimal, or fundamental, for a particular bau plan and ecological strategy.     

BIODIVERSITY RESEARCH: Nestedness for different reasons: the distributions of birds,lizards and small mammals on islands of an inundated lake

Aim We examined whether the community compositions of birds, lizards and small mammals were nested in a fragmented landscape in the Thousand Island Lake, China. We also assessed whether the mechanisms influencing nestedness differed among these taxonomic groups. Location Thousand Island Lake, China. Methods Presence/absence matrices were compiled for birds (42 islands) and lizards (42 islands) using line‐transect methods, and for small mammals (14 islands) using live‐trapping methods from 2006 to 2009. Nestedness was analysed using BINMATNEST, and statistical significance was assessed using the conservative null model 3. We used Spearman rank correlations and partial Spearman rank correlations to examine associations of nestedness and habitat variables (area, isolation, habitat diversity and plant richness) as well as life‐history traits (body size, habitat specificity, geographical range size and area requirement) related to species extinction and immigration tendencies. Results The community compositions of birds, lizards and small mammals were all significantly nested, but the causal factors underlying nestedness differed among taxonomic groups. For birds, island area, habitat specificity and area requirement were significantly correlated with nestedness after controlling for other independent variables. For lizards, habitat heterogeneity was the single best correlate of nestedness. For small mammals, island area, habitat heterogeneity and habitat specificity were significantly correlated with nestedness. The nested patterns of birds, lizards and small mammals were not attributable to passive sampling or selective colonization. Main conclusions The processes influencing nested patterns differed among taxonomic groups. Nestedness of bird assemblages was driven by selective extinction, and lizard assemblage was caused by habitat nestedness, while nestedness of small mammals resulted from both selective extinction and habitat nestedness. Therefore, we should take taxonomic differences into account when analysing nestedness to develop conservation guidelines and refrain from using single taxa as surrogates for others.     

Area, isolation and body size evolution in insular carnivores

Body sizes of insular mammals often differ strikingly from those of their mainland conspecifics. Small islands have reduced numbers of competitor and predator species, and more limited resources. Such reductions are believed to select for predictable changes in body sizes, with large mammals growing progressively smaller as island area decreases, while small ones grow progressively larger. Medium-sized mammals are thought to be largest on intermediate-sized islands. Increased isolation is seen as promoting insular gigantism. We searched for such patterns using a large database of insular carnivore specimens. Neither small nor large carnivores show a consistent area/body size relationship. Medium-sized carnivores are no more likely to attain large size on medium-sized islands then they are to be small there. We found no consistent patterns of body size variation in relation to isolation.     

Australian Snakes Do Not Follow Bergmann’s Rule

Bergmann’s Rule (i.e., the tendency of body size to increase with decreasing environmental temperature) was originally explained by a mechanism that is unique to endotherms. Nevertheless, geographic variation of body size of ectotherms, including snakes, is increasingly studied, and some claim that the rule should apply to ectotherms, or to thermoregulating ectotherms. Such studies usually focus on assemblages or on species in a region, but mostly ignore species’ ecological and biological traits when seeking biogeographic patterns. We examined the relationship between environmental temperatures and body size of 146 Australian snake species. We examined this relationship while considering the effects of ecological traits (activity time and habitat use), climatic variables which are thought to influence snake body size, and shared ancestry. Our finding suggest that Bergmann’s Rule is not a valid generalization across species of Australian snakes. Furthermore, ecological traits greatly influence the relationship between snake body size and environmental temperature. Body size of fossorial species decreases with environmental temperature, whereas body size of nocturnal, surface active species increases. Body size of diurnal, surface active species is not related to environmental temperature. Our results indicate that lumping all species in a clade together is misleading, and that ecological traits profoundly affect the geographic variation of snake body size. Though environmental temperature generally does not exert a strong selective force on snake body size, this relationship differs for taxa exhibiting different ecological traits.     

COLOR PATTERN POLYMORPHISM IN THE LAKE ERIE WATER SNAKE,NERODIA SIPEDON INSULARUM

Populations of the water snake, Nerodia sipedon, on islands in western Lake Erie are polymorphic for color pattern. These populations include banded, intermediate, and unbanded morphs while surrounding mainland populations consist solely of the banded morph. The hypothesis that this polymorphism is maintained by strong selection and migration pressures is widely accepted. Unbanded morphs are apparently more cryptic along island shorelines while banded morphs are more cryptic on the mainland. Migration of banded morphs from the mainland explains their persistence in island populations. Data collected in a capture-mark-recapture program on six islands provide no evidence of differential selection among morphs; morph frequencies do not differ among age classes, between once-captured and multiply-captured snakes, or between scarred and unscarred snakes. Furthermore, herring gulls, the most common snake predators in the island area, appear to detect banded and unbanded model snakes with equal ease. High site fidelity of water snakes and the distribution of morphs among islands suggest that migration from the mainland is not common. However, islands close to each other are similar in morph frequency, and water snakes have colonized islands elsewhere in the Great Lakes, indicating that some migration does occur. Recently, the frequency of banded morphs has increased in island populations while adult population sizes have declined. This increase in banded morphs is interpreted as reflecting an increased impact of migration from the mainland into these reduced populations. One scenario for the evolution and maintenance of this polymorphism is that selection was important in establishing unbanded morphs in island populations as they became isolated from the mainland. As populations declined to their present size, the impact of migration from the mainland increased and is now swamping the effect of selection. Further declines in island population size may result in fixation of the banded morph.     

Biogeography of body size in Pacific island birds

Many insular vertebrates have undergone rapid and dramatic changes in body size compared to their mainland counterparts. Here we explore the relationship between two well known patterns of island body size – the tendency for large‐bodied species to dwarf and small‐bodied species to get larger on islands, known as the “island rule”, and the scaling of maximum and minimum body size of island assemblages with island area. Drawing on both fossil and modern data, we examined the relationship between body size and island area in Pacific island birds, both within clades and at the island assemblage level. We found that the size of the smallest bird on each island decreased with island area while the maximum body size increased with island area. Similarly, within clades the body size of small‐bodied groups decreased and large‐bodied groups increased from small to large islands, consistent with the island rule. However, the magnitude of size change within clades was not sufficient to explain the overall scaling of maximum size with island area. Instead, the pattern was driven primarily by the evolution of very large, flightless birds on large islands. Human‐mediated extinctions on islands over the past few millennia severely impacted large, flightless birds, to the effect that this macroecological pattern has been virtually erased. After controlling for effects of biogeographic region and island area, we found island productivity to be the best predictor of maximum size in flightless birds. This result, and the striking similarities in maximum body size between flightless birds and island mammals, suggests a common energetic mechanism linking body size and landmass area in both the island rule and the scaling of island body size extremes.     

Rapid and repeated origin of insular gigantism and dwarfism in Australian tiger snakes

It is a well-known phenomenon that islands can support populations of gigantic or dwarf forms of mainland conspecifics, but the variety of explanatory hypotheses for this phenomenon have been difficult to disentangle. The highly venomous Australian tiger snakes (genus Notechis) represent a well-known and extreme example of insular body size variation. They are of special interest because there are multiple populations of dwarfs and giants and the age of the islands and thus the age of the tiger snake populations are known from detailed sea level studies. Most are 5000-7000 years old and all are less than 10,000 years old. Here we discriminate between two competing hypotheses with a molecular phylogeography dataset comprising approximately 4800 bp of mtDNA and demonstrate that populations of island dwarfs and giants have evolved five times independently. In each case the closest relatives of the giant or dwarf populations are mainland tiger snakes, and in four of the five cases, the closest relatives are also the most geographically proximate mainland tiger snakes. Moreover, these body size shifts have evolved extremely rapidly and this is reflected in the genetic divergence between island body size variants and mainland snakes. Within south eastern Australia, where populations of island giants, populations of island dwarfs, and mainland tiger snakes all occur, the maximum genetic divergence is only 0.38%. Dwarf tiger snakes are restricted to prey items that are much smaller than the prey items of mainland tiger snakes and giant tiger snakes are restricted to seasonally available prey items that are up three times larger than the prey items of mainland tiger snakes. We support the hypotheses that these body size shifts are due to strong selection imposed by the size of available prey items, rather than shared evolutionary history, and our results are consistent with the notion that adaptive plasticity also has played an important role in body size shifts. We suggest that plasticity displayed early on in the occupation of these new islands provided the flexibility necessary as the island's available prey items became more depauperate, but once the size range of available prey items was reduced, strong natural selection followed by genetic assimilation worked to optimize snake body size. The rate of body size divergence in haldanes is similar for dwarfs (h(g) = 0.0010) and giants (h(g) = 0.0020-0.0025) and is in line with other studies of rapid evolution. Our data provide strong evidence for rapid and repeated morphological divergence in the wild due to similar selective pressures acting in different directions.     

Body size of insular carnivores: little support for the island rule

Large mammals are thought to evolve to be smaller on islands, whereas small mammals grow larger. A negative correlation between relative size of island individuals and body mass is termed the "island rule." Several mechanisms--mainly competitive release, resource limitation, dispersal ability, and lighter predation pressure on islands, as well as a general physiological advantage of modal size--have been advanced to explain this pattern. We measured skulls and teeth of terrestrial members of the order Carnivora in order to analyze patterns of body size evolution between insular populations and their near mainland conspecifics. No correlations were found between the size ratios of insular/mainland carnivore species and body mass. Only little support for the island rule is found when individual populations rather than species are considered. Our data are at odds with those advanced in support of theories of optimal body size. Carnivore size is subjected to a host of selective pressures that do not vary uniformly from place to place. Mass alone cannot account for the patterns in body size of insular carnivores.     

Classification tree methods provide a multifactorial approach to predicting insular body size evolution in rodents

Many hypotheses have been proposed to explain size changes in insular mammals, but no single variable suffices to explain the diversity of responses, particularly within Rodentia. Here in a data set on insular rodents, we observe strong consistency in the direction of size change within islands and within species but (outside of Heteromyidae) little consistency at broader taxonomic scales. Using traits of islands and of species in a classification tree analysis, we find the most important factor predicting direction of change to be mainland body mass (large rodents decrease, small ones increase); other variables (island climate, number of rodent species, and area) were significant, although their roles as revealed by the classification tree were context dependent. Ecological interactions appear relatively uninformative, and on any given island, the largest and smallest rodent species converged or diverged in size with equal frequency. Our approach provides a promising framework for continuing examination of insular body size evolution.     

Testing the ‘island rule’ for a tenebrionid beetle (Coleoptera,Tenebrionidae)

Insular populations and their closest mainland counterparts commonly display body size differences that are considered to fit the island rule, a theoretical framework to explain both dwarfism and gigantism in isolated animal populations. The island rule is used to explain the pattern of change of body size at the inter-specific level. But the model implicitly makes also a prediction for the body size of isolated populations of a single species. It suggests that, for a hypothetical species covering a wide range of island sizes, there exists a specific island size where this species reaches the largest body size. Body size would be small (in relative terms) in the smallest islets of the species range. It would increase with island size, and reach a maximum at some specific island size. However, additional increases from such a specific island size would instead promote body size reduction, and small (in relative terms) body sizes would be found again on the largest islands. The biogeographical patterns predicted by the island rule have been described and analysed for vertebrates only (mainly mammals), but remain largely untested for insects or other invertebrates. I analyse here the pattern of body size variation between seven isolated insular populations of a flightless beetle, Asida planipennis (Coleoptera, Tenebrionidae). This is an endemic species of Mallorca, Menorca and a number of islands and islets in the Balearic archipelago (western Mediterranean). The study covers seven of the 15 known populations (i.e., there are only 15 islands or islets inhabited by the species). The populations studied fit the pattern advanced above and we could, therefore, extrapolate the island rule to a very different kind of organism. However, the small sample size of some of the populations invites some caution at this early stage.     

Of mice and mammoths: evaluations of causal explanations for body size evolution in insular mammals

Aim We investigated the hypothesis that the insular body size of mammals results from selective forces whose influence varies with characteristics of the focal islands and the focal species, and with interactions among species (ecological displacement and release). Location Islands world‐wide. Methods We assembled data on the geographic characteristics (area, isolation, maximum elevation, latitude) and climate (annual averages and seasonality of temperature and precipitation) of islands, and on the ecological and morphological characteristics of focal species (number of mammalian competitors and predators, diet, body size of mainland reference populations) that were most relevant to our hypothesis (385 insular populations from 98 species of extant, non‐volant mammals across 248 islands). We used regression tree analyses to examine the hypothesized contextual importance of these factors in explaining variation in the insular body size of mammals. Results The results of regression tree analyses were consistent with predictions based on hypotheses of ecological release (more pronounced changes in body size on islands lacking mammalian competitors or predators), immigrant selection (more pronounced gigantism in small species inhabiting more isolated islands), thermoregulation and endurance during periods of climatic or environmental stress (more pronounced gigantism of small mammals on islands of higher latitudes or on those with colder and more seasonal climates), and resource subsidies (larger body size for mammals that utilize aquatic prey). The results, however, were not consistent with a prediction based on resource limitation and island area; that is, the insular body size of large mammals was not positively correlated with island area. Main conclusions These results support the hypothesis that the body size evolution of insular mammals is influenced by a combination of selective forces whose relative importance and nature of influence are contextual. While there may exist a theoretical optimal body size for mammals in general, the optimum for a particular insular population varies in a predictable manner with characteristics of the islands and the species, and with interactions among species. This study did, however, produce some unanticipated results that merit further study – patterns associated with Bergmann’s rule are amplified on islands, and the body size of small mammals appears to peak at intermediate and not maximum values of latitude and island isolation.     

Phylogeny and co‐occurrence of mammal species on Southeast Asian islands

Aim Islands have often been used as model systems in community ecology. The incorporation of information on phylogenetic relatedness of species in studies of island assemblage structure is still uncommon, but could provide valuable insights into the processes of island community assembly. We propose six models of island community assembly that make different predictions about the associations between co‐occurrences of species pairs on islands, phylogenetic relatedness and ecological similarity. We then test these models using data on mammals of Southeast Asian islands. Location Two hundred and forty islands of the Sundaland region of Southeast Asia. Methods We quantified the co‐occurrence of species pairs on islands, and identified pairs that co‐occur more frequently (positive co‐occurrence) or less frequently (negative co‐occurrence) than expected under null models. We then examined the distributions of these significantly deviating pairs with respect to phylogenetic relatedness and ecological differentiation, and compared these patterns with those predicted by the six community assembly models. We used permutation regression to test whether co‐occurrence patterns are predicted by relatedness, body size difference or difference in diet quality. Separate co‐occurrence matrices were analysed in this way for seven mammal families and four smaller subsets of the islands of Sundaland. Results In many matrices, average numbers of negative co‐occurrences were higher than expected under null models. This is consistent with assemblage structuring by competition, but may also result from low geographic overlap of species pairs, which contributes to negative co‐occurrences at the archipelago‐wide level. Distributions of species pairs within plots of phylogenetic distance × ecological differentiation were consistent with competition, habitat filtering or within‐island speciation models, depending on the taxon. Regressions indicated that co‐occurrence was more likely among closely related species pairs within the Viverridae and Sciuridae, but in most matrices phylogenetic distance was unrelated to co‐occurrence. Main conclusions Simple deterministic models linking co‐occurrence with phylogeny and ecology are a useful framework for interpreting distributions and assemblage structure of island species. However, island assemblages in Sundaland have probably been shaped by a complex idiosyncratic set of interacting ecological and evolutionary processes, limiting the predictive power of such models.     

Patterns in the geographical range sizes of ectotherms in North America

The distributions of homeothermic mammals and birds in continental North America show a distinct pattern in the configuration of their geographical ranges. Smaller ranges tend to be elongated north-south while larger ranges tend to be elongated east-west. To examine the generality of this pattern in ectotherms, we analyzed the distribution on continental North America of 139 species of mosquitoes, 164 amphibians, and 221 reptiles. Unlike birds and mammals, small ranges of ectotherms were not elongated north-south and the small ranges of snakes were elongated east-west. The distribution of ectotherms with small ranges does not appear to be affected by the major topographic features of North America which tend to run north-south. Like birds and mammals, large ranges of mosquitoes and reptiles but not amphibians are elongated east-west. The east-west orientation of mosquitoes with large ranges is not attributable to the three largest genera in North America taken singly, Aedes, Culex, or Anopheles, but appears only when all genera are pooled. The east-west orientation of reptiles with large ranges is attributable to turtles and snakes but not lizards. Climatic zones may thus affect the distribution of mosquitoes, turtles, and snakes with large ranges but are not the major determinants of range dimensions among ectotherms in general. Received: 1 September 1997 / Accepted: 8 February 1998     

The Costs of Evaluating Species Densities and Composition of Snakes to Assess Development Impacts in Amazonia

Studies leading to decision-making for environmental licensing often fail to provide accurate estimates of diversity. Measures of snake diversity are regularly obtained to assess development impacts in the rainforests of the Amazon Basin, but this taxonomic group may be subject to poor detection probabilities. Recently, the Brazilian government tried to standardize sampling designs by the implementation of a system (RAPELD) to quantify biological diversity using spatially-standardized sampling units. Consistency in sampling design allows the detection probabilities to be compared among taxa, and sampling effort and associated cost to be evaluated. The cost effectiveness of detecting snakes has received no attention in Amazonia. Here we tested the effects of reducing sampling effort on estimates of species densities and assemblage composition. We identified snakes in seven plot systems, each standardised with 14 plots. The 250 m long centre line of each plot followed an altitudinal contour. Surveys were repeated four times in each plot and detection probabilities were estimated for the 41 species encountered. Reducing the number of observations, or the size of the sampling modules, caused significant loss of information on species densities and local patterns of variation in assemblage composition. We estimated the cost to find a snake as $ 120 U.S., but general linear models indicated the possibility of identifying differences in assemblage composition for half the overall survey costs. Decisions to reduce sampling effort depend on the importance of lost information to target-issues, and may not be the preferred option if there is the potential for identifying individual snake species requiring specific conservation actions. However, in most studies of human disturbance on species assemblages, it is likely to be more cost-effective to focus on other groups of organisms with higher detection probabilities.     

Body size and coexistence in gamasid mites parasitic on small mammals: null model analyses at three hierarchical scales

We studied body size ratio in gamasid mites (Acari: Mesostigmata) parasitic on Palearctic small mammals at 3 hierarchical scales, namely infracommunities (an assemblage of mites harboured by an individual host), component communities (an assemblage of mites harboured by a host population), and compound communities (an assemblage of mites harboured by a host community). We used null models and asked a) whether body size distributions in these communities demonstrate non‐random patterns; b) whether these patterns indicate segregation or aggregation of body sizes of coexisting species; and c) whether patterns of body size distribution are scale‐dependent, that is, differ among infracommunities, component communities, and compound communities. In most mite assemblages, the observed pattern of body size distribution did not differ from that expected by chance. However, meta‐analyses demonstrated that component and compound communities of gamasid mites consistently demonstrated a tendency to reduced body size overlap, while we did not find any clear trend in mite body size distribution across infracommunities. We discuss reasons for scale‐dependence of body size distribution pattern in parasite communities and propose ecological and evolutionary mechanisms that allowed the reduced body size overlap in component and compound communities of ectoparasites to arise.     

Phylogenetic relationships within laticaudine sea snakes (Elapidae)

The sea snake subfamily Laticaudinae consists of a single genus with eight named species, based on morphological characters. We used microsatellite and mitochondrial DNA (mtDNA) data to clarify the adaptive radiation of these oviparous sea snakes in the South Pacific, with special reference to New Caledonia and Vanuatu. A mitochondrial DNA data set (ND4 gene 793 bp) was obtained from 345 individuals of the five species of Laticauda sp. sea snakes endemic to the region. Maximum likelihood and Bayesian approaches yielded the same optimal tree topology, identifying two major clades (yellow-banded and blue-banded sea snakes). Although all laticaudine sea snakes rely on small islands as oviposition sites, the two lineages differ in their use of marine vs. terrestrial habitats. A highly aquatic species (Laticauda laticaudata) shows a strong pattern of genetic isolation by distance, implying that the patchy distribution of terrestrial habitats has had little impact on gene flow. The more terrestrial clade (Laticauda colubrina, Laticauda frontalis, Laticauda guineai, Laticauda saintgironsi) shows stronger geographic differentiation in allelic frequencies, associated with island groups rather than with geographic distance. Microsatellites and mtDNA suggest that L. frontalis (restricted to Vanuatu) represents a recent founder-induced speciation event, from allopatric migrants of the New Caledonian taxon L. saintgironsi. A major divergence in speciation patterns between the two major clades of laticaudine snakes thus correlates with (and perhaps, is driven by) differences in the importance of terrestrial habitats in the species' ecology.     

First fossil record of Vipera Laurenti 1768 “Oriental vipers complex” (Serpentes: Viperidae) from the Early Pliocene of the western Mediterranean islands

The first fossil record of Vipera “Oriental vipers complex” (Serpentes: Viperidae) in the western Mediterranean islands is presented. Two large-sized vertebrae were found in an Early Pliocene karstic deposit located on the eastern coast of Mallorca, close to Caló den Rafelino (Manacor). The centrum length of the trunk vertebra (12.7 mm) represents the largest-sized known specimen of the European vipers belonging to the “Oriental vipers complex” and it suggests a body length close or greater than 200 cm. The arrival of this snake to Mallorca probably took place during the Messinian Salinity Crisis of the Mediterranean Sea (Late Miocene, 5.6-5.32 My ago) and it should be considered as one of the largest predators in Mallorca during the Early Pliocene. Although patterns of body size change in island snakes are unclear, some considerations about the large size of the Caló den Rafelino viper and co-evolution with endothermic preys are proposed.     

The island rule: made to be broken?

The island rule is a hypothesis whereby small mammals evolve larger size on islands while large insular mammals dwarf. The rule is believed to emanate from small mammals growing larger to control more resources and enhance metabolic efficiency, while large mammals evolve smaller size to reduce resource requirements and increase reproductive output. We show that there is no evidence for the existence of the island rule when phylogenetic comparative methods are applied to a large, high-quality dataset. Rather, there are just a few clade-specific patterns: carnivores; heteromyid rodents; and artiodactyls typically evolve smaller size on islands whereas murid rodents usually grow larger. The island rule is probably an artefact of comparing distantly related groups showing clade-specific responses to insularity. Instead of a rule, size evolution on islands is likely to be governed by the biotic and abiotic characteristics of different islands, the biology of the species in question and contingency.