Convergence in reduced body size,head size,and blood glucose in three island reptiles

Many oceanic islands harbor diverse species that differ markedly from their mainland relatives with respect to morphology, behavior, and physiology. A particularly common morphological change exhibited by a wide range of species on islands worldwide involves either a reduction in body size, termed island dwarfism, or an increase in body size, termed island gigantism. While numerous instances of dwarfism and gigantism have been well documented, documentation of other morphological changes on islands remains limited. Furthermore, we lack a basic understanding of the physiological mechanisms that underlie these changes, and whether they are convergent. A major hypothesis for the repeated evolution of dwarfism posits selection for smaller, more efficient body sizes in the context of low resource availability. Under this hypothesis, we would expect the physiological mechanisms known to be downregulated in model organisms exhibiting small body sizes due to dietary restriction or artificial selection would also be downregulated in wild species exhibiting dwarfism on islands. We measured body size, relative head size, and circulating blood glucose in three species of reptiles—two snakes and one lizard—in the California Channel Islands relative to mainland populations. Collating data from 6 years of study, we found that relative to mainland population the island populations had smaller body size (i.e., island dwarfism), smaller head sizes relative to body size, and lower levels of blood glucose, although with some variation by sex and year. These findings suggest that the island populations of these three species have independently evolved convergent physiological changes (lower glucose set point) corresponding to convergent changes in morphology that are consistent with a scenario of reduced resource availability and/or changes in prey size on the islands. This provides a powerful system to further investigate ecological, physiological, and genetic variables to elucidate the mechanisms underlying convergent changes in life history on islands.     

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.     

The generality of the island rule reexamined

Aim  M.V. Lomolino and colleagues have recently reviewed the island rule in mammals and other vertebrates, claiming it is a general pattern. They have portrayed our recent analysis as weakly supporting the island rule, seeing weakness in our use of what they considered to be inadequate size indices (skulls and teeth, rather than mass or body length) and in our use of large islands. They argue that size evolution on islands points to a bauplan-specific fundamental size. We aim to test the generality of the rule and the adequacy of some of the data used to support it.
Location  Insular environments world-wide.
Methods  We collate and analyse data on skull sizes of carnivores and body masses of mammals in general to see whether there is a graded trend from dwarfism in large species to gigantism in smaller ones.
Results  The island rule is not supported with either the carnivore or the mammal data sets. Island area does not influence size change.
Main conclusions  Our results suggest that data recently advanced in support of the island rule are inadequate and that the island rule is not a general pattern for all mammals.     

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.     

Mainland size variation informs predictive models of exceptional insular body size change in rodents

The tendency for island populations of mammalian taxa to diverge in body size from their mainland counterparts consistently in particular directions is both impressive for its regularity and, especially among rodents, troublesome for its exceptions. However, previous studies have largely ignored mainland body size variation, treating size differences of any magnitude as equally noteworthy. Here, we use distributions of mainland population body sizes to identify island populations as ‘extremely’ big or small, and we compare traits of extreme populations and their islands with those of island populations more typical in body size. We find that although insular rodents vary in the directions of body size change, ‘extreme’ populations tend towards gigantism. With classification tree methods, we develop a predictive model, which points to resource limitations as major drivers in the few cases of insular dwarfism. Highly successful in classifying our dataset, our model also successfully predicts change in untested cases.     

Size evolution in island lizards

Aim  The island rule, small animal gigantism and large animal dwarfism on islands, is a topic of much recent debate. While size evolution of insular lizards has been widely studied, whether or not they follow the island rule has never been investigated. I examined whether lizards show patterns consistent with the island rule.
Location  Islands worldwide.
Methods  I used literature data on the sizes of island–mainland population pairs in 59 species of lizards, spanning the entire size range of the group, and tested whether small insular lizards are larger than their mainland conspecifics and large insular lizards are smaller. I examined the influence of island area, island isolation, and dietary preferences on lizard size evolution.
Results  Using mean snout–vent length as an index of body size, I found that small lizards on islands become smaller than their mainland conspecifics, while large ones become larger still, opposite to predictions of the island rule. This was especially strong in carnivorous lizards; omnivorous and herbivorous species showed a pattern consistent with the island rule but this result was not statistically significant. No trends consistent with the island rule were found when maximum snout–vent length was used. Island area had, at best, a weak effect on body size. Using maximum snout–vent length as an index of body size resulted in most lizard populations appearing to be dwarfed on islands, but no such pattern was revealed when mean snout–vent length was used as a size index.
Main conclusions  I suggest that lizard body size is mostly influenced by resource availability, with large size allowing some lizard populations to exploit resources that are unavailable on the mainland. Lizards do not follow the island rule. Maximum snout–vent length may be biased by sampling effort, which should be taken into account when one uses this size index.     

Climate change and size evolution in an island rodent species: new perspectives on the island rule

As stated by the island rule, small mammals evolve toward gigantism on islands. In addition they are known to evolve faster than their mainland counterparts. Body size in island mammals may also be influenced by geographical climatic gradients or climatic change through time. We tested the relative effects of climate change and isolation on the size of the Japanese rodent Apodemus speciosus and calculated evolutionary rates of body size change since the last glacial maximum (LGM). Currently A. speciosus populations conform both to Bergmann's rule, with an increase in body size with latitude, and to the island rule, with larger body sizes on small islands. We also found that fossil representatives of A. speciosus are larger than their extant relatives. Our estimated evolutionary rates since the LGM show that body size evolution on the smaller islands has been less than half as rapid as on Honshu, the mainland-type large island of Japan. We conclude that island populations exhibit larger body sizes today not because they have evolved toward gigantism, but because their evolution toward a smaller size, due to climate warming since the LGM, has been decelerated by the island effect. These combined results suggest that evolution in Quaternary island small mammals may not have been as fast as expected by the island effect because of the counteracting effect of climate change during this period.     

Body Size Evolution in Insular Speckled Rattlesnakes (Viperidae: Crotalus mitchellii)

Background

Speckled rattlesnakes (Crotalus mitchellii) inhabit multiple islands off the coast of Baja California, Mexico. Two of the 14 known insular populations have been recognized as subspecies based primarily on body size divergence from putative mainland ancestral populations; however, a survey of body size variation from other islands occupied by these snakes has not been previously reported. We examined body size variation between island and mainland speckled rattlesnakes, and the relationship between body size and various island physical variables among 12 island populations. We also examined relative head size among giant, dwarfed, and mainland speckled rattlesnakes to determine whether allometric differences conformed to predictions of gape size (and indirectly body size) evolving in response to shifts in prey size.

Methodology/Principal Findings

Insular speckled rattlesnakes show considerable variation in body size when compared to mainland source subspecies. In addition to previously known instances of gigantism on Ángel de la Guarda and dwarfism on El Muerto, various degrees of body size decrease have occurred frequently in this taxon, with dwarfed rattlesnakes occurring mostly on small, recently isolated, land-bridge islands. Regression models using the Akaike information criterion (AIC) showed that mean SVL of insular populations was most strongly correlated with island area, suggesting the influence of selection for different body size optima for islands of different size. Allometric differences in head size of giant and dwarf rattlesnakes revealed patterns consistent with shifts to larger and smaller prey, respectively.

Conclusions/Significance

Our data provide the first example of a clear relationship between body size and island area in a squamate reptile species; among vertebrates this pattern has been previously documented in few insular mammals. This finding suggests that selection for body size is influenced by changes in community dynamics that are related to graded differences in area over what are otherwise similar bioclimatic conditions. We hypothesize that in this system shifts to larger prey, episodic saturation and depression of primary prey density, and predator release may have led to insular gigantism, and that shifts to smaller prey and increased reproductive efficiency in the presence of intense intraspecific competition may have led to insular dwarfism.     

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.     

The island rule in large mammals: paleontology meets ecology

The island rule is the phenomenon of the miniaturization of large animals and the gigantism of small animals on islands, with mammals providing the classic case studies. Several explanations for this pattern have been suggested, and departures from the predictions of this rule are common among mammals of differing body size, trophic habits, and phylogenetic affinities. Here we offer a new explanation for the evolution of body size of large insular mammals, using evidence from both living and fossil island faunal assemblages. We demonstrate that the extent of dwarfism in ungulates depends on the existence of competitors and, to a lesser extent, on the presence of predators. In contrast, competition and predation have little or no effect on insular carnivore body size, which is influenced by the nature of the resource base. We suggest dwarfism in large herbivores is an outcome of the fitness increase resulting from the acceleration of reproduction in low-mortality environments. Carnivore size is dependent on the abundance and size of their prey. Size evolution of large mammals in different trophic levels has different underlying mechanisms, resulting in different patterns. Absolute body size may be only an indirect predictor of size evolution, with ecological interactions playing a major role.     

Rapid evolution of great kiskadees on Bermuda: an assessment of the ability of the island rule to predict the direction of contemporary evolution in exotic vertebrates

Aim To determine whether an exotic bird species, the great kiskadee (Pitangus sulphuratus), has diverged in morphology from its native source population, and, if so, has done so in a manner predicted by the island rule. The island rule predicts that insular vertebrates will tend towards dwarfism or gigantism when isolated on islands, depending on their body size. For birds, the island rule predicts that species with body sizes below 70–120 g should increase in size. The great kiskadee has a mean mass of c. 60 g in its native range, therefore we predicted that it would increase in size within the exotic, and more insular, Bermudan range. Location The islands of Bermuda (exotic population) and Trinidad (native source population). Methods We took eight morphological measurements on 84 individuals captured in the exotic (Bermudan) population and 62 individuals captured in the native source (Trinidadian) population. We compared morphological metrics between populations using univariate and principal components analyses. We assessed whether the effects of genetic drift could explain observed differences in morphology. We calculated divergence rates in haldanes and darwins for comparison with published examples of contemporary evolution. Finally, we used mark–recapture analysis to determine the effects of the measured morphological characters on survivorship within the exotic Bermudan population. Results Individuals in the exotic Bermudan population have larger morphological dimensions than individuals in the native source population on Trinidad. The degree of divergence in body mass (g) and bill width (mm) is probably not due to genetic drift. This rate of divergence is nearly equal to that observed amongst well‐documented examples of contemporary bird evolution, and is within the mid‐range of rates reported across taxa. There is no clear effect of body size on survivorship as only one character (bill width) was found to have an influence on individual survivorship. Main conclusions Exotic species provide useful systems for examining evolutionary predictions over contemporary time‐scales. We found that divergence between the exotic and native populations of this bird species occurred over c. 17 generations, and was in the direction predicted by the island rule, a principle based on the study of native species.     

The evolution of island gigantism and body size variation in tortoises and turtles

Extant chelonians (turtles and tortoises) span almost four orders of magnitude of body size, including the startling examples of gigantism seen in the tortoises of the Galapagos and Seychelles islands. However, the evolutionary determinants of size diversity in chelonians are poorly understood. We present a comparative analysis of body size evolution in turtles and tortoises within a phylogenetic framework. Our results reveal a pronounced relationship between habitat and optimal body size in chelonians. We found strong evidence for separate, larger optimal body sizes for sea turtles and island tortoises, the latter showing support for the rule of island gigantism in non-mammalian amniotes. Optimal sizes for freshwater and mainland terrestrial turtles are similar and smaller, although the range of body size variation in these forms is qualitatively greater. The greater number of potential niches in freshwater and terrestrial environments may mean that body size relationships are more complicated in these habitats.     

The island rule and the evolution of body size in the deep sea

Aim  Our goal is to test the generality of the island rule – a graded trend from gigantism in small-bodied species to dwarfism in large-bodied species – in the deep sea, a non-insular but potentially analogous system.
Location  Shallow-water and deep-sea benthic habitats in the western Atlantic Ocean from the North to South Poles.
Methods  We conducted regression analyses of body size of deep-sea gastropods species relative to their shallow-water congeners using measurements from the Malacolog ver. 3.3.3 database.
Results  Our results indicate that, consistent with the island rule, gastropod genera with small-bodied, shallow-water species have significantly larger deep-sea representatives, while the opposite is true for genera that are large-bodied in shallow water. Bathymetric body size clines within the deep sea are also consistent with predictions based on the island rule.
Main conclusions  Like islands, the deep sea is characterized by low absolute food availability, leading us to hypothesize that the island rule is a result of selection on body size in a resource-constrained environment. The body size of deep-sea species tends to converge on an optimal size for their particular ecological strategy and habitat.     

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.     

EVOLUTION OF GIGANTISM IN NINE‐SPINED STICKLEBACKS

The relaxation of predation and interspecific competition are hypothesized to allow evolution toward “optimal” body size in island environments, resulting in the gigantism of small organisms. We tested this hypothesis by studying a small teleost (nine‐spined stickleback, Pungitius pungitius) from four marine and five lake (diverse fish community) and nine pond (impoverished fish community) populations. In line with theory, pond fish tended to be larger than their marine or lake conspecifics, sometimes reaching giant sizes. In two geographically independent cases when predatory fish had been introduced into ponds, fish were smaller than those in nearby ponds lacking predators. Pond fish were also smaller when found in sympatry with three‐spined stickleback (Gasterosteus aculeatus) than those in ponds lacking competitors. Size‐at‐age analyses demonstrated that larger size in ponds was achieved by both increased growth rates and extended longevity of pond fish. Results from a common garden experiment indicate that the growth differences had a genetic basis: pond fish developed two to three times higher body mass than marine fish during 36 weeks of growth under similar conditions. Hence, reduced risk of predation and interspecific competition appear to be chief forces driving insular body size evolution toward gigantism.     

‘On being the right size’ – Do aliens follow the rules?

Aim

To assess whether mammalian species introduced onto islands across the globe have evolved to exhibit body size patterns consistent with the ‘island rule,’, and to test an ecological explanation for body size evolution of insular mammals.

Location

Islands worldwide.

Methods

We assembled data on body mass, geographical characteristics (latitude, maximum elevation) and ecological communities (number of mammalian competitors, predators and prey) for 385 introduced populations across 285 islands, comprising 56 species of extant, non‐volant mammals. We used linear regression, ANCOVA and regression tree analyses to test whether introduced populations of mammals exhibit the island rule pattern, whether the degree of body size change increased with time in isolation and whether residual variation about the general trend can be attributed to the geographical and ecological characteristics of the islands.

Results

Introduced populations follow the predicted island rule trend, with body size shifts more pronounced for populations with greater residence times on the islands. Small mammals evolved to larger body sizes in lower latitudes and on islands with limited topographic relief. Consistent with our hypothesis on the ecology of evolution, body size of insular introduced populations was influenced by co‐occurring species of mammalian competitors, predators and prey.

Conclusion

The island rule is a pervasive pattern, exhibited across a broad span of geographical regions, taxa, time periods and, as evidenced here, for introduced as well as native mammals. Time in isolation impacts body size evolution profoundly. Body size shift of introduced mammals was much more pronounced with increasing residence times, yet far less than that exhibited by native, palaeo‐insular mammals (residence times > 10,000 years). Given the antiquity of many species introductions, it appears that much of what we view as the natural character and ecological dynamics of recent insular communities may have been rendered artefacts of ancient colonizations by humans and commensals.     

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.     

Convergent evolution of gigantism in the flora of an isolated archipelago

Animals often evolve conspicuous differences in body size after colonising isolated islands. However, far less is known about repeated patterns in the evolution of plant size on islands. We tested for convergent evolution of leaf area, seed size and plant stature in the flora of the Chatham Islands (i.e. Rēkohu), which are located 700 km off the east coast of New Zealand. First, we measured leaf area, seed size and plant stature from populations of 22 plant taxa on the Chatham Islands. These data were then compared to analogous measurements from their sister taxa on the mainland to test for evidence of island gigantism, and whether size changes differ between leaves, seeds and stature. Second, we collated data from the literature to test whether size changes in endemic taxa differed among plant growth forms or were correlated with divergence times. Results showed that all three plant traits tended to increase in size on the Chatham Islands. However, field data showed that size increases tended to be more consistent in seeds and leaves than in stature. Data from the literature indicated that size increases also differed among growth forms. Herbaceous species showed the strongest evidence of gigantism, followed by woody plants, while graminoids showed a weak trend towards dwarfism. Insular size increases in seeds and leaves were also positively related to divergence times, indicating that taxa which have resided on the Chatham Islands for longer periods show stronger evidence of gigantism than taxa which arrived more recently. Overall results illustrate that gigantism is a hallmark of the Chatham Island flora, providing a remarkable example of convergent evolution.     

Distribution, variation, and systematics of the Seychelles treefrog, Tachycnemis seychellensis (Amphibia: Anura: Hyperoliidae)

The endemic Seychelles treefrog, Tachycnemis seychellensis (Duméril & Bibron), is restricted to four of the granitic islands of the Seychelles: La Digue, Mahe, Praslin and Silhouette. Megalixalus infrarufus Gunther 1869 is a junior synonym of Eucnemis ( Tachycnemis ) seychellensis Dumeril Bibron, 1841. Significant variation in colour and morphometric characteristics exists within and between island populations. The patterns of geographic variation revealed support the hypothesis that the distribution of Tachycnemis seychellensis in the granitic Seychelles reflects vicariance through fragmentation of the Seychelles Microcontinent 10,000 years B. P. by marine transgression. However, the possibility of low rates of postfragmentation dispersal between islands cannot be ruled out. The close relationship of the nearby Mahe and Silhouette populations probably reflects prefragmentation gene flow over relatively short distances and postfragmentation stasis due to large population size and similar environments. The small body size and colour similarities of the Praslin and La Digue populations may result from prefragmentation gene flow between these close populations, but the relatively great differences in morphometric traits suggest rapid divergence in isolation perhaps as a result of genetic drift and strong selection. It is argued that the four island populations represent a single species and that subspecies should not be named.     

Of mice and mammoths: generality and antiquity of the island rule

Aim

We assessed the generality of the island rule in a database comprising 1593 populations of insular mammals (439 species, including 63 species of fossil mammals), and tested whether observed patterns differed among taxonomic and functional groups.

Location

Islands world‐wide.

Methods

We measured museum specimens (fossil mammals) and reviewed the literature to compile a database of insular animal body size (S_i = mean mass of individuals from an insular population divided by that of individuals from an ancestral or mainland population, M). We used linear regressions to investigate the relationship between S_i and M, and ANCOVA to compare trends among taxonomic and functional groups.

Results

S_i was significantly and negatively related to the mass of the ancestral or mainland population across all mammals and within all orders of extant mammals analysed, and across palaeo‐insular (considered separately) mammals as well. Insular body size was significantly smaller for bats and insectivores than for the other orders studied here, but significantly larger for mammals that utilized aquatic prey than for those restricted to terrestrial prey.

Main conclusions

The island rule appears to be a pervasive pattern, exhibited by mammals from a broad range of orders, functional groups and time periods. There remains, however, much scatter about the general trend; this residual variation may be highly informative as it appears consistent with differences among species, islands and environmental characteristics hypothesized to influence body size evolution in general. The more pronounced gigantism and dwarfism of palaeo‐insular mammals, in particular, is consistent with a hypothesis that emphasizes the importance of ecological interactions (time in isolation from mammalian predators and competitors was 0.1 to > 1.0 Myr for palaeo‐insular mammals, but < 0.01 Myr for extant populations of insular mammals). While ecological displacement may be a major force driving diversification in body size in high‐diversity biotas, ecological release in species‐poor biotas often results in the convergence of insular mammals on the size of intermediate but absent species.