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.     

The island syndrome and population dynamics of introduced rats

The island syndrome predicts directional changes in the morphology and demography of insular vertebrates, due to changes in trophic complexity and migration rates caused by island size and isolation. However, the high rate of human-mediated species introductions to some islands also increases trophic complexity, and this will reduce the perceived insularity on any such island. We test four hypotheses on the role of increased trophic complexity on the island syndrome, using introduced black rats (Rattus rattus) on two isolated coral atolls in the Mozambique Channel. Europa Island has remained relatively pristine and insular, with few species introductions, whereas Juan de Nova Island has had many species introductions, including predators and competitors of rats, anthropogenically increasing its trophic complexity. In the most insular environments, the island syndrome is expected to generate increases in body size and densities of rodents but decreases in the rates of reproduction and population cycling. Morphology and reproduction were compared using linear regression and canonical discriminant analysis, while density and population cycling were compared using spatially explicit capture–recapture analysis. Results were compared to other insular black rat populations in the Mozambique Channel and were consistent with predictions from the island syndrome. The manifestation of an island syndrome in rodents depends upon the trophic composition of a community, and may not relate to island size alone when many species additions, such as invasions, have occurred. The differing patterns of rodent population dynamics on each island provide information for future rodent eradication operations.     

Adaptation and plasticity in insular evolution of the house mouse mandible

Morphometric methods allow the quantification of directions of phenotypic changes and their statistical comparison in a morphometric space. We applied this approach to investigate several candidate factors to explain changes in mandible shape occurring in house mice (Mus musculus domesticus, Mammalia, Rodentia) in Corsica and a nearby islet. The role of niche widening and of the concomitant change in diet was evaluated by comparing the micro‐evolutionary insular change to the macro‐evolutionary difference between omnivorous and herbivorous rodents. Phenotypic plasticity, which may contribute to rapid insular evolution, was assessed by breeding laboratory mice on hard versus soft food. The related change in mandible shape was compared with differences between continental and insular populations. The role of allometry was evaluated by assessing shape change related to size within the continental population and comparing this direction of change with differences on islands. Finally, evolution may be facilitated along the direction of the greatest phenotypic variance. This hypothesis was tested by computing in wild populations vectors corresponding to this direction and by comparing these vectors with those corresponding to estimates of shape changes related to plasticity, micro‐ and macro‐evolutionary processes. In Corsica, the congruence in directions of macro‐ and micro‐evolutionary phenotypic vectors (Corsican/continental mice versus omnivorous/herbivorous rodents) supports the hypothesis of adaptation in mandible shape evolution. By contrast, on the islet, phenotypic divergence follows directions of plastic response to food consistency as well as within‐population allometry. Thus, results suggest differences in the relative importance of processes which may influence rodent mandibular shape depending on the size of the islands they colonized. Faster evolution and plasticity may be more evident in small and often ephemeral populations living on small islands, whereas micro‐evolutionary processes may have enough time and genetic variability to progressively ‘align’ with macro‐evolutionary trends in large populations from big islands.     

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.     

Dwarfism in insular sloths: biogeography,selection, and evolutionary rate

The islands of Bocas del Toro, Panama, were sequentially separated from the adjacent mainland by rising sea levels during the past 10,000 years. Three-toed sloths (Bradypus) from five islands are smaller than their mainland counterparts, and the insular populations themselves vary in mean body size. We first examine relationships between body size and physical characteristics of the islands, testing hypotheses regarding optimal body size, evolutionary equilibria, and the presence of dispersal in this system. To do so, we conduct linear regressions of body size onto island area, distance from the mainland, and island age. Second, we retroactively calculate two measures of the evolutionary rate of change in body size (haldanes and darwins) and the standardized linear selection differential, or selection intensity (i). We also test the observed morphological changes against models of evolution by genetic drift. The results indicate that mean body size decreases linearly with island age, explaining up to 97% of the variation among population means. Neither island area nor distance from the mainland is significant in multiple regressions that include island age. Thus, we find no evidence for differential optimal body size among islands, or for dispersal in the system. In contrast, the dependence of body size on island age suggests uniform directional selection for small body size in the insular populations. Although genetic drift cannot be discounted as the cause for this evolution in body size, the probability is small given the consistent direction of evolution (repeated dwarfism). The insular sloths show a sustained rate of evolution similar to those measured in haldanes over tens of generations, appearing to unite micro- and macroevolutionary time scales. Furthermore, the magnitude and rate of this example of rapid differentiation fall within predictions of theoretical models from population genetics. However, the linearity of the relationship between body size and island age is not predicted, suggesting that either more factors are involved than those considered here, or that theoretical advances are necessary to explain constant evolutionary rates over long time spans in new selective environments.     

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.     

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.     

‘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.     

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.     

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.     

Rodent eradications as ecosystem experiments: a case study from the Mexican tropics

For effective and efficient pest management it is essential to understand the ecology of the target species and recipient ecosystems. The use of rodent eradication as a restoration tool is well established in temperate regions, but less common in the tropics, presenting an opportunity to undertake scientific learning in tandem with rodent eradications. On a dry tropical archipelago, we used a Before-After-Control-Impact framework to document (1) fluctuations in the abundance and demography of invasive Rattus rattus and Mus musculus on three different islands, (2) the trophic niche of all three invasive rodent populations, and (3) changes in the invertebrate community before and after rodent eradication, also comparing with two rodent free islands. While rat density was high and relatively stable throughout the year, the two mouse populations greatly differed in body size and seasonal dynamics, despite their proximity. The rodents in all three populations were generalist and opportunistic feeders, although stable isotope analyses results indicated major differences among them, driven by food availability and rodent species. Seasonal fluctuations in invertebrate communities depended on rodent invasion status, but recovery in the invertebrate communities one year after rodent removal was limited for all islands. Predictions for other tropical ecosystem biomes require long-term research on more tropical islands. Improving our understanding of island and species-specific contexts of rodent eradications can advance island restoration projects and assist the selection of indicator species for ecosystem recovery.     

Energy expenditure in Crocidurinae shrews (Insectivora): is metabolism a key component of the insular syndrome?

A cascade of morphological, ecological, demographical and behavioural changes operates within island communities compared to mainland. We tested whether metabolic rates change on islands. Using a closed circuit respirometer, we investigated resting metabolic rate (RMR) of three species of Crocidurinae shrews: Suncus etruscus, Crocidura russula, and C. suaveolens. For the latter, we compared energy expenditure of mainland and island populations. Our measurements agree with those previously reported for others Crocidurinae: the interspecific comparison (ANCOVA) demonstrated an allometric relation between energy requirements and body mass. Energy expenditure also scaled with temperature. Island populations (Corsica and Porquerolles) of C. suaveolens differed in size from mainland (gigantism). A GLM showed a significant relationship between energy expenditure, temperature, body mass and locality. Mass specific RMR allometrically scales body mass, but total RMR does not significantly differ between mainland and island, although island shrews are giant. Our results are consistent with other studies: that demonstrated that the evolution of mammalian metabolism on islands is partially independent of body mass. In relation to the insular syndrome, we discuss how island selective forces (changes in resource availability, decrease in competition and predation pressures) can operate in size and physiological adjustments.     

Insular shifts in body size of rice frogs in the Zhoushan Archipelago, China

1. Differences in body size between mainland and island populations have been reported for reptiles, birds and mammals. Despite widespread recognition of insular shifts in body size in these taxa, there have been no reports of such body size shifts in amphibians. 2. We provide the first evidence of an insular shift in body size for an amphibian species, the rice frog Rana limnocharis. We found significant increases in body size of rice frogs on most sampled islands in the Zhoushan archipelago when compared with neighbouring mainland China. 3. Large body size in rice frogs on islands was significantly related to increased population density, in both breeding and non-breeding seasons. Increases in rice frog density were significantly related to higher resource availability on islands. Increased resource availability on islands has led to higher carrying capacities, which has subsequently facilitated higher densities and individual growth rates, resulting in larger body size in rice frogs. We also suggest that large body size has evolved on islands, as larger individuals are competitively superior under conditions of harsh intraspecific competition at high densities. 4. Increases in body size in rice frogs were not related to several factors that have been implicated previously in insular shifts in body size in other taxa. We found no significant relationships between body size of rice frogs and prey size, number of larger or smaller frog species, island area or distance of islands from the mainland. 5. Our findings contribute to the formation of a broad, repeatable ecological generality for insular shifts in body size across a range of terrestrial vertebrate taxa, and provide support for recent theoretical work concerning the importance of resource availability for insular shifts in body size.     

Body size of insular carnivores: evidence from the fossil record

Aim Our goals here are to: (1) assess the generality of one aspect of the island rule – the progressive trend towards decrease in size in larger species – for fossil carnivores on islands; (2) offer causal explanations for this pattern and deviations from it – as far as fossil carnivores are concerned; and (3) estimate the speed of this trend. Location Oceanic and oceanic‐like islands world‐wide. Methods Body size estimates of fossil insular carnivores and of their phylogenetically closest mainland relative were obtained from our own data and the published literature. Our dataset consisted of 18 species from nine islands world‐wide. These data were used to test whether the body size of fossil insular carnivores varies as a function of body size of the mainland species in combination with characteristics of the island ecosystem. Results Dwarfism was observed in two canid species. Moderate decrease in body mass was observed in one hyena species. Gigantism was observed in one otter species. Moderate body mass increase was observed in two otter species, one galictine mustelid and perhaps one canid. Negligible or no change in body mass at all was observed in five otter species, three galictine mustelids and one genet. Size changes in teeth do not lag behind in comparison to skeletal elements in the dwarfed canids. The evolutionary speed of dwarfism in a canid lineage is low. Main conclusions Size change in fossil terrestrial insular carnivores was constrained by certain ecological conditions, especially the availability of prey of appropriate body size. When such alternative prey was not available, the carnivores retained their mainland size. The impact of competitive carnivores seems negligible. The case of (semi‐)aquatic carnivores is much less clear. The species that maintained their ancestral body mass may have changed their diet, as is evidenced by their dentition. Among the otters, one case of significant size increase was observed, perhaps best explained as being due to it entering the niche of an obligate aquatic otter. Dwarfism was not observed in otters. The island rule seems to apply to fossil carnivores, but with exceptions. The dependency of the island rule on resource availability is emphasized by the present study.     

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.     

Primates follow the 'island rule': implications for interpreting Homo floresiensis

When the diminutive skeleton of Homo floresiensis was found on the Indonesian island of Flores, it was interpreted as an island dwarf, conforming to the 'island rule' that large animals evolve smaller size on islands, but small animals tend to get larger. However, previous studies of the island rule have not included primates, so the extent to which insular primate populations undergo size change was unknown. We use a comparative database of 39 independently derived island endemic primate species and subspecies to demonstrate that primates do conform to the island rule: small-bodied primates tend to get larger on islands, and large-bodied primates get smaller. Furthermore, larger species undergo a proportionally greater reduction in size on islands.     

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.