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.     

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.     

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.     

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.     

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.     

Local endemism and within‐island diversification of shrews illustrate the importance of speciation in building Sundaland mammal diversity

Island systems are important models for evolutionary biology because they provide convenient, discrete biogeographic units of study. Continental islands with a history of intermittent dry land connections confound the discrete definitions of islands and have led zoologists to predict (i) little differentiation of terrestrial organisms among continental shelf islands and (ii) extinction, rather than speciation, to be the main cause of differences in community composition among islands. However, few continental island systems have been subjected to well‐sampled phylogeographic studies, leaving these biogeographic assumptions of connectivity largely untested. We analysed nine unlinked loci from shrews of the genus Crocidura from seven mountains and two lowland localities on the Sundaic continental shelf islands of Sumatra and Java. Coalescent species delimitation strongly supported all currently recognized Crocidura species from Sumatra (six species) and Java (five species), as well as one undescribed species endemic to each island. We find that nearly all species of Crocidura in the region are endemic to a single island and several of these have their closest relative(s) on the same island. Intra‐island genetic divergence among allopatric, conspecific populations is often substantial, perhaps indicating species‐level diversity remains underestimated. One recent (Pleistocene) speciation event generated two morphologically distinct, syntopic species on Java, further highlighting the prevalence of within‐island diversification. Our results suggest that both between‐ and within‐island speciation processes generated local endemism in Sundaland, supplementing the traditional view that the region's fauna is relictual and primarily governed by extinction.     

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.     

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.     

Stranded on an island: consequences of forest fragmentation for body size variations in an arboreal mammal, the edible dormouse (Glis glis)

The island rule states that small mammals isolated on islands have the evolutionary tendency to become larger, while large mammals tend to become smaller. However, the underlying mechanisms and life history consequences of these insular shifts in body size still remain speculative. The aim of this study was to investigate whether an arboreal mammal, the edible dormouse (Glis glis), showed shifts in body size when inhabiting isolated forest fragments. We analysed a data set of 541 individuals captured between 2005 and 2010 in four different forest fragments and one continuous forest, which served as a reference area. Sex, age, body mass, and size of all individuals were known. We used linear mixed-effect models to investigate whether individuals differed in their body size and mass between forest fragments and continuous forest. Our study revealed that edible dormice inhabiting forest fragments were significantly larger and heavier than individuals in the continuous forest, in accordance with patterns described by the island rule for small mammals. Because edible dormice frequently use nest boxes to rest during the day and to rear offspring, the life history strategies of this rodent can be easily investigated under evolutionary relevant conditions in the field. Thus the edible dormouse represents an excellent model organism for studying the mechanisms underlying shifts in body size as a response to habitat fragmentation and to investigate the consequences of these shifts on their life history strategies.     

The mid‐domain effect matters: simulation analyses of range‐size distribution data from Mount Kinabalu,Borneo

Aim In simulation exercises, mid‐domain peaks in species richness arise as a result of the random placement of modelled species ranges within simulated geometric constraints. This has been called the mid‐domain effect (MDE). Where close correspondence is found between such simulations and empirical data, it is not possible to reject the hypothesis that empirical species richness patterns result from the MDE rather than being the outcome (wholly or largely) of other factors. To separate the influence of the MDE from other factors we therefore need to evaluate variables other than species richness. The distribution of range sizes gives different predictions between models including the MDE or not. Here, we produce predictions for species richness and distribution of range sizes from one model without the MDE and from two MDE models: a classical MDE model encompassing only species with their entire range within the domain (range‐restricted MDE), and a model encompassing all species with the theoretical midpoint within the domain (midpoint‐restricted MDE). These predictions are compared with observations from the elevational pattern of range‐size distributions and species richness of vascular plants. Location Mount Kinabalu, Borneo. Methods The data set analysed comprises more than 28,000 plant specimens with information on elevation. Species ranges are simulated with various assumptions for the three models, and the species simulated are subsequently subjected to a sampling that simulates the actual collection of species on Mount Kinabalu. The resulting pattern of species richness and species range‐size distributions are compared with the observed pattern. Results The comparison of simulated and observed patterns indicates that an underlying monotonically decreasing trend in species richness with elevation is essential to explain fully the observed pattern of richness and range size. When the underlying trend is accounted for, the MDE model that restricts the distributions of theoretical midpoints performs better than both the classical MDE model and the model that does not incorporate geometric constraints. Main conclusions Of the three models evaluated here, the midpoint‐restricted MDE model is found to be the best for explaining species richness and species range‐size distributions on Mount Kinabalu.     

Evolutionary size changes in plants of the south‐west Pacific

Aim To investigate evolutionary changes in the size of leaves, stems and seeds of plants inhabiting isolated islands surrounding New Zealand. Location Antipodes, Auckland, Campbell, Chatham, Kermadec, Three Kings and Poor Knights Islands. Methods First, we compared the size of leaves and stems produced by 14 pairs of plant taxa between offshore islands and the New Zealand mainland, which were grown in a common garden to control for environmental effects. Similar comparisons of seed sizes were made between eight additional pairs of taxa. Second, we used herbarium specimens from 13 species pairs to investigate scaling relationships between leaf and stem sizes in an attempt to pinpoint which trait might be under selection. Third, we used herbarium specimens from 20 species to test whether changes in leaf size vary among islands located at different latitudes. Lastly, we compiled published records of plant heights to test whether insular species in the genus Hebe differed in size from their respective subgenera on the mainland. Results Although some evidence of dwarfism was observed, most insular taxa were larger than their mainland relatives. Leaf sizes scaled positively with stem diameters, with island taxa consistently producing larger leaves for any given stem size than mainland species. Leaf sizes also increased similarly among islands located at different latitudes. Size changes in insular Hebe species were unrelated to the average size of the respective subgenera on the mainland. Main conclusions Consistent evidence of gigantism was observed, suggesting that plants do not obey the island rule. Because our analyses were restricted to woody plants, results are also inconsistent with the ‘weeds‐to‐trees’ hypothesis. Disproportionate increases in leaf size relative to other plant traits suggest that selection may favour the evolution of larger leaves on islands, perhaps due to release from predation or increased intra‐specific competition.     

One size does not fit all: no evidence for an optimal body size on islands

Aim Optimal body size theories predict that large clades have a single, optimal, body size that serves as an evolutionary attractor, with the full body size spectrum of a clade resulting from interspecific competition. Because interspecific competition is believed to be reduced on islands, such theories predict that insular animals should be closer to the optimal size than mainland animals. We test the resulting prediction that insular clade members should therefore have narrower body size ranges than their mainland relatives. Location World‐wide. Methods We used body sizes and a phylogenetic tree of 4004 mammal species, including more than 200 species that went extinct since the last ice age. We tested, in a phylogenetically explicit framework, whether insular taxa converge on an optimal size and whether insular clades have narrow size ranges. Results We found no support for any of the predictions of the optimal size theory. No specific size serves as an evolutionary attractor. We did find consistent evidence that large (> 10 kg) mammals grow smaller on islands. Smaller species, however, show no consistent tendency to either dwarf or grow larger on islands. Size ranges of insular taxa are not narrower than expected by chance given the number of species in their clades, nor are they narrower than the size ranges of their mainland sister clades – despite insular clade members showing strong phylogenetic clustering. Main conclusions The concept of a single optimal body size is not supported by the data that were thought most likely to show it. We reject the notion that inclusive clades evolve towards a body‐plan‐specific optimum.     

Environmentally‐ and human‐induced body‐size responses in Macropus robustus and Macropus rufus,two widespread kangaroo species with largely overlapping distributions

Progressive body‐size dwarfing of animal populations is predicted under chronic mortality stress, such as that inflicted by human harvesting. However, empirical support for such declines in body size due to elevated mortality is lacking. In fact, the size of three macropodid species ─ the two grey kangaroo species, Macropus fuliginosus and M. giganteus, and the Red‐necked Wallaby, M. rufogriseus ─ appears to have increased since European settlement in Australia, despite these species being subjected to size‐selective harvesting over this period. To test whether this unexpected trend also characterises other species, we sought evidence of human‐induced body‐size changes in the two most widely distributed kangaroo species, the Euro Macropus robustus and Red Kangaroo M. rufus, from the late 19th Century onwards. Spatial autoregressive models controlling for age, sex and island effects were first used to identify environmental predictors of body size and to evaluate multi‐causal explanations for spatial body‐size patterns. Primary productivity emerged as the key driver of body size in both species, while heat conservation was supported as a further mechanism explaining the large body size of M. robustus in cold climatic regions. After controlling for these environmental factors, we find that the size of M. rufus has been stable over time and limited support for a small increase in the size of M. robustus. Hence, there is no empirical evidence that contemporary size‐selective harvesting has reduced body size in these species. Rather, the latter result supports the possibility that pasture improvement and/or dingo control (and associated reduction in predation pressure) facilitated body‐size increases following European settlement in Australia.     

Rule reversal: Ecogeographical patterns of body size variation in the common treeshrew (Mammalia,Scandentia)

There are a number of ecogeographical “rules” that describe patterns of geographical variation among organisms. The island rule predicts that populations of larger mammals on islands evolve smaller mean body size than their mainland counterparts, whereas smaller‐bodied mammals evolve larger size. Bergmann's rule predicts that populations of a species in colder climates (generally at higher latitudes) have larger mean body sizes than conspecifics in warmer climates (at lower latitudes). These two rules are rarely tested together and neither has been rigorously tested in treeshrews, a clade of small‐bodied mammals in their own order (Scandentia) broadly distributed in mainland Southeast Asia and on islands throughout much of the Sunda Shelf. The common treeshrew, Tupaia glis, is an excellent candidate for study and was used to test these two rules simultaneously for the first time in treeshrews. This species is distributed on the Malay Peninsula and several offshore islands east, west, and south of the mainland. Using craniodental dimensions as a proxy for body size, we investigated how island size, distance from the mainland, and maximum sea depth between the mainland and the islands relate to body size of 13 insular T. glis populations while also controlling for latitude and correlation among variables. We found a strong negative effect of latitude on body size in the common treeshrew, indicating the inverse of Bergmann's rule. We did not detect any overall difference in body size between the island and mainland populations. However, there was an effect of island area and maximum sea depth on body size among island populations. Although there is a strong latitudinal effect on body size, neither Bergmann's rule nor the island rule applies to the common treeshrew. The results of our analyses demonstrate the necessity of assessing multiple variables simultaneously in studies of ecogeographical rules.     

How island size and isolation affect bee and wasp ensembles on small tropical islands: a case study from Kepulauan Seribu,Indonesia

Aim To study the effects of isolation and size of small tropical islands on species assemblages of bees (superfamily Apoidea) and wasps (superfamily Vespoidea). Location Twenty islands in the Kepulauan Seribu Archipelago off the coast of west Java, Indonesia. The size of surveyed islands ranges between 0.75 and 41.32 ha; their distance from the coast of Java varies between 3 and 62 km. Methods Field work was conducted from February to May 2005. Bees and wasps were caught with a sweep net during sampling units of 15 min, continuing until four consecutive samples revealed no new species. Total species richness was quantified by the estimators Chao 2, first‐order jackknife and Michaelis–Menten. The software binmatnest was used to test for nestedness of species assemblages. Similarities of species composition between islands were quantified by Sørensen’s similarity index. Results Eighty‐two species were recorded on the 20 surveyed islands. Species richness declined with increasing isolation of islands from the source area, Java. Although the size of the largest island exceeded that of the smallest island by a factor of almost 60, island size only very weakly affected species richness of bees; no effect of island size was found for wasps. Mean body size of species decreased with increasing island isolation. Nestedness of island faunas was only weakly developed. Species composition of both superfamilies was affected by island isolation, but not by island size. Main conclusions While the species–isolation relationship on the very small islands of Kepulauan Seribu followed the prediction of MacArthur and Wilson’s equilibrium theory, the absence of a species–area relationship indicated a weak ‘small‐island effect’, at least in wasps. The combination of an only weakly developed pattern of nested species subsets, the shift in species compositions and the decline of mean body size with increasing island isolation from the source area indicates that biotic interactions and different species traits contribute to the shaping of communities of bees and wasps within the archipelago. The potential of biotic interactions for generating distribution patterns of species within the archipelago is also emphasized by the observed restriction of some species with apparently high dispersal abilities to outer islands.     

Environmental correlates of body size distributions of European springtails (Hexapoda: Collembola)

Aim Species–body size distributions (SBDs) are plots of species richness across body size classes. They have been linked to energetic constraints, speciation–extinction dynamics and to evolutionary trends. However, little is known about the spatial variation of size distributions. Here we study SBDs of European springtails (Collembola) at a continental scale and test whether minimum, average and maximum body size and the shapes of size distributions change across latitudinal and longitudinal gradients and whether SBDs of islands and mainlands differ. We also test whether the island rule and the positive body size–range size relationship of vertebrates also holds for Collembola. Location Europe. Methods We use a unique data set on the spatial distributions of 2102 species of European springtails across 52 countries and larger islands together with associated data on body size, area, climate variables, longitude and latitude. Differences in the central moments of SBDs are inferred from simultaneous spatial autoregression models. Results The SBD of the European Collembola and its largest suborder Entomobryomorpha is unimodal and symmetrical. Average, minimum and maximum body weight and the skewness of the mainland/island SBDs peaked at intermediate latitudes. We could not find simple latitudinal gradients in minimum and maximum body weight. Average and maximum body size increased with country/island area in accordance with the island rule in vertebrates, while minimum body size did not significantly differ between islands and mainlands. Finally, we found a weak but statistically significant positive correlation of range size and body size. Main conclusions We provide evidence for differences in body size distributions between islands and mainlands that are in part in line with the island rule in invertebrates. We also find evidence for an interspecific body size–range size relationship similar to that of vertebrates although the vertebrate pattern is much stronger than the springtail pattern. Our results on latitudinal gradients of maximum and average body size imply the need to account for species richness and area effects in the study of latitudinal gradients in body size. We recommend implementing sample size and area effects in the study of body size distributions on islands and mainlands.     

Effect of temperature and social environment on worker size in the ant Temnothorax nylanderi

Warm temperatures decrease insect developmental time and body size. Social life could buffer external environmental variations, especially in large social groups, either through behavioral regulation and compensation or through specific nest architecture. Mean worker size and distribution of worker sizes within colonies are important parameters affecting colony productivity as worker size is linked to division of labor in insect societies. In this paper, we investigate the effect of stressful warm temperatures and the role of social environment (colony size and size of nestmate workers) on the mean size and size variation of laboratory-born workers in the small European ant Temnothorax nylanderi. To do so, we reared field-collected colonies under medium or warm temperature treatments after having marked the field-born workers and removed the brood except for 30 first instar larvae. Warm temperature resulted in the production of fewer workers and a higher adult mortality, confirming that this regime was stressful for the ants. T. nylanderi ants followed the temperature size rule observed in insects, with a decreased developmental time and mean size under warm condition. Social environment appeared to play an important role as we observed that (i) larger colonies buffered the effect of temperature better than smaller ones (ii) colonies with larger workers produced larger workers whatever the rearing temperature and (iii) the coefficient of variation of worker size was similar in the field and under medium laboratory temperature. This suggests that worker size variation is not primarily due to seasonal environmental fluctuations in the field. Finally, we observed a higher coefficient of variation of worker size under warm temperature. We propose that this results from a disruption of social regulation, i.e. the control of nestmate workers over developing larvae and adult worker size, under stressful conditions.