Phylogeny and adaptation shape the teeth of insular mice

Pests or prey? Micromammal species within an ancient anthropic environment at the Norse settlement site of Tuquoy (Westray, Orkney)

Micromammals, like rodents and shrews, adapt rapidly to take advantage of new food sources, habitats and ecological niches, frequently thriving in anthropogenic environments. Their remains, often retrieved during archaeological investigations, can be a valuable source of information about the past environmental conditions as well as interspecies interactions and human activity. However, the research on such finds rarely covers multiple approaches, often relying on single species or data type (e.g. identification/information for proxy studies). Here we investigate micromammal remains from the Norse and medieval (AD tenth-fourteenth centuries) archaeological site at Tuquoy, Orkney, to elucidate the relationships between micromammals, humans and other species present using a variety of data. Four micromammal species were identified, and their species dynamics as well as relationships with humans could be inferred by tracking changes in spatial and temporal location of remains, from their taphonomic history and by age estimation for individual animals. A larger, predatory assemblage was also identified, with species composition differing from that in the rest of the archaeological assemblage, and possibly therefore representing small mammal species composition in the wild. The assemblage was probably deposited by a diurnal raptor, though identification to species is not certain due to post-depositional processes.

Microevolutionary variation in molar morphology of Onychomys leucogaster decoupled from genetic structure

In neutral models of quantitative trait evolution, both genetic and phenotypic divergence scale as random walks, producing a correlation between the two measures. However, complexity in the genotype-phenotype map may alter the correlation between genotypic and phenotypic divergence, even when both are evolving neutrally or nearly so. Understanding this correlation between phenotypic and genetic variation is critical for accurately interpreting the fossil record. This study compares the geographic structure and scaling of morphological variation of the shape of the first lower molar of 77 individuals of the northern grasshopper mouse Onychomys leucogaster to genome-wide SNP variation in the same sample. We found strong genetic structure but weak or absent morphological structure indicating that the scaling of each type of variation is decoupled from one another. Low P_ST values relative to F_ST values are consistent with a lack of morphological divergence in contrast to genetic divergence between groups. This lack of phenotypic structure and the presence of notable within-sample phenotypic variance are consistent with uniform selection or constraints on molar shape across a wide geographic and environmental range. Over time, this kind of decoupling may result in patterns of phenotypic stasis masking underlying genetic patterns.

Skull morphological evolution in Malagasy endemic Nesomyinae rodents

Madagascar is a large island to the south-east of Africa and in many ways continental in size and ecological complexity. Here we aim to define how skull morphology of an endemic and monophyletic clade of rodents (sub-family Nesomyinae), that show considerable morphological variation, have evolved and how their disparity is characterized in context of the geographical and ecological complexity of the island. We performed a two-dimensional geometric morphometric analysis on 370 dorsal and 399 ventral skull images of 19 species (comprising all nine extant endemic genera) and tested the influence of three ecological parameters (climate, locomotor habitat and nychthemeral cycle) in a phylogenetic context on size and shape. The results indicate that skull shape appears to importantly reflect phylogeny, whereas skull size does not carry a significant phylogenetic signal. Skull shape is significantly influenced by climate while, skull size is not impacted by any of the ecological factors tested, which is controversial to expectations in an insular context. In conclusion, Nesomyinae must have evolved under unusual types of local constraints, preventing this radiation from demonstrating strong ecological release.

Wild versus lab house mice: Effects of age, diet, and genetics on molar geometry and topography

Molar morphology is shaped by phylogenetic history and adaptive processes related to food processing. Topographic parameters of the occlusal surface, such as sharpness and relief, can be especially informative regarding diet preferences of a species. The occlusal surface can however be deeply modified by wear throughout an animal's life, potentially obliterating other signals. Age being difficult to assess in wild populations, especially small rodents, experimental studies of wear through age in laboratory populations may constitute a powerful way to assess its impact on molar geometry and topography, and to validate descriptors of molar morphology that could mitigate this issue. Molar morphology was therefore quantified using 3D geometric morphometrics and topographic estimates in four groups of house mice: wild-trapped mice, lab-bred offspring of these wild mice, typical laboratory mice, and their hybrids. Three descriptors of the molar morphology were considered: the surface of the whole molar row, the surface of the first upper molar, and a truncated template of the first upper molar mimicking advanced wear. Increasing wear with age was demonstrated in the different groups, with a more pronounced effect in the wild-trapped population. The geometry of the molar row is not only modified by wear, but also by the relative position of the late developing molars on the jaw due to loading during mastication. As a consequence, the alignment of the molars is modified in wild mice, showing a qualitative difference between wild animals and their lab-bred offspring. Results obtained from the lab should thus be transferred with caution to the interpretation of differences in wild populations. Topographic estimates computed for the first upper molar seems to provide more stable parameters than those based on the whole molar row, because issues related to non-planar occlusal surface along the molar row are discarded. The truncated template was proven efficient in discarding the wear effect to focus on genetic differences, allowing an efficient characterization of the hybridization signature between wild and lab mice. Dominance of the wild phenotype for the first molar shape supports that the lab strain evolved in a context of relaxation of the selective pressures related to nutrition.

Size changes in miR‑21 knockout mice: Geometric morphometrics on teeth, alveolar bone and mandible

MicroRNA‑21 (miR‑21) is a small non‑coding RNA that is differentially expressed during tooth development, particularly during amelogenesis. Although orthodontic tooth movement and the innate immune response are impaired, miR‑21 knockout mice demonstrate no obvious skeletal phenotype. However, the consequence of miR‑21 knockout on tooth phenotype and corresponding alveolar bone is unknown. The current study utilized landmark‑based geometric morphometrics to identify anatomical dissimilarities of the three lower and upper molars, and the corresponding alveolar bone, in miR‑21 knockout and wild‑type control mice. The anatomical structures were visualized by microcomputer tomography. A total of 36 and 38 landmarks were placed on mandibular and maxillary molars, respectively. For the alveolar bone, 16 landmarks were selected on both anatomical sites. General Procrustes analysis revealed significantly smaller molars and dimensions of the alveolar bone in the mandible of the miR‑21 knockout mice when compared with wild‑type controls (P=0.03 and P=0.04, respectively). The overall dimension of the mandible was reduced by the lack of miR‑21 (P=0.02). In the maxilla, the dimension of the alveolar bone was significant (P=0.02); however, this was not observed in the molars (P=0.36). Based on principal component analysis, no changes in shape for any of the anatomical sites were observed. Dental and skeletal jaw length were calculated and no prognathism was identified. However, the fluctuating asymmetry of the molars in the mandible and the maxilla was reduced in the miR‑21 knockout mice by 38 and 27%, respectively. Taken together, the results of the present study revealed that the molars in the mandible and the dimension of the respective alveolar bone were smaller in miR‑21 mice compared with wild‑type littermates, suggesting that miR‑21 influences tooth development.

Evaluating the island effect on phenotypic evolution in the Italian wall lizard, Podarcis siculus (Reptilia: Lacertidae)

Islands are compelling natural laboratories for studying evolutionary processes. Nevertheless, the existence of general rules underlying morphological evolution on islands remains an unresolved issue. In this study, we investigated the insular phenotypic variability of the Italian wall lizard (Podarcis siculus) on a large geographical scale, in order to assess the putative existence of an island effect on three morphological head traits: shape, size and degree of sexual dimorphism. A geometric morphometric analysis was performed on 30 island and 24 mainland populations, involving a total of 992 specimens, and we analysed differences in both mean trait values and variances (disparity). We found increased shape disparity in insular lizards with respect to mainland ones. On the other hand, both size disparity and mean head dimensions of males decreased on islands, leading to a reduction in sexual dimorphism. Our results provide evidence for a multidirectional morphological diversification on islands concerning head shape of both sexes, while directional and canalizing selection likely occurred for head size, but only in males. Our findings improve our knowledge on the effect of insularity in Podarcis siculus, and highlight the need for an exstensive sampling scheme and a multi-trait methodological approach.

Genetic structure in Orkney island mice: isolation promotes morphological diversification

Following human occupation, the house mouse has colonised numerous islands, exposing the species to a wide variety of environments. Such a colonisation process, involving successive founder events and bottlenecks, may either promote random evolution or facilitate adaptation, making the relative importance of adaptive and stochastic processes in insular evolution difficult to assess. Here, we jointly analyse genetic and morphometric variation in the house mice (Mus musculus domesticus) from the Orkney archipelago. Genetic analyses, based on mitochondrial DNA and microsatellites, revealed considerable genetic structure within the archipelago, suggestive of a high degree of isolation and long-lasting stability of the insular populations. Morphometric analyses, based on a quantification of the shape of the first upper molar, revealed considerable differentiation compared to Western European populations, and significant geographic structure in Orkney, largely congruent with the pattern of genetic divergence. Morphological diversification in Orkney followed a Brownian motion model of evolution, suggesting a primary role for random drift over adaptation to local environments. Substantial structuring of human populations in Orkney has recently been demonstrated, mirroring the situation found here in house mice. This synanthropic species may thus constitute a bioproxy of human structure and practices even at a very local scale.

Identifying early stages of reindeer domestication in the archaeological record: a 3D morphological investigation on forelimb bones of modern populations from Fennoscandia

Reindeer herding probably developed during the Late Iron Age onwards and is still an important part of the subsistence and culture of many peoples in northern Eurasia. However, despite the importance of this husbandry in the history of these Arctic people, the period and place of the origin as well as the spread of domestic reindeer is still highly debated. Besides the existence of different breeding methods in these territories, identifying domesticated individuals in the archaeological record is complicated because reindeers are considered to still be in the early phases of the domestication process. Indeed, the traditional morphological markers used in zooarchaeology to decipher the domestication syndrome are hardly perceptible in these early stages. In this work, we propose solutions for identifying domestic reindeer bones using 3D geometric morphometrics on isolated elements from the long bones of the forelimb (i.e. humerus, radio-ulna and metacarpal). These bones are important to understand both the feeding behaviour and the mobility of reindeer, and the potential effect of load-carrying or draught in the case of domestic reindeer. We analysed 123 modern specimens from Fennoscandia, including the two interbreeding subspecies currently present in these territories: mountain reindeer (Rangifer tarandus tarandus) and forest reindeer (R.t. fennicus); and where the sex and the lifestyle were known (i.e. free-ranging, racing or draught and captive individuals). A good level of discrimination between the size and shape variables of the bones of the forelimb was found among both subspecies and sexes. Moreover, individuals bred in captivity had smaller bone elements and a thinner and more slender morphology than free-ranging individuals. This demonstrates that the long bones of the forelimb can provide information on changes in feeding and locomotor behaviour prompted by the domestication process, like control and/or reduction of mobility and food of individual reindeer by humans. This also demonstrates that analysis in 3D geometric morphometrics is useful in detecting reindeer incipient domestication markers. Our results can be used by archaeologists to trace the early stages of domestication from fossil reindeer remains, and aid in reconstructing the socio-economic changes of past Arctic populations over time.

Morphological variation in two sigmodontine rodents along the mainland and the Fuegian archipelago in Chilean southern Patagonia

Background

Analysis of morphological variation in archipelagos has been essential to understand the evolution of terrestrial vertebrates. In particular, these natural scenarios allow to assess morphological changes experienced by insular fauna compared to their mainland counterparts. In mammals, morphological changes of insular forms have been observed in size and shape of body and cranial traits. The southern Patagonia of Chile represents a unique scenario to analyze morphological variation in two of the most widely distributed small rodents along western southern South America: Abrothrix olivacea and Oligoryzomys longicaudatus.

Methods

We applied linear and three-dimensional geometric morphometric tools to analyze the variation in cranial morphology of A. olivacea (N = 80) and O. longicaudatus (N = 49). Sampled localities were distributed in the mainland and islands of the Fuegian archipelago in Chilean Patagonia. Standard multivariate methods, as Principal Components Analysis, and Logistic Regression models were carried out to evaluate differences in size and shape of crania.

Results

We detected higher levels of morphological variation in the crania of A. olivacea if compared to O. longicaudatus. The variation was associated to cranial size instead of shape.

We observed significant differences between insular and mainland individuals in A. olivacea, being the cranium size of this species significantly bigger on islands. Indeed, specimens of A. olivacea from “Isla Wellington”, exhibited the higher increment in cranium size compared to any other mainland and insular site sampled for this species. In contrast, insular and mainland forms of O. longicaudatus did not show significant differences in cranial morphology.

Conclusions

We suggest that the patterns of morphological variation observed in these two small rodents along Patagonia, could be explained in terms of the historical biogeography of the region, and the different ecological features of the studied species.

Morphometrics and genetics highlight the complex history of Eastern Mediterranean spiny mice

Spiny mice of the Acomys cahirinus group display a complex geographical structure in the Eastern Mediterranean area, as shown by previous genetic and chromosomal studies. To better elucidate the evolutionary relationships between insular populations from Crete and Cyprus and continental populations from North Africa and Cilicia in Turkey, genetic and morphometric variations were investigated, based on mitochondrial D-loop sequences, and the size and shape of the first upper molar. The Cypriot and the Cilician populations show idiosyncratic divergence in molar size and shape, while Cretan populations present a geographical structure with at least three differentiated subpopulations, as shown by congruent distributions of haplogroups, Robertsonian fusions and morphometric variation. A complex history of multiple introductions is probably responsible for this structure, and insular isolation coupled with habitat shift should have further promoted a pronounced and rapid morphological evolution in molar size and shape on Crete and Cyprus.

The amylase gene cluster in house mice (Mus musculus) was subject to repeated introgression including the rescue of a pseudogene

Background

Amylase gene clusters have been implicated in adaptive copy number changes in response to the amount of starch in the diet of humans and mammals. However, this interpretation has been questioned for humans and for mammals there is a paucity of information from natural populations.

Results

Using optical mapping and genome read information, we show here that the amylase cluster in natural house mouse populations is indeed copy-number variable for Amy2b paralogous gene copies (called Amy2a1 - Amy2a5), but a direct connection to starch diet is not evident. However, we find that the amylase cluster was subject to introgression of haplotypes between Mus musculus sub-species. A very recent introgression can be traced in the Western European populations and this leads also to the rescue of an Amy2b pseudogene. Some populations and inbred lines derived from the Western house mouse (Mus musculus domesticus) harbor a copy of the pancreatic amylase (Amy2b) with a stop codon in the first exon, making it non-functional. But populations in France harbor a haplotype introgressed from the Eastern house mouse (M. m. musculus) with an intact reading frame. Detailed analysis of phylogenetic patterns along the amylase cluster suggest an additional history of previous introgressions.

Conclusions

Our results show that the amylase gene cluster is a hotspot of introgression in the mouse genome, making it an evolutionary active region beyond the previously observed copy number changes.

Size matters: micro-evolution in Polynesian rats highlights body size changes as initial stage in evolution

Microevolutionary patterns in populations of introduced rodent species have often been the focus of analytic studies for their potential relevance to understanding vertebrate evolution. The Polynesian rat (Rattus exulans) is an excellent proxy species because of its wide geographic and temporal distribution: its native and introduced combined range spans half the globe and it has been living for at least seven centuries wherever it was introduced. The objective of this study was to assess the effects of long-term isolation (insularity; up to 4,000 years) and geographic variables on skull shape variation using geometric morphometrics. A sample of 513 specimens from 103 islands and four mainland areas was analysed. This study, to my knowledge the first to extensively sample introduced rats, analysed 59 two-dimensional landmarks on the skull. Landmarks were obtained in three separate aspects (dorsal, lateral, ventral skull view). The coordinate data were then subjected to a multivariate ordination analysis (principal components analysis, or PCA), multivariate regressions, and a canonical variates analysis (CVA). Three measures of disparity were evaluated for each view. The results show that introduced Polynesian rats evolve skull shapes that conform to the general mammalian interspecific pattern of cranial evolutionary allometry (CREA), with proportionally longer snouts in larger specimens. In addition, larger skulls are more tubular in shape than the smaller skulls, which are more balloon-shaped with a rounder and wider braincase relative to those of large skulls. This difference is also observed between the sexes (sexual dimorphism), due to the slightly larger average male size. Large, tubular skulls with long snouts are typical for Polynesia and Remote Oceania, where no native mammals occur. The greater disparity of Polynesian rats on mammal species-poor islands ('exulans-only' region) provides further insight into how diversity may affect diversification through ecological release from predators and competitors.

Exerting an influence on evolution

Experiments on mice have shown that developmental processes are influencing the generation of phenotypic variation in a way that shapes evolution.

Developmental variability channels mouse molar evolution

Do developmental systems preferentially produce certain types of variation that orient phenotypic evolution along preferred directions? At different scales, from the intra-population to the interspecific, the murine first upper molar shows repeated anterior elongation. Using a novel quantitative approach to compare the development of two mouse strains with short or long molars, we identified temporal, spatial and functional differences in tooth signaling center activity, that arise from differential tuning of the activation-inhibition mechanisms underlying tooth patterning. By tracing their fate, we could explain why only the upper first molar reacts via elongation of its anterior part. Despite a lack of genetic variation, individuals of the elongated strain varied in tooth length and the temporal dynamics of their signaling centers, highlighting the intrinsic instability of the upper molar developmental system. Collectively, these results reveal the variational properties of murine molar development that drive morphological evolution along a line of least resistance.

Over time species develop random mutations in their genetic sequence that causes their form to change. If this new form increases the survival of a species it will become favored through natural selection and is more likely to get passed on to future generations. But, the evolution of these new traits also depends on what happens during development.

Developmental mechanisms control how an embryo progresses from a single cell to an adult organism made of many cells. Mutations that alter these processes can influence the physical outcome of development, and cause a new trait to form. This means that if many different mutations alter development in a similar way, this can lead to the same physical change, making it ‘easy’ for a new trait to repeatedly occur. Most of the research has focused on finding the mutations that underlie repeated evolution, but rarely on identifying the role of the underlying developmental mechanisms.

To bridge this gap, Hayden et al. investigated how changes during development influence the shape and size of molar teeth in mice. In some wild species of mice, the front part of the first upper molar is longer than in other species. This elongation, which is repeatedly found in mice from different islands, likely came from developmental mechanisms.

Tooth development in mice has been well-studied in the laboratory, and Hayden et al. started by identifying two strains of laboratory mice that mimic the teeth seen in their wild cousins, one with elongated upper first molars and another with short ones. Comparing how these two strains of mice developed their elongated or short teeth revealed key differences in the embryonic structures that form the upper molar and cause it to elongate. Further work showed that variations in these embryonic structures can even cause mice that are genetically identical to have longer or shorter upper first molars.

These findings show how early differences during development can lead to small variations in form between adult species of mice. This study highlights how studying developmental differences as well as genetic sequences can further our understanding of how different species evolved.

Morphological diversity of wild rabbit populations: implications for archaeology and palaeontology

Morphometric characteristics of the European rabbit (Oryctolagus cuniculus) are regularly used in archaeological and palaeontological studies to explore aspects of prehistoric human hunting behaviour, to reconstruct past environments or to define new species. However, the variability of these characteristics is still both poorly understood and under-documented due to a lack of population-level data that are essential for reliably interpreting the fossil record of this species. Here we address the morphometric diversity of wild rabbits in seven current populations from south-western Europe. Size variations in different skeletal parts were analysed to explore the potential impact of sexual dimorphism. A geometric morphometric analysis of the third lower premolar (p3) – a tooth commonly used to distinguish leporid species – was used to evaluate the relative effects of size, phylogeny, geographical location and climate on shape variation. The results show a negligible impact of sexual dimorphism, contradicting previous studies. We also demonstrate geography and climate to be the main factors driving variation in p3 shape, potentially calling into question criteria typically used to identify rabbit species. These results are valuable not only for palaeobiologists studying the taxonomy and the evolutionary history of the leporid family but also for archaeologists interested in the socio-economic and behavioural aspects of Palaeolithic human groups.

Divergent in shape and convergent in function: Adaptive evolution of the mandible in Sub-Antarctic mice

Convergent evolution in similar environments constitutes strong evidence of adaptive evolution. Transported with people around the world, house mice colonized even remote areas, such as Sub-Antarctic islands. There, they returned to a feral way of life, shifting towards a diet enriched in terrestrial macroinvertebrates. Here, we test the hypothesis that this triggered convergent evolution of the mandible, a morphological character involved in food consumption. Mandible shape from four Sub-Antarctic islands was compared to phylogeny, tracing the history of colonization, and climatic conditions. Mandible shape was primarily influenced by phylogenetic history, thus discarding the hypothesis of convergent evolution. The biomechanical properties of the jaw were then investigated. Incisor in-lever and temporalis out-lever suggested an increase in the velocity of incisor biting, in agreement with observations on various carnivorous and insectivorous rodents. The mechanical advantage related to incisor biting also revealed an increased functional performance in Sub-Antarctic populations, and appears to be an adaptation to catch prey more efficiently. The amount of change involved was larger than expected for a plastic response, suggesting microevolutionary processes were evolved. This study thus denotes some degree of adaptive convergent evolution related to changes in habitat-related changes in dietary items in Sub-Antarctic mice, but only regarding simple, functionally relevant aspects of mandible morphology.

Seabirds fighting for land: phenotypic consequences of breeding area constraints at a small remote archipelago

Identifying associations between phenotypes and environmental parameters is crucial for understanding how natural selection acts at the individual level. In this context, genetically isolated populations can be useful models for identifying the forces selecting fitness-related traits. Here, we use a comprehensive dataset on a genetically and ecologically isolated population of the strictly marine bird, the brown booby Sula leucogaster, at the tropical and remote Saint Peter and Saint Paul Archipelago, mid-Atlantic Ocean, in order to detect phenotypic adjustments from interindividual differences in diet, foraging behaviour, and nest quality. For this, we took biometrics of all individuals of the colony breeding in 2014 and 2015 and tested their associations with nest quality, diet parameters, and foraging behaviour. While body size was not related to the foraging parameters, the body size of the females (responsible for nest acquisition and defence) was significantly associated with the nest quality, as larger females occupied high-quality nests. Our findings suggest that the small breeding area, rather than prey availability, is a limiting factor, emphasizing the role of on-land features in shaping phenotypic characteristics and fitness in land-dependent marine vertebrates.

Genomic regions controlling shape variation in the first upper molar of the house mouse

Numerous loci of large effect have been shown to underlie phenotypic variation between species. However, loci with subtle effects are presumably more frequently involved in microevolutionary processes but have rarely been discovered. We explore the genetic basis of shape variation in the first upper molar of hybrid mice between Mus musculus musculus and M. m. domesticus. We performed the first genome-wide association study for molar shape and used 3D surface morphometrics to quantify subtle variation between individuals. We show that many loci of small effect underlie phenotypic variation, and identify five genomic regions associated with tooth shape; one region contained the gene microphthalmia-associated transcription factor Mitf that has previously been associated with tooth malformations. Using a panel of five mutant laboratory strains, we show the effect of the Mitf gene on tooth shape. This is the first report of a gene causing subtle but consistent variation in tooth shape resembling variation in nature.

Ecomorphological characterization of murines and non-arvicoline cricetids (Rodentia) from south-western Europe since the latest Middle Miocene to the Mio-Pliocene boundary (MN 7/8-MN13)

Rodents are the most speciose group of mammals and display a great ecological diversity. Despite the greater amount of ecomorphological information compiled for extant rodent species, studies usually lack of morphological data on dentition, which has led to difficulty in directly utilizing existing ecomorphological data of extant rodents for paleoecological reconstruction because teeth are the most common or often the only micromammal fossils. Here, we infer the environmental ranges of extinct rodent genera by extracting habitat information from extant relatives and linking it to extinct taxa based on the phenogram of the cluster analysis, in which variables are derived from the principal component analysis on outline shape of the upper first molars. This phenotypic “bracketing” approach is particularly useful in the study of the fossil record of small mammals, which is mostly represented by isolated teeth. As a case study, we utilize extinct genera of murines and non-arvicoline cricetids, ranging from the Iberoccitanian latest middle Miocene to the Mio-Pliocene boundary, and compare our results thoroughly with previous paleoecological reconstructions inferred by different methods. The resultant phenogram shows a predominance of ubiquitous genera among the Miocene taxa, and the presence of a few forest specialists in the two rodent groups (Murinae and Cricetidae), along with the absence of open environment specialists in either group of rodents. This appears to be related to the absence of enduring grassland biomes in the Iberian Peninsula during the late Miocene. High consistency between our result and previous studies suggests that this phenotypic “bracketing” approach is a very useful tool.

A potential pitfall in studies of biological shape: Does size matter?

The number of published studies using geometric morphometrics (GM) for analysing biological shape has increased steadily since the beginning of the 1990s, covering multiple research areas such as ecology, evolution, development, taxonomy and palaeontology. Unfortunately, we have observed that many published studies using GM do not evaluate the potential allometric effects of size on shape, which normally require consideration or assessment. This might lead to misinterpretations and flawed conclusions in certain cases, especially when size effects explain a large part of the shape variation. We assessed, for the first time and in a systematic manner, how often published studies that have applied GM consider the potential effects of allometry on shape. We reviewed the 300 most recent published papers that used GM for studying biological shape. We also estimated how much of the shape variation was explained by allometric effects in the reviewed papers. More than one-third (38%) of the reviewed studies did not consider the allometric component of shape variation. In studies where the allometric component was taken into account, it was significant in 88% of the cases, explaining up to 87.3% of total shape variation. We believe that one reason that may cause the observed results is a misunderstanding of the process that superimposes landmark configurations, i.e. the Generalized Procrustes Analysis, which removes isometric effects of size on shape, but not allometric effects. Allometry can be a crucial component of shape variation. We urge authors to address, and report, size effects in studies of biological shape. However, we do not propose to always remove size effects, but rather to evaluate the research question with and without the allometric component of shape variation. This approach can certainly provide a thorough understanding of how much size contributes to the observed shaped variation.