Left, right or both? Estimating and improving accuracy of one‐side‐only geometric morphometric analyses of cranial variation

Procrustes-based geometric morphometric analyses of bilaterally symmetric structures are often performed using only one side. This is particularly common in studies of cranial variation in mammals and other vertebrates. When one is not interested in quantifying asymmetry, landmarking one side, instead of both, reduces the number of variables as well as the time and costs of data collection. It is assumed that the loss of information in the other half, on which landmarks are not digitized, is negligible, but this has seldom been tested. Using 10 samples of mammalian crania and a total of more than 500 specimens, and five different landmark configurations, I demonstrate that this assumption is indeed easily met for size. For shape, in contrast, one-side landmarking has potentially more severe consequences on the estimates of similarity relationships in a sample. In this respect, microevolutionary analyses of small differences are particularly affected, whereas macroevolutionary studies are fairly robust. In almost all instances, however, a simple preliminary operation improves accuracy by making one-side-only shape data more similar to those obtained by landmarking both sides. The same operation also makes estimates of allometry more accurate and improves the visualization. This operation consists in estimating the missing side by a mirror reflection of bilateral landmarks. In the Supporting Information, I exemplify how this can be easily done using free user-friendly software. I also provide an example data set for readers to repeat and learn the steps of this simple procedure

Unsustainable development goals

More than halfway between their proposal and the 2030 deadline for their achievement, none of the Sustainable Development Goals (SDGs) is on track to be attained. Meanwhile, we are increasingly crossing planetary boundaries and nearing tipping points of global environmental change. Failure is inevitable because solutions to the crisis cannot come from the same anthropocentric world-view that has created it. The growth of an already supersized consumerist economy and human population is incompatible with long-term functioning ecosystems, but it is precisely this unsustainable perspective that permeates the SDGs. Its rejection in favour of one that has the planetary ecosystem at its heart is urgent and crucial to rebalance life on Earth, prevent further environmental destruction, slow the pace of climate change, and rapidly reduce inequality and injustice within humanity and across the biosphere

Missing the third dimension in geometric morphometrics: how to assess if 2D images really are a good proxy for 3D structures?

Procrustean geometric morphometrics has made large use of 2D images for studying three-dimensional structures such as mammalian bones or arthropod exoskeleta. This type of use of 2D data is still widespread today and will likely remain common for several years due to its simplicity, efficiency and low cost. However, using 2D pictures to measure morphological variation in a 3D object is an approximation that inevitably implies measurement error. Despite this being an obvious problem, which was emphasized since the early days of the first applications of geometric morphometrics to biology, whether 2D is a good proxy for 3D has been a rather neglected topic in the literature until very recently. In this paper, using marmot mandibles and crania as an example, I show how to assess the potentially crucial impact of 'missing the third dimension' in 2D landmarks and suggest a new method to test the accuracy of these data: the method is simple and can be easily performed in a user-friendly free software such as MorphoJ. This test is complimentary to other more exploratory analyses, that can also be performed using free programs and might offer a routine protocol to estimate the goodness of the 2D to 3D approximation in geometric morphometrics. Example data and a fully worked out MorphoJ project are provided for readers to learn how to replicate the analysis

Lost in the Other Half: Improving Accuracy in Geometric Morphometric Analyses of One Side of Bilaterally Symmetric Structures

Systematists and evolutionary biologists have widely adopted Procrustes-based geometric morphometrics for measuring size and shape in biology. Many structures, and in fact most animals, are bilaterally symmetric with an internal plane of symmetry (also called object symmetry). Often, when quantifying asymmetric variation is not an aim, only one or the other side is measured and analyzed. This approach has been used in hundreds of studies. Its implicit assumption is that the information on the other side is redundant and a single side will, therefore, produce results mirroring those one would have obtained from the analysis of the entire structure with all its left and right landmarks. However, the extent to which this assumption is met has, to my knowledge, never been explored. Using two example data sets, I will show that congruence may be high in analyses at a macroevolutionary level but much lower at a microevolutionary one, and inaccuracies might especially affect shape. I will discuss some of the other factors that may influence results and will suggest a simple expedient that can improve both the visualization and accuracy of shape analyses in one-side-only studies

Piracy strikes back on Lake Maggiore (Northen Italy): first report of Common Merganser Mergus merganser kleptoparasitizing Great Crested grebe Podiceps cristatus

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Virtual Morphology and Evolutionary Morphometrics in the new millenium.

EDITED VOLUME; abstract N/

On growth and form in the "computer era": from geometric to biological morphometrics.

Almost 100 years after the publication of Thompson's seminal book "On growth and form", the study of animal morphology is becoming again central to biology. This is also thanks to the development of powerful computerized quantitative methods for statistical shape analysis, collectively known as geometric morphometrics (GM). GM was announced as a revolution just two decades ago. The "revolution" is now a standard tool in numerical analyses of phenotypic variation in mammals and other organisms. Hundreds of studies are published every year that take advantage of GM (e.g., more than 800 entries in Google Scholar only for 2012). We celebrate the 20t anniversary of the "revolution in morphometrics" (Rohlf and Marcus, 1993, p. 129) with the publication of a "Yellow Book", a special issue of Hystrix dedicated to Evolutionary Morphometrics and Virtual Morphology. A series of 14 papers by leading morphometricians summarizes the main achievements in GM (surface methods, comparative shape analysis, phenotypic trajectories quantification, modularity/integration, the use of R in morphometrics), describes its most innovative developments (ecometrics, eigensound analysis, biomechanical GM), and discusses common misunderstandings of well extablished methods (visualization of shape differences). Besides celebrating the success of statistical shape analysis in biology, this issue aims at introducing to GM readers unfamiliar with or intimidated by its strong numerical background. This is why, as Editors, we asked all contributors to provide concise and accurate but also clear and simple descriptions of techniques and applications. We hope that we succeeded in this aim, and wish that this Yellow Book may help to tighten the connection between biologists and statisticians for a truly "biological" GM

A 'long-fuse domestication' of the horse? Tooth shape suggests explosive change in modern breeds compared with extinct populations and living Przewalski's horses

Archaeological and molecular data suggest that horses were domesticated comparatively recently, the genetic evidence indicating that this was from several maternal haplotypes but only a single paternal one. However, although central to our understanding of how humans and environmental conditions shaped animals during domestication, the phenotypic changes associated with this idiosyncratic domestication process remain unclear. Using geometric morphometrics on a sample of horse teeth including Pleistocene wild horses, modern Icelandic and Thoroughbred domestic horses, Przewalski’s wild horses of recent age and domestic horses of different ages through the Holocene, we show that, despite variations in size likely related to allometry (changes to bone size in proportion to body size), natural and artificial selective pressures and geographic and temporal heterogeneity, the shape of horse teeth has changed surprisingly little over thousands of years across Eurasia: the shapes of the premolars of prehistoric and historic domestic horses largely resemble those of Pleistocene and recent wild horses. However, this changed dramatically after the end of the Iron Age with an explosive increase in the pace and scale of variation in the past two millennia, ultimately resulting in a twofold jump in the magnitude of shape divergence in modern breeds. Our findings indicate that the pace of change during domestication may vary even within the same structure with shape, but not size, suggesting a ‘long-fuse’ model of phenotypic modification, where an initial lengthy period of invariance is followed by an explosive increase in the phenotypic change. These observations support a testable model that is applicable to other traits and species and add a new layer of complexity to the study of interactions between humans and the organisms they domesticated. Funding was provided to GB from the Leverhulme Trust project grant scheme (F/09 757/B) and to KS and AC from the Lang Fund for Human-Environmental Anthropology, Department of Anthropology, Stanford.This is the author accepted manuscript. The final version is available from Sage via http://dx.doi.org/10.1177/095968361663843