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

Abstract

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.

Wing morphology and migration status, but not body size, habitat or Rapoport's rule predict range size in North‐American dragonflies (Odonata: Libellulidae)

“…Shape variables contain allometric variation (Outomuro and Johansson 2017). However, since wing centroid size was not found to be significant in any of the models (see Results), we decided to keep the shape variables without removing the allometric component.…”

Altitude and life-history shape the evolution ofHeliconiuswings

“…To select the most supported model given the available data, that is, one that improves model fit while penalizing complexity, we used the Aikaike Information Criteria corrected for small sample sizes (AICc, Hurvich and Tsai 1989), where the best models had the lowest AICc values, implemented with the package MuMin (Bartón 2019). Maximal PGLS models included species mean altitude and distance from the Equator (to control for potential latitudinal clines), sex ratio in our samples interacting with either wing aspect ratio or wing size, to control for potential allometric and sexual dimorphism relationships, which could be different among closely related taxa (Outomuro and Johansson 2017). Most species are found in the Andean mountains or the Amazonian region near the Equator, so we did not have much power to examine variation with latitude in wing aspect ratio and size across species, but we included distance from the Equator as an explanatory variable in the PGLS models to account for it.…”

A tale of two tails: asymmetry in Great Grey Shrike (Lanius excubitor)

“…In order to obtain the measurement error associated with landmark digitisation, Procrustes ANOVA test was carried out to analyse whether the variation between double distributions of landmarks on the same individual's tail was greater than that between samples. Since size effects can be an important component of shape variation in biological studies (Outomuro and Johansson 2017), the association between the tail shape and centroid size was analysed using regression analysis.…”