Marked Directional Skull Asymmetry in the Araucan Horse

Abstract

Background Deviations from the perfect symmetry of normally bilateral symmetrical characters occur during individual development due to the influence of multiple factors. Fluctuating asymmetry (FA) is the random developmental variation of a trait (or character) that is perfectly symmetrical, on average, across a population. Directional asymmetry (DA) occurs when one side of the pair of body sides is strongly more marked. Objective We investigated the presence and level of skull FA and DA in the Araucan horse, a breed from East Colombia. Study Design A sample of 21 skulls belonging to adult animals was studied by means of standard geometric–morphometric methods using 16 landmarks on the dorsal aspect of the crania. Results Measurements showed a significant DA with a consistent rightward shift of the splanchnocrania. Conclusions The results of this study raise questions about the influence of masticatory biomechanics on the asymmetric development of the skull, and also about how management and ingesta-specific properties (such as abrasiveness) may influence this asymmetry.

Breaking Symmetry: Fluctuating Asymmetry and Geometric Morphometrics as Tools for Evaluating Developmental Instability under Diverse Agroecosystems

“…This was attributed to the effects of mechanical forces. The dominance of one side may determine differential mastication and bone remodeling, contributing to directional asymmetry [80].…”

Developmental instability in domesticated mammals

“…Initial population bottlenecks associated with the domestication process, causing inbreeding and an accumulation of deleterious mutations (i.e., genetic stress) (e.g., Marsden et al, 2016), may also result in increased FA among domesticated animals. Domesticates have been the subject of several studies using FA, including for the cranium of pigs (Parés Casanova & Esteve‐Puig, 2014), horses (Parés Casanova et al, 2020), sheep (Parés Casanova & Bravi, 2014; Parés Casanova, 2019), and dogs (Drake & Klingenberg, 2010), as well as the metacarpals of sheep (Parés‐Casanova, 2014), the horns of mountain goats (Côté & Festa‐Bianchet, 2001) and hooves and forelimbs of horses (Wilson et al, 2009). With the exception of Drake and Klingenberg (2010), who compared wild and domestic forms, these studies were focused on small comparisons within domestic populations and revealed that FA accounts for a moderate amount of shape variance, consistent with the results across species examined here (Figure S2).…”

“…The direct comparison of FA in domestic forms with that in their wild relatives has received comparatively less attention than analyses of disparity in form among domesticated species (but see Drake & Klingenberg, 2010; Pulcini et al, 2015). Nevertheless, several studies have focused exclusively on within‐population assessments of FA for small populations of individual domestic species and these indicate that a moderate magnitude of asymmetry in the cranium is common (Parés Casanova, 2019; Parés Casanova & Bravi, 2014; Parés Casanova & Esteve‐Puig, 2014; Parés Casanova et al, 2020).…”

“…Nevertheless, several studies have focused exclusively on withinpopulation assessments of FA for small populations of individual domestic species and these indicate that a moderate magnitude of asymmetry in the cranium is common (Parés Casanova, 2019;Parés Casanova & Bravi, 2014;Parés Casanova & Esteve-Puig, 2014;Parés Casanova et al, 2020). Parsons (1990) likened domestication to an environmental stress with human-specified parameters, hypothesizing that FA may manifest to a larger degree among domesticates compared to their wild relatives.…”

Basicranial Modular Organization. A Study in the Araucanian Horse of Colombia

“…The Araucanian horse constitutes an equine population typical of the ecosystem of the Araucanian plains in NE Colombia. Of great rusticity, it has hardly been studied in its ecological environment until recently, when investigations have been initiated aimed at its morphological description, functional study, and genetic management [13][14][15]. The Araucanian horse represents a cultural and genetic heritage adapted to the environment of the flooded savanna; therefore, a morphological study might reveal its evolutionary traits of adaptation to the environment.…”