Why ruminating ungulates chew sloppily: Biomechanics discern a phylogenetic pattern

Abstract

There is considerable debate regarding whether mandibular morphology in ungulates primarily reflects phylogenetic affinities or adaptation to specific diet. In an effort to help resolve this debate, we use three-dimensional finite element analysis (FEA) to assess the biomechanical performance of mandibles in eleven ungulate taxa with well-established but distinct dietary preferences. We found notable differences in the magnitude and the distribution of von Mises stress between Artiodactyla and Perissodactyla, with the latter displaying lower overall stress values. Additionally, within the order Artiodactyla the suborders Ruminantia and Tylopoda showed further distinctive stress patterns. Our data suggest that a strong phylogenetic signal can be detected in biomechanical performance of the ungulate mandible. In general, Perissodactyla have stiffer mandibles than Artiodactyla. This difference is more evident between Perissodactyla and ruminant species. Perissodactyla likely rely more heavily on thoroughly chewing their food upon initial ingestion, which demands higher bite forces and greater stress resistance, while ruminants shift comminution to a later state (rumination) where less mechanical effort is required by the jaw to obtain sufficient disintegration. We therefore suggest that ruminants can afford to chew sloppily regardless of ingesta, while hindgut fermenters cannot. Additionally, our data support a secondary degree of adaptation towards specific diet. We find that mandibular morphologies reflect the masticatory demands of specific ingesta within the orders Artiodactyla and Perissodactyla. Of particular note, stress patterns in the white rhinoceros ( C . simum ) look more like those of a general grazer than like other rhinoceros’ taxa. Similarly, the camelids (Tylopoda) appear to occupy an intermediate position in the stress patterns, which reflects the more ancestral ruminating system of the Tylopoda.

Geometric morphometrics of mandibles for dietary differentiation of Bovidae (Mammalia: Artiodactyla)

“…These explanations have not been rigorously tested or applied to inferring grazing adaptions in extinct species. A finite element analysis may be able to address this question (e.g., Fletcher et al 2010 ; Zhou et al 2019 ). It is worth noting that reduction of the premolar row is not always correlated with percent grass consumption in ungulate groups ( Codron et al 2019 ).…”

The diet of early birds based on modern and fossil evidence and a new framework for its reconstruction

“…These percentages can then be compared directly in a histogram or plotted into principal component space. The intervals method has proved effective in subsequent feeding studies (Zhou et al ., 2019; Coatham et al ., 2020; Miller et al ., 2020), and is recommended here.…”

Many-to-one function of cat-like mandibles highlights a continuum of sabre-tooth adaptations

“…Within felines, the von Mises stress measured in the mandible was higher in smaller taxa within the same clade, which is not especially the case in other previous FEA studies, e.g. in otters [25], ungulates [54] or lizards [55]. It was already noted [11] that the largest amount of skull shape variation in felines is driven by allometry over functional or phylogenetic factors while in sabre-tooths the best predictor was canine length over body size [11].…”