Morphometric models for estimating bite force in Mus and Rattus: mandible shape and size do better than lever-arm ratios

Abstract

Morphological traits are frequently used as proxies for functional outputs such as bite force performance. This allows researchers to infer and interpret the impacts of functional variation, notably in adaptive terms. Despite their mechanical bases, the predictive power of these proxies for performance is not always tested. In particular, their accuracy at the intraspecific level is rarely assessed, and they have sometimes been shown to be unreliable. Here, we compared the performance of several morphological proxies in estimating in vivo bite force, across five species of murine rodents, at the interspecific and intraspecific levels. Proxies used included the size and shape of the mandible, as well as individual and combined muscular mechanical advantage (temporalis, superficial masseter and deep masseter). Maximum voluntary bite force was measured in all individuals included. To test the accuracy of predictions allowed by the proxies, we combined linear regressions with a leave-one-out approach, estimating an individual's bite force based on the rest of the dataset. The correlations between estimated values and the in vivo measurements were tested. At the interspecific and intraspecific levels, size and shape were better estimators than mechanical advantage. Mechanical advantage showed some predictive power at the interspecific level, but generally not within species, except for the deep masseter in Rattus. In a few species, size and shape did not allow us to predict bite force. Extrapolations of performance based on mechanical advantage should therefore be used with care, and are mostly unjustified within species. In the latter case, size and shape are preferable.

Insectivory leads to functional convergence in a group of Neotropical rodents

“…Our results are in agreement with the conclusions of Ginot et al. (2019) on the importance of the temporal in the production of the bite force on rodents incisors. This muscle had the lowest predictive power of all muscles when assessed individually.…”

“…The masseter was divided in two portions, deep and superficial, based on functional interpretations (Hiiemäe, 1971). We calculated the out‐lever as the linear distance between the mandibular joint and the tip of the incisors and, for the in‐levers, we considered the linear distance of the mandibular joint to the point of insertion of each muscle (Figure 1), based on previous works of muscle dissections (Fabre et al., 2017; Satoh & Iwaku, 2006; Voss, 1988), CTscan images (Baverstock et al., 2013; Cox et al., 2012; Ginot, Herrel, et al, 2018), and biomechanical and mechanical advantage models of rodents (Becerra et al., 2014; Fabre et al., 2017; Ginot et al., 2019; Herring & Herring, 1974; Swiderski & Zelditch, 2010).…”

The contribution of functional traits to the understanding of palaeoenvironmental changes

“…The mechanical potential as used in this study is the ratio of the muscle moment arm to the jaw outlever. As such it is dependent on the geometry of the skull and mandible and the insertion of the masticatory muscles (Herrel et al, 2008;Chazeau et al, 2013;Manhães, Nogueira, & Monteiro, 2017;Ginot et al, 2018Ginot et al, , 2019. In particular, mandible shape is known for being a good estimator of bite force (Brassard et al, 2020a,b).…”

“…Moreover, it only considers a single force -the temporalis muscle -which is applied uniformly (Young et al, 2007). All these parameters are important drivers of the mechanical potential, in particular size (Wroe et al, 2005;Freeman & Lemen, 2008;Herrel et al, 2008;Chazeau et al, 2013;Manhães et al, 2017;Ginot et al, 2018Ginot et al, , 2019Brassard et al, 2020a). In order to increase the accuracy of our estimate of bite force, we corrected MP to take size into account.…”

“…Corrected MP (cMP) was subsequently expressed as: cMP = MP + f(size) (1), where MP is the mechanical potential and f(size) the part of cMP due to size. It is also known that log 10 bite force is linearly correlated to size (Wroe et al, 2005;Chazeau et al, 2013;Manhães et al, 2017;Ginot et al, 2019;Brassard et al, 2020a). Thus, for cMP to be a good estimator of bite force, log-transformed cMP must also be linearly correlated to size.…”