Inter‐relationships among body mass, body dimensions, jaw musculature and bite force in reptiles

Abstract

Bite force is important in understanding how feeding biomechanics impacts the life-history of vertebrates. Bite force has been measured in many lizards and crocodilians but fewer turtles and snakes. Few comparative interspecific studies exist of the inter-relationships between body size, jaw musculature and bite force. This study collated data reported in the literature for bite force and mass of the jaw musculature in reptiles and explored the relationships between these variables and body mass and size. Two hypotheses were explored: (1) bite force and mass of the jaw musculature scale with body size irrespective of order (as a broad taxonomic grouping) when controlled for phylogeny, and (2) bite force and jaw musculature mass would be directly related and be unrelated to order. Phylogenetically controlled Bayesian analyses showed that in relation to body mass there were different relationships with bite force for testudines compared to saurians and crocodilians. For snout-vent length, squamates and crocodilians exhibited a single relationship, but for head length, all reptile orders exhibited different allometric patterns. Although there was a strong phylogenetic signal in all cases, indicating that there are clusters of closely related species for a measure of size, there was no effect of order on the relationships between measures of body size and mass of the jaw musculature. The relationship between bite force and jaw muscle mass was isometric and did not differ among reptile orders. Several ecological factors may correlate with head morphology and bite force in reptiles. Further studies could explore the evolutionary drivers that shape the relationship between head size, jaw musculature and bite force. Similarities for the relationships between bite force and body size in different reptilian orders suggests that this could be a model system to demonstrate fundamental biomechanical principles of how jaw muscles generate a bite force.

Allometric inter-relationships between jaw musculature mass, skull size and body mass in Psittaciformes

“…The parrots varied in the relative composition of the muscle in the jaw apparatus with genera varying in their arrangement of the adductor and protractor muscles. Unlike lizards, whose head size scales isometrically with body mass (Deeming 2022), as Psittaciformes get larger, their heads become a proportionally larger part of their body. The jaw muscles, however, scaled isometrically with the size of these proportionally larger heads.…”

“…This study, therefore, explored allometry in the different jaw muscle masses from a variety of parrot species ranging in body mass by 40-fold. Given the similarity in skull and beak morphology seen in the Psittaciformes (Zusi 1993), because earlier studies in reptiles showed that muscle mass isometrically scaled with body mass and showed marginal positive allometry with skull size (Deeming 2022), we hypothesized that this would also be true in Psittaciformes. The results will provide an insight into how body size impacts on the myology of the jaw apparatus in this order and how this may impact on the functional properties of the jaw apparatus, i.e.…”

“…Although this difference may reflect taxonomic variability, one possible reason for this discrepancy could be the limited and biased range of species represented in the avian dataset compared to that for reptiles. Deeming (2022) had values for over 275 species of reptile but for birds, 46 of the 122 species were from the Passeriformes and were no larger than 70 g (Deeming et al 2022).…”

“…In addition, skulls can be dissected to reveal muscle masses, which can be used to calculate bite force in birds (Sustaita 2008, Soons et al 2015. Recently, Deeming et al (2022) showed that the relationship between muscle mass and bite force in birds exhibited positive allometry but in reptiles this relationship was isometric (Deeming 2022). Although this difference may reflect taxonomic variability, one possible reason for this discrepancy could be the limited and biased range of species represented in the avian dataset compared to that for reptiles.…”

Strong positive allometry of bite force in leaf-cutter ants increases the range of cuttable plant tissues

“…In sharp contrast to the magnitude of bite force, the positive static allometry appears to be less remarkable: around 70% of available intraspecific studies reported significant positive allometry of bite forces, if often less pronounced [including one study that also reported negative allometry for other phenotypes of the same species, 100]). Remarkably, and with the notable exception of bats and finches [89, 108–111], bite force studies involving different genera typically report isometric scaling [89, 112, 113, and see meta-analysis above], or even modest negative allometry [85, 103, 114–117]. There thus appears to be a systematic difference between ontogenetic, static and evolutionary scaling of bite forces.…”