Comparative cranial biomechanics in two lizard species: impact of variation in cranial design

Abstract

Cranial morphology in lepidosaurs is highly disparate and characterised by the frequent loss or reduction of bony elements. In varanids and geckos, the loss of the postorbital bar is associated with changes in skull shape, but the mechanical principles underlying this variation remain poorly understood. Here, we sought to determine how the overall cranial architecture and the presence of the postorbital bar relate to the loading and deformation of the cranial bones during biting in lepidosaurs. Using computer-based simulation techniques, we compared cranial biomechanics in the varanid Varanus niloticus and the teiid Salvator merianae, two large, active foragers. The overall strain magnitude and distribution across the cranium were similar in the two species, despite lower strain gradients in V. niloticus In S. merianae, the postorbital bar is important for resistance of the cranium to feeding loads. The postorbital ligament, which in varanids partially replaces the postorbital bar, does not affect bone strain. Our results suggest that the reduction of the postorbital bar impaired neither biting performance nor the structural resistance of the cranium to feeding loads in V. niloticus Differences in bone strain between the two species might reflect demands imposed by feeding and non-feeding functions on cranial shape. Beyond variation in cranial bone strain related to species-specific morphological differences, our results reveal that similar mechanical behaviour is shared by lizards with distinct cranial shapes. Contrary to the situation in mammals, the morphology of the circumorbital region, calvaria and palate appears to be important for withstanding high feeding loads in these lizards.

Keywords: Feeding; Finite element analysis; Lepidosauria; Multibody dynamic analysis; Skull; Squamata.

Fig. 1.

Skull anatomy of Salvatormerianae and Varanusniloticus.

Fig. 2.

Strain in the cranial bones of S. merianae and V. niloticus. (A) Average first (ε₁) and third (ε₃) principal strain magnitude and standard deviation in 10 sections along the cranium. (B) Principal strain magnitude and corresponding |ε₁:ε₃| ratio averaged from the anterior bilateral and posterior unilateral maximal bite for selected bones. Strain magnitude is in microstrain (με).

Fig. 3.

Strain pattern in the cranium of S. merianae and V. niloticus and the impact of the postorbital bar (POB) on bone strain. First (ε₁) and third (ε₃) principal strain calculated during an anterior bilateral and posterior unilateral bite. Results are presented for the actual and digitally altered cranial morphology (i.e. postorbital bar removed in S. merianae and added in V. niloticus). Strain magnitude is in microstrain (με); areas in grey correspond to out-of-range strain values.

Fig. 4.

Relative difference in principal strain between models with and without a postorbital bar. Negative values (cold colours) correspond to higher strain when the postorbital bar is present, while positive values (warm colours) correspond to higher strain when the postorbital bar is absent. Areas in grey correspond to out-of-range strain values.

Fig. 5.

Effect of the postorbital ligament on bone strain in V. niloticus. (A) Average first (ε₁) and third (ε₃) principal strain value and standard deviation in 10 sections along the cranium calculated for different values of the Young's modulus of the postorbital ligament. (B) Contour plots in dorsal view showing the effect of varying the Young's modulus of the postorbital ligament on bone strain during a posterior unilateral bite. Areas in grey correspond to out-of-range strain values.

Fig. 6.

Strain in the frontals of V. niloticus. A dorsal view of the cranium and a ventral view of a transverse section of the cranial roof are shown. (A) Effect of the subolfactory process (SP) on strain in the frontal. The subolfactory processes are outlined when included in the analyses but hidden in the rendering to observe strain on the ventral side of the frontals. (B) Effect of the interfrontal suture (IFS) on bone strain. Areas in grey correspond to out-of-range strain values.