Functional Evolution of the Feeding System in Rodents

TLDR

The masticatory musculature of rodents has evolved to enable both gnawing at the incisors and chewing at the molars, with the masseter muscle extending anteriorly from the rostrum; all living rodents achieve this masseteric expansion in one of three ways—sciuromorph, hystricomorph, or myomorph—matching their dietary habits of nuts and seeds for squirrels and grasses for guinea pigs. Here, we used finite element analysis to investigate the biomechanical implications of these three morphologies, aiming to determine whether each is better adapted for gnawing or chewing. We applied finite element analysis to a squirrel, guinea pig, and rat. The analysis revealed that squirrels excel at incisor gnawing, guinea pigs excel at molar chewing, and rats outperform both as efficient, versatile feeders with no compromise in biting efficiency, making them high‑performance generalists.

Abstract

The masticatory musculature of rodents has evolved to enable both gnawing at the incisors and chewing at the molars. In particular, the masseter muscle is highly specialised, having extended anteriorly to originate from the rostrum. All living rodents have achieved this masseteric expansion in one of three ways, known as the sciuromorph, hystricomorph and myomorph conditions. Here, we used finite element analysis (FEA) to investigate the biomechanical implications of these three morphologies, in a squirrel, guinea pig and rat. In particular, we wished to determine whether each of the three morphologies is better adapted for either gnawing or chewing. Results show that squirrels are more efficient at muscle-bite force transmission during incisor gnawing than guinea pigs, and that guinea pigs are more efficient at molar chewing than squirrels. This matches the known diet of nuts and seeds that squirrels gnaw, and of grasses that guinea pigs grind down with their molars. Surprisingly, results also indicate that rats are more efficient as well as more versatile feeders than both the squirrel and guinea pig. There seems to be no compromise in biting efficiency to accommodate the wider range of foodstuffs and the more general feeding behaviour adopted by rats. Our results show that the morphology of the skull and masticatory muscles have allowed squirrels to specialise as gnawers and guinea pigs as chewers, but that rats are high-performance generalists, which helps explain their overwhelming success as a group.