• squirrels;
  • bite force;
  • granivory;
  • sympatry


This study explores the relationship between feeding habit and craniodental morphology in two closely related sympatric callosciurine squirrels in south-east Asia: Callosciurus erythraeus and Dremomys rufigenis. Both C. erythraeus and D. rufigenis prefer fruits and insects, as generally seen in other tree squirrels. However, C. erythraeus also feeds extensively on hard seeds and tough tree bark, which are rarely included in the diet of D. rufigenis. We tested the hypothesis that C. erythraeus shows structural features in craniodental morphology that provide an advantage for feeding on hard seeds and tough tree bark. Results show that C. erythraeus possesses a greater mechanical advantage of the masseter and temporalis muscles for chewing, and a more robust zygomatic arch, compared with D. rufigenis. Callosciurus erythraeus is capable of generating a greater bite force at a given muscle force at the cheek teeth and its zygomatic arch is more resistant against the stresses generated by the masseter muscle, suggesting adaptation for feeding on hard seeds. Callosciurus erythraeus also exhibits greater leverage of the masseter and temporalis for incision and has a wider skull at the interorbital region, suggesting the capability for efficiently generating a greater bite force and a greater mechanical resistance to increased loads when biting into wood with its anterior dentition. Our findings indicate that the craniodental divergence between C. erythraeus and D. rufigenis may be linked functionally to the differences in their diets. We suggest that their morphological divergence may correlate with their dietary differences and contribute to the sympatric distribution of these closely related squirrels.