Craniodental divergence associated with bite force between hybridizing pine squirrels (Tamiasciurus)

Area of Science:

• Evolutionary biology
• Ecomorphology
• Mammalogy

Background:

• Bite force is crucial for mammal foraging, competition, and fitness.
• Tamiasciurus squirrels (Douglas and red squirrels) rely on strong bite forces to access conifer seeds.
• These squirrel species inhabit forests with varying conifer cone hardness and overlap in a North Cascades hybrid zone.

Purpose of the Study:

• To investigate interspecific divergence in craniodental traits related to diet.
• To compare these traits in allopatry, sympatry within the hybrid zone, and between hybrids and parental species.
• To understand how ecological pressures drive morphological divergence at shallow evolutionary timescales.

Main Methods:

• Focused on three craniodental traits: incisor-strength index (proxy for bite force), cranial-suture complexity, and mandible shape.
• Examined squirrels in allopatry, sympatry, and hybrid zones.
• Compared morphological traits between the two species and their hybrids.

Main Results:

• Douglas and red squirrels exhibit differences in bite force and cranial-suture complexity in both allopatry and sympatry.
• Mandible shape varied with expected food hardness but showed no significant interspecific difference.
• Hybrids' craniodental morphology overlapped with red squirrels in the hybrid zone, but not with Douglas squirrels.

Conclusions:

• Ecological processes significantly impact morphological trait divergence in closely related species.
• Bite force and suture complexity are key differentiating traits between these squirrel species.
• Hybridization dynamics can result in asymmetrical morphological contributions from parental species.