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

Bite force can be a limiting factor in foraging and can significantly
affect the competitive ability and lifetime fitness of mammals.
Tamiasciurus squirrels feed primarily on conifer seeds and have a strong
bite force to mechanically extract seeds from conifer cones with their
mouths. In the North Cascades region, Douglas squirrels (Tamiasciurus
douglasii) and red squirrels (T. hudsonicus) occupy ecologically different
forests with different hardnesses in conifer cones. The ranges of these
species overlap in a narrow hybrid zone where these forests meet near the
crest of the North Cascades. We examined interspecific divergence in
dietary ecomorphology in allopatry, in sympatry within the hybrid zone,
and between hybrids and each parental species. We focused on three
craniodental traits, including the incisor-strength index as a proxy
measure for maximal bite force, cranial-suture complexity, and mandible
shape. We find that these sister squirrel species differ in bite force and
suture complexity in allopatry and sympatry and that mandible shape
changes with the expected hardness of accessed food items, but is not
significantly different between species. Furthermore, we find that hybrids
display morphologies that overlap with hybrid zone red squirrels but not
with hybrid zone Douglas squirrels. This work shows how important
ecological processes at shallow evolutionary timescales can impact the
divergence of morphological traits in taxa with extreme conservation of
craniomandibular shape.