title: Rudimentary pedal grasping in mice and implications for terminal branch arboreal quadrupedalism.
abstract: We use an outbred laboratory mouse strain (ICR/CD-1, Charles River Laboratories, Inc.) to model a type of preprimate locomotion associated with rudimentary pedal grasping. Ten male mice were assigned to either control or climbing groups (n = 5 per group). Climbing mice lived within a specialized terrarium that included ∼7.5 m of thin branches (5 and 10 cm long) with a thickness of 3.3 mm, arranged in a reticulated canopy. Food, water, and a nest site were placed among the branches. To discourage mice from palmigrade or digitigrade locomotion, the floor of the terrarium was flooded with a few centimeters of water. Climbing mice were placed in this setting upon weaning and reared for 3 months until they were mature in size. Litter, and age-matched controls were also maintained for comparison with climbers. Climbing mice quickly acclimated to the requirements of the fine-branch model using the foot and tail for grasping and balance. At maturity, climbing and control mice exhibited minor, but significant, morphological plasticity. For climbers, this includes a greater angle of the femoral neck, larger patellar groove index, relatively shorter talar neck length, and more circular talar head aspect ratio (P < 0.10). Climbers also exhibit increased curvature of the distal third metacarpal, decreased talar head angle, and relatively longer caudal vertebrae transverse processes (P < 0.05). These results in a small-bodied eutherian mammal suggest that facultative hallucial opposability and coordinated tail use enable a kind of grasping active arboreal quadrupedality relevant to the latest stages of pre-euarchontan evolution. In light of these data, we hypothesize that a unique advantage of mouse-sized mammals is that they exhibit a highly flexible body plan allowing them to engage in a diverse array of anatomical positions without requiring specific limb morphologies.
authors.affiliation: Department of Biology, Mercer University, Macon, Georgia 31207, USA. byron_cd@mercer.edu
authors.forename: Craig
authors.initials: C
authors.lastname: Byron
authors.forename: Hawley
authors.initials: H
authors.lastname: Kunz
authors.forename: Heather
authors.initials: H
authors.lastname: Matuszek
authors.forename: Stephanie
authors.initials: S
authors.lastname: Lewis
authors.forename: Daniel
authors.initials: D
authors.lastname: Van Valkinburgh
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001842:Bone and Bones / Q000033:anatomy & histology*; D005121:Extremities / Q000033:anatomy & histology; D005528:Foot / Q000033:anatomy & histology; D008124:Locomotion; D008297:Male; D050279:Metacarpal Bones / Q000033:anatomy & histology; D051379:Mice; D008813:Mice, Inbred ICR; D009043:Motor Activity; D011187:Posture; D013623:Tail / Q000502:physiology; D013628:Talus / Q000033:anatomy & histology