Rudimentary pedal grasping in mice and implications for terminal branch arboreal quadrupedalism

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.