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

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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 3.3mm, 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. J. Morphol.,2011.

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