The transition from quadrupedal to bipedal locomotion involves a shift in the magnitude and concentration of ground reaction forces (GRF). As the most derived and taxonomically rich group of bipedal rodents, jerboas (family Dipodidae) are a model system for examining the functional morphological response to this shift in a phylogenetic context. Previous studies reveal that quadrupedal rodents vary in body size, but facultatively bipedal rodents (Napeozapus sp., Zapus sp.) and early diverging bipedal jerboas (Cardiocranius sp., Salpingotus sp.) are small. Body size increase in bipedal jerboas correlates with fusion of the central three metatarsals into a single cannon bone. We hypothesize that fusion structurally reinforces the hind foot to accommodate the higher GRF necessary for bipedal hopping. We analyzed micro-CT scans from five species in Dipodidae that span the extant size range. We standardized metatarsal length and cortical thickness. Using Finite Element Analysis, we modeled a dynamic collision between jerboa metatarsals and a flat plate representing the substrate, using experimentally measured parameters. The area of greatest stress spread across the dorsal surface of the unfused central metatarsus, taking up half of the length and all of the width. With fusion, the area of greatest stress is significantly smaller. Our results suggest that unfused metatarsals of facultatively bipedal and early diverging bipedal rodents likely presented a biomechanical constraint, limiting the body size to reduce overall GRF, which was gradually lifted as fusion evolved. Other animals with fused metapodials, such as horses and chickens, may have also experienced selective pressures favoring structural reinforcement.