Why do humans have short fingers and long arm bones, and how did the elongated fingers of bats evolve to support powered flight? My laboratory seeks the answers to these profoundly important questions about the form and function of limbs by studying the closest relative of the laboratory mouse with the most unique limb skeleton, the bipedal lesser Egyptian jerboa. This rodent has extremely long hindlimbs with disproportionately long feet that propel it through the sparsely vegetated deserts of Africa and Central Asia. Using multiple comparative methods and the strengths of mouse genetic engineering, my laboratory is identifying genes and the associated cis-regulatory mechanisms that establish differential growth rates throughout the skeleton and therefore determine the development and evolution of limb proportion.