Another major difference in bat forelimbs is in the density of their skeletal limbs. The bones found in their forelimbs are reduced to achieve a light body weight required for flight. In particular, their ulna is reduced in width and fused to the other zeugopod element, the radius. One of the possible molecular pathways involved in reduction of bat skeletal forelimb thickness is differences in SHH expression. Mice with shh null mutations lose their ulna structure. Another good candidate for bat bone reduction is Hox-d13, a gene belonging to the Hox gene family. In situ hybridization studies have found that the Hoxd13 expression domain in bat limbs has been shifted posteriorly in comparison to mouse. This observed difference in the expression pattern of Hoxd13 may also explain reduced size and density of the ulna found in bats. Overall, these studies suggest that the molecular changes responsible for the evolution of wings in bats is due to genetic regulatory changes.
