Comparative morphology and scaling of the femur in yangochiropteran bats

Better known by their remarkable forelimb morphology, bats are also unique among mammals with respect to their hindlimbs. Their legs are rotated through 180°, generally reduced in size, and in some extant taxa particular bones (e.g. fibula) can even be absent. The femur is the main leg bone, but to date few bat studies have considered its morphology in detail, none in a wide-scale comparative study. Yangochiroptera is the largest bat taxon, spans nearly three orders of magnitude in body mass, and is highly diverse both in ecology and behavior, representing a good model for comparative analyses. Here, we describe the anatomy of the femur in a large sample of yangochiropteran bats (125 species, 70 genera, and 12 families), and explore major trends of morphological variation and scaling patterns in this bone. We used 13 categorical characters in the anatomical description and five linear dimensions in the quantitative analyses. Based on the categorical data, each family studied here was diagnosed, and those from the Neotropical region were included in an identification key. From the phylogenetic principal component analysis (pPCA) we showed that, in addition to size, major axes of variation in bat femur are related to robusticity and head morphology, features that are clearly distinct among some families. We also generated a phylomorphospace based on pPCA scores, highlighting convergences in femur shape. Molossidae, Mystacinidae, and Desmodontinae were grouped based on their greater robusticity, a pattern that was also recovered from categorical data. In these families, we found anatomical features (e.g. presence of tubercles and posterior ridges on the greater trochanter, long or medially/distally displaced lateral ridges on the shaft) that are well-known from their functional link with quadrupedal locomotion. Using phylogenetic regressions, we found out that compared with body mass, femur length scaled with negative allometry, as expected, but that femur width scaled isometrically, counter to expectations. As a result, robusticity index (the ratio of width to length), scaled with positive allometry - larger bats tended to have more robust hindlimbs. At species level, our most remarkable finding was related to Myotis simus, which presented the most robust femur (for its size) among yangochiropterans. Our results reinforce the informative potential of the chiropteran femur from both taxonomic and functional perspectives. Furthermore, the allometric trends seen in this bone may help understand the strategies adopted by flying vertebrates to deal with the high energetic cost of flight and, at the same time, evolve diversified foraging behaviors.