External and Skeletal Morphology of Molossus fluminensis Lataste, 1891 (Chiroptera, Molossidae) with Notes on Quadrupedal Locomotion and Habitat Use

Molossus fluminensis is a species of the wide diversified family Molossidae. Representants of this family have evolved under pressures associated to two different behaviors — a high speed and efficient flight and a good quadrupedal ability. The aim of this study is to describe in detail the morphological characters of M. fluminensis, focusing on its skeletal features, and to point out the main characters that could represent functional responses to the evolution of the quadrupedal ability and habitat use in the group. Furthermore, we aim to contribute to the knowledge of a recently revalidated and cryptic species of Molossus, giving a morphometric characterization for the species and notes on sexual dimorphism. Four adult specimens of M. fluminensis, one male and three females, were prepared as skin and skeleton, and five external and 146 linear measurements of the skeleton were taken. The anatomical characters were described following current literature and included the axial and appendicular skeleton. We found that the male presented greater measurements than females in most linear characters, and had a more developed sagittal and lambdoid crests, while the ischiatic arcade was more caudally projected in females. A narrow rib cage, a long, thin sternum, and a cranially oriented ventral process of manubrium make the M. fluminensis body more compact dorsoventrally without compromising the area of insertion of the flight musculature. These appear to be adaptations for roosting in narrow crevices. The expanded sacral vertebrae, the deep gluteal fossa of the ilium, and the presence of well-developed processes on the first caudal vertebra, point to a greater area for muscle origin in this region, while the well-developed trochanters and ridges of the femur and tibia provide surfaces for muscle attachment. Together, these characteristics point to a great range of movement of the limbs and the tail, important factors during quadrupedal locomotion. The results obtained here may shed light on the evolution of quadrupedalism in molossids and other quadrupedal bats.