Variation in cross-sectional shape and biomechanical properties of the bat humerus under Wolff's law

Abstract

Bats use their forelimbs in different ways, but flight is the most notable example of morphological adaptation. Foraging and roosting specializations beyond flight have also been described in several bat lineages. Understanding postcranial evolution during the locomotory and foraging diversification of bats is fundamental to understanding bat evolution. We investigated whether different foraging and roosting behaviors influenced humeral cross-sectional shape and biomechanical variation, following Wolff's law of bone remodeling. The effect of body size and phylogenetic relatedness was also tested, in order to evaluate multiple sources of variation. Our results suggest strong ecological signal and no phylogenetic structuring in shape and biomechanical variation in humeral phenotypes. Decoupled modes of scaling of shape and biomechanical variation were consistently indicated across foraging and roosting behaviors, suggesting divergent allometric trajectories. Terrestrial locomoting and upstand roosting species showed unique patterns of shape and biomechanical variation across all our analyses, suggesting that these rare behaviors among bats place unique functional demands on the humerus, canalizing phenotypes. Our results suggest that complex and multiple adaptive pathways interplay in the postcranium, leading to the decoupling of different features and regions of skeletal elements optimized for different functional demands. Moreover, our results shed further light on the phenotypical diversification of the wing in bats and how adaptations besides flight could have shaped the evolution of the bat postcranium.

Keywords: Chiroptera; biomechanics; cross-sectional shape; foraging behavior; humerus; mammal flight; postcranial morphology.