Functional Anatomy and Evolution of a Novel Skeletal Element in Bat Feet

Abstract

The striking postcranial anatomy of bats reflects their specialized ecology; they are the only mammals capable of powered flight. Bat postcranial adaptations include a series of membranes that connect highly-modified, or even novel, skeletal elements. While most studies of bat postcranial anatomy have focused on their wings, bat hindlimbs also contain many derived and functionally important, yet less studied, features. In this study, I investigate the anatomy, evolution, and function of the calcar, a novel skeletal element found in bat feet. In the first chapter, I introduce calcar anatomy with a detailed study of three bat species with different flight and foraging ecologies. I found more complex muscle arrangements in the species that exhibit more maneuverable flight, suggesting that they have more control over calcar movement. This first study inspired the rest of the dissertation, by suggesting that calcar morphology is functionally-relevant. In the second chapter, I present a thorough overview of calcar skeletal anatomy throughout Chiroptera. Through evolutionary modeling of calcar length, I find that the calcar exhibits an early burst of morphological evolution, indicating that the calcar anatomically diversified as bats initially radiated through the aerosphere. In the third chapter, I again narrow the focus and conduct an analysis of calcar motion during free, forward-flight in a laboratory population of Seba’s short-tailed fruit bat (Carollia perspicillata). I find that the calcar does rotate about its joint with the calcaneus and that this rotation is greater about one axis than another. The muscles inserting on the calcar may act to stabilize it in one plane of motion. These chapters provide the most complete study of calcars to-date, particularly with regard to the quantitative tests of calcar evolutionary patterns and kinematics. Four data tables and one video are provided as Electronic Supplementary Materials for Chapters 2 and 3. Collectively, these three chapters demonstrate that novel skeletal additions can become integrated into vertebrate body plans and subsequently evolve into a variety of forms, potentially impacting clade diversification by expanding the available morphological space into which organisms can evolve.