INTRODUCTION

In this chapter, I place the extraordinary diversity in microchiropteran skull size and shape within a rather simplistic framework. That is, despite all other craniodental adaptations, the microchiropteran head must function as an efficient acoustical horn during echolocation. This truism becomes infinitely more interesting when one considers that echolocatory calls are either emitted directly from the open mouth (oral-emitters), or forced through the confines of the nasal passages (nasal-emitters). Given that oral-emission is the primitive state, the advent of nasal-emission is viewed as a complex morphological innovation that required a substantial redesign of the microchiropteran rostrum: 1) the nasal passages must be reoriented and aligned with the direction of flight, and 2) the nasal passages must exhibit dimensions that provide for the efficient transfer of sound (resonance) through the adult skull. In the following treatment, I draw examples from developmental studies and functional morphology to illustrate how evolution has solved this intriguing design problem.

Spatial competition and the packaging of the fetal head

The dynamic nature of the developing skeletal system is all too frequently overlooked in the classroom where the skull is often presented as an immutable structure into which the brain, ears, and eyes are stuffed during development. Rather, the converse is a more accurate view; cranial growth and form are ‘soft tissue’ phenomena affected only secondarily by osteological development. Indeed, early in development, the differential growth of the brain and pharynx governs the shape of the chondrocranium and influences the forms of the embryonic neuro- and viscerocrania (Ranly 1980; Klima 1987; Hanken & Thorogood 1993).