Echolocation, evo-devo and the evolution of bat crania

Abstract

The geneticists are trying to make evolution fit the genes rather than to make the genes fit evolution. (Osborn, 1932) Introduction Despite all other cranio-dental adaptations (Covey and Greaves, 1994; Dumont and Herrel, 2003), the microchiropteran head must function as an efficient acoustical horn during echolocation. This becomes infinitely more interesting when one considers that echolocation 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 (Starck, 1954; Wimberger, 1991; Schneiderman, 1992), the advent of nasal emission is viewed as a complex morphological innovation that required a substantial redesign of the microchiropteran rostrum: the nasal passages must be reoriented and aligned with the direction of flight, and they must have dimensions that provide for the efficient transfer of sound (resonance) through the adult skull. Once the acoustical axis of the head is established, bats emit a remarkable array of echolocation calls that reflect a great deal of behavioral plasticity. In the following treatment, we draw examples from developmental studies and functional morphology to illustrate how evolution has solved this intriguing design problem associated with nasal emission of the echolocation call. Terminology: operational definitions The term echolocation has been broadly applied to the Microchiroptera and to some members of the Megachiroptera. Despite evidence that shows that Rousettus aegyptiacus is able to navigate quite well by tongue clicking (Waters and Vollratch, 2003), there is no clear neuroanatomical, dental, developmental or physiological data whatsoever suggesting that pteropodids ever had the capacity for laryngeal echolocation or were derived from bats that did echolocate. Herein, the term “echolocation” will refer only to ultrasound produced by the larynx. It is our opinion that to do otherwise will confuse the understanding of the evolution of chiropteran communication, navigational skills and neural processing, i.e., ultrasound and tongue clicking should be considered separately during taxonomic analyses.