By studying function in a comparative, phylogenetic context, I have been able to trace the historical development of specialized feeding behaviors. My attention in graduate school was first drawn to the pharyngeal jaw apparatus of fishes, a series of modified gill arch bones that is used for prey processing (Figure 1) and which, in many groups, is associated with marked ecological diversification.

Figure 1. Pharyngeal prey processing in a surfperch revealed by X-ray cinematography. UPJ, upper pharyngeal jaw; LPJ, lower pharyngeal jaw; P, prey item.

In the surfperches (Teleostei: Embiotocidae), I examined the mechanics of winnowing (see reference 1 below), a feeding strategy in which food items and non-nutritive debris ingested together are separated within the oropharynx. The goal was to determine the mechanism underlying this specialized behavior that allows competitively subordinate species to exploit food resources unavailable to sympatric taxa. Study of the patterns of recruitment of the pharyngeal muscles during feeding using electromyography, coupled with visualization of movements of the skeleton of the head and the prey via X-ray cinematography, provided evidence that winnowing is accomplished through increased mobility of the pharyngeal jaws, not through anatomical specialization of the jaws themselves. A comparison between surfperches and distantly related winnowers revealed a distinct convergence in prey-handling mechanics, indicating that the behavior imposes very specific functional requirements regardless of the phyletic context in which it evolves.

To explore more broadly the evolutionary origin of specialized pharyngeal jaw processing behaviors, of which winnowing is an example, it was necessary to establish the ancestral condition, a problem not previously explored. In collaboration with Dr. Frietson Galis (University of Leiden), I characterized the structure and function of the pharyngeal musculature (Figure 2) in a number of representative basal percoid fishes (centrarchids) as well as species exhibiting the derived pharyngeal mechanics (cichlids) (ref. 2). Muscle function was assessed by electrical stimulation in intact, living specimens. This comparative work led to a six-step transformation hypothesis, involving anatomical, physiological and behavioral changes, to explain how the structurally decoupled and functionally versatile pharyngeal jaw apparatus arose in advanced perciform fishes.

Figure 2. Pharyngeal anatomy of a cichlid. Muscles effecting movement of the pharyngeal jaws represented by black bars.

Relevant Publications

1. Drucker, E. G. and J. S. Jensen. 1991. Functional analysis of a specialized prey processing behavior: winnowing by surfperches (Teleostei: Embiotocidae). Journal of Morphology 210: 267-287.

2. Galis, F. and E. G. Drucker. 1996. Pharyngeal biting mechanics in centrarchid and cichlid fishes: insights into a key evolutionary innovation. Journal of Evolutionary Biology 9: 641-670.

Complete publication list