Doctoral Dissertations

Enrique Font

Date of Award

12-1988

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Life Sciences

Major Professor

Neil Greenberg

Committee Members

Gordon M. Burghardt, Arthur C. Echternacht, Robert C Switzer III, Lawrence C. Rome

Abstract

Dewlap extension is an important component of inter- and intraspecific displays of Anolis lizards. The dewlap, an extendible flap of skin ordinarily folded under the throat, is supported by the hyoid apparatus. The mechanism and neural pathway for dewlap extension were investigated in Anolis ecruestris (Sauria, Iguanidae) , a large arboreal lizard with a prominent dewlap.

Specializations of the hyoid skeleton for dewlap extension include elongated second ceratobranchials and highly movable joints between the ceratohyals and the hypohyals and between the first ceratobranchials and the body of the hyoid. A well developed M. ceratohyoideus extends between the ceratohyals and the first ceratobranchials. The mechanism of dewlap extension was studied using X-rays, electrical stimulation, and denervation and electromyography of key hyoid muscles. During dewlap extension, the hyoid apparatus acts as a first order lever. Contraction of M. ceratohyoideus pulls the ceratohyals posterio-laterally causing the hypohyals and the body of the hyoid to rotate dorsally around the first ceratobranchial/body joints. This movement results in the second ceratobranchials swinging forward and down, unfolding the dewlap. A comparison is made of dewlap extension and other hyoid displays.

The brain stem origins of efferent fibers to M. ceratohyoideus were investigated by retrograde transport of horseradish peroxidase (HRP). Following application of HRP to M. ceratohyoideus or to its nerve supply on one side of the throat, large polygonal motoneurons were found ipsilaterally in the vagal (Amb X) and glossopharyngeal (Amb IX) parts of the medullary nucleus ambiguus. Labelled neurons were more abundant and more heavily labelled in Amb X than in Amb IX. In addition, small spindle-shaped cells were labelled ipsilaterally in three parasympathetic nuclei that innervate glandular structures in the pharynx and the larynx.

HRP injections of the larynx labelled cells in many of the same locations, including Amb X and Amb IX. Thus, nucleus ambiguus in A. eguestris contains motoneurons for supply of striated muscles in the hyoid (i.e., M. ceratohyoideus) and the larynx. The distribution of these motoneuronal pools shows no evidence of topographical organization.

Afferent sources to Amb X were tentatively identified by injection of HRP or wheat germ agglutinin-HRP conjugate (WGAHRP) into the caudal medulla. These injections labelled cells in several nuclei in the diencephalon, mesencephalon and rhombencephalon. Comparison of locations of HRP labelled cells and results of previous experiments using electrical stimulation of the brain suggests candidate nuclei subserving dewlap extension. In particular, it is proposed that nuclei in the reticular formation and the mesencephalic laminar nucleus of the torus semicircularis (Tori) are part of the neural network for control of dewlap extension. The implications of these findings for the evolutionary origins of dewlap displays are discussed.

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