干旱区生态环境知识资源中心

The internal structure of the conspicuous proboscis of three male Guenther’s dikdiks (Rhynchotragus guentheri) was investigated. The proboscis consisting of connective tissue and musculature is attached to the rostral part of the head. This head region is characterized by a reduction and caudal displacement of bony and cartilaginous components. The ventral concha in particular is small and reduced in length. The length of the nose amounts to 60-70% of skull length. Taking into account the anatomically-determined curvature the length of the air passage through the nose may reach 80% of skull length. Proboscis tubes are supported by a muscular pad originating from the incisive bone. Evolution of the proboscis tubes caused a rostral separation of the ventral nasal meatus from both the middle and dorsal nasal meatus. The three passages remain separated up to the caudal end of the proboscis tubes.

The structure of the respiratory region is conspicuously different from a typical bovid nose. It consists of an umbrella-shaped cartilage with its concave surface rostrally oriented and a posterior cartilaginous chamber the interior of which is accessible only through a small medial opening. A thick bundle of longitudinally-oriented, anastomosing, thick-walled veins passes along the dorsal surface. In addition, larger longitudinally-oriented fascicles of veins run along the ventral surface of the cartilaginous chamber, along the medial side of the dorsal concha, and within the plica recta; all of these vessels underlie the delicate nasal mucosa. The longitudinal fascicles of veins facilitate a countercurrent exchange between air stream and blood stream only during the expiratory phase. During inspiration, air and blood flow in the same direction. The rostral opening of the nasolacrimal duct is situated at the medial side of the cartilaginous umbrella. Below its mucosa the nasal septum is equipped with a rostral and a caudal cavernous body each consisting of numerous longitudinally-oriented veins. The caudal one extends laterally into the opening of the cartilaginous chamber.

The relevance of these anatomical results with respect to thermoregulation and water conservation is discussed using physiological data from the literature. At ambient temperatures below 30 degrees C (either at night or in the shade) the exchange functions of the nose are employed to recover effectively water. At ambient temperatures above 30 degrees C (during daytime in the sun) the evaporative cooling functions of the nose (and of the oral cavity) have to be employed to a greater extent accompanied by an increased loss of water. Water shortage causes a partial dehydration of the large amount of mucus covering the nasal mucous membrane surfaces. This might allow the recovery of water vapour during exhalation.

Fat deposits in honeycomb-shaped cavities of the skull base probably serve to insulate the complex system of blood vessels consisting of the carotid rete (= rete mirabile epidurale) and its surrounding venous network (= sinus cavernosus).

The cartilaginous elements of the respiratory region and the accompanying mucous membrane folds as well as the contiguous muscles can be clearly homologized with structural elements of a typical bovid nose. In the evolution of the proboscideal nose, rostrally located parts became reduced in length and were displaced caudally. This resulted in the cartilaginous elements of the lateral nasal wall and of the nostril, together with the fascicles of veins, undergoing change to a thermoregulative function (heat exchange, water recovery). The proboscis tubes lined with cutaneous mucosa and the circular nostrils directly in contact with each other and pointing downward are recently evolved structures. Presumably they improve the insulation of the vapour exchanger. Of all the Madoquini, R. guentheri lives in the most arid habitats and is largely independent of drinking water. Thus, the development of a proboscideal nose and the transformation of the respiratory region have evolved as adaptations for the enhancement of water recovery.