EVOLUTIONARY MORPHOLOGY OF THE PROBOSCIDEAL NOSE OF GUENTHER DIKDIK (RHYNCHOTRAGUS-GUENTHERI THOMAS, 1894) (MAMMALIA, BOVIDAE)

The internal structure of the conspicuous proboscis of three male Guen ther's dikdiks (Rhynchotragus guentheri) was investigated. The probosc is consisting of connective tissue and musculature is attached to the rostral part of the head. This head region is characterized by a reduc tion and caudal displacement of bony and cartilaginous components. The ventral concha in particular is small and reduced in length. The leng th of the nose amounts to 60-70% of skull length. Taking into account the anatomically-determined curvature the length of the air passage th rough the nose may reach 80% of skull length. Proboscis tubes are supp orted by a muscular pad originating from the incisive bone. Evolution of the proboscis tubes caused a rostral separation of the ventral nasa l meatus from both the middle and dorsal nasal meatus. The three passa ges 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 i ts concave surface rostrally oriented and a posterior cartilaginous ch amber the interior of which is accessible only through a small medial opening. A thick bundle of longitudinally-oriented, anastomosing, thic k-walled veins passes along the dorsal surface. In addition, larger lo ngitudinally-oriented fascicles of veins run along the ventral surface of the cartilaginous chamber, along the medial side of the dorsal con cha, and within the plica recta; all of these vessels underlie the del icate nasal mucosa. The longitudinal fascicles of veins facilitate a c ountercurrent exchange between air stream and blood stream only during the expiratory phase. During inspiration, air and blood flow in the s ame direction. The rostral opening of the nasolacrimal duct is situate d at the medial side of the cartilaginous umbrella. Below its mucosa t he 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 thermoregula tion 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 employe d to recover effectively water. At ambient temperatures above 30 degre es 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 ext ent accompanied by an increased loss of water. Water shortage causes a partial dehydration of the large amount of mucus covering the nasal m ucous membrane surfaces. This might allow the recovery of water vapour during exhalation. Fat deposits in honeycomb-shaped cavities of the s kull base probably serve to insulate the complex system of blood vesse ls consisting of the carotid rete (= rete mirabile epidurale) and its surrounding venous network (= sinus cavernosus). The cartilaginous ele ments of the respiratory region and the accompanying mucous membrane f olds 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 an d were displaced caudally. This resulted in the cartilaginous elements of the lateral nasal wall and of the nostril, together with the fasci cles of veins, undergoing change to a thermoregulative function (heat exchange, water recovery). The proboscis tubes lined with cutaneous mu cosa and the circular nostrils directly in contact with each other and pointing downward are recently evolved structures. Presumably they im prove 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 th e transformation of the respiratory region have evolved as adaptations for the enhancement of water recovery.