The snakes of Somaliland and the Sokotra Islands

The systematic status of the snakes recorded from the area has been reinvestigated, often in the light of new material. Allometric growth is shown to occur in several species and taxonomic errors arising from its previous non-recognition are rectified. The same feature is also used to determine the respective ontogenetic levels of closely allied forms. The fauna of the Sokotra group is shown to have greater relationships with the Palaearctic and Oriental regions than with the Ethiopian and to consist of mainly primitive types; they are assumed to have been isolated in the islands since the mid-Miocene. The mainland fauna, though predominantly Ethiopian, contains a strong northern admixture and this element predominates in the northern coastal plain which is apparently part of the route by which the northern forms have reached the peninsula. The northern component of the fauna shows a higher degree of endemism in the peninsula than the Ethiopian component and this is ascribed to the fact that they are completely or partly isolated from their northern relatives. Where the isolation is incomplete the connexion is tenuous and the distributional evidence suggests that the isolating factor is desiccation; any intensification of the present aridity would result in complete isolation. From this it is concluded that the elements retaining a connexion with the north have not been subjected to an epoch more arid than that of the present time and must, therefore, have entered the area since the last inter-pluvial, late in the Middle, or early in the Upper, Pleistocene; on the other hand those that are completely isolated probably lost their connexion with the north during this inter-pluvial. If Zeuner's dating of the Mount Carmel cave deposits is accepted, this implies that, in the absence of complete isolation these forms have undergone at most infraspecific change in a period of up to 120,000 years whereas other, often congeneric, forms that have been isolated for a minimum of 125-175,000 years have become differentiated to specific level. The extreme richness of the Somali fauna is due to its geographical and ecological circumstances. In addition to Palaearctic elements associated with the desert fringes and Ethiopian elements from the Savannah regions, the uplands of the west and north contain an element associated with the damp uplands of the Abyssinian mountains. Amongst the endemics with Savannah affinities are some with analogues in the arid regions of South-west Africa. The taxonomic status of these suggests that the present arid phase in the latter area is of longer standing than that in Somaliland. This is explicable on the basis of alternations of climatic change north and south of the tropical region, which accords with the theory that the changes are associated with oscillating movements of the caloric equator; this cannot, however, be the sole cause since the number of these oscillations far exceeds the number of climatic changes for which there is any evidence.