Evolutionary transformation of the cervicobrachial plexus in the colugo (Cynocephalidae: Dermoptera) with a comparison to treeshrews (Tupaiidae: Scandentia) and strepsirrhines (Strepsirrhini: Primates).

ABSTRACT

Four cervicobrachial plexuses from two colugos (Dermoptera), which are gliding mammals with semi-elongated necks, were dissected with imaging analysis and compared with those in its relatives, 12 sides of six treeshrews (Scandentia) and 32 sides of 16 strepsirrhines (Primates), for considering of its evolutionary constraint and functional adaptation. (1) The relative cervical length in the colugos was significantly longer than those in the others, regardless of the number and proportion of vertebrae. (2) In all examined colugos, the cervical plexus exhibited broader cervical root segments comprising the hypoglossal (N. XII) and first to fifth cervical (C1-C5) nerves, whereas the brachial plexus exhibited concentrated segments comprising C6 to the first thoracic nerve (T1) and part of T2. (3) On the other hand, the cervical plexus composed of N. XII and C1-C4 and the brachial plexus composed of C5-T1(2) were formed in all treeshrews (12/12 sides, 100.0%) and most strepsirrhines (27/32 sides, 84.4%) as seen in most terrestrial placental mammals. (4) Similar root segments of broader cervical and concentrated brachial plexuses were found in five sides of three strepsirrhines (15.6%), which are species with somewhat longer necks than the other strepsirrhines and treeshrews. Based on present and previous reports on elongated and shortened neck mammals, the modified root segments of the cervicobrachial plexus in the colugo appears to be related more to neck length than to its ecological habit, specialized locomotion, or any phylogenetic constraint.