Morphology of the nucho-dorsal glands and related defensive displays in three species of Asian natricine snakes

Many animals are equipped with specialized defensive systems that function in a coordinated manner involving morphological structure, physiological processes and behaviour. Nucho‐dorsal glands, unusual organs known in a few Asian natricine snakes, are believed to function in avoidance of predation, based on the defensive function of similar organs in a related Japanese species that sequesters prey toxins and stores them in the glands. We examined the arrangement of the nucho‐dorsal glands of Rhabdophis nuchalis, R. pentasupralabialis and Macropisthodon plumbicolor and tested behavioural responses to tapping stimulation to investigate the spatial distribution of glands on the body and related defensive displays respectively. We confirmed the presence of glands that extend from the neck along the length of the body in all three species. The spatial arrangement of the glands was similar between the two Rhabdophis species, but it differed substantially in M. plumbicolor. In M. plumbicolor, there were two uninterrupted rows of glands throughout the full length of the body, whereas in the two Rhabdophis species, the position and size of the glands differed between the neck and trunk regions, with the two series separated by a spatial gap. In spite of these structural differences, M. plumbicolor and R. pentasupralabialis exhibited a similar defensive display, which we refer to as body lift, in response to a tapping stimulus on the body. Our study shows detailed morphological features of the nucho‐dorsal glands and a novel display that are consistent with the presumed predator deterrent function of the glands, which have evolved as a unique defensive system in this lineage of snakes. We describe the detailed morphological features of unique defensive organs, called mucho‐dorsal glands, in three species of Asian natricine snakes, Rhabdophis nuchalis, R. pentasupralabialis and Macropisthodon plumbicolor. We also report a novel display, body lift, which is considered to enhance the defensive function of the glands. Correlated evolution of these morphological and behavioural traits are discussed.