My daughter-in-law and grand-daughter were treated to this sighting in the ladies’ lavatory of an outdoor play centre in the New Territories of Hong Kong last weekend. This a is a Red-necked Keelback, a snake that is both venomous and poisonous, but one which was thought to be neither for decades.
The species, now Rhabdophis subminiatus, was lumped in with our Grass Snake in the genus Natrix as harmless colubrid snakes. It is now known that it is a rear-fanged, venomous snake. There has been at least one fatality and a number of cases of severe haemorrhage after people have been bitten. Then it was discovered that the snake is not only venomous, it is, like all members of the genus, poisonous too. It accumulates toxins from the toads it eats and stores them in glands in the neck, the nuchal glands. Any attack or approach by a predator is met by the snake adopting one of three defensive posture in which the neck is pointed towards or moved to contact the predator. For example, the mouth is pointed downwards with the neck arched upwards.
The nuchal glands were discovered in a Japanese species of the genus by Nakamura in 1935. Nakamura also realised the glands, which lie just under the skin, produce something noxious. He was sprayed in the eye with yellow droplets as he decapitated the snake. He could not fail to notice the great pain. Others, previously, had noticed sore eyes after handling these snakes. Malcolm Smith (1875-1958), after he retired as house physician to the royal household of Siam, studied the nuchal glands in a number of species. They are paired structures which in some species extend the length of the back as well as the neck.
The glands are unusual in that they have no lumen for secretion to accumulate nor a duct to the exterior. The yellow, foul-smelling secretion is formed by breakdown of the secretory cells and also contains lymphatic and pigment cells. It is easy to see how a predatory bird grasping the snake by the neck break the skin and be confronted, in effect, by toad toxins. However, breaking the skin and damaging the nuchal glands may not be the only way for the secretion to be released.
In 2011 Kevin Messenger, Daniel Rosenberg, Kevin Caldwell and William Sargent came across a Red-necked Keelback in a hole on the side of a water catchment on Lantau island in Hong Kong. The snake after it had been caught, but not by the neck, arched its neck against a glove and then secretion began to ooze from the region of the nuchal glands. How, without ducts to the exterior this was accomplished is not known. Would contraction of the neck muscles suffice? Is there a myoepithelium surrounding the glands which might contract? Is there a mechanically weak pathway to the exterior for the secretion to reach the surface? What are the mechanisms providing defence against, and transport from the gut to the glands, of toad toxins? Much remains to be discovered about the Red-necked Keelback and others of its genus.
My grand-daughter was very lucky to see the scenario of killing, feeding and getting hold of defensive toxins. The toad, Duttaphrynus melanostictus, was not so lucky.
Messenger KR, Rosenberg D, Caldwell KK, Sargent WL. 2012. Rhabdophis subminiatus helleri (Red-necked Keelback). Defensive Behavior. Herpetological Review 43, 497.
Mori A, Burghardt GM, Savitzky AH, Roberts KA, Hutchinson DA, Goris RC. 2012. Nuchal glands: a novel defensive system in snakes. Chemoecology 22, 187-198 DOI 10.1007/s00049-011-0086-2
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