Let me introduce you to the red-necked Keelback (Rhabdophis subminiatus), a beautiful snake that has the distinction of being one of only a handful of snakes in the world that are known to posses both venom and poison, and it can be found here in Thailand.
When talking about snakes many people confuse the terms venom and poison much to this dismay of many herpetologists. So, lets clear things up: poison is a substance that is harmful when eaten or absorbed through the skin, but venom is a harmful substance that must be injected into your body via fangs, stingers or spines. Here’s an image to help you remember the two.
Poison in snakes is still being investigated, but so far reserchers have found 13 species that can produce poison from special glands called nuchal glands. Nine of these species come from just one genus, Rhabdophis 1. and there are thought to be three species with these glands here in Thailand: the Red-necked Keelback (R. subminiatus), the Black-striped keelback (R. nigrocinctus), and the Specklebelly keelback (R. chrysargos).2
So where does the posion come from?
The poison itself,which consists of chemicals called bufadienolides comes from the snake’s diet of toads whose skins contain the poison.
The origin of the poison was demonstrated by experiments in which snakes of the same species from areas without toxic toads, or those fed on a non toxic diet were compared to snakes from areas where the toxic toads were present. The poison was found to be absent in the glands of snakes whose diet lacked toxic prey. It was also demonstrated that mothers who possessed the toxins in their glands could pass it down to their hatchlings.3.
So how do they use the poison?
The poison is stored in special defensive glands called Nuchal glands which are located under the skin of snake to be used as a defense against predators. They glands are pretty unsusual as they have no ducts or tubes from which the poison can flow 4. It is thought that the snake can rupture these glands with strong muscular contractions or maybe an attacking predator may accidentally rupture them.
These snakes present these glands to would be predators when they feel threatened in a beheaviour called ‘neck butting’ which you can see really well below.
So these snakes can manufacture poison, but what about their venom?
These snakes were previously listed as non-venemous but they have recently classed as venomous after a man was bitten by one he was keeping as a pet which lead to him being hospitalised with major complications.5. Their venom is potentially lethal and has a similar rating to that of the deadly Banded Krait (Bungarus fasciatus) which is also found here in Thailand.2
So, in conclusion you’d would be correct 90% of the time in saying that a snake is venomous and not poisonous, but, once again mother nature provides another remarkable exception to one of her rules. I can’t look at one of these beauties without remembering that.
1.Discovery of Nucho-Dorsal Glands in Rhabdophis adleri. By:Akira Mori, Teppei Jono, Li Ding, Guang-Xiang Zhu, Jichao Wang, Hai-Tao Shi, and Yezhong Tang. In: Current Herpetology Feb 2016 : Vol. 35, Issue 1, pg(s) 53-58 doi: 10.5358/hsj.35.53
2.Red Necked Keelback–Venomous –Dangerous, By: Vern Lovic, In: Thailand Snakes (https://www.thailandsnakes.com/venomous/rear-fanged/red-necked-keelback-venomous-mildly-dangerous/), June 23 2015
3.Dietary sequestration of defensive steroids in nuchal glands of the Asian snake Rhabdophis tigrinus. By: Hutchinson DA1, Mori A, Savitzky AH, Burghardt GM, Wu X, Meinwald J, Schroeder FC. In: Proc Natl Acad Sci U S A. 2007 Feb 13;104(7):2265-70. Epub 2007 Feb 6.
4. Nuchal glands: a novel defensive system in snakes. By:
5.Morbidity After a Bite from a ‘Non-Venomous’ Pet Snake, By: E Seow, P Kuperan, S K Goh, P Gopalakrishnakone, In: Singapore Medical Journal, 2000; Vol 41(1)