How Some Snakes Don't Kill Themselves With Their Own Venom

If this king cobra looks like it is sulking it's probably because some of the snakes it would like to prey on have evolved resistance to its toxins, a technique the cobra had to get to first in order to survive. Image credit: Kailash kumbhkar CC BY-SA 4.0

Being a highly venomous creature means either evolving the antidote to your own toxins or spending your entire life in mortal fear of biting your cheek. Not surprisingly, snakes slithered their way up the first path. The nature of this immunity varies by venom and is poorly understood. A new study reveals two families of venomous snakes manage it with a neat electrostatic trick, and shows many non-venomous snakes have also caught on.

Neurotoxins represent one of the more common ways for snakes to subdue their prey. Certain snakes are known to avoid being affected themselves by blocking their own receptors. Dr Bryan Fry of the University of Queensland compares this to putting a twig in a docking site, preventing the venom molecule from entering. Fry knew only a minority of snakes use this method, however, and is studying what others do.

In Proceedings of the Royal Society B, Fry and Richard Harris provide part of the answer: α-neurotoxins used by some snakes include positively charged amino acids. This makes them particularly effective against most animals because the part of the receptor they target is negatively charged. However, these snakes have evolved to flip their receptor's charge. Trying to lock the two positive charges together, Fry told IFLScience; “Is like trying to bring together the matching poles of magnets.” Without connecting to the receptor, the venom cannot trigger the nerves.

Unfortunately for cobras and their ilk, they're not the only ones to find this protective mechanism. Harris and Fry found this type of charge resistance has evolved at least ten times independently among snakes. Fry told IFLScience they can tell this because; “We see it in types of snakes that are unrelated, with it not appearing in a number of intervening species.”

The common feature of all the snakes with this immunity – other than those that are venomous themselves – is that they all live around snakes that use α-neurotoxins to hunt.

Burmese pythons in their home range would be very vulnerable to being eaten by cobras, had they not evolved resistance to the cobra's venom. You can drop any plans to introduce cobras to the Florida everglades, which have been overslithered by the pythons. Image credit: U.S. Fish and Wildlife Service Headquarters. CC BY 2.0

The obvious question is why, if snakes have evolved this defense mechanism so often, other species haven't done the same thing and put the venom out of business entirely. Fry, confirming honey badgers use something similar, is yet to see how many other animals are also immune, but told IFLScience; “There must be a cost. In a previous paper we showed that where vipers were no longer being predated by cobras they lost the [other form of] resistance, so there must be some selection pressure against it.”

Until recently, research like this would have been almost impossible since it would have been necessary to kill specimens of every snake studied. Even ignoring the ethical issues, the cost and effort involved would have made this impractical. However, the authors used new technology Fry has pioneered, creating artificial nerves that replicate those of each different species, testing the response to any molecule.

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