Most venomous snakes evolved their venom to kill prey before it could run away. Scaring off potential predators was secondary. Cobras are different, however. Mixed with their prey-killing venoms they have peptides that purely serve to repel threats. New discoveries about how these defensive molecules evolved, and how those of different species relate, could help us produce better anti-venoms, and even lead to possible anti-cancer drugs.
Dr Bryan Fry of the University of Queensland studied 29 species of cobras and their relatives. He found the cobra's defensive venoms evolved together with other features, such as the ability to spread their hoods to look larger, and warning markings. “For the longest time it was thought that only spitting cobras had these defensive toxins in high amounts in their venoms, however, we’ve shown that they are widespread in cobras,” Fry said in a statement.
This makes sense, Fry explained to IFLScience, since there is no point evolving the capacity to spit if you don't have the payload to make it count. Instead, ancestral cobras evolved potent pain-inducing venoms, and three distinct groups of their descendants independently found a way to deliver it at a distance. Venomous spit cannot penetrate the skin, but if it reaches the eyes can cause extreme pain and blindness, a powerful deterrent to anything that might consider a cobra a potential meal.
For predatory purposes snakes want venoms that will kill, or at least immobilize, prey as quickly as possible. Killing other predators isn't necessary, however, and since they are larger than prey can be difficult. Instead, defensive venoms need to cause as much pain as possible. Unfortunately, these toxins don't just hurt, they destroy tissue. If not treated, bite victims often lose the use of limbs or need amputations.
image in text: Dr Bryan Fry communes with a cobra, which over millions of years developed venom specifically to deal with annoying creatures like him. Sean McCarthy
Among African Cobras, Fry reports in Toxins, spitting species have more powerful venoms than their non-spitting cousins. Nevertheless, even the non-spitters have defense-specific venom. Among Asian cobras, there is no relationship between spitting capacity and the strength of the venom.
Fry told IFLScience the capacity to spit gives a snake an incentive to make its defensive venoms more powerful, but that the Asian snakes had already gone as far as they could go with the strength of their defensive venoms when they developed spitting. Any further refinement would have been at the expense of their prey-controlling molecules. Those African cobras, on the other hand, on evolving spitting, still had room to improve the potency of their new long range weapon.
Meanwhile king cobras, which despite the name are not true cobras, never developed the capacity to spit because their venom molecules are too large to penetrate the eyeballs, a demonstration of the contingency of evolution.
“Globally, snakebite is the most neglected of all tropical diseases and antivenom manufacturers are leaving the market,” Fry said. Fry's work demonstrates the challenge of making an anti-venom that will work against different forms of cobra venom, but may also help achieve that goal.
Although most venoms kill healthy and cancerous cells equally, Fry told IFLScience somewhere among the vast array of cell-killing molecules he has explored it is likely there is one that is more selective, and could eventually be used to stop tumors while having less impact on healthy cells than existing chemotherapies.
Expanding the hood allows a cobra to look more fearsome, like a human puffing up their chest, while coloring says "don't tread on me". Gowri Mallapur
