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Why are some snakes venomous?

Viper venom kills and digests. Photo: Janaki Lenin   | Photo Credit: mail pic

“Both ratsnakes and Russell’s vipers eat the same thing — rats. But why is one species venomous and the other not?” asked 11-year-old Abhinav.

A group of children and adults, who had landscaped a waterhole on the farm, were about to leave, when the sharp, young lad posed this question. I don’t think Rom and my answers were adequate in the limited time, so here is a more elaborate explanation.

Overpowering prey is a challenge for limbless creatures. Some species inject venom like Russell’s viper. Some others opt for an alternative non-chemical method — rat snakes, for instance, grab and push their prey against the ground, while pythons use their brawn to squeeze their quarry to death. But snakes can’t be neatly divided into venomous and non-venomous categories.

Rom and I watched a bullfrog whose head was caught in the vice-like jaws of a slender vine snake. Although the amphibian seemed too big for the snake, it wasn’t strong enough to escape. Could the snake’s mild venom kill the large frog? If it did, could the serpent swallow its quarry?

An hour later, the frog became sluggish as a bright yellow spot slowly grew larger on its head. When the amphibian went limp, the snake struggled to swallow it. We grew frustrated watching its attempts to wrap its pointed, narrow mouth around the frog’s head. Perhaps tired or realising the futility of the effort, it dropped the frog and slithered into the bushes. We examined the dead amphibian and concluded the yellow spot may have been caused by the vine snake’s venom.

When a vine snake latched on to Rom’s middle finger, he patiently pried it off with a pen even as it chewed on him. If he yanked his hand away, its teeth would break and squeezing out each tooth is painful business. His finger swelled up and itched for 24 hours. Since their bites aren’t fatal to humans, these snakes are called ‘mildly venomous’. But to their quarry, they can be lethal.

Even species listed as non-venomous aren’t completely devoid of venom. The common sand boa, for instance, produces secretions particularly toxic to birds. So the species doesn’t hedge its bets — it constricts its prey and injects venom for good measure.

Do vipers need venom potent enough to kill hundreds of rats with just one drop? After all, they eat only one or two at a time.

While predators try their darndest to kill most efficiently, their prey use any trick to avoid becoming a meal, such as developing immunity to venom. For instance, Californian ground squirrels are resistant to Northern Pacific rattlesnake venom. Venomous snakes need to maintain their edge by evolving venom toxic enough to override immunity. We don’t know if any of our Indian rodents use this trick.

Competition with prey is not the only thing driving snakes to evolve more and more toxic venom. Snakes also struggle to avoid becoming prey themselves.

Some snake predators have partial immunity to venom. Famously, mongooses are highly resistant to cobra venom, and with their speed and agility, kill snakes with impunity. It would be the death of cobras as a species if they didn’t evolve more toxic venom to immobilise mongooses.

Venom has another important role. It’s an extreme meat tenderiser; specific enzymes disintegrate the innards of prey. Normally, a reptile depends on the sun’s warm rays to aid digestion. Venomous snakes have an advantage: enzymes in venom digest the meal from the inside before it rots in their guts. These same enzymes cause considerable tissue damage in human snakebite victims. I agree it’s awful to contemplate.

But I wonder if we cannot use venom in our favour. In remote parts of India, local hospitality often involves leather-tough meat. I chew and chew until my jaws ache. If I spit it out or refuse, our hosts would be offended. Eventually, I swallow like a python stuffing a deer down its throat and hope I don’t choke.

If only I had venom.

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Printable version | Jan 26, 2021 8:52:41 AM |

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