It was a mystery to venom experts how cobras evolved these flesh-eating chemicals in the first place.
The toxins seemed of little use except as defensive spittle. Cobras on the hunt inject their meals with strong neurotoxins, knocking out birds and small mammals before consumption begins. Skin-melting agony would have the opposite effect.
"Predatory toxins are honed for killing prey. Defensive toxins are all about inflicting pain," as Bryan Fry, a biologist at the University of Queensland in Australia, told the Washington Post.
But according to a new study that Fry and his colleagues published this week in the journal Toxins, the development of cobras' flesh-eating venom was a tale of snakes backing up their bluffs with defensive weapons.
First came the snakes' hoods, used to scare off predators. This was followed by the painful toxins. Finally, three separate times in the cobra family tree, some of the snakes evolved the unusual ability to emit blinding spit at perceived threats.
The study was "a refreshingly fresh look at the evolution of venom compositions," said evolutionary biologist Kartik Sunagar, a venom expert at the Hebrew University of Jerusalem, who was not involved with the new research. The relationship between the flesh-eating venom and physical traits, he said, was "very surprising and intriguing".
This painful flesh-eating venom was something of a biological oddity. "Venom, one of nature's most complex biological cocktails, is energetically expensive to produce," Sunagar said. The chemicals, called cytotoxins after their cell-killing properties, are relatively weak - up to 100 times less lethal than the neurotoxic sedatives. Why, then, would cobras waste the energy producing the less-potent stuff?
To trace the origins of snake cytotoxins, Fry and his colleagues compared evolutionary traits of 29 cobra species and their close serpent cousins. In their analysis of the cobra evolutionary tree, the researchers determined that the snakes first evolved hoods or brightly coloured warning bands.
These hoods and bands sent a similar message: Don't tread on me or in my general vicinity. "It's like the guy puffing up his chest at the bar," Fry said.
It was inevitable, though, that an eater of snakes would have called the cobra's bluff. Evolutionary pressure gave way to what Fry called "chemical krav maga": a liquid cocktail, like the one found in the glands of modern cobras, of painful cytotoxins and lethal neurotoxins. (Snakes need a mixture because too much defensive venom would impede the ability to hunt. Only a few animals, such as cone snails, have the ability to switch their venom between chemical defence and offense.)
The researchers found that cytotoxicity increased in brightly banded or brightly hooded cobras as well as spitting cobras. This was contrary to long-held belief that only spitters produced the painful, but less lethal, venom.
A snake species called the king cobra helped drive the theory home. Although these snakes have cobra-like warning hoods, they are cobras in name only. Thirty million years and a large number of snake species separate the king cobras from the true cobras.
But some king cobras shared remarkably similar venom with their namesake snakes. The scientists found a cytotoxin within the glands of Malaysian king cobras, a species with the colourful warning hoods.
It should be noted that the study was more than an idle academic curiosity. Although spitting cobras have venom that burns eye tissue, several non-spitting cobras have harmful amounts of the cell-killer toxins, too. And these cobras have a dramatic impact on human health.
On the Indian subcontinent, for instance, snakes bite about 1 million people a year. About 10 per cent of the bites are fatal. Of those who survive, nearly 400,000 to 500,000 will have lasting injuries, many from cytotoxic venom.
