5 days ago
Snakes may have once faced a vicious enemy: the humble ant
Some snakes are well-known for injecting prey with venom from their fangs. What's less well known is that they produce toxic stuff at the other end of their bodies, too. Located at the base of the tail in venomous and nonvenomous snakes alike are glands that generate foul-smelling secretions. The point of these glands has long been a mystery, but new research suggests they could stem from a time when snakes were much less impressive and needed to protect themselves from a vicious enemy: the humble ant.
Scientists have known since at least the 1960s that some tail secretions are bug-repellent. One snake, a teeny, worm-like thing called the Texas blindsnake, which when coiled is no larger than a 50-pence piece, smears itself in its tail poison when raiding ant and termite nests for food, for example. Yet until now it has been unclear why all snake species, even those that seemingly never interact with ants, produce this noxious concoction.
To get to the bottom of the issue, Paul Weldon of the Smithsonian Conservation Biology Institute in Virginia and Robert Vander Meer of the Centre for Medical, Agricultural and Veterinary Entomology in Florida collected secretions from snakes on all family branches of the serpentine evolutionary tree. The collection included a boa constrictor, a middle American burrowing python, a ball python, a timber rattlesnake, a king cobra and a unicolour cribo (a large, nonvenomous snake known as the 'lord of the forest'). The team then set up enclosures with red fire ants that have large underground colonies and make aggressive stinging attacks on intruders. In one chamber, the team allowed the stench of the snake gunk to waft in, to see if it would put the ants off. But they entered the chamber undeterred.
Drs Weldon and Vander Meer next questioned whether directly interacting with the secretions would have an effect. They presented the ants with both a droplet of ordinary water and a droplet of water tainted with 200 microlitres of snake secretion. Though the ants readily encircled and drank from the ordinary water droplets, they rarely even approached the tainted droplets. Fascinated, the researchers then tested placing tiny amounts of secretions from four different species directly on a small handful of unlucky ants. No matter which snake provided the poison, the ants almost always became paralysed and half usually died within four hours.
The researchers interpret these findings, reported recently in the Science of Nature, a journal, to mean that tail secretions from snakes probably evolved for insect defence long ago. Since both ants and snakes occupied subterranean environments during the Cretaceous period when dinosaurs still roamed the Earth, Drs Weldon and Vander Meer propose that the secretion appeared in the earliest snakes, which were probably similar to the modern Texas blindsnake. It would allow them to respond to angry ants defending themselves from attack or predatory ants looking for their next meal.
As for why formidable snakes like king cobras still produce these chemicals, the team believes that they could have come to serve a dual purpose. Past work in other labs shows that carnivorous mammals steer clear of meat streaked with snake-tail secretions. Since carnivorous mammals evolved millions of years after snakes, there is little chance that pressure from mammal predators encouraged the rise of the adaptation. What is more likely is that this built-in insecticide, just by happenstance, tasted so terrible to mammals that it put them off eating snakes. When you have no limbs, you might as well make both ends count.
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