A ring neck spits a stream of poison in self-defense.
© The trustees of the Natural History Museum, London; Callum Mair
By Mitch Leslie
Spitting cobras protect themselves by shooting poison jets into their attackers’ eyes. A new study suggests that over the course of several million years, all three groups of spitters adapted the chemical of their toxins in the same way to cause pain in a prospective predator. The work provides a new example of co-evolution that ‘deepens our understanding of this unique system’ for the delivery of poison, says Timothy Jackson, an evolutionary toxicologist at the University of Melbourne.
Like other cobras, spitting cobras will bite attackers in self-defense. Spitting, however, is their characteristic move, and the snakes are shots. They can aim a poison poison more than 2 meters away in the face of an attacker and aim at the eyes. The behavior is such a formidable defense that it has evolved independently three times: in Asian cobras, African cobras, and a cobra cousin called the ring-necked (Hemachatus haemachatus) living in southern Africa.
Scientists have previously found the venom of some other snakes evolved to better subdue their prey. The poison biologist Nicholas Casewell of the Liverpool School of Tropical Medicine and colleagues analyzed the poison from 17 spitting and non-spitting species – and measured its effect – to see if the composition of spitting cobra poison also changed over time to become a more effective defense. .
The most common compounds in cobra venom are the so-called three-finger poisons – proteins named after their 3D chemical form, not the number of digits you can expect to lose if a snake bites your hand. Three-finger toxins are equally abundant in the toxins of spitters and non-spitters, Casewell and colleagues found, making up about 60% of the toxic molecules. However, the venom of the spitting species contained higher levels of another group of proteins known as phospholipase A2 toxins, which non-pitters produce only in small quantities, or not at all.
To investigate the effects of the extra phospholipase A2 proteins, the scientists placed different combinations of snakes’ toxins on isolated mouse nerves that are sensitive to pain. The more neurons a toxin stimulates, the more pain it will cause. The researchers found that three-finger toxins cause more pain when combined with phospholipase A2 toxins than alone. For example, when the researchers applied both types of ringworm toxins to mouse nerves, the mixture stimulated approximately twice as many nerve cells as the three-fingered ringworm toxins alone, they reported today in Science.
The work suggests that natural selection refined the composition of the snake’s venom to make it a better defense, Casewell says. That the three groups of peaks independently obtained the same solution – increased amount of phospholipase A2 toxins – is an example of converging evolution, in which species that are not closely related but have similar survival challenges get similar adaptations. “Evolution can be very repetitive,” Casewell says.
The evolutionary logic of the study makes sense, says toxicologist Stephen Mackessy of the University of Northern Colorado, Greeley, who was not involved in the research. Increasing the venom force that the venom causes will help the snakes repel predators, because ‘one of the best learning tools is the production of pain’, he says. But Joe Alcock, a researcher in evolutionary medicine at the University of New Mexico, Albuquerque, says it is possible that damage to the attacker’s eyes was the driving force behind the development of unique chemistry. “If you can blind a predator, you can prevent an attack independent of pain,” he says.
Why some cobras start spraying venom rather than just delivering it through bites is unclear. Some researchers argue that the behavior protects the snakes from ungulates. But the eyes that look at buffalo, zebras and other heavy-footed mammals will be hard hit with a single jet of poison, Casewell says. Instead, he and his colleagues postulate that early humans motivated the origin of spitting behavior. Our ancestors would have been a threat to the snakes, and they had comfortably forward-looking eyes that would make good targets for a stream of harmful venom.