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News

Spitting cobras optimise their accuracy by rapid head movements

University Of Bonn (Universit : 31 March, 2007  (Technical Article)
Spitting cobras spit their venom into the faces of potential attackers, according to some reports even across a distance of several metres. This venomous cocktail of toxins hits the victim's eyes surprisingly often and may leave them blinded. University of Bonn zoologists have discovered how the snakes optimise their accuracy rate: while they are squirting the venom out of their fangs at high speed, they move their heads to and fro in a circling or bobbing movement.
The whole process lasts on average only a twentieth of a second and is not visible to the naked eye. The head movement has the effect of spreading the venom over the target. In this way one species of cobra managed to hit at least one eye each time it spat venom.

The red Mozambique spitting cobra stiffens, fixing its gaze on the victim's face, which is moving backwards and forwards in front of it. For several seconds it remains erect like this; then its head flashes forwards. For an instant the fangs in front of its pale pink throat are visible in its wide-open mouth, as they squirt the venom at high pressure towards the victim. On the plastic visor two red spiral patterns appear. The eyes behind it look surprisingly unperturbed. 'I sprayed the visor beforehand with rhodamine,' Katja Tzschätzsch calmly explains, 'It's a pigment which dyes liquids red. This makes the traces of venom easier to see.'

In her undergraduate dissertation the trainee teacher investigated what spitting cobras aim at when spitting. 'In the literature it often says: they aim at the eyes,' her supervisor Dr. Guido Westhoff, junior lecturer in Professor Horst Bleckmann's team, explains. 'However, up to now nobody has investigated it.' The cocktail of toxins partly consists of nerve poisons, but also contains components which are harmful to tissue. Through a narrow channel in their fangs the snakes can spray the liquid at high pressure, similar to a bullet in the barrel of a gun. If they manage to hit an eye, the sensitive cornea reacts with severe stinging pain. In the worst case these burns can ultimately lead to blindness.

As guinea pigs Katja Tzschätzsch used four Mozambique and six black-necked spitting cobras from the animal house in Schloss Poppelsdorf. In her experiments she either stood face to face with them herself, protected only by a plastic visor, or she used photos. In addition, for both species she recorded the spitting process using a high-speed video camera. 'The snakes really do spit only at moving faces,' was her first finding; 'movements involving the hand elicited no response from any of the snakes.' Only two cobras reacted to the photos. These even spat when Katja touched up the photo, taking out one eye. And even when both eyes were removed, one of the black-necked cobras still remained aggressive. The conclusion: 'For really reliable results we would need a larger sample.'

Always straight at the eyes
The evaluation of the traces of venom on the photos and the visor revealed how accurate the aim of both species of snake was: the black-necked spitting cobras hit at least one eye eight out of ten times, with the red Mozambique spitting cobras even reaching the target in 100 per cent of cases. However, there was a clear difference in the traces left by the two species: whereas the black-necked cobra sprayed its venom, the attack by the red Mozambique cobra is reminiscent of something shot from a double-barrelled water pistol.

What is decisive for the high degree of accuracy is a pattern of behaviour which researchers were able to observe in both species. 'In super slow motion it is clearly visible that the snakes move their heads rapidly when squirting the toxin,' Dr. Westhoff explains. 'Rather like we do when we wish to use a garden hosepipe to water the flowers of an entire flowerbed.' In this way the venom is spread out over a larger area; the chance that it will hit one eye increases.

However, Dr. Westhoff would like to scotch one prejudice: 'Cobras only spit when they feel threatened, not to kill their prey,' he says; 'anything else is a myth.' They kill their prey like other poisonous snakes do, by biting them and thereby injecting the venom into their bloodstream, which then proves fatal. Human beings are not on their list of potential prey; even so, these snakes are dangerous, even when they are still very young. Dr. Westhoff reveals, 'I was once attacked by a spitting cobra which had just emerged from its shell, it practically spat at me out of its shell.'
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