Spitting cobras keep constant venom viscosity

Abstract

Sometimes a bite just isn't good enough. Turning your teeth on a predator won't always be sufficient warning to back off, so some species throw in a dash of venom to get the message across. But one group of snakes take their toxic warning a stage further. They don't wait until a threat is within fang reach; spitting cobras eject their venom, sometimes over several metres, aiming at their victim's face and eyes. To ensure that their venom hits the spot, spitters have evolved specialised fangs that eject the venom forward at high pressure, but Arie van der Meijden and Ignazio Avella from the Research Center in Biodiversity and Genetic Resources (CIBIO), Porto, Portugal, wondered whether the snakes’ alternative lifestyle may also have affected the venom itself. Previous research had suggested that the venom of spitters might be a specialised fluid – known as a non-Newtonian fluid – which becomes less viscous as it flows through the fine channel connecting the venom gland to the fangs before increasing in viscosity as the venom sprays out to extend the range of the jet. But no one had thoroughly tested the theory on a variety of venoms, so Avella travelled to the Liverpool School of Tropical Medicine (LSTM), UK, home to some of the most venomous species on the planet.‘LSTM houses one of the richest collections of venomous snakes in Europe. Nicholas Casewell and Robert Harrison, the heads of LSTM's Centre for Snakebite Research and Interventions, allowed us to analyse the venoms of 13 different cobra species including spitters and non-spitters. I don't know of many other places that would be able to provide such a nice sample!’ exclaims Avella. During Avella's visit, Paul Rowley and Edouard Crittenden carefully collected venom samples by gently holding each snake behind the head and allowing it to bite a Parafilm-covered Petri dish, to catch the venom as it flowed from the fangs. ‘It doesn't matter how many venomous snakes you have dealt with, when a cobra raises its head and inflates its hood at you, it always sends a shiver down your spine’, Avella chuckles. Then he transported the lethal samples – ranging from 0.13 g from a Naja atra to 1.4 g from a Naja nigricollis – to Edgar Barajas-Ledesma and Chris Holland at the University of Sheffield, UK, where they took tiny droplets of each venom to measure the fluid's viscosity and how that altered as the liquids flowed.However, when the team compared the cobra venoms, they were surprised that the viscosity of the spitters’ venom didn't alter at different flow speeds and was remarkably similar to the viscosity of the venom collected from non-spitting snakes. ‘When our measurements disproved something that we considered a given based on previous studies, it was both surprising and exhilarating’, says Van der Meijden. In addition, Barajas-Ledesma calculated the impact of venom viscosity and fang shape on the fluid pressure inside the fang and found that the pressure in the non-spitting Naja nivea fangs was higher than the pressure in the spitting N. nigricollis fangs. However, Barajas-Ledesma adds ‘Spitting and non-spitting cobras still use the same pressure as a firehose to eject their venom, so perhaps it's not so surprising they need it to flow as consistently as possible’.Given that spitting cobras only do so for defence, Van der Meijden is puzzled as to why more of the fearsome serpents haven't opted for this fangs-off approach to self-defence. He suspects that spitters may have evolved the strategy to keep fast-moving predators at bay, especially out in the open.