While these may not always be lethal, they can cause severe damage. Many primates will pre-emptively kill a snake if they feel threatened, often using projectile weapons or tools, like rocks and sticks. We believe several factors make human ancestors the most likely selective agent. This kind of defence must have been stimulated by a very strong selective pressure. Yet this projectile defence system, and the specific mixture of toxins that cause more pain, evolved three times independently, only within this small group. Venom spitting is a unique behaviour found only in a small handful of closely related snake species. Could human ancestors have prompted this evolution? The threefold independent evolution of spitting was accompanied on each occasion by the same complex, synergistic changes in venom composition. This suggests spitting cobras increased the abundance of PLA2s in their venom over time, to make the already present cytotoxins much more effective as a pain inducer.
However, the mixture of PLA2s and cytotoxins caused this activity to increase dramatically. It was the cytotoxins, the toxins widespread in spitting and non-spitting cobras alike, that caused activation of the neurons. To our surprise, PLA2s were ineffective on their own. We suspected spitting cobra PLA2s might activate these neurons and potentially cause pain. To assess pain causing activity, we tested cobra venom on isolated mouse neurons, responsible for sensations in the eyes and face. We set out to find out whether defensive spitting cobra venom would be especially painful on contact. Many animals that use venom defensively do so by inflicting rapid, severe pain on their aggressors.
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Because these cobras spit for defensive reasons, this is the first evidence of a defensive driver of venom evolution in snakes. Our results found spitting cobras have increased the abundance of a different toxin family, called phospholipase A2 (PLA2s), in their venom compared to their non spitting counterparts. However, cobras have a unique type of three-finger toxins that destroy cells, rather than block neurotransmission. This is due to an abundance of toxins called neurotoxic three-finger toxins, which stop neurotransmission, signals being sent from the nervous system to the prey’s muscles. Venom in fixed front-fanged snakes, including cobras, tends to cause paralysis. While snakes do use their venom in self defence, for example in the case of human snakebites, most of the evidence suggests venom composition has been evolved for foraging, not defence. Snake venoms are complex mixtures of proteins, used primarily in foraging to efficiently incapacitate prey. Wolfgang Wüster, Author provided Unique toxin cocktails Spitting cobras can hit the eyes of a target up to 2.5 metres away with venom. The results show it might have been to ward off attacks from our human ancestors. In a new study, we tested the venom of spitting cobras to see what toxins could be found, to work out what might have caused this defensive behaviour to evolve. While most snakes use venom for preying on other animals, spitting cobras use it purely for defence.
For this behaviour, they are known as spitting cobras.īizarrely, this unique adaptation has evolved three times independently in a small group of Afro-Asian snakes: once in African cobras, once in Asian cobras, and once in the related rinkhals, also known as the ring-necked spitting cobra. These allow them to forcibly eject venom as a spray or “spit”, which can hit the eyes of a target up to 2.5 metres away. Some species of cobra have modified fangs with small, front facing orifices. However, hooding isn’t the only defensive behaviour in a cobra’s arsenal. These snakes are most well known for their characteristic defence mechanism called hooding, when the sides of their neck flare out in a dramatic display. Cobras are fascinating and frightening creatures.
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