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Cobras are fascinating and frightening creatures. These snakes are best known for their characteristic defense mechanism called a balaclava, when the sides of their necks flare out in a dramatic display.
Hooding isn’t the only defensive behavior in a cobra’s arsenal, however. Some cobra species have modified fangs with small openings facing the front. These allow them to forcefully eject venom in the form of a spray or “spit,” which can reach the eyes of a target up to 2.5 meters away. For this behavior, they are known as spitting cobras.
Oddly, this unique adaptation independently evolved three times in a small group of Afro-Asian snakes: once in African cobras, once in Asian cobras, and once in related rinkhals, also known as the spitting cobra. necklace.
While most snakes use venom to attack other animals, spitting cobras only use it for self-defense. In a new study, we tested the venom of spitting cobras to see what toxins could be found, to determine what might have evolved this defensive behavior. The results show that it could have been to ward off attacks from our human ancestors.
Wolfgang wüster, Author provided
Unique toxin cocktails
Snake venoms are complex mixtures of proteins, used primarily in foraging to effectively neutralize prey. While snakes use their venom for self-defense, for example in the case of human snakebites, most of the evidence suggests that the composition of the venom was developed for foraging, not defense.
The venom of snakes with fixed front fangs, including cobras, tends to cause paralysis. This is due to an abundance of toxins called three-fingered neurotoxins, which stop neurotransmission, with signals being sent from the nervous system to the muscles of the prey. However, cobras have a unique type of three-fingered toxins that destroy cells rather than block neurotransmission. These are called cytotoxins.
Our results revealed that spitting cobras increased the abundance of a different family of toxins, called phospholipase A2 (PLA2), in their venom compared to their non-spitting counterparts. Because these cobras spit for defensive reasons, this is the first evidence of a defensive conductor of the evolution of venom in snakes.
Toxic teamwork
Many animals that use venom defensively do so by inflicting rapid and severe pain on their attackers. We investigated whether the defensive-spitting cobra venom would be particularly painful on contact.
To assess pain activity, we tested cobra venom on isolated mouse neurons, responsible for sensations in the eyes and face. We suspect that spitting cobra PLA2s could activate these neurons and cause pain.
To our surprise, the PLA2s alone were ineffective. It was the cytotoxins, the toxins prevalent in both spitting and non-spitting cobras, that caused the neurons to activate. However, the mixing of PLA2 and cytotoxins resulted in a dramatic increase in this activity.
This suggests that spitting cobras increased the abundance of PLA2 in their venom over time, to make the cytotoxins already present much more effective as an inducer of pain. The triple independent evolution of sputum was accompanied each time by the same complex and synergistic changes in the composition of the venom.
Could human ancestors have caused this evolution?
Venom-spitting is a unique behavior that is only found in a small handful of closely related snake species. Yet this projectile defense system and the specific mixture of toxins that cause more pain have evolved three times independently, only within this small group.
This type of defense must have been stimulated by very strong selective pressure. We believe that several factors make human ancestors the most likely selective agent.
Many primates will preemptively kill a snake if they feel threatened, often using projectile weapons or tools, such as stones and sticks. Although these are not always fatal, they can cause serious damage. The bipedal hominins, human ancestors who walked on two legs with the forelimbs released, almost certainly posed an even greater threat at long distances compared to their four-legged relatives. This requires long-range defense from their serpentine enemies, like spit.
The timing of the evolution of venom sputum coincides with key dates in the evolution of early human ancestors. The emergence of sputum in African cobras occurred around the same time as the separation of hominids from the chimpanzee and bonobos lineage, about 7 million years ago. The evolution of sputum in Asian cobras occurred in parallel with the arrival of Man standing in Asia about 2.5 million years ago.
Read more: Why do snakes produce venom? Not for self-defense, study finds
Additionally, spitting cobra fang fossils have been found in ancient hominid sites such as the Cradle of Humankind in Africa. The current evidence is circumstantial, which means we need more evidence. However, sputum of venom in response to trampling by animals in the herd or to the prey of birds or mammals is much less supported.
Additional fossils could support or refute our hypothesis. In particular, finding the fossilized remains of spitting cobras predating the divergence between hominins and chimpanzees would refute our hypothesis.
If the tendency of our distant ancestors to attack snakes with rocks or sticks triggered the evolution of a specific defensive adaptation in snakes, an adaptation that persists to this day, we should reflect on our own place in it. history of the Earth. Rather than being a separate lineage, our human ancestors could have had a direct impact on how these animals evolved.
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