Cobras are fascinating and terrifying creatures. These snakes are best known for their distinctive defense mechanism called hood, when the sides of their neck flare up in a dramatic display.
However, hooding is not the only defensive behavior in a cobra’s arsenal. Some species of cobra have modified tusks with small, openings on the front. This allows them to forcibly spray poison as a syringe or a spit, which can strike up to 2.5 meters away in the eyes of a target. For this behavior, they are known as spitting cobras.
Oddly enough, this unique adaptation developed three times independently in a small group of Afro-Asian snakes: once in African cobras, once in Asian cobras, and once in the related ring-neck, also known as the ring-necked spitting cobra.
While most snakes use venom to prey on other animals, spitting cobras use it merely for defense. In a new study, we tested the venom of spitting cobras to see what toxins could be found, to find out what this defensive behavior might have developed. The results show that it was possible to repel attacks from our human ancestors.
Wolfgang Wüster, Author provided
Unique Toxin Cocktails
Snake venom is complex mixtures of proteins, which are mainly used in feed to make prey efficiently unfit. Although snakes use their venom in self-defense, for example in the case of human snake bites, most evidence suggests that venom composition was developed to feed, not defense.
Poison in solid snakes with solid fangs, including cobras, tends to cause paralysis. This is due to an abundance of toxins, neurotoxic three-finger toxins, which stop neurotransmission, and signals sent from the nervous system to the prey’s muscles. However, cobras have a unique type of three-fingered toxin that destroys cells rather than blocking neurotransmission. These are called cytotoxins.
According to our results, spitting cobras increased the occurrence of another toxin family, called phospholipase A2 (PLA2s), in their venom compared to their non-spitting counterparts. Because these cobras spit for defensive reasons, it is the first evidence of a defensive driver of venom evolution in snakes.
Toxic teamwork
Many animals that use poison use it defensively by inflicting rapid, severe pain on their attackers. We determined to determine if defensive spitting cobra venom would be particularly painful during contact.
To determine pain-causing activity, we tested cobra venom on isolated mouse neurons responsible for sensations in the eyes and face. We suspect that spitting cobra PLA2s may activate these neurons and possibly cause pain.
To our surprise, PLA2s were ineffective on their own. It is the cytotoxins, the toxins commonly found in spitting and non-spitting cobras, that have caused the activation of the neurons. However, the mixture of PLA2s and cytotoxins caused this activity to increase dramatically.
This suggests that spitting cobras have increased the amount of PLA2s in their venom over time, making the already present cytotoxins much more effective as a pain inducer. The triple independent evolution of saliva was accompanied on each occasion by the same complex, synergistic changes in toxin composition.
Could human ancestors have spurred this evolution?
Poison saliva is a unique behavior that occurs only 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 developed three times independently, just within this small group.
This type of defense had to be stimulated by a very strong selective pressure. We believe that several factors human ancestors are the most likely selective means.
Many primates will kill a snake if they feel threatened, and often use projectile weapons or tools, such as rocks and sticks. While it may not always be fatal, it can cause serious damage. Bipedal hominins, human ancestors that walked on two legs with freed forelegs, almost certainly posed an even greater long-distance threat compared to their four-legged relatives. It requires a long-range defense from their snake-like enemies, such as spitting.
The timing of the evolution of poison spit coincides with key dates in the evolution of early human ancestors. The emergence of spit in African cobras occurred at about the same time as the separation of hominins from chimpanzees and bonobos, about 7 million years ago. The evolution of spitting in Asian cobras took place along with the advent of Homo erectus about 2.5 million years ago in Asia.
Read more: Why do snakes produce poison? Not for self-defense, study shows
In addition, fossils of spitting cobra tusks have been found in ancient hominin sites such as the cradle of mankind in Africa. Current evidence is controversial, which means we need more evidence. However, poison spitting as a response to trampling by herd animals or by preying on birds or mammals is much less supported.
Additional fossils can support or refute our hypothesis. In particular, we would refute hypothesis to find the fossilized remains of spitting cobras that preceded the divergence between hominins and chimpanzees.
If the tendency of our distant ancestors to attack snakes with stones or sticks does cause the development of a specific defensive adaptation in snakes, which continues to this day, we need to think about our own place in Earth history. Instead of being a lineage, our human ancestors might have had a direct influence on the development of these animals.