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Spitting cobra leads to Science publication

Spitting cobra venom composition has been influenced by defensive behaviour according to an international research team. This may have originated from the arrival of human ancestors. Students at the Institute of Biology Leiden made an important contribution to the study. Publication in Science on 22 January.

Some snake species are known for their ability to spit venom. An international research team writes in Science that venom spitting in cobras originated as a defence mechanism, which influenced the composition of their venom. This has never been demonstrated before; previous studies suggest that the evolution of venom composition is driven by capturing prey. Also, the researchers show in the article that the venom composition has evolved three times in the same way in three groups of spitting cobras, independent from each other.

Roel Wouters (left) and Jory van Thiel milking common adders for research (photo: Joey Markx)
Roel Wouters (left) and Jory van Thiel milking common adders for research (photo: Joey Markx)

Spat in the eyes

Masters students and reptile enthusiasts Jory van Thiel and Roel Wouters are the Leiden co-authors of the publication, together with Professor of Evolutionary Developmental Zoology Michael Richardson. Jory and Roel conducted a research internship with Richardson at the Institute of Biology Leiden during their bachelor studies. The research idea came from Jory, who had been spit in the eyes by a spitting cobra in Thailand some years before. He turned out fine because he was able to wash away the venom immediately. It gave him the idea to investigate the difference in eye irritation between the venom from spitting and non-spitting cobras.

Extra painful venom

The students tested seventeen different types of venom from spitting cobras, non-spitting cobras and related species on the membrane of chicken embryos. This is a commonly used model to test eye irritation. The tests showed that the venom from all spitting cobras and some non-spitting cobras resulted in increased eye irritation. 

The findings turned out to be complementary to research being conducted at the Liverpool School of Tropical Medicine (LSTM) at the same time, at a research group with which Professor Richardson collaborated. The team led by Professor Casewell from LSTM tested the toxins for pain and found the venom from spitting cobras to be extra painful. Further research showed that this is caused by a higher amount of the substance PLA2 in spitting cobra venom. The PLA2 has a synergistic effect on the toxic substances already found in all cobras.

Co-evolution with human ancestors

But why did some cobras start spitting? The publication suggests that the spitting behaviour evolved during the emergence of bipedal hominins - the ancestors of humans, chimpanzees and bonobos. That happened about 7 million years ago in Africa and later also in Asia when Homo erectus arrived there about 2.5 million years ago. 

Hominins have been known to use branches and stones to kill snakes. By elevating their posture and spitting, a cobra could cover a greater distance, up to 2.5 meters, and aim better at the eyes. The extra painful venom in the eyes of the attacker gave the cobra time to get to safety.

The publication appeared 22 January in Science.

Header image: Mozambique spitting cobra Naja mossambica (by Wolfgang W├╝rster)

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