How did the venom of spitting cobras evolve?

A team of CSIC researchers has published the first evidence that the defensive behavior of ‘Naja’ spitting cobras influenced the composition of their venom.

Snakes don’t just use their venom to hunt down prey. Some types of cobra use it as part of their defensive strategy, spitting it from a distance of up to 2 meters with great precision towards the eyes of their potential predator.

An international research group in which the Valencia Institute of Biomedicine (IBV), a center of the Higher Council for Scientific Research (CSIC), participates, publishes in Science the first evidence that the evolution of this behavior has decisively influenced the composition of the venom of spitting cobras of the genus najawhich inhabit Africa and Asia.

The study reveals a greater production in the poison of these species of phospholipases, a type of toxin that can cause serious damage to the eye and even blindness.

The work, led by the University of Liverpool (United Kingdom) and published on the cover of the magazine Scienceis based on analyzes carried out at the IBV’s Evolutionary and Translational Venomics Laboratory in Valencia, a unique laboratory in Spain directed by CSIC researcher Juan José Calvete.

Using complementary mass spectrometry strategies, the teams of Calvete and Daniel Petras (University of California at San Diego, USA) have participated in determining the composition of the venom of three lineages of cobras that independently developed their ability to spit it out . They are snakes of the genus naja living in Africa and Asia.

spitting cobras

The analyzes showed that the three different groups of spitting cobras had independently increased the production of PLA2 toxins or phospholipases, a type of enzyme present in the venom of snakes, spiders and other insects, and which has toxic effects in mammals.

“We observed that spitting cobras have a higher abundance of phospholipases in their venom, which, along with other cytotoxins common to all cobras, enhance the defensive capabilities of their venom.”explains Juan José Calvete.

convergent evolution

The work highlights that the evolution of the most painful venom allows this type of cobra to defend itself more effectively from predators or aggressors by spitting venom into the eyes, which causes pain, inflammation and even blindness.

“This reveals that the composition of the venom of spitting cobras has been modified to favor the defensive function”Calvete points out. That each independent lineage has developed the same solution to defend itself represents an exemplary case of “convergent evolution” in the natural world, the researchers note.

The research team links the ability to spit venom to two important preadaptations. First, cobras can raise the front third of their body, giving them an ideal posture for defensively spitting venom into their eyes with great precision.

Second, cytotoxins already existed in cobra venom before the ability to spit evolved; these toxins probably caused a low level of pain that was the precursor to the highly painful poison found today.

‘Arms race’ between snakes and hominids

The research team also wonders about the origin of this defensive strategy in snakes, finding it related to the evolution of our ancestors. They hypothesize that larger-brained bipedal hominins likely posed a major threat to snakes, exerting selective pressure to favor such defensive behaviors.

Thus, the study shows that the origin of the venom spit by snakes was first in Africa and later in Asia, which would correspond to the divergence of our ancestors from chimpanzees and bonobos in Africa and their subsequent migration to Asia. However, more data is required to strongly test this hypothesis, the researchers acknowledge.

“It would be like an arms race, where snakes developed an ability to spit out increasingly toxic venom, and humans a more developed eye to detect these dangers.”Calvete summarizes.

“Snake venom evolves very quickly. There are no two species of snakes that have the same venom, so there is no universal antidote”reflects the CSIC researcher, who won the highest award in 2019 from the International Society of Toxinology (IST), a scientific organization that studies the poisons and toxins of all kinds of poisonous organisms.

Spitting cobras: developing better remedies against this type of poison

Thus, one of the main consequences of this study will be to develop better remedies against this type of poison, whose potential victims are farmers and children in rural areas of Africa and Asia.

According to the World Health Organization, between 1.8 and 2.7 million cases of poisoning caused by snakes occur each year, mainly in Africa, Asia and Latin America. This represents between 81,410 and 137,880 deaths, and approximately triple the number of amputations and other permanent disabilities, condemning the victims of these accidents and their families to social marginalization and a future of poverty.

Font: Isidoro García / CSIC Communication Valencian Community

Reference article: https://www.csic.es/es/actualidad-del-csic/el-veneno-de-las-cobras-escupidoras-evolved-towards-a-defensive-function