Of the nearly 4,000 species known to science, about 600 are venomous, and the vast majority of these use venom to immobilize and digest their prey. But some cobras have independently evolved the ability defend themselves by spitting super-toxic flesh-eating venom at predators. New research reveals the genetic basis of these adaptations, and the driving force behind them.
Snake venoms consist of a complex cocktail of hundreds or thousands of proteins, which together have three main effects. Neurotoxins in the venom paralyze prey by blocking the signals from nerves to muscles; hemotoxins destroy red blood cells, causing hemorrhaging; and cytotoxins damage tissue to cause swelling and blistering around the bite site. The exact composition differs widely, both between and within species, and is thought to be adapted to a specific diet: venoms from Northeast African vipers, which regularly feed on arthropods, are far more toxic to scorpions than those from West African carpet vipers, which do so only occasionally, but these in turn are more toxic than venoms from painted carpet vipers, whose diets consist primarily of small mammals and birds.
All snakes have highly specialized skulls, and their fangs represent teeth that have been modified to dispense venom. Fangs vary widely in shape and size, but all contain a hollow canal running from the venom-producing glands under the eyes to a discharge orifice near the fang tip. Rattlesnakes have long fangs that lie against the roof of the mouth, which tilt forward and downward into a biting position by means of a slide and hinge action. Cobras lack this hinge mechanism, and have shorter fangs that extend into pockets in the gum tissue of the lower jaw when their mouth is closed.
When attacking prey, some snakes use a rapid stabbing action, while others bite or chew, and then inject venom hypodermically. Spitting cobras have another adaptation that enables them to project venom outwards and upwards. In non-spitters, the venom canal follows the curvature of the fang, and the discharge orifice points downwards, but in spitting cobras, the canal meanders to a front-facing orifice, enabling them to eject twin jets of venom perpendicular to the length of the fang.
Many snakes, cobras in particular, also exhibit a behavioural adaptation called hooding, or neck flattening. This is a defensive visual display, in which the snake raises its head and flares its neck sideways when confronted, to appear bigger to its opponent. In this raised posture, a spitting cobra can project venom over distances of up to two-and-a-half meters. Whereas most snake venoms are rich in neurotoxins, those of spitting cobras are enriched in cytotoxins that break down flesh. After rising up to a threat and flaring its neck, a spitting cobra will project venom into the predator’s eyes, causing intense pain and inflammation which can lead to permanent blindness.
Taline Kazandjian of the Liverpool School of Tropical Medicine and her colleagues analyzed and compared venoms, genes and proteins in three lineages of spitting cobras and a number of non-spitters. Their results, published today in the journal Science, reveal the molecular events underlying the production of super-toxic defensive venom, and hint at their evolutionary origins.
One key finding is that the venom from each spitting cobra species has a unique composition, but all differ from those of non-spitters in one important respect. Many hooding snakes synthesize three-finger toxins as a major cytotoxic component of their venom, and are found in similar abundance i n spitting and non-spitting species. Yet, experiments showed that venoms from spitting cobras activate isolated mammalian sensory neurons much more effectively than those from non-spitters.
Genetic analyses showed that this is due to the duplication of a gene encoding phospholipase A2 toxins, which have enzymatic activity that breaks down cell membranes. Thus, spitting cobras produce larger amounts of these toxins, which potentiate the activity of other components in the venom, and enhance the pain it causes. The analyses also suggest that venom spitting emerged in African spitting cobras some time between 6.7 and 10.7 million years ago, and again in Asian species, about 4 million years later. Spitting probably evolved after the emergence of three-finger toxin-rich venoms, with the gene duplication event and hooding.
Kazandjian, T. D., et al. (2021). Convergent evolution of pain-inducing defensive venom components in spitting cobras. Science, 371: 386-90 [Abstract]
Header image: The Mozambique spitting cobra, Naja mossambica, from eSwatini, by Wolfgang Wüster.