Cobras are snakes of the genus Naja, which inject highly toxic venom into their prey's bodies through specialised fangs with an internal groove, which function exactly like a hypodermic syringe. Among these are the 'spitting' cobras, capable of using venom not only for predation but also for defence, spraying it from their fangs into the eyes of their attackers, up to more than 3 metres away. When the venom makes contact with the yes, the effect is devastating as it causes intense pain and, in the most severe cases, blindness.

This unique behaviour was thought to have led the spitting cobras to evolve a venom with fluid-dynamic properties better suited to being projected at a distance. Today, a new study involving researchers from Sapienza University of Rome, published in the Journal of Experimental Biology, has shown that it is the morphological characteristics of the spitting cobras' fangs, such as the external outlet of the canal being more anteriorly oriented than in non-spitting cobras, that favour the directional release of venom.

The discovery was made by a young graduate of the Department of Biology and Biotechnology "Charles Darwin" at Sapienza University, Ignazio Avella, winner of a scholarship from the Journal of Experimental Biology and the first name of the work carried out in collaboration with Riccardo Castiglia, from the same Department, and researchers from the universities of Porto (Portugal), Sheffield (England) and Bangor (Wales), and the Liverpool School of Tropical Medicine (England).

Specifically, to investigate the properties of venom flow along the fangs, the team compared the viscosity of venoms produced by 13 species of spitting and non-spitting cobras from around the world. Surprisingly, although the fangs of spitting cobras have developed to allow the venom to flow through the internal canal faster and with less pressure than in non-spitting cobras, there appears to be no significant difference in the physical properties of the venoms produced by the two types of cobras.

The researchers also came up with other unexpected results. Compared to previous studies on a smaller scale, which had assumed that the poison changed its properties depending on other factors such as speed (behaving like a pseudoplastic non-Newtonian fluid), this work showed that poisons behave like water, i.e. like a normal Newtonian fluid that does not change its properties, calling into question previous work.

Therefore, excluding the role of venom, the study results suggest that the particular morphology of the spitting cobras' fangs contributes to the aerial ejection of venom more than the physical properties of the venom itself.

References:

Unexpected lack of specialisation in the flow properties of spitting cobra venom - Avella I., Barajas-Ledesma E., Casewell N. R., Harrison R. A., Rowley P. D., Crittenden E., Wüster W., Castiglia R., Holland C., van der Meijden A. (2021). Journal of Experimental Biology. doi:10.1242/jeb.229229

Further Information

Riccardo Castiglia
Department of Biology and Biotechnology "Charles Darwin"
riccardo.castiglia@uniroma1.it