3D Flow in the Venom Channel of a Spitting Cobra: Do the Ridges in the Fangs Act as Fluid Guide Vanes?

Abstract

The spitting cobra Naja pallida can eject its venom towards an offender from a distance of up to two meters. The aim of this study was to understand the mechanisms responsible for the relatively large distance covered by the venom jet although the venom channel is only of micro-scale. Therefore, we analysed factors that influence secondary flow and pressure drop in the venom channel, which include the physical-chemical properties of venom liquid and the morphology of the venom channel. The cobra venom showed shear-reducing properties and the venom channel had paired ridges that span from the last third of the channel to its distal end, terminating laterally and in close proximity to the discharge orifice. To analyze the functional significance of these ridges we generated a numerical and an experimental model of the venom channel. Computational fluid dynamics (CFD) and Particle-Image Velocimetry (PIV) revealed that the paired interior ridges shape the flow structure upstream of the sharp 90° bend at the distal end. The occurrence of secondary flow structures resembling Dean-type vortical structures in the venom channel can be observed, which induce additional pressure loss. Comparing a venom channel featuring ridges with an identical channel featuring no ridges, one can observe a reduction of pressure loss of about 30%. Therefore it is concluded that the function of the ridges is similar to guide vanes used by engineers to reduce pressure loss in curved flow channels.

How the Cobra Got Its Flesh-Eating Venom: Cytotoxicity as a Defensive Innovation and Its Co-Evolution with Hooding, Aposematic Marking, and Spitting

“…Most prior research has focussed on the spitting behavior itself, treating it as a peculiar oddity in nature without investigating associated evolutionary trends (e.g., venom composition) [16], and it has been described at the morphological [17,18], mechanical [12,19,20] and behavioural levels [13,21]. Hooding is a distinctive characteristic of the genera Hemachatus , Naja and Ophiophagus and members of these genera are found in Africa and Asia [22].…”

X-ray microtomography in herpetological research: a review

“…Vertebrate biomechanics often rely on finiteelement analysis or modelling to reconstruct stress, strain and deformation, explore fluid dynamics and identify patterns of heat transfer (reviewed in Rayfield, 2007). Examples hereof include the mechanical strength of turtle shell shapes (Rivera and Stayton, 2011;Stayton, 2011), venom flow through snake fang canal (Triep et al, 2013) and the thermoregulatory capacity of lizard and crocodilian osteoderms (Broeckhoven et al, 2017a; Clarac et al, 2017). Scans obtained through x-ray microtomography can be readily converted to (meshbased) models and provide a significant advantage over more traditional methods, such as three-dimensional laser surface scanning, because internal voids and spaces can be easily incorporated.…”

Hunting in archerfish – an ecological perspective on a remarkable combination of skills

“…Various forms of spitting slime, glue and venom in various patterns and at different distances are found in many taxa, and many of these abilities might be far more adjustable than initially thought. Spitting cobras use the same mechanism they use for injecting prey to spit venom at the face or eyes of a predator that is over 2 m away, can adapt the area over which they distribute their jets and can even track moving targets (Young et al, 2004;Westhoff et al, 2010;Berthé et al, 2013;Triep et al, 2013). Velvet worms (Onychophora) can immobilize prey by squirting a jet of slime at it.…”