Monday, April 18, 2011

Snake venom: Groovy, baby!

A team of six scientists from the University of Massachusetts Lowell and the Technische Universit√§t M√ľnchen (the Technical University of Munich, Germany) used biophysics to explain how snakes use grooved fangs to deposit venom in victims.

[These images show grooves (white arrows) on the fangs of (A) a banded snake (Bothryum lentiginosum) and (B) a mangrove snake (Boiga dendrophila) . In (B), the fang is embedded in the tissue of the snake's prey with only the base of the fang visible. The prey's tissue has separated from the fang, creating a venom tube (yellow arrow) with three walls defined by the fang and one wall defined by the tissue.]

Some snakes have tubing inside their fangs that distributes globules of venom in prey like a syringe. Most venomous snakes, however, along with many other reptiles, deposit venom in prey via a groove that runs down the middle of each fang.

Bruce Young, from the University of Massachusetts Lowell and his German colleagues, Florian Herzog, Paul Friedel, Sebastian Rammensee, Andreas Bausch and J. Leo van Hemmen, all of the Technical University of Munich, studied how the viscosity of the venom and the fang-prey interaction affected the venom delivery during a snake bite.

Snake venom is a non-Newtonian fluid, meaning it behaves sometimes more like a solid and sometimes more like a liquid. Examples of other non-Newtonian fluids include ketchup, oobleck - the two parts corn starch, one part water gooey concoction - and Silly Putty.

The flow of liquids is affected by liquids' interactions with surfaces. How quickly they slide against a surface is called viscosity. Water has a low viscosity - it flows quickly over substances. Honey, on the other hand, has a high viscosity. It's stickier and flows more slowly, behaving almost like a solid. Snake venom also has a high viscosity, flowing about 500 times more slowly than water. Even so, it's fast enough to flow down a fang and into a victim at a pace of about one centimeter per second (compared to water which flows as fast as 7,000 centimeters a second).

[Sequential photographs taken over a period of less than 400 milliseconds show a drop of snake venom penetrating a puncture wound in a euthanized lizard.]

As a non-Newtonian fluid, snake venom changes its viscosity. When sliding down a fang, the venom has a high viscosity, clinging to the fang as the snake prepares to bite. When a snake sinks its fangs into a victim, however, the three walls of the grooved fang are sealed by a fourth wall - the prey's tissue - forming a hollow venom tube that leads down into the prey's deep tissue. Only in that deep tissue, and not at the surface, can the venom be effective.

The increase in surface area (by the addition of a fourth wall) combined with the friction caused by the fang sliding into the tissue changes the viscosity of the venom in the groove. It becomes less viscous, flowing faster, and accelerates into the prey, usually in less than a second.

Snakes with grooved fangs hold on to and often repeatedly penetrate their prey in order to give the slower-moving venom time to set in. This is in contrast to snakes with tubular fangs that deposit venom rapidly.

The authors' research is due to appear in the APS Physical Review Letters journal.

1 comment:

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