For venom-spitting cobras, an accurate shot is the difference between slithering away and getting trampled to death by an elephant.

A new study finds that the snakes adjust the trajectory of their squirts to deliver venom right to the faces of animals that tread too closely.

"We know they spit on elephants, hyenas, just about anything that passes by that's big enough to trample on them or even eat them," says Guido Westhoff, a herpetologist at the University of Bonn, Germany, who led the study.

Instead of bringing unwieldy elephants and hyenas into his laboratory, Westhoff equipped graduate students with large plastic visors as they approached two species of spitting cobras, Naja pallida and Naja nigricollis.

Cobras can shoot venom up to 3 metres, he says, but the laboratory raised snakes didn't spit until the students were far closer than that - between 6 and 88 centimetres.

Bull's eye!

Immediately after the fake attack, the researchers recorded the distance between the cobra and its would-be victim. They also drew a rectangular box around the spitting pattern left on the visor and estimated the trajectory of the venom using trigonometry.

Across 178 spitting patterns dished out by 11 adult and juvenile cobras, Westhoff's team found that cobras reduce the vertical angles of their venomous spits and widen the spread of the spray as the student's distance from them increases. This keeps the target of the spit relatively constant in size and roughly equal to the dimensions of a human head, Westhoff says.

That's pretty good accuracy - but not perfect. Westhoff's lab previously found that cobras aim their spits toward the eyes of an intruder, which are most sensitive to a tissue-destroying enzyme in cobra venom. His team is now working to determine whether cobras use geometric, reflective or other properties of the eye to target their spits.

Journal reference: Journal of Comparative Physiology A (DOI: 10.1007/s00359-0451)