Spitting cobras track first, predict later

Most venomous snakes are legendary for their lethal bites, but not all. Some
spit defensively. Bruce Young, from the University of Massachusetts Lowell,
explains that some cobras defend themselves by spraying debilitating venom into
the eyes of an aggressor.

Getting the chance to work with spitting cobras in
South Africa, Young took the opportunity to record the venom spray tracks aimed
at his eyes. Protected by a sheet of Perspex, Young caught the trails of venom
and two things struck him: how accurately the snakes aimed and that each track
was unique.

This puzzled Young. For a start the cobra's fangs are fixed and they
can't change the size of the venom orifice, 'so basic fluid dynamics would lead
you to think that the pattern of the fluid should be fixed,' explains Young.

But
Young had also noticed that the snakes 'wiggled' their heads just before letting
fly. 'The question became how do we reconcile those two things,' says Young. 
His  study found suggested that snakes initially track their victim's
movement and then switch to predicting where the victim is going to be 200ms in
the future.

Young remembers that Guido Westhoff had also noticed the spitting cobra's
'head wiggle', so he and his research assistant, Melissa Boetig, travelled to
Horst Bleckmann's lab in the University of Bonn, Germany, to find out how
spitting cobras fine-tune their venom spray. The team had to find out how a
target provokes a cobra to spit, and Young was the man for that job, 'I just put
on the goggles and the cobras start spitting all over,' laughs Young.

Wearing a visor fitted with accelerometers to track his own head movements
while Boetig and Westhoff filmed the cobra's movements at 500 frames/s, Young
stood in front of the animals and taunted them by weaving his head about. Over a
period of 6 weeks, the team filmed over 100 spits before trying to discover why
Young was so successful at provoking the snakes.

Analysing Young's movements, only one thing stood out; 200 ms before the
snake spat, Young suddenly jerked his head. The team realised that Young's head
jerk was the spitting trigger. They reasoned that the snake must be tracking
Young's movements right up to the instant that he jerked his head and that it
took a further 200 ms for the snake to react and fire off the venom.

But Young was still moving after triggering the snake into spitting and the
snake can't steer the stream of venom, so how was the cobra able to successfully
hit Young's eyes if it was aiming at a point where the target had been 200 ms
previously? Realigning the data to the instant when Young jerked his head, the
team compared all of the snakes' head movements and noticed that the cobras were
all moving in a similar way. They accelerated their heads in the same direction
that Young's eyes were moving. 'Not only does it speed up but it predicts where
I am going to be and then it patterns its venom in that area,' explains Young.

So spitting cobras defend themselves by initially tracking an aggressor's
movements. However, at the instant that an attacker triggers the cobra into
spitting, the reptile switches to predicting where the attacker's eyes will be
200 ms in the future and aims there to be sure that it hits its target. -
Biologists