The researchers came to the conclusion after designing a series of experiments that involved observing wild chimps in the Budongo Forest of Uganda. The scientists hid a realistic fake snake — a model of a venomous gaboon viper — in the forest and watched how the chimps reacted.
"My observations from wild chimpanzees show that while unexpectedly encountering a snake or snake model may trigger a startle response, they often then investigate the snake, but do not necessarily show signs of fear," Crockford said. "This is different in the case of pythons, which are the only snakes likely to prey on chimpanzees."
In the first experiment, the researchers recorded how wild chimps used their bodies to indicate the location of the snake to other chimps. This behavior is known as marking. Crockford said marking involves repositioning oneself to have direct visual access to an object or individual — in this case, the seemingly real snake — and the receiver. Eye movement can then "tell" the receiver where something, or someone, is.
In one third of the cases, the chimps marked the location of what they thought was a real viper, alternating their gaze between other chimps and the snake.
In the second experiment, the scientists evaluated two different scenarios involving pre-recorded calls from other chimps. In one scenario, a hidden speaker played a call that suggested a nearby chimp knew about the snake. In the second, the speaker played a chimpanzee call indicating ignorance.
When the actual chimps came across the snake, those that had heard the call suggesting ignorance from a troop member gave more warnings, through both vocal “alert hoos” and body language.
"Alert hoos are produced to a specific set of threats, such as snakes, snares, and leopard scat," Crockford said. "They can perhaps be classified as 'hidden threats.' Other threats elicit other calls, such as bush pigs, leopards and, out-group chimpanzees."
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Not only do the complex calls of chimps contain specific information about what they are observing, but their visual cues can add additional information. The primates take into account what the recipient knows, and adjust their effort accordingly.
The researchers eliminated the possibility that the marking behavior had some other significance, such as the "marker" chimp trying to self-protect by keeping an eye on both the threat and the troop mate.
"One reason we can rule out that the marking behavior is partially about self-protection is that while signalers are marking the snake, apparently waiting for the receiver to approach and see the snake, the signalers often lose interest in the snake and look elsewhere," Crockford said. "When the receiver finally shows interest in trying to see the snake, then the signalers' attention refocuses on the snake."