Chimpanzees moving through the forest take into account other chimps' ignorance or knowledge of a threat when they raise the alarm.The apes were more likely to make warning calls when they spotted a venomous snake if others in their troop had not seen the danger, researchers found. As chimps in the know arrived at the scene, they passed the warning on to others who lagged behind but were still within earshot. Video footage of wild chimps foraging in Uganda's Budongo forest show apes at the front of their groups jumping with surprise on spotting a model snake lying camouflaged in the undergrowth.