On a June day in 2015, primatologist Aaron Sandel was quietly observing a small cluster of the Ngogo chimpanzee group in Uganda’s Kibale national park when he noticed something strange. As other members of the chimpanzees’ wider group moved closer through the forest, the chimpanzees in front of him began to display nervous behaviour. They grimaced and touched each other for reassurance, acting more like they were about to meet strangers than close companions.
In hindsight, Sandel said, that moment was the first sign of what would become a years-long bloody conflict between a once close-knit group of chimps.
