It suggests that this is more than just familiarity, that they're keeping track of aspects of the quality of these social relationships." Using a non-invasive eye-tracking device, the team measured where the apes looked and for how long, speculating they'd look longer at apes they recognized. The apes looked significantly longer at former groupmates, no matter how long they'd been apart. The work also raises the questions of whether the apes are missing individuals they're no longer with, especially their friends and family. The team would next like to explore whether these long-lasting social memories are special to great apes or something experienced by other primates.