Giant otters carry a pale cream to white marking across the throat and upper chest. Its outline is irregular and, in this species, individually distinctive, in the same way a whale's fluke edge or a zebra's flank pattern is. Because otters lift their heads clear of the water when they periscope to look around, the patch is presented to an observer at exactly the moment it can be photographed.
Field projects have used this to build long-running individual histories: who belongs to which group, how long animals live, which lakes they move between. The method needs no capture, no anaesthesia and no telemetry collar, which matters for an endangered species where handling carries real risk.
The otters themselves are highly social. Groups typically number from around three to eight and can be larger, sharing dens, hunting cooperatively and raising a single breeding pair's pups collectively. They are also strikingly noisy, with a vocal repertoire that researchers have divided into numerous distinct call types; the precise number and the meaning attached to each is still debated and depends heavily on how calls are classified.