To escape danger, a Proboscis may dive into water from up to 16m and stay underwater for 30 seconds. The whole troop may dive into the water when threatened. They are rarely more than 600m away from the river. The groups appear to have little structure. There are suggestions that these large groups comprise smaller subgroups of a male with his harem of 2-7 females and their offspring.
Females freely change membership among the harems, and males are kicked once they reach adolescence.
These harems feed together and gather together for the night and roost together, resulting in the large groups often sighted. To keep other males off his females, the dominant male will make loud spectacular leaping displays which seldom result in actual contact. In fact, while the group feeds, one of the larger males will keep a lookout at the top of a tall tree. But it is the females that lead the group in foraging for food. The females sounds more like a goose.