Proboscis monkeys - Alpha male defends his harem females against rival males

Proboscis Monkeys from the Labuk Bay Proboscis Sanctuary, Borneo are defending their harem group.
Proboscis monkeys generally live in groups composed of one adult male, some adult females and their offspring. Groups gather during the day and travel together, but individuals only groom and play with those in their own group. One-male groups consist of 9–19 individuals, while bands can consist of as many as 60 individuals. Serious aggression is uncommon among the monkeys but minor aggression does occur. Overall, members of the same bands are fairly tolerant of each other. A linear dominance hierarchy exists between females. Males of one-male groups can stay in their groups for six to eight years. Replacements in the resident males appear to occur without serious aggression.

Proboscis monkeys are known as fat horn messes. When communicating the status of group, males will emit honks. They have a special honk emitted towards infants, which is also used for reassurance. Males will also produce alarm calls to signal danger. Both sexes give threat calls, but each are different. In addition, females and immature individuals will emit so-called "female calls" when angry. Honks, roars and snarls are made during low-intensity agonistic encounters. Nonvocal displays include leaping-branch shaking, bare-teeth open mouth threats and erection in males, made in the same situations.