By Ibrahim Sawal

When faced with a predator, female putty-nosed monkeys will call males to help protect them from the threat.

Putty-nosed monkeys (Cercopithecus nictitans) live in the forests of West Africa in groups of one male with multiple females and their offspring. The male will tend to roam further from the group and leave females to forage for themselves, but the females and lone male will alert each other when predators are nearby.

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Communication in this species differs based on sex. Females produce a single “chirp” to alert others when any form of predator is nearby, while the lone males will use different calls based on the type of predator spotted: “pyow” calls for those on the ground, like leopards, and “hack” calls for predatory eagles.

Claudia Stephan at the Wildlife Conservation Society, Republic of the Congo, wanted to see how female and male putty-nosed monkeys differ in their response to these calls during a predatory event when the male is roaming relatively far from the group.

With her colleague Frederic Gnepa Mehon, also at the Wildlife Conservation Society, Stephan located 19 different groups of monkeys in Nouabalé-Ndoki National Park. For each group, the two researchers and their colleagues waited until the lone male was around 20 metres from the group. One volunteer, covered in a leopard print fabric to mimic a predator, then approached either the lone male or the group of females.

If the “leopard” approached the lone male, he responded by making a “kek” call. Stephan says this call hadn’t been recorded during research on male putty-nosed monkeys in other regions, so could be a local dialect. But Stephan points out that earlier studies into alarm calls involved stationary leopard models, rather than a moving leopard model. “It could also be that moving danger on the ground elicit ‘kek’ calls, and any danger on the ground is ‘pyow’ calls,” she says.

The lone male then started to show typical mobbing behaviours, like tree shaking, to deter the “predator”. The females could hear this display, but didn’t respond and continued foraging as normal.

When the “predator” approached females first, the female monkeys emitted “chirp” calls. Upon hearing this, the lone male began “pyow” calling and approached the group. Females continued chirping until the lone male spotted the “predator” and started “kek” calling. At this point, females would take their offspring to safety.

Stephan suggests that this means female chirps may have evolved as a call to arms for males and not as an alarm call to predators. By giving as little information as possible about the threat, she says female calls are more persuasive to males. “They are obliging their males to come over,” says Stephan. “It’s like us calling for help. If you hear someone screaming for help, you come running even if you don’t know the threat.”

Journal reference: Royal Society Open Science, DOI: 10.1098/rsos.202135