Alarm calls are pretty useful in the animal world. Not only do they help the group (which often includes the alarm caller's relatives), but they also help the individual making the call, by offering a more privileged position in the group. In the case of putty-nosed monkeys (Cercopithecus nictitans, a type of guenon monkey), males but not females possess loud alarms that serve both as a warning and as predator deterrents.
Researchers with the Wildlife Conservation Society's (WCS) Congo Program and the Nouabalé-Ndoki Foundation analyzed how these monkeys use these calls not just when reacting to predators, but also to interact with one another. The results suggest that females produce alarm calls mostly to call the males and assemble a defense group. When females assess the nature of the threat and issue anti-predator calls, the females cease their alarm. In other words, the females recruit the males as hired guards.
Males, for their part, advertise the commitment by producing "pyow" calls, which serve as a signature for individual males. Men whose anti-predator calls are more efficient develop a strong reputation as good defenders on their "signature" calls, while those with less reputation are forced to leave the group earlier. Lead author Frederic Gnepa Mehon of WCS's Congo Program and the Nouabalé-Ndoki Foundation explains:
"Our observations on other forest guenons suggest that if males do not prove to be good group protectors, they likely have to leave groups earlier than good defenders. To date, it remains unclear whether female guenons have a saying in mate choice, but our current results strongly suggest this possibility."
It's a cultural thing
The researchers also observed another call among males: a "kek." Whenever the researchers would present a leopard model, the males would use the "kek" -- which is not surprising in itself, but this type of sound has never been reported in previous studies. This suggests the existence of dialects and cultural variations across different populations, a matter fiercely debated in animal research.
Ultimately, it may all boil down to sexual selection, says co-author Claudia Stephan Wildlife Conservation Society's (WCS) Congo Program and the Nouabalé-Ndoki Foundation:
"Sexual selection might play a far more important role in the evolution of communication systems than previously thought. In a phylogenetic context, what strategies ultimately drove the evolution of communication in females and in males? Might there even be any parallels to female and male monkeys' different communication strategies in human language?"
The way females manipulate males and convince them to offer protection is remarkable -- but as remarkable as these interactions are, researchers suspect there's still far more we've yet to discover. Males have more complex vocal repertoires and could convey even more information with them.
The study has been published in Royal Society Open Science.