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Klaus Zuberbühler and team published his report of blue monkey vocalizations in the Royal Society’s journal Biology Letters. The paper is titled, “Male blue monkeys alarm call in response to danger experienced by others.” I don’t have access to the journal, nor am I willing to pay $50 for the privileged to read it. If anyone has access to Biology Letters and is kind enough to send me a copy of the paper, I’d be more than grateful. In lieu of the primary source, I’ll be summarizing what Lauren Cahoon, from ScienceNOW Daily News, reported on it.

Blue Monkey

Blue Monkeys (Cercopithecus mitis stuhlmanni)

Klaus and team observed and recorded the behaviors of male blue monkeys (Cercopithecus mitis stuhlmanni) which live in the Budongo Forest, Uganda. Blue monkeys make two vocalizations. Lauren describes them as,

“predator-specific calls: the “hack,” a low, gagging sound that warns about eagles, and the “pyow,” which sounds a bit like a laser gun and warns about more general dangers on the ground, such as leopards.”

The meanings behind the hack and pyow have been largely interpreted as a basic “watch out” warning call. In this new paper, Klaus and team have shown that the frequency of calls indicate the urgency of the danger. In other words, male blue monkeys yelled more frequently if members of their group were close to a crowned eagle. But, if they were further away from the danger the monkeys would lessen the number of calls. They found this out by playing the

“recordings of hacks and pyows from a loudspeaker near blue monkey troops, which are usually made up of a lead male and about 10 to 40 females and young. The recorded sounds prompted the lead male to follow up with his own alarm call, and he typically repeated the cry about 23 times. However, if a female or baby was close to the loudspeaker–the “predator”–the males gave an average of 42 cries. It didn’t matter how close the male was to the danger; he sounded the red-alert alarm only when the females and young appeared to be at risk.”

This behavior suggest that blue monkey males are concerned of any threat to their group, regardless of the impact it to his own direct well-being. Gregory Radick generally agrees but suggests that its less of a group specific reaction and more of a highten level of fear. Of course that hasn’t come without criticisms, because many don’t believe monkeys to be so selfless. Michael Owren thinks that the males are simply more emotional because they are in the proximity of females.

Two years ago, Klaus Zuberbühler and Kate Arnold reported on this behavior in male putty-nosed monkeys (Cercopithecus nictitans) , a close relative of the blue monkey. Their paper was published in Nature, under the title, “Language evolution: Semantic combinations in primate calls.” Male putty-nosed monkeys also make two distinct vocalizations. The frequency and sequence of these calls was also understood to be a danger specific response.

If you’re curious to listen to what the hacks and pyows of male putty-nosed monkeys sound like, listen to these sound bites:


A series of ‘pyow’ calls: these can function as an alarm in response to a nearby leopard, but are also used in other contexts.


A series of ‘hack’ calls: mostly functions as an alarm in response to a nearby eagle.


A ‘pyow–hack’ sequence: sometimes produced in response to eagles or leopards; normally results in significant movement by the monkey troop in a variety of contexts.

    Arnold, K., Zuberbühler, K. (2006). Language evolution: Semantic combinations in primate calls. Nature, 441(7091), 303-303. DOI: 10.1038/441303a

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