Stephan, C. and Zuberbuhler, K. (2008). Predation increases acoustic complexity in primate alarm calls. Biology Letters. 4: 641-644.

The purpose of this research article is to address potential differences in acoustic complexity in alarm calls made by primates in response to predators. Basically they wanted to know if there was a relationship between ontogenetic predator experiences and the alarm calling behavior of West African Diane monkeys (Cercopithecus diana). Males from two different populations of West African Diane monkeys were observed in order to make comparisons between their vocal behaviors. One population was from the Tai forest while the other was from the Tiwai forest. In the Tai forest the monkeys frequently encounter both leopards and crowned eagles while in the Tiwai forest the monkeys only encounter crowned eagles, leopards have not been seen there for at least 30 years.

In terms of the experimental setup, 15 second recordings of crowned eagles and leopards were played at each site in order to observe the alarm call responses from the male West African Diane monkeys. The vocal responses of the males to the predator vocalizations were recorded. The researchers measured number of calls of each type, number of call series, and number of calls of each type per series.

The results showed that the male monkeys produced similar alarm calls types at each research site. In response to leopard growls, male monkeys produced alarm calls with strong frequency transitions while in response to eagle shrieks, male monkeys produced alarm calls with weak frequency transitions. There were no differences in the total number of calls produced by male monkeys at each site. The major difference that was observed involved how the males organized their calls into various sequences. In the Tiwai forest, males produced more calls per call series in response to leopard growls than males from the Tai forest. There were no differences in the organization of call types into sequences for males from both forests in response to eagle shrieks. In the Tai forest male monkeys were able to differentiate acoustically between their responses to leopards and general disturbances while the male monkeys from the Tiwai forest produced call sequences that were not different acoustically in response to leopards and general disturbances.

Thus the presence of predators seems to influence the alarm calls produced by West African Diane monkeys. It is believed that since males from the Tai forest have grown up in the presence of leopards that they are able to produce different sounds in terms of acoustics when they hear a leopard growl versus when they hear a general disturbance. However the males from the Tiwai forest have not grown up in the presence of leopards thus their alarm calls did not differ acoustically when hearing a leopard growl versus hearing a general disturbance. Basically increased threats of leopard predation on the monkeys of the Tai forest have led the production of alarm calls with greater acoustical complexity.

In terms of the relevance of this article to what we have been studying in lecture, in a previous lecture we discussed the importance of alarm calls produced by various animal species. We addressed the functions of alarm calls which are to deter predators from coming any closer and to warn members of the same species about the upcoming danger. We also discussed the different types of predator alarm calls which include predator inspection and mobbing signals and distress signals. I am assuming that the monkeys in this article were producing both types of predator alarm calls.