According to Robert Seyfarth, Dorothy Cheney, and Peter Marler, the semantics of animal communication is a “central but neglected issue” (Seyfarth et al. 801). Therefore, they conducted research on predator classification to provide the necessary attention to such matters. Their focus was on the systematic use of distinguishing signals which represented a distinction of objects and a sorting of objects into groups. Essentially, they wanted to see how animas “categorize” objects in their external world, specifically speaking, how free-ranging vervet monkeys categorize predators. In order to do so, they studied three groups of vervet monkeys comprised of adult males, adult females, juveniles, and infants. Previous studies showed that vervets give “acoustically different alarm calls to at least three different predators: leopards, martial eagles, and pythons” in the following manner (Seyfarth et al. 802).
Leopard: short, tonal calls produced in a series on both exhalation and inhalation
Eagle: low-pitched, staccato grunts
Snake: high-pitched, stutters
These alarm calls were distinct from nonalarm vocalizations and associated with different types of vervet monkey responses. Alarm calls produced by separate vervet monkeys were recorded previously. Through a hidden speaker, Seyfarth et al. presented each type of alarm call to the groups of monkeys and observed their reactions. (By using a speaker, they eliminated the confound of monkeys reacting due to visual cues or sightings of the predator.) Equal presentations of each alarm were played in a systematically varied fashion. Fifty trials were conducted when monkeys were on the ground, and thirty-eight trials were conducted when the monkeys were in trees.
From the table, we see that these responses were specific to each alarm presented and “seemed to represent adaptive strategies for coping with the hunting behavior of the predators involved” (Seyfarth et al. 802). When leopard alarms were played, the monkeys most often ran up into trees. In a natural setting, this would locate them in an area safe from the ambush style of attack characteristic of leopards. When eagle alarms were played, the monkeys most often looked upward, ran into dense bushes for cover, or executed both behaviors. In a natural setting, this would allow the monkeys to avoid attacks from the air. When snake alarms were played, the monkeys most often looked down towards the ground, which in a natural setting, would allow them to locate and avoid their predator (Seyfarth et al. 802). For all calls, all monkeys looked toward the speaker, the source of the alarm, and scanned their surroundings as if searching for additional cues; they responded “as though each type of alarm call designated different external objects or events” (Seyfarth et al. 802).
Therefore, Seyfarth’s et al. findings (that acoustically distinct alarms were assigned to different predators) support the notion that vervet monkeys can effectively categorize other species, particularly their predators. What’s interesting is that there is an age discrimination for this behavior. Adults were the most selective such that leopard alarms were primarily performed to leopards, eagle alarms to eagles, and snake alarms to pythons. Younger monkeys, however, did not distinguish between the different predators as well as adult monkeys did. According to Seyfarth et al., infants gave leopard alarms to a wide range of terrestrial mammals, eagle alarms to many different kinds of birds, and snake alarms to snakes or long objects on the ground, therefore failing to distinguish between particular predator species within such classes (Seyfarth et al. 802). In addition, young monkeys were more likely to give alarms to things that posed no danger. However as they matured and gained more experience they “sharpened the association between predator species and the type of alarm call” (Seyfarth et al. 803).
The findings of Seyfarth et al. are interesting because this systematic use of signals means that the animals must understand contextual communication cues. An alarm signal can elicit general behavior to remove oneself from danger. However, vervet monkeys show much more complexed behavior; they have to know how exactly to remove themselves from danger. It brings into discussion discriminative stimulus, where the specific type of alarm call is the discriminative stimulus, and the response outcome relationship varies. If the wrong response is performed based on the context, they will not effectively avoid the predator and therefore will not experience a desired outcome. However, if the alarm call is different, that same response can allow avoidance of the predator and produce a desire outcome. It all depends on the discriminative stimulus, or specific alarm call, that is produced by the vervet monkeys.
Seyfarth, R., Cheney, D., & Marler, P. (1980) Monkey responses to three different alarm calls: evidence of predator classification and semantic communication. Science, 210(14), 801-803.