Written by: R. Reeder
One of our tasks this field season was to try and define the vocalizations from the diverse vocal repertoire of woolly monkeys. After six months of data collection we have been able to name and define thirteen distinct vocalizations. The majority of these vocalizations are some sort of variation of two main types: a chirp and a trill. A chirp is a single, short vocalization, whereas a trill is a longer vocalization that has more vibrato. These two main calls can vary in length, strength (loud, quiet, or regular), or number of times per unit time. Each variation has a different meaning. Stevenson (1997) notes that the vocalizations of woolly monkeys at Tinigua National Park in Colombia can be grouped in to three general types: contact, alarm, and social interactions. The chirps and trills that I described earlier are contact calls. A woolly usually produces them when they are trying to say, “I’m over here” or “where are you?” The shorter length of a chirp restricts this vocalization to closer distances, whereas a trill is more often used when, for example, the group is spread apart over several hundred meters and they are trying to coordinate their movements.
Variations of chirps and trills are by far the vocalizations we hear most often, and their diversity may best be exemplified during intergroup encounters. An intergroup encounter is often a chaotic affair involving many individuals running around in the canopy, often chasing and displaying at one another. As an observer, the most reliable way to first detect an intergroup encounter is usually not by the presence of new individuals, but by the vocalizations emitted from the group. Oftentimes just as an intergroup encounter begins, there will be a sudden increase in the number of chirps emitted by the group, and they become higher in pitch. These vocalizations sound excited and anxious, just as the group probably is as they discover the presence of strangers in their group. These excited chirps reach a climax when males from the two groups start chasing each other, during which screams usually ensue. These encounters don’t usually last very long, and end as the groups calm down and go their separate ways. However, this can also be chaotic if the groups have mixed and individuals are trying to find their own groups again so they can leave. In order to find members of their group during this time individuals often employ the trill, which carries for longer distances and so is a more efficient method of finding family and friends.
Woollies also emit vocalizations that we describe as barks, and they are usually produced in the presence of a predator. The single and repeated bark are the two types of alarm calls we most often hear. Stevenson (1997) found that “Terrestrial predator alarms (barking) were characterized by a higher number of vocalizations per unit time”. I have most often observed the woollies emit an alarm call in the presence of an aerial predator, when, for example, a hawk or an eagle flies above them. In this case an individual will emit a high-pitched bark, which usually causes other individuals to repeat this vocalization. The barks help to alert the group of potential predators, and subsequently direct their next movements to safety if necessary.
Play vocalizations are probably some of the strangest sounds woollies emit, and hence the hardest to describe. These sounds can be heard when, for example, a juvenile male approaches an adult male, hangs from his tail, and starts to harass the adult male by grabbing his face. This usually provokes the adult male enough that he will start to wrestle with the juvenile, and thus start a play session. Either one, or both of them, will then be making a very guttural, yet quiet, “ach”-ing sound. I would imagine it is the human equivalent of laughing, or something similar to that.
While I’ve mostly described how their vocalizations are influenced by social factors, they are also influenced by ecological factors. The foraging habits of the woollies play a large role in how they vocalize; their vocalizations change in almost perfect harmony with the seasonal variations of fruiting trees. During times of high fruit productivity in the forest, we would hear more vocalizations in general. We would also hear a variation of the chirp that we called the “happy chirp”, which has a more cooing sound to it, and is often heard when the woollies found a tree with fruit they really liked, and would subsequently spend many minutes foraging in it. During time of low fruit productivity, the woollies dedicated more time searching and foraging for insects. This foraging strategy was accompanied by far fewer vocalizations. The woollies would be as quiet in their movements and vocalizations as possible to better find and sneak up on their prey.
In the midst of the symphony of sounds these woollies produce, is it at all possible to distinguish one from another, or even associate it with a particular individual? Stevenson (1997) noted that it might be possible. He was able to “…distinguish the contact vocalizations emitted by the biggest and oldest male in the group from the sounds of other males…”. Furthermore, his results “…suggest the possibility that the great majority of calls involve specific individuals to maintain spacing between them”, which would mean that the individuals would need to be able to differentiate calls in order to do that. The topic continues to be debated, and requires much more research to be solved.
Stevenson P. 1997. Vocal Behavior of Woolly Monkeys (Lagothrix lagothrichai) at Tinigua National Park, Colombia. Field Studies of Fauna and Flora at La Macarena, Colombia 10: 17-28.