|Mario Di Bitetti, Ph.D.|
|A Comparative Study of Two Howler Monkey Species Living in Sympatry (Alouatta guariba and Alouatta caraya) in El Piñalito Provincial Park, Argentina|
|Location: El Piñalito Provincial Park, Argentina|
|Abstract: Comparative studies of closely related taxa living in sympatry have illustrated how interspecific competition has shaped species differences. Howler monkeys (Alouatta) are folivorous-frugivorous Neotropical primates. The six species recognized have almost no overlap in their geographic distributions. The black howler (Alouatta caraya) and the brown howler (Alouatta guariba) show a small area of sympatry in the Atlantic Forest of Argentina. This brings a unique opportunity for understanding the degree to which the species are competing for resources and how competition may have shaped species differences. Our study is aimed at 1) evaluating the population status of the nearly threatened brown howler in one of the few areas of Argetina where is present; and 2) carrying out the first comparative study on the ecology and social organization of two howlers living in sympatry. The study area is El Piñalito Privincial Park, a strictly protected area in Misiones, Argentina. We will conduct line-transect surveys on an important portion of the park to obtain data on the density, composition and distribution of groups of the two species. We will determine habitat use and and the distribution of each species within El Piñalito by building a GIS map of the park. We will select two groups of each species to conduct a comparative study on the ecology and social behavior of these species. This will be the first comparative study of two howler species living in sympatry and the first one conducted in Argentina on the threatened brown howler.|
In January, field work began in El Piñalito Provincial Park. Between January and December 2005, 141 days of field work was completed. The base camp was established in a small cabin that was used as a field station.
The first two months of the project were spent getting familiar with the area and establishing line-transects for the surveys. With assistance of the field team, the park staff opened an old road to access some remote areas of the park. Each transect was measured and marked in 50m intervals.
Howler monkey surveys
The first reaction of un-habituated howlers when surveyors are detected is to stay quiet. This cryptic behavior makes the detection of howler groups difficult. The low rate of group encounters could be the result of both a very low population density for both species as well as the difficulty to detect these cryptic monkeys during transects walking.
Despite the higher sampling effort in areas of native forest, most observations of howler groups (both species combined) took place in the NW portion of the park, in areas with pine plantations. The higher encounter rate with groups of howler monkeys in areas with pine plantations could result from: 1) the better visibility that characterizes pine plantations, or 2) a real preference of howlers for this type of forest.
and ecological data
During the first two months of data gathering the monkey groups were not habituated to the observers and it was difficult to keep contact and follow the monkeys. The monkeys tried to escape and were frequently lost by the observers. Two groups of black howlers and two groups of brown howlers have been habituated. Another group of each species has been identified in the study area and all individuals in each group have been counted.
Although a detailed analysis of the sample scan data has not been completed, groups of both species of howlers spent most of their time in the pine plantations, resting and eating leaves of several species of vines that climb the pines. Most of the feeding was spent eating vines of several species growing on pines. This could be the result of a preference for these food items or a preference for this type of habitat. A larger number of black howlers have been recorded than brown howlers. However, the results are still preliminary and the larger number of food items eaten by the black howlers may be the result of the larger number of contact hours spent with this species.
In addition to the scan sample protocol an all-occurrence sampling technique to record any type of social interactions (aggression, sex, grooming, play, vocal contests, etc.) that may occur among individuals within groups and between groups. Particular importance is given to interactions among groups that could reveal the relative importance of within- vs between-species aggressive competition.
We have observed 2 intra- and 4 inter-specific group encounters. Most of these encounters were not openly aggressive and usually involve the slow movement of one group leaving the area, in a clear avoidance behavior.
We observed a lot of overlap in home-range use between a group of black howlers (Orixas) and a group of brown howlers (Gitanos). These two groups also use similar routes to move through the canopy layer. There seems to be less home range overlap between groups of the same species (black howlers: Orixas and Reviro ; brown howlers : Gitanos and Piratas). This suggests that howlers at our study site are not strictly territorial. However, these results are preliminary and long-term data may provide a different picture of home-range use.