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
There were surveys conducted in morning and afternoon, in the same transect. Five encounters with groups of brown howlers (A. guariba) and seven encounters with groups of black howlers (A. caraya), have occurred to date. With this low number of encounters we are still not able to run program Distance to get reliable absolute density estimates. However, the low rate of group encounters (brown howlers: 0.3 groups * 10km-1; black howlers: 0.4 groups * 10km-1) indicate a very low density of howlers in the study area, especially when compared with other study sites. Black howlers and brown howlers have similar encounter rates.
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.
Demographic, behavioral and ecological data
At least three groups of each species have been observed and identified in the study area. After the initial survey phase, groups of both species of howlers were followed to obtain information on home-range use, activity budgets and diet. Behavioral and ecological data is being collected, using the scan sampling method. In each scan sample, activities of all individuals are recorded (resting, moving, feeding, socializing, and other).
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.