Introduction

The genus Alouatta occurs from the south of Mexico to northern Argentina and from the South American Pacific Coast to the Brazilian Atlantic Forest (Defler, 2003). A. caraya is the southernmost species, inhabiting central and southern Brazil, eastern Bolivia, Paraguay, and northern Argentina (Thorington et al., 1984). Although there are adequate demographic data from populations in the forests of the Río Paraná in Argentina (Pope, 1968; Thorington et al., 1984; Rumiz, 1990; Arditi and Placci, 1994; Brown and Zunino, 1994; Zunino et al., 1996, 2001), the data are comparatively limited for populations in the Argentinean Chaco (Arditi and Placci, 1990; Brown and Zunino, 1994; Dvoskin et al., 2004).

In this preliminary study we describe the composition and structure of wild troops of A. caraya that inhabit the gallery forests along the Riacho Pilagá in Formosa Province and compare them with data obtained from this area two decades ago.

Methods

The A. caraya population we studied occupies forests on the Guaycolec Ranch (58°13′W, 25°54′s), a spread of 25,000 ha, 25 km northeast of the city of Formosa. The ranch covers a mosaic of grasslands, savannas, dry forest, and semideciduous gallery forests. Our study area consisted of approximately 300 ha of gallery forest along the banks of the Riacho Pilagá. We mapped the area using a transect grid, marked every 50 m with colored tape.

In July and August 2001, two people surveyed the study area for black howler troops from 07:30 h to 17:00 h working together for 11 days and independently for 13 days, making a total of 24 days. Troops were found by following the loud, characteristic vocalizations, or by spotting resting individuals or moving branches. All visible individuals in each troop were classified into age and sex categories as suggested by Rumiz (1990):

Results

We made contact with troops on 19 occasions, for a total observation time of 24.5 h; each troop was observed for an average of 1.1 h. The two observers encountered monkeys on nine occasions while surveying together and on ten occasions while working independently.

We identified 111 individuals in thirteen mixed-gender reproductive troops (Table 1). Two additional individuals were found ranging alone, but we found no temporary associations of males or females in the area. The reproductive troops ranged in size from 5 to 15 individuals (mean 8.5, SD ± 3.4). The study population consisted of mixedgender troops with one or several adult males. Uni-male troops were more common (n = 10) than multi-male troops (n = 3). Uni-male troops were also smaller than multi-male troops (mean of 7.5 ± 2.7 vs. 12.0 ± 3.6 individuals). There were a relatively large number of infants (n = 22) compared to adult females (n = 28).

Table 1.

Size and composition of troops of Alouatta caraya in Formosa Province, Argentina.

Approximately 40% of the individuals were adults (Table 1), while juveniles — the second largest age-class — made up slightly less than a third of the population (31%). Subadults (7%) and infants (38%) comprised the rest. Overall, the numbers of females and males in the population were very similar (48 and 45, Table 1). In the adult class, the sex ratio was strongly biased in favor of females (28 and 17). The numbers of female and male juveniles were very similar for all three juvenile categories as well as for all juvenile categories combined (18 and 17). The small sample sizes of subadults and infants prevented any statistical analysis.

Discussion

The average group size of A. caraya has apparently increased during the last two decades: Brown and Zunino (1994) estimated a mean group size of 6.3 individuals during censuses conducted in 1981, and Arditi and Placci (1990) found no groups with more than eight individuals during a one-year study of the population in 1989–1990. Both estimates are significantly smaller than ours (range: 5 to 15 individuals, mean = 8.5). An increase in troop size may suggest that the population is expanding, as has been observed in red howlers in Venezuela (Rudran and Fernandez-Duque, 2003). In our population, the three multi-male groups were significantly larger than uni-male troops. The age structure of the troops also suggests an expanding population, as indicated by a relatively large number of juveniles and infants and by the number of reproducing adult females.

Our estimates of sex ratio should be considered tentative due to the small number of groups and the potential for age misclassification. It would be necessary to determine the sex of the large number of unsexed infants before any conclusions could be reached. Rudran and Fernandez-Duque (2003) found that the ratio of male to female infants changed in a linear association with density. In other words, more females than males were born at low population densities. If the observed trend of more male than female births were to be confirmed, it might indicate that the population is expanding. On the other hand, the strongly biased sex ratio in the subadult category needs to be considered with some caution. Although it is relatively easy to identify subadult males because they have started to show signs of the characteristic black coat of adult males, it is more difficult to classify subadult females who cannot be differentiated from adult females by coat color. The number of subadult females may, therefore, have been underestimated.

In conclusion, our preliminary data suggest that the population has expanded during the last two decades, in agreement with a previous analysis of changes in population density in the howler troops of this region (Dvoskin et al., 2004). Our conclusion is mainly supported by the observed increase in average troop size and by the relatively large number of infants and juveniles. Other parameters of population structure may confirm and detail this expansion, but longer-term demographic records will be required to construct the necessary dataset. For example, the proportion of uni-male troops may also indicate the status of the population, as in Venezuela, where the proportion of uni- and multi-male troops changed dramatically over 30 years in a population of red howler monkeys (Rudran and Fernandez-Duque, 2003). Uni-male troops accounted for a relatively larger proportion of the population when the population was declining than when it was expanding. Although the reasons for the change in the proportion of uni-male troops were not clear, if confirmed this factor could become a convenient tool for population management, helping us anticipate, with relative accuracy, changes in population size that would take decades to observe.

The vast amount of data on the population biology of howler monkeys, spanning several decades of fieldwork, has convincingly shown the limitations of short-term studies (Rudran and Fernandez-Duque, 2003; Estrada et al., 1999; Fedigan and Jack, 2001). Thus, although our preliminary study offers a few solid results and several tentative findings worth examining in the future, it is imperative that we expand the database if we want to examine how social and environmental factors may be shaping the observed demographic features.