Introduction

Sympatry of two species belonging to the same genus occurs rarely in Neotropical primates. It has been observed regularly for Cebus apella and Cebus albifrons (Terborgh, 1983), and for Saguinus fuscicollis and Saguinus sp. (Heymann and Buchanan-Smith, 2000). In both cases, the species occupy different ecological niches, through the use of different forest strata or individual foraging strategies. For species of the genus Alouatta there are several limited cases of sympatry, for example Alouatta palliata and Alouatta pigra in Tabasco, Mexico and at the southern Belize-Guatemala border (Horwich and Johnson, 1986), Alouatta caraya and Alouatta guariba in southern Brazil (Júlio César Bicca-Marques, pers. comm.) and the El Piñalito Provincial Park in Misiones, Argentina (Di Bitetti, 2004), and A. palliata and Alouatta seniculus in northwestern Colombia (Hernández-Camacho and Cooper, 1976). Here we report for the first time a clearly sympatric occurrence of the two howler monkey species Alouatta sara and A. caraya.

Study area and Methods

The Río Yacuma is a small tributary of the Río Mamoré, approximately 10 km upstream of the village of Santa Rosa (14°10′S, 66°52′W, Fig. 1). The Río Yacuma flows through the alluvial plain of the Río Mamoré at an elevation of approximately 150 m a.s.l. Heavy floods during the rainy season transform the region into a vast swamp. Mean annual temperature is 26°C, and mean annual precipitation is approximately 1,800 mm (Montes de Oca, 1997; Navarro and Maldonado, 2002). The region is part of the biogeographic sector of the Moxos lowlands, characterized by tree savannahs and Várzea forests along the watercourses (Navarro and Maldonado 2002). Observations were made during a boat trip between April 10 and 12.

Results and Discussion

In April 2006, while travelling northeast of the city of Rurrenabaque in the Beni Department in northern Bolivia (Fig. 1), we observed two distinct howler species, A. caraya and Alouatta sara (taxonomy following Groves, 2001), foraging and resting in close proximity (at a minimum distance of approx. 100 m and in the range of vision of each other on the bank of a river). The two species were clearly distinguished by the different coloration of the fur (black in male and yellowish in female A. caraya, red in A. sara). Presumably, the Río Yacuma is not a natural barrier to the dispersal of the Alouatta species as both species were seen in the gallery forest on the left and right bank of the river. Water levels decline considerably during the dry season, probably enabling the howler monkeys to cross the river. Groups of A. caraya were observed three times. The observations included: an adult male and an adult female; an adult male and two adult females, one carrying a baby; and three adult females with a male. Alouatta sara was seen two times. Once we observed a group of three individuals—two adult males and an adult female—foraging in a Cecropia concolor tree. On another occasion, we saw a single adult male of A. sara resting in the tree canopy. Several minutes of howling of Alouatta groups were heard repeatedly in the morning and in the late afternoon on both sides of the river.

In Bolivia, A. caraya has been observed at two localities in the Santa Cruz Department and at various localities in the Beni Department including the mouth of the Rio Yacuma. Distributional notes on A. sara in Bolivia include localities in the Beni, Cochabamba, La Paz, Pando, and Santa Cruz departments. There have been no reports for the central Beni or Rio Yacuma region, though (Anderson, 1997). Both Alouatta species observed are assessed as Least Concern on a global level by IUCN—The World Conservation Union (2004) although declines have occurred in many parts due to hunting pressure and habitat loss. The sympatric occurrence of the two Alouatta species at the Rio Yacuma raises the following questions: 1) Is the sympatry of A. caraya and A. sara a result of ecological differences (i.e., the use of different forest strata, different foraging strategies or activity patterns)?, 2) Is interspecific territoriality the same as within species territoriality?, and 3) Is the observed sympatry a recent phenomenon (i.e., caused by habitat loss and hunting pressure in the surrounding area) or has it existed for a longer time?

Figure 1.

Location of the study area (black box) at the Río Yacuma in the Beni Department, Northern Bolivia.