Ranging patterns of two syntopic howler monkey species (Alouatta guariba and A. caraya) in Northeastern Argentina

Abstract

To avoid competition, ecologically similar and closely related species tend to differ in their patterns of habitat use when they live in sympatry. We compared ranging patterns of brown howler (Alouatta guariba) and black and gold howler (A. caraya) monkeys living syntopically, i. e., co-occurring and overlapping their ranges in the same habitat within the zone of sympatry, in the Atlantic Forest of northeastern Argentina with the objective of evaluating whether their use of space contributes to the avoidance of interspecific competition for food resources. During 12 mo we collected data on the ranging behavior of 2 groups of each howler species. We analyzed annual and seasonal daily path lengths and movement rates, home range size, use and overlap, habitat and vertical strata use, and intergroup encounters. Black and gold howlers traveled farther and faster during the time of relative food abundance (abundant season) than during the time of relative food shortage (lean season), and their movement rates were affected by group identity and increased with the proportion of fruits in the diet. Brown howlers' traveling patterns were not affected by any of these factors. Home ranges for both species (95% fixed kernel; brown howlers: 31-70 ha, black-and-gold howlers: 17-112 ha) were among the largest recorded for Alouatta. For both species, core areas (50% fixed kernel) were larger for larger versus smaller groups, and decreased in the lean season compared to the abundant season. Both species showed similar patterns of habitat use, except for a slight vertical stratification. Groups of different species overlapped their ranging areas consistently more and responded to one another less aggressively during encounters than groups of the same species, suggesting that interspecific spatial niche separation for these two syntopic species is not occurring. The vertical stratification, as well as a day-to-day avoidance strategy, may be the only responses of species to one another that could reduce the potentially high levels of competition for food suggested by their elevated trophic niche overlap. A high degree of niche overlap may explain the parapatric distribution of howlers and other closely related and ecologically similar species of primates.