Socioecological Factors Affecting Range Defensibility Among Howler Monkeys

Behavioral responses of mantled howler monkeys to neighbor long-distance vocalizations

OBJECTIVES

Long-distance vocalizations are used by primates in a variety of contexts and may have different functions. The long-distance vocalizations of howler monkeys (Alouatta spp.) underlie the spatial regulation of neighboring groups and could be associated with the defense of food resources. Here, we test the hypothesis that the behavioral responses of mantled howler monkeys (A. palliata) to neighbor long-distance vocalizations are influenced by the potential for range defensibility while accounting for location within the home range and food availability.

METHODS

We studied two groups for 13 months and a total of 888 h at La Flor de Catemaco (Mexico). Group 1 had a 92-ha home range and Group 2 had a 24-ha home range. We recorded vocalizations (N = 178 calls) and movements (N = 74 movements) of focal groups following long-distance vocalizations produced by their neighbors.

RESULTS

Movement responses, but not vocal responses, were predicted by range defensibility, location, and food availability. As predicted, the group living in the smaller and more defendable range showed stronger movement responses than the group in the larger home range. These movement responses had a shorter latency and longer duration in more valuable spatial and temporal contexts (i.e., the core area and during periods of low food availability).

CONCLUSIONS

These results suggest that the tradeoff between the costs and benefits of range defense varies according to the interactions between home range size and both the spatial (core areas) and temporal (food availability) abundance of resources. Thus, the responses of mantled howler monkeys to neighbor long-distance vocalizations could be related to home range defensibility.

Lone males: Solitary and group-living male howler monkey (Alouatta palliata) behavioral ecology in a Costa Rican rainforest

OBJECTIVES

Many group-living primate species have evolved the capacity for some individuals to live alone for part of their lives, but this solitary life stage has rarely been the subject of focused research. The mantled howler monkey (Alouatta palliata) is a social primate species with bisexual dispersal that lives in mixed-sex groups with low male-to-female sex ratios. Consequently, males often spend a long period of their lives as solitary individuals. This study compares the tree use, feeding, and long-distance vocalization behavior of solitary and group-living mantled howler monkey males living within a fragmented rainforest in Costa Rica, La Suerte Biological Research Station. Based on differences in competitive ability between solitary and group-living males, we predicted that lone males would be found in significantly smaller feeding and resting trees, consume more low-quality foods, and produce shorter howling bouts made at lower rates than group-living males.

MATERIALS AND METHODS

We collected data on tree use and feeding during 30-min focal samples on male focal animals, recording data at 2-min intervals. We measured the trees in which the monkeys fed and rested for two or more intervals, and recorded the plant parts consumed. We recorded howling behavior using all-occurrences sampling.

RESULTS

Lone males used significantly smaller feeding and resting trees, consumed more low-quality foods, and howled at lower rates but had longer howling bouts triggered by anthropogenic noise more than group-living males.

DISCUSSION

Our results demonstrate that lone males differ in their behavioral ecology compared to group-living males, thus improving understanding of the solitary male life stage in primates.

Western gorilla space use suggests territoriality

The evolutionary origins of how modern humans share and use space are often modelled on the territorial-based violence of chimpanzees, with limited comparison to other apes. Gorillas are widely assumed to be non-territorial due to their large home ranges, extensive range overlap, and limited inter-group aggression. Using large-scale camera trapping, we monitored western gorillas in Republic of Congo across 60 km². Avoidance patterns between groups were consistent with an understanding of the “ownership” of specific regions, with greater avoidance of their neighbours the closer they were to their neighbours’ home range centres. Groups also avoided larger groups’ home ranges to a greater extent, consistent with stronger defensive responses from more dominant groups. Our results suggest that groups may show territoriality, defending core regions of their home ranges against neighbours, and mirror patterns common across human evolution, with core areas of resident dominance and larger zones of mutual tolerance. This implies western gorillas may be a key system for understanding how humans have evolved the capacity for extreme territorial-based violence and warfare, whilst also engaging in the strong affiliative inter-group relationships necessary for large-scale cooperation.