Most comparative models of primate behavioral ecology assume that the behaviors of wild primates are adaptive, or are the products of selection pressures under ecological conditions in the past. Yet, the fast pace at which human activities alter the environment results in new ecological challenges and makes it difficult to distinguish evolutionary adaptations from more rapidly acquired behavioral responses that may or may not be adaptive over time. Understanding this plasticity is fundamental to analyses of evolutionary mechanisms and represents an urgent priority for assessments of the future of endangered primates living in disturbed habitats.
This study builds on a unique historical database from the long-term field study of wild northern muriqui monkeys (Brachyteles hypoxanthus) in the Atlantic Forest of southeastern Brazil to evaluate how ecological change drives behavioral innovations and to identify the social pathways by which new behavior patterns spread within and between groups. The northern muriqui is critically endangered, and with 281 individuals confined to a 957-hectare protected forest fragment, the study population represents more than 25% of the entire species. During the past 27 years, the population has quintupled in size and the population's density is now the highest known for the species. Previous behavioral adjustments to population pressures have included shifts from cohesive to fluid grouping patterns. More recently, and despite their arboreal lifestyle, the largest group in the population has undergone a 20-fold increase in its use of the ground and all three of the other smaller groups now engage in terrestrial feeding. The greater risks from terrestrial predators and pathogens imply that this change in the muriquis' habitat use is an extreme manifestation of an overall expansion of their ecological niche necessitated by the population's growth within a confined area. With the collection of systematic behavioral data on all four muriqui groups in this crowded population, this study will quantify the ecological implications of niche expansion and the social processes by which newly acquired behavior patterns are diffused between groups by dispersing females and subsequently spread within groups along established social networks. The intellectual merits of the study include the new comparative perspectives it will provide into the ecological conditions associated with ground use in other primates, and the critical insights it will provide into the adaptive potentials of contemporary primates and their prospects for survival in a rapidly changing world.
In addition to its scientific contributions, this study will provide essential continuity for the only long-term study on this critically endangered species and one of the longest-running field studies of any primate. It will contribute to the conservation and protection of one of the last remaining strongholds for northern muriquis, and maintain the researcher's longstanding tradition of providing training opportunities for Brazilian students. The established record of collaborative research and conservation activities, and success with capacity building in Brazil, extend the broader impacts of this study to include an international conservation and educational agenda.
Overview: This study provides new insights into the ecological, demographic, and social processes associated with the adoption and spread of novel behavior patterns in wild primates. Specifically, we documented an increase in the frequency of novel terrestrial behavior by the northern muriquis in our study population, which has now grown to include nearly 350 individuals distributed among four social groups. Increasing population density in this isolated forest fragment in southeastern Brazil continues to be the most likely stimulus for the muriquis’ shift from strictly arboreal to increasingly terrestrial feeding behavior, and is consistent with the positive relationship predicted between ground use and group size. We confirmed our prediction that females born in largest group with the strongest terrestrial tradition would continue to engage in ground use after they disperse into other groups prior to puberty, and thereby demonstrate the vital role of females to the spread of new behavior patterns throughout their population. Greater risks from terrestrial predators and pathogens are implicated in the significant increase in mortality; however, the even greater benefits associated with the muriquis’ expansion of their vertical niche are implicated in the corresponding increase in female fertility. Intellectual merits: These findings, which reflect both the causes and consequences of behavioral innovation, are consistent with the hypothesis that the capacity for behavioral flexibility provides a buffer that permits primates to cope with rapid environmental changes. As such, this study contributes original comparative perspectives into the kinds of ecological, social, and demographic conditions that may have played a role in shaping human behavioral evolution, as well as those shaping the behavioral flexibility of other primates in contemporary times. By mapping how behavioral innovations spread within and among groups, and in particular, the role of dispersing females to behavioral transmission, this study advances our understanding of learned traditions in primates. Additionally, by tracking the ways in which novel behaviors can affect population demography, this study extends our understanding of the interactions between primate adaptability and the potential of populations of critically endangered species to persist over time. Broader impacts: Through its support of the long-term monitoring of one of the only viable populations of this critically endangered primate species, the impact of this project extends beyond its scientific contributions in a number of ways. First, our discoveries about muriquis have been influential in informing conservation and management programs for this species, and in stimulating involvement in conservation and the development of conservation education programs in the local community. Second, this study provided important training opportunities and experiences for a new generation of scientists and conservationists in Brazil and in the United States. A total of 15 Brazilian university graduates, including 8 women, received training in data collection and methods while participating on the field project during this study period, and four of these participants have either completed or are now enrolled in graduate programs. In addition, 14 undergraduates, including 11 women, gained experience working with original data and images from the field study while volunteering in the Muriqui Behavioral Ecology Data (MBED) lab. Finally, by collaborating with journalists, science writers, and television and film crews through interviews and sharing of information, our scientific discoveries and their implications for conservation have become accessible to the general public in both the U.S. and abroad. Nationally televised documentaries have appeared on Brazilian T.V., and popular articles featuring components of this study have appeared in major newspapers and magazines in both Brazil and the U.S., including an article featured in the September 2013 issue of Smithsonian Magazine. Additional outreach through numerous public lectures and talks to high school students has complemented the scientific venues at which this research has been presented by bringing our findings to broader, more general audiences of all ages.
