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INFLUENCES OF MANTLED HOWLERS (ALOUTTA PALLIATA) ON FEEDING

Many correlational studies have been conducted on mixed species primate groups and sympatric species, and evolutionary theory has been advanced to understand interspecies interactions. However, few experimental field studies have been conducted to determine causal influences of one primate species on another. In this paper, we describe an experimental field study focused on the effects of mantled howler monkeys (Alouatta palliata) on rufous-naped tamarins (Saguinus geoffroyi). Two juvenile howler monkeys were placed on a small island with 5 tamarins in an A1-B1A2-B2 experimental design: A1-baseline with no howlers present, B1-experimental condition with howlers present, A2-second baseline with no howlers, and B2-second experimental condition with howlers present. Systematic focal sample data were collected on the tamarins’ feeding patterns, their height in the forest, their distance from the nearest howler, the height of the howlers, and agonistic interactions between the two species. The presence of the howlers appeared to slightly decrease the tamarins’ rate of feeding on provisioned and natural food. This effect diminished during the second experimental condition. There were few agonistic interactions between the species and no wounding or deterioration in the condition of either species was observed. The tamarins shifted early morning and mid-day feeding patterns later when the howlers were present. There was a faint tendency for the tamarins to be higher in the trees when the howlers were present. Analyses of the relationships between the variables suggest the tamarins and the howlers were simultaneously attracted to the same provisioned and natural food items. This led to synchrony in their use of vertical space. Early in the day, when provisioned food was more frequently accessed, the tamarins and howlers tended to be closer to each other. The two species did not seem to form a mixed species group; instead, they were drawn together by use of common resources. The introduction of the howlers to the island was deemed a success as neither species appeared to be compromised by the presence of the other. It is possible that that they could both profit from sharing the same island since they could alert each other of the presence of potential dangers and resources. Introduction Previous field studies on mixed species groups have been correlational or differential (in the sense of GRAZIANO and RUALIN, 1997) research: interactions and relationships between two or more species are either qualitatively or quantitatively described (e.g. TERBORGH, 1983). Therefore the causal influence of one species on another can only be inferred: despite intricate evolutionary theory, one species can seldom be definitively said to have either a positive or negative impact on the other Primate Report 65, January 2003 39 D.R. Rasmussen and I. Broekema: Influences of Howlers on Tamarin Feeding