Comparison of Foraging Behavior Between Howler Monkeys, Spider Monkeys, and Squirrel Monkeys

Different monkey species have different foraging behaviors based on their preferred food quality and its availability. We compared foraging behavior of focal individuals of 3 monkey species: Alouatta palliata (howler monkey; a folivore), Ateles geoffroyi (spider monkey; a frugivore) and Saimiri oerstedii (squirrel monkey; an omnivore on fruits and insects). We predicted that howler monkeys would spend a larger proportion of time foraging, and a smaller proportion of time moving, than the other two species. We found that squirrel and spider monkeys spent the greatest proportion of time foraging, the greatest proportion of time moving, and traveled farther than howler monkeys. Proportion of time spent moving, and the total distance traveled by each species, appears to be driven by food availability. However, proportion of time spent foraging appears to depend on overall energy demands and not food quality. We suggest that specific energetic demands and food availability be considered in explaining mechanisms that drive monkey foraging behavior. Introduction Monkeys spend a large proportion of their time foraging to obtain energy for other activities. However, different species of monkeys prefer different food types. Alouatta palliata (howler monkey), Ateles geoffroyii (spider monkey) and Saimiri oerstedii (squirrel monkey) are three species of New World monkeys commonly found in Corcovado National Park, Costa Rica. Each species has different food preferences: howler monkeys are selective folivores (1), spider monkeys are selective frugivores, and squirrel monkeys consume a wide variety of foods such as fruits, insects, and nectar (2). In this study, we compared foraging behavior of these three species. We hypothesized that each species would exhibit different foraging behaviors based on differences in food quality and availability, which we expect to be inversely related. Leaves (a low-quality food) are abundant, while fruit (a better-quality food) is less abundant, and insects (a high-quality food) are the least abundant. Based upon the theoretical premise of an inverse relationship between food quality and quantity, we predicted that: 1) time spent foraging is lowest in howler monkeys and highest in squirrel monkeys; 2) time spent moving is lowest in howler monkeys and highest in squirrel monkeys. Alternatively, food availability might not vary according to food quality if not all leaves and fruit are equally edible, and if monkeys are discriminating foragers. In this case, each species would spend an equal amount of time searching for food, and we would not Howler, Spider, and Squirrel monkeys in their native habitats in Costa Rica Images Courtesy of Katherine Amato ‘07 ECOLOGY Comparison of Foraging Behavior Between Howler Monkeys, Spider Monkeys, and Squirrel Monkeys KATHERINE R. AMATO ‘07, D. DÜNYA ÖNEN ‘07, SARAH L. EMEL ‘06, AND CHRISTINA H. MAY ‘05 DARTMOUTH UNDERGRADUATE JOURNAL OF SCIENCE 28 expect to see differences in time spent foraging, moving, or in distance traveled. A relationship also exists between food quality and metabolic rate of the consumer, where low food quality may select for lower metabolic rates. Thus, species that depend on high-quality food should be more active, or spend less time resting, than species with lower metabolic rates. Under this hypothesis, howler monkeys would rest the most, and squirrel monkeys would rest the least. Methods We observed howler, spider, and squirrel monkeys from February 1 to February 4, 2006, along Naranjos, Rio Claro, Espaveles, Ollas, Guanacaste, and Pavo trails in Corcovado National Park, Costa Rica. We observed the monkeys intermittently between 07:30 and 17:30 during two 3-hour intervals on each day. We recorded number of individuals per troop, chose focal individual monkeys within troops, and observed them for five minutes. Although we made an effort to choose in an unbiased manner, focal individuals were usually the most visible individuals in the troop. Using five one-minute increments, we categorized animal behavior as foraging (searching for and manipulating food), watching (cautiously scanning surrounding ground and vegetation), resting (sitting, lying, or sleeping without any other activity), moving (walking and climbing not accompanied by other activities), or performing other activities (behaviors not fitting into the above categories). We also estimated the total distance each individual moved during the observation period. We collected and identified samples of each species’ food resource when possible (Table A). We examined the effect of species on proportion of time spent foraging, moving, watching, resting, and other behaviors, and total distance traveled using a twoway ANOVA, with troop nested within species. We examined the directionality of results using general linear contrasts. Also, we recorded the number of individuals in each troop and tested the effects of species on troop size using a one-way ANOVA (Table B). We performed all analyses with JMP 5.0.1. Results During the 5-day sampling period, we observed 99 focal individuals from 39 monkey troops (6 howler, 24 spider, 9 squirrel) varying in size from 2 25 individuals. No one monkey species spent significantly more time foraging than any other monkey species (Figure 1). Squirrel monkeys spent a greater proportion of time moving than spider monkeys, which spent a greater proportion of time moving than howler monkeys (Figure 2). Squirrel monkeys also moved further than spider monkeys, which moved further than howler monkeys (Figure 3). No one species spent a significantly greater amount of time resting than another, but a linear trend in the data suggested that howler monkeys spent the Monkey Species Plant Family Species Portion of Plant