Behavioral Ecology of Monkey Spatial Movements

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Non-technical Summary: Janson, C. Proposal IBN-9511642: 'Behavioral Ecology of Monkey Spatial Movements' The goal of this project is to understand the rules animals use to decide where to move within a known home range. Much evidence from studies of animal foraging suggests that they attempt to maximize food intake relative to foraging costs. When costs are proportional to distance moved between known resources, the problem is essentially similar to the human 'Traveling Salesman Problem' , in which the goal is to travel among N known cities in the shortest possible distance. This problem has resisted precise mathematical solution despite great efforts, and thus the possible rules that animals might use are far from obvious -- even the approximate solutions to this problem offered in the economics literature typically involve vast numbers of calculations on powerful computers. Do animals use simple or complex rules, do they 'look ahead' to more than the next immediate goal, how closely do their rules approach the best possible solutions available on computers today? A wild population of fruit-eating primates (capuchin monkeys) were chosen as the study animals because they need to solve such spatial problems every day. Their major resources, fruit trees, are scarce and fixed in space, while the cost of traveling between fruit patches in rainforests is very high compared to moving on level ground or flying. Some data in captivity suggest that fruit-eating primates can learn and remember the locations of large numbers of hidden food sources, so it is plausible that they can use such abilities to solve spatial foraging problems in the wild. In addition, primate groups can be followed continuously for long periods (weeks), thus allowing for large sample sizes of natural spatial movements. However, because human researchers can rarely know everything (such as location, productivity, and palatability) about all the food patches available to a primate group, the results of previous observational studies are not easy to interpret. To reduce these problems of interpretation, this project uses a field-experimental approach in which fruit (tangerines) is provided on a series of wooden platforms suspended from trees -- the investigator can completely control the location, amount of food reward, and food type on the platforms. Confounding influences of other, natural food sources are minimized in this subtropical Argentine study site by working only in winter, when nearly all natural fruit production stops. The spatial locations of the platforms are chosen (using a computer program) so that the sequences of movement choices from one platform to the next will differ depending on the possible decision rules used. The specific rules tested will include ones that (1) place differing amounts of emphasis on delayed vs. immediate rewards, (2) trade off distance vs. amount of food, and (3) examine rewards individually by platform or integrate over the entire spatial array of platforms. Investigating the rules capuchins use to move between known resources will help us to understand (1) a largely neglected aspect of foraging behavior, (2) the likely mental organization of spatial knowledge in primates and possibly humans, and (3) what aspects of the resource base of an animal species may favor the evolution of complex mental maps, which have been implicated in the evolution of intelligence in man's ancestors. The knowledge about spatial decisions of these monkeys will also provide important information about their spatial needs and how they may react to habitat disturbance, results which may help in conserving this species in the wild as increasing numbers are taken for laboratory use.