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The Relationship of Size of Feeding Aggregate to Size of Food Patch: Howler Monkeys (Alouatta palliata) Feeding in Trichilia cipo Fruit Trees on Barro Colorado Island
Mark Leighton and Donna R. Leighton
Vol. 14, No. 2 (Jun., 1982), pp. 81-90
Published by: Association for Tropical Biology and Conservation
Stable URL: http://www.jstor.org/stable/2387735
Page Count: 10
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The influence of one socioecological variable, food patch size, on grouping patterns of animals is examined. Food patch size is measured by the number of feeding spaces, S, a patch holds for a given animal species. The values of S should be determined by structural or spatial criteria if the size of the feeding aggregate (the animals feeding together within a patch) is limited by the availability of space or by interference behavior. Otherwise, S is limited by the abundance and density of food items within the patch, and may vary, depending on the relative profitability of alternative patches available to the animal, a result deduced from simple optimal foraging theory. If the feeding aggregate sizes favored by selection for efficient foraging differ from those favored by other selective pressures, aggregate sizes will be adjusted to reflect the relative strengths of these counteracting selective pressures. Data from Barro Colorado Island on the sizes of howler monkey feeding aggregates seen in different-sized Trichilia cipo fruit trees are applied to test the hypothesis that: food patch size limits the sizes of feeding aggregates. This hypothesis is supported by demonstrating a positive correlation between howler feeding aggregate size and Trichilia DBH (diameter at breast height), since DBH is shown, in turn, to predict accurately both the spatial sizes and crop sizes (numbers of ripe fruits) of Trichilia patches. Some observations support the explanation that howlers were sensitive to the densities and abundances of ripe fruits within patches and not to the spatial size of patches, per se. A X2 analysis is used to establish that small feeding aggregates were underrepresented in large patches, indicating that, in this instance, feeding aggregate size is adjusted to and not merely limited by patch size. To account for this unexpected result, a hypothesis is proposed: that howler feeding aggregates are often part of larger foraging units (the group of animals maintaining contact while ranging) which travel between clumps of food patches, and upon reaching a clump, the monkeys apportion themselves among the patches according to the sizes of the patches. The result that howlers were seen feeding more frequently in Trichilia that are near to Quararibea asterolepsis trees (the other major food source at this time) than in those located farther from Quararibea supports this explanation. This manner of ranging exemplifies one potential way that groups of animals can maintain cohesion while using patches too small for the entire foraging unit.
Biotropica © 1982 Association for Tropical Biology and Conservation