Scramble Competition: An Examination of Day Range Length Variation

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A major goal of studies in primate behavior has been to understand the causes of competitive relationships among females. Historically, attempts to explain variation in competitive relationships have focused on what has been called "contest competition," which can lead to dominance hierarchies, a major organizing theme in describing female relationships. However, field studies have now shown that greater variation in female relationships exists than is accommodated or recognized by models of contest competition alone. Also, predictions of day range size from competition models have proved inadequate to date. This project will focus on the complementary relationship of contest competition and "scramble" competition, which has not been so widely examined in the past. It has been widely accepted that intragroup competition increases as group size increases. One result of this increased competition is that in many primate species larger groups travel farther per day than smaller groups. However, not all species respond to increases in group size by traveling longer daily distances. In fact, primates can be separated clearly by whether they do or do not increase their day ranges with increasing group size. This division also separates clearly along the strength of the dominance hierarchy within groups (a form of contest competition). Isbell argues that in those species that increase day range length with increasing group size, dominance hierarchies among females within groups are strong and easily detected. In contrast, in those species that do not increase day range length with increasing group size, dominance hierarchies are weak or difficult to detect. This study promises to clarify some fundamental issues regarding behavioral expressions of competition in social groups of primates.