Range utilization slopes as a measure of central tendency and intergroup overlap in primates

Abstract: Animals that occupy stable home ranges tend to unevenly exploit different areas in their efforts to find fitness-limiting resources, while also reducing the risks of intergroup conflict. Most analyses of these extrinsic forces identify their effects on movement paths and home range geometry, but not on the interaction of these responses or how movements might be centrally constrained as a result of competition with neighbors. The range utilization slope is a measure of central tendency and consists of space use plotted against distance from the center of the range. Slopes tend to be linear, concave-up, or concave-down and are predicted to change as a function of feeding competition from neighbors. To test this prediction and determine the spatio-temporal scales over which the central tendency might vary, we calculated utilization slopes and an index of range overlap for grey-cheeked mangabeys (Lophocebus albigena), blue monkeys (Cercopithecus mitis), and red-tailed monkeys (C. ascanius) in Uganda, which consume similar diets but experience varying intensities of intergroup conflict. As predicted, we find variation in utilization slopes across and within species, which corresponds with the extent of range overlap among conspecific groups. Significance statement: How animals use different parts of the home range provides clues to the constraints they experience, such as food availability, predation risk, and competition from neighbors. Despite its importance in behavioral ecology, the role that intergroup competition plays on home range geometry is not well understood. We propose that the range utilization slope, which evaluates spatial use as a function of distance from the center of the range, is a useful measure of central tendency and indicates how animals are compressed into the center of the range by neighbors. In an analysis of monkey groups of three species, we find that utilization slopes vary across space and time, but generally correspond with the intensity of resource limitation. These slopes provide a rapid assessment of resource access at multiple spatial scales.