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

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.

Methods
Members of the Ngogo Monkey Project intermittently collected observational data on monkey groups from January 2008 through August 2016 at the Ngogo research station (0°29' N 30°25' E) in Kibale National Park (0°13-41' N 30°19-32' E), Uganda. Monkey groups were followed from dawn until dusk for 5–14 consecutive days per month. In 2008–2009, 3–4 red-tailed monkey groups were followed simultaneously for 1 week each month, and alternated with week-long follows of 3–6 grey-cheeked mangabey groups. In 2012–2015, pairs or trios of red-tailed monkey groups were observed for 1–2 weeks each month for periods of (mean ± SD) 4.9 ± 1.1 months (N = 7 periods), with 11–17 months between successive periods in which a group was followed. The blue monkey groups were followed monthly from Aug 2015 through Aug 2016. The field team recorded the centroid position of the majority of group members every 30 min using a 50 x 50 m gridded map of the trail system and by pacing to the nearest trails (2008–2015), or by using a hand-held GPS unit (2015–2016) and converting the UTM coordinates to the grid cell format.