The energetic costs and benefits of intergroup conflicts over feeding sites are widely hypothesized to be significant, but rarely quantified. In this study, we use short-term measures of energy gain and expenditure to test whether winning an intergroup encounter is associated with greater benefits, and losing with greater costs. We also test an alternative perspective, where groups fight for access to large food sources that are neither depletable nor consistently monopolizable: in this case, a group that has already fed on the resource and is willing to leave first (the loser) is supplanted by a newly arrived group (the winner). We evaluate energy balance and travel distance during and after encounters for six groups of red-tailed monkeys in Kibale National Park, Uganda. We find that winning groups experience substantial energetic benefits, but do so to recoup from earlier deficits. Losing groups, contrary to predictions, experience minimal energetic costs. Winners and losers are predictable based upon their use of the contested resource immediately before the encounter. The short-term payoffs associated with these stressful conflicts compensate for any associated costs and support the perception that between-group contests are an important feature of social life for species that engage in non-lethal conflicts.

We collected the data for this analysis as part of a broader study on intergroup conflict in six neighboring groups of red-tailed monkeys from January 2012 through June 2015. All mature animals are individually recognized using the shape of the white nose spot, nipple characteristics (for females), and scars or other injuries. We followed 2-3 groups simultaneously for 1-2 weeks each month for multi-month periods (mean 4.9 ± SD 1.1 months, N = 7 periods), with 11-17 months between successive periods for a group. On every follow day, 2-4 observers tracked each group from dawn until dusk and noted the presence or absence of each individual, as well as whether each female was carrying and nursing an infant. We recorded the location of the group every 30 min as the point around which the majority of group members were clustered; we determined location using a 50 x 50 m gridded map of the trail system and by pacing to the nearest trails, or by using a hand-held GPS unit and later converting the UTM coordinates to the grid cell format. We recorded all foraging activity by group members during a 5 min window every half hour, including the plant species and part eaten.

Observers recorded details of intergroup encounters, which we define as periods when the edges of two groups are ≤ 50 m apart, using a pre-printed template to ensure consistency across observers [17]. These details include the start and end times, the identity of the opposing group (if known), whether any chases or physical contact occurred between groups, and the location of the encounter. Observers spaced themselves out along the leading edge of the focal group as well as behind this edge in order to maximize our ability to track the events of these sometimes chaotic encounters. We also attempted to follow the opposing group for 60 minutes after the end of the encounter to track its movements.

We collected fresh urine samples from mature individuals opportunistically throughout the day by pipetting droplets from low-lying vegetation immediately after excretion. We stored the samples on ice until 1700 hrs, at which time they were transferred to a -12°C freezer at the camp site. MB transported samples 1-2 times per year to the Hominoid Reproductive Ecology Laboratory at the University of New Mexico and we measured C-peptide levels with commercial radioimmunoassay kits (Millipore Sigma, Burlington, MA) using the manufacturer instructions.

We defined four types of intergroup encounter outcome based upon the movements of the groups, relative to each other and/or to their pre-encounter travel direction [13, 14, 18]. ‘Displacements’ occurred when one group (the winner) stayed in the IGE location for at least 30 min after the departure of the other, losing, group (N = 38 encounters). ‘Deflections’ occurred when both groups moved away after the encounter (N = 7), but the winner continued moving within 45° of its original travel direction while the loser turned around to retreat into its home range. ‘Mutual avoid’ outcomes occurred when both groups turned around and retreated (N = 38), and ‘mutual ignore’ occurred when both groups continued moving forward (N = 1). Both displacements and deflections are considered ‘decided’ outcomes while mutual avoid and ignore are ‘draw’ outcomes.

For each urine sample, we determined whether an intergroup encounter occurred on the same or following day; for those samples with an associated encounter, we calculated the difference between the time at which the sample was voided and the start of the encounter. Samples produced before and after an encounter had negative and positive values, respectively. We did not have enough urine samples from both interacting groups per encounter to compare their changes in C-peptide levels, so our analyses investigate general changes experienced by groups on days that they won, lost, or experienced a draw outcome. We also included samples from control periods: these were 2-day periods throughout the study in which we had collected multiple urine samples on each day, and in which there was no intergroup encounter. The control samples allowed us to determine the normal patterning of C-peptide responses and served as a useful comparison with the intergroup encounter-associated samples. To facilitate this comparison, we assigned a fake intergroup encounter number to each 2-day control period and designated the fake encounter start time as noon on the first day. This was a reasonable approximation because the encounters were approximately normally distributed around noon: 50% of encounters occurred before noon, and 41% occurred between 1100 and 1359 hrs.

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