Inter‐unit contests within a provisioned troop of Sichuan snub‐nosed monkeys (Rhinopithecus roxellana) in the Qinling Mountains, China

Abstract

Numerical superiority does not always ensure victory in intergroup contests. Although group size is likely to determine the maximum resource holding potential (RHP) of a group, the realized RHP is the collective outcome of individual group members' choices about participation in any given contest. For any group member, the choice about participation should be based on the assessment of costs and benefits that are affected by both ecological and social factors. In this study, we studied inter-unit contests in a provisioned troop of Sichuan snub-nosed monkeys (Rhinopithecus roxellana). We spent 368 hr in contact with 9 one-male units sharing the same home range, during which we recorded 148 inter-unit contests at a provisioning site. Inter-unit contests always started as inter-individual contests. Contests escalated only when the two individuals were of different age-sex classes and one was an adult male. When a contest escalated, additional individuals were likely to get involved, and the outcome of the contest depended on unit members' choices about participation. The superiority in the number of participants rather than the superiority in unit size led to victory in inter-unit contests, given that the difference in unit size did not predict a difference in the number of participants. Unit members were more likely to support others in inter-unit contests in winter when food was sparse than in spring when food was abundant. In addition, unit members were more likely to support others in escalated contests than in those resulting in displacement, indicating that they tended to alter the outcome of a contest to gain immediate benefit. Although males initiated most inter-unit contests, a clear win-loss was most likely when females joined the fight. This sex difference may reflect the benefits to males vs. females of living in a multi-level society.

Patch Occupation Time Predicts Responses by Grey-Cheeked Mangabeys (Lophocebus albigena) to Real and Simulated Neighboring Groups

“…It is possible that, for mangabeys, group size is not an adequate measure of RHP; male mangabeys participate in all aggressive IGIs and are the primary participants, so perhaps the size, strength, or aggressiveness of group males is a better determinant of RHP than the number of participants (Batchelor and Briffa 2010). Alternatively, the number of actual participants may represent RHP more accurately than the number of potential participants (Zhao and Tan 2010), or RHP may only be influential in determining other contest aspects (duration, intensity, or outcome). Further exploration is needed to test these possibilities, and more broadly, a theoretical framework is needed to explain why species vary in the relative importance of RHP and payoff asymmetries across contest phases.…”

“…Evolutionary game theory (Maynard Smith 1974;Parker 1974) posits that animal contests are settled by asymmetries, e.g., in competitive ability (resource-holding potential [RHP]; Zhao and Tan 2010), the fitness-related benefits to be gained by winning the contest (payoffs; Harrison 1983;Pride et al 2006;Putland and Goldizen 1998), or both RHP and payoffs (Crofoot et al 2008;Harris 2010;Kitchen et al 2004). Weaker or less motivated contestants are generally reluctant to initiate contests that they are likely to lose, whereas stronger or more motivated contestants do not avoid competitors (Furrer et al 2011;McComb et al 1994;Wilson et al 2012).…”

“…Lastly, group RHP is likely to affect patterns of contest initiation (Hypothesis 7), and should be approximated by the number of group members in the age-sex classes that participate aggressively (Kitchen 2003;Zhao and Tan 2010). If the resource at stake is valued by both groups and asymmetries in group RHP are the primary determinant of group responses, then call-and-response sequences should be invariant for each group dyad; in other words, for a particular dyad, the larger group will always approach and the smaller group will never approach (Prediction 7a).…”

Male social rank and food competition in a primate multi‐level society

“…In tufted capuchins Sapajus nigritus , dominant groups are better at defending their core area, and may thus have higher per capita access to food (Scarry, 2013). There is also evidence of competition for food among OMUs within this same R. roxellana BB (Zhao & Tan, 2011) but the effects of rank are unknown. Overall, we show that social rank among adult males can affect access to high‐quality food within this MLS.…”

“…There is also a distinct male hierarchy that results from dyadic interactions between adult males (Zhang, Watanabe, Li, & Qi, 2008). Such agonistic events often occur due to disputes over food items (Zhao & Tan, 2011), and may also include the participation of some female members of the same OMUs as the competing males (Zhao, Chen, Li, & Romero, 2013). Higher‐ranking males, and their associated OMU group members, can therefore be expected to gain preferential access to favored food items, and this may enable higher‐ranking males to spend more time on alternative activities, such as engaging in affiliative behaviors with the other members of their OMU (sensu Huntingford & Turner, 1987; Lehmann et al, 2007; Dunbar et al, 2009; Dunbar & Lehmann, 2013).…”