Sex Differences in Hierarchical Stability in a Formation of a Mixed-sex Group of Rhesus Macaques

Forming groups of captive rhesus macaques (Macaca mulatta) is a common management practice. New formations of unfamiliar macaques can be costly, with high levels of trauma, particularly as intense aggression is used to establish a dominance hierarchy. Combining previous subgroups into one new group may be beneficial, as some individuals already have established dominance relationships. We tested this hypothesis by forming a new mixed-sex group of rhesus macaques that combined an established group of females with an established group of males. Prior to the mixed-sex group formation, both the female and male hierarchies had been stable for 3 y; after mixed-sex group formation these hierarchies were maintained by the females and were initially maintained by the males for 3 wks. However, the temporary hospitalization (due to a laceration caused by aggression) of the alpha male destabilized the male hierarchy. Age and weight then predicted male rank. Temporary hospitalizations resulted in rank changes for the males, evidenced by reversals in subordination signals. This study indicates that using established groups of familiar individuals may maintain female hierarchical stability in a mixed-sex group formation, but further research is needed to understand how to maintain and predict male hierarchical stability to reduce trauma. Improved knowledge of hierarchical stability would be invaluable to managers of large rhesus macaque groups and would help improve the welfare of captive rhesus macaques.

Social group formations of captive nonhuman primates (NHPs) occur frequently for a variety of reasons (for example, breeding, research purposes, permanent housing arrangements). Rhesus macaques are the most used NHP in biomedical research.¹⁵ Classified as highly despotic, rhesus macaques rely on a heavy use of aggression to maintain and reinforce their dominance hierarchy,⁴⁷ which ultimately governs access to resources and mates.⁵⁰ Extensive aggression can result in significant traumas and even death.^12,51 Reducing trauma is therefore an important goal both to maintain the animals’ welfare and to minimize the associated cost of care. Thus, this research is aimed at improving the success of forming groups with minimal trauma.One strategy to potentially improve the success of group formations and decrease aggression is to mix familiar individuals with an already established dominance relationship.^18,53 However, dominance ranks are not static and can change if aspects of the social environment change,^1,11,17 such as the loss of keystone individuals³⁴ or addition of new allies.¹⁷ As many NHPs are housed in pairs or small social groups, substantial research has been aimed at understanding pairing success,^14,31,38 but less is known about factors that contribute to successful formations of large, naturalistic social groups. Because high levels of aggression and trauma can occur even in well-established groups,^8,45 gauging social stability in large group formations of rhesus macaques is difficult, as aggression does not necessarily equate to incompatibility or trauma.^8,31,38 Understanding whether individuals maintain their previous hierarchies is critical information for behavioral managers as they form new social groups.Wild female rhesus macaques remain in their natal groups and acquire adjacent ranks to their mothers through coalitionary support.¹⁰ Males also retain their ranks near their mothers while in their natal groups,¹⁶ but after dispersal at sexual maturity and entry into a new breeding group, males mainly enter the group at the bottom of the hierarchy and move up after dispersals or deaths (see²⁶ for general background information and an unusual case of rank acquisition). Because males emigrate from multiple natal groups, many of them will be unfamiliar to one another. Whether male dominance relationships in their natal groups affect their rank in subsequent groups as adults is uncertain, although some research suggests that postnatal nepotism occurs in long-tailed macaques (Macaca fascicularis), with males maintaining a high dominance rank longer in non-natal groups when other male kin are present.²⁵ In many groups in which males originate from multiple natal groups, tenure in the new social group often dictates male dominance rank.²⁶The current research sought to determine whether female and male rhesus macaques would maintain their established same-sex hierarchies during a group formation in which an established group of females was introduced to an established group of males. If previous relationships are maintained, this could indicate that using established groups of familiar same-sex individuals could lessen the trauma and associated costs of a group formation, as individuals would not need to use high levels of aggression to sort out their hierarchy. We predicted that previous social rank, rather than individual attributes, would predict social rank in the new mixed-sex social group.