Succession Dominates Alpha Male Replacement in Despotic Rhesus Monkeys: Insights from a Long-Term Study in the Taihang Mountains, Henan Province, China

Alpha male replacement occurs in many group-living non-human primates, but its characteristics remain under-explored. Species of the genus Macaca live in multi-male, multi-female groups and are structured into four levels of social styles, which may impact alpha male replacement. Rhesus monkeys (Macaca mulatta) are classified as Grade 1, yet little is known about alpha male replacements in this species. This study investigated the occurrence and characteristics of alpha male replacements in rhesus monkeys inhabiting the southern end of Taihang Mountains, China. The types of alpha male replacements included succession (8/11), Rank Reversal (1/11), and Group Fission (2/11). The average age at which adult males took the alpha rank was 10.2 (±4.1, n = 11) years of age. Their average social rank prior to attaining alpha rank was 3.91 (±3.05, n = 11). The average tenure of alpha males after 2017 was 2.6 (±1.4, n = 5) years, which appeared shorter than that before 2017 (>7.5 ± 2.9 years, n = 4). The occurrence of alpha male replacements did not significantly correlate with group sizes, natality, the ratio of adult males to adult females, or the proportion of immatures. Compared with tolerant Macaca species, alpha male replacement in rhesus monkeys tend to be biased toward Succession, a pattern that may be linked to their extremely despotic social style. This study suggests that social style could interact with changes in social structure, deepening our understanding of the evolution of primate social systems.