Succession in ape social groups refers to the dynamic process by which a new individual replaces the current dominant member, often the alpha male or, in some species, the alpha female. This power transition is a fundamental feature of the social structure across all great ape species. Great apes exist within a dominance hierarchy, a ranking system that dictates access to resources, mating opportunities, and social privileges within the group. The position an ape holds in this hierarchy significantly influences its lifetime reproductive success.
Mechanisms of Power Transfer
The ascent to the dominant position typically involves a combination of physical and psychological strategies. The most direct mechanism involves a physical challenge, where a contender directly confronts the current alpha in a fight. These aggressive encounters can be brutal, sometimes resulting in serious injury or the death of the loser, but they serve to clearly establish the superior physical condition of the winner.
Challengers often begin with sustained display behaviors designed to intimidate rivals. They engage in ritualized aggression, such as charging runs, loud vocalizations, and piloerection—the ruffling of hair to appear larger—to demonstrate fitness and assert their presence. These displays are a form of psychological warfare, testing the confidence of the ruling individual without necessarily engaging in a full-scale physical battle.
A more sophisticated strategy, particularly common in chimpanzees, is the formation of coalitions or alliances. A challenger may not be able to defeat the alpha alone, but with the support of lower-ranking males, they can collectively overthrow the established leader. This requires complex social intelligence, as the challenger must invest time and energy into reciprocal relationships, such as sharing food or offering support in minor conflicts, to secure the necessary backing.
Species-Specific Succession Strategies
In chimpanzee communities, succession is often a volatile and bloody affair, relying heavily on male-male cooperation. The alpha male position frequently turns over as ambitious young males form temporary, yet powerful, alliances to violently depose the current leader. The success of a chimpanzee alpha hinges not just on strength, but on maintaining these fragile political alliances through strategic grooming and support.
Succession among gorillas, particularly the one-male, multi-female groups led by a silverback, is a more straightforward physical contest. A challenger, typically a younger silverback from outside the group or a mature male who grew up within it, must directly challenge the ruling silverback for control. If the challenger wins, the former leader often leaves the group or is killed, and the victor gains immediate control over the females. The long-term tenure of a silverback is determined by his ability to maintain physical superiority and protect the group from outside threats.
Bonobos exhibit a succession strategy where female alliances exert substantial influence over male dominance. Males often derive their status from their mothers, and high-ranking females actively support their sons in conflicts, effectively determining the male hierarchy. Since a coalition of females can collectively dominate any male, a challenger must secure the support of the alpha female or a group of high-ranking females to achieve the alpha position.
For orangutans, succession is less about group politics due to their semi-solitary nature. Adult male orangutans are categorized as either “flanged” (dominant, cheek-padded) or “unflanged” (subordinate, non-padded). Succession occurs through physical competition between flanged males for control over a large territory that encompasses several female home ranges. While an unflanged male may attempt to mate, a flanged male aggressively defends his territory, making succession primarily a matter of establishing spatial control.
Social and Reproductive Consequences
One immediate consequence of a new alpha taking over is the redistribution of reproductive access. The new dominant male gains priority mating rights, meaning he is responsible for siring the majority of the subsequent offspring in the group. This is the primary biological reward for achieving dominance.
A change in leadership often destabilizes the group’s social cohesion, especially after a violent takeover, such as those common in chimpanzees. The new alpha must work to establish his authority, which can lead to increased aggression and stress hormones among lower-ranking individuals. However, in some species, a peaceful transition or the establishment of a new, strong leader can quickly restore order and stability to the community.
Infanticide is an increased risk, particularly in gorillas and chimpanzees. The new alpha male may kill the unweaned offspring of the previous leader to eliminate genetic competition. This action causes the mothers to cease lactation, bringing them back into estrus and making them available for the new alpha to impregnate sooner.
Beyond mating, the new dominant individual often assumes the role of group coordinator, influencing movement and foraging patterns. The new alpha may lead the group to better feeding grounds or change the patrol routes to better defend the territory from rivals. Therefore, the success of the new leader impacts the entire group’s survival, from their social stability to their daily ecology.