From learning ‘who monkeys support’, we infer ‘What they know’

Abstract

Though understanding what monkeys see and know without inferring anthropomorphically is virtually impossible, the more detail and concrete the ground of this inference we have, the more promising our surmise would be. A closer look into the male-male relation in cercopithecines, a genus of old world monkeys, confirms what scientists have theorized: in the absence of inclusive fitness advantage, the males cooperate closely with nonkins following the norm of reciprocity, resulting in a stratified rather than linear, continuous dominance hierarchy, as a function of seniority and age. This feature is ubiquitous and found in 3 species of macaques and yellow baboons; and it is reliably robust across 4 continents, 5 study sites, 17 troops, and 297 monkeys. In other words, although new members of a group are often at the prime age of belligerence, because senior males share a longer history of cohabitation and reciprocal relation with one another, they intervene in each other’s fight and outcompete the new members who are often not supported. This implies that, in cercopithecines, the support to nonkins may be guided by a mental scorebook, which enforces them to reciprocate in the future when their reciprocal partners are in need.