Abstract
The Machiavellian/Social Intelligence Hypothesis proposes that a complex social environment selected for advanced cognitive abilities in vertebrates. In primates it has been proposed that sophisticated social strategies like obtaining suitable coalition partners are an important component of social intelligence. Knowing the rank relationships between group members is a basic requirement for the efficient use of coalitions and the anticipation of counter-coalitions. Experimental evidence for such knowledge currently exists in only few species. Here, we conducted rank reversal playback experiments on adult females belonging to three different groups of free-ranging vervet monkeys (Chlorocebus aethiops pygerythrus) to test their knowledge of the female hierarchy. Playbacks simulating rank reversals (subordinate aggressing a dominant) induced longer looking times than playbacks simulating a dominant aggressing a subordinate. Vervet monkey females therefore seem to compute the rank relationships between other females. Our results suggest that detailed social knowledge about rank relationships may be widespread in primates and potentially also in other species living in stable groups.
Highlights
There is enormous variation in both absolute and relative vertebrate brain size [1]
While current explicitly experimental evidence in primates is restricted to two large brained species [22,23], observational evidence suggests that detailed knowledge about the relationships between other group members may well be widespread in primates [16,18] and more generally in species living in large stable groups [24]
Our results demonstrate that female vervet monkeys distinguish between playbacks that suggest a female conflict congruent with the hierarchy and playbacks that suggest a rank reversal
Summary
There is enormous variation in both absolute and relative vertebrate brain size [1]. The complexity of a species’ social life has been identified as one important factor promoting the evolution of large brains [2,3], though exactly which aspects of social life require larger brains remains largely unknown. Individuals of many species obtain such information through ‘eavesdropping’ on social interactions in a communication network [8] Evidence for this capacity has been provided in a variety of vertebrate taxa [9]. While current explicitly experimental evidence in primates is restricted to two large brained species [22,23], observational evidence suggests that detailed knowledge about the relationships between other group members may well be widespread in primates [16,18] and more generally in species living in large stable groups [24]. We predicted that if vervet monkey females know all rank relationships between other females they should look longer at speakers if ranks are reversed
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