The complex, graded vocal repertoire of the social Rüppell's vulture

The evolutionary origins of complex communication remain contentious, with social, ecological, and environmental factors proposed as selective drivers. The Social Complexity Hypothesis for Communicative Complexity (SCHCC) predicts that the cognitive demands of navigating socially intricate groups favour the evolution of complex communication. Although several studies provide support for the SCHCC, its empirical basis remains limited and inconsistent, as evidence is drawn predominantly from a small number of well-studied taxa (primarily primates), often neglects species' complete vocal repertoires, and employs inconsistent approaches for assessing social and vocal complexity. This narrow focus limits its generalisability across animals, underscoring the need to characterise the full communicative complexity of additional model species to expand its evolutionary relevance. Vultures, with their varied social structures, provide a unique system to test this hypothesis, yet foundational work on communication in this guild is limited. Here, we present the first quantitative analysis of the vocal repertoire of the critically endangered, highly social Rüppell's vulture, based on 2,500 calls recorded from two captive populations (35 individuals) across diverse social contexts. Using a fuzzy clustering approach, we reveal a repertoire far more complex than previously assumed, comprising multiple discrete and graded call types, with gradations likely conveying additional information. Calls serve diverse functions, including mediating parent-offspring interactions, manipulating conspecifics, and resolving conflicts. The Rüppell's vulture repertoire exceeds the average call repertoire size observed across birds and is likely an adaptation to this species' social life. Our results further establish vultures as a promising model for studying the vocal evolution and provide a framework for comparative analyses that can illuminate the selective pressures shaping complex communication.