The social functions of complex vocal sequences in wild geladas

Several studies show that highly social taxa produce relatively more complex vocalizations. Yet, very few of these cases have demonstrated the function that vocal complexity plays within a highly social setting. Here, we assess potential functions of vocal complexity in male geladas (Theropithecus gelada) living in the Simien Mountains National Park, Ethiopia. Geladas are known for both their diverse vocalizations (routinely produced in long sequences) and their complex social structure (extremely large groups and long-term male-female bonds). We tested whether sequence complexity (i.e., including elaborate derived calls that are unique to geladas and absent in closely related taxa) or size (i.e., number of calls) may function (1) to counteract the challenges of living in a large group (overcoming conspecific noise and crowding, maintaining cohesion), or (2) to maintain social bonds with females. We found that an increase in conspecific noise contributed to the use of longer and more complex sequences. However, behavioral contexts in which the risk of separation was highest (i.e., traveling) were associated with only longer (but not more complex) sequences. We also found that sequence complexity (but not size) was associated with male-female bonding as complex call sequences were produced primarily when males were in close proximity to and approached females, and they led to males being groomed by females. Together, these findings suggest that, while a noisy backdrop of conspecific vocalizations might contribute to vocal complexity, the potential driver of gelada vocal complexity is the need to maintain cross-sex bonds.Significance statementWhy do some animals make many diverse sounds while others make only a few simple sounds? Broad comparisons suggest that sociality may be important as more social species (e.g., those with large group size and social bonding) tend to make more types of sounds. Yet, it remains unclear why gregarious species need an expanded call repertoire. Here, we take advantage of previous work on a highly social primate (geladas) that identified several complex vocalizations that contribute to gelada's expanded vocal repertoire. To better understand why geladas evolved an expanded set of calls, we focus on the context where complex calls are produced and the responses those calls elicit. We found that the potential driver of the use of more call types is the need to maintain cross-sex bonds, suggesting an important role for male-female bonds in the evolution of vocal complexity.