Screams in the Night: Pilot Study Reveals Moderate Evidence for Individual Differences in Lorisoid Vocalizations

Abstract

Social complexity is argued to be a driving factor in the evolution of communicative complexity. Complex social systems require individuals to form relationships with many conspecifics and interact in a wide variety of contexts over time, thus selecting for acoustic communication systems complex enough to facilitate these relationships. To better understand the evolution of such social and communicative complexity, we investigated a nocturnal, solitary forager, Garnett’s bushbaby (Otolemur garnettii), as a lorisoid model for the ancestral primate social systems from which more complex systems evolved. We hypothesized that it would be advantageous for solitary foragers to have individual differences in long-distance calls, as this could be crucial to the maintenance of their dispersed social networks. We tested for individual differences in the long distance bark vocalization. We measured 6 frequency and temporal parameters for 120 barks (15 barks from each of 8 individuals housed at the University of Southern Mississippi). Principal component and discriminant function analyses assigned the calls to the respective individuals at a rate that was moderately accurate and higher than chance (binomial test: 54.2% correct, P < 0.001, chance = 12.5%). This pilot work provides moderate evidence for individual differences and is the first such study to be conducted on lorisoids. Because individual differences have been documented in the vocalizations of solitary foraging lemuroids, we suggest that moderate individual differences may have been present in ancestral primates and contributed to the dispersed social system that is thought to have been the foundation from which increased social complexity evolved in primates.