Anthropology, University of Texas at Austin
Friday All day, Plaza Level
Group living species often rely on vocal communication to maintain spatial and social cohesion among group mates. These vocalizations become increasingly important during travel when individuals experience the greatest risk of becoming separated from the group. Several authors have suggested that male howler monkeys produce glottal vocalizations to help facilitate and coordinate group travel. However, quantitative assessments of the use and function of these calls are limited. To better understand the role of glottal vocalizations in traveling contexts, we analyzed over 250 hours of spatial and behavioral data collected on a single group of mantled howler monkeys (3 adult males, 6 adult females) in June and July of 2010 on Barro Colorado Island, Panama. We found that glottal vocalizations occurred in 74 percent of observed group movements, with the greatest proportion of calls occurring at the end of a long rest period and/or near the start of a travel progression (χ2 = 50.5, df = 3, p < 0.01). All three males produced glottal vocalizations with equal frequency, and in all successful initiations of group travel, the azimuth of the groups’ final trajectory was significantly clustered around the caller (Rayleigh Test of Uniformity for Circular Variance, R = 0.2146, N = 33, p < 0.05). These data support the hypothesis that glottal pop vocalizations by male howlers function as an auditory signal to both motivate and direct group travel.