Spatial and Diurnal Distribution of Loud Calling in Black Howlers (Alouatta pigra)

Loud calling, i.e., howling, is the single most distinctive behavioral attribute of the social system of howler monkeys (Alouatta spp.), yet no general consensus has been established regarding its function. During a 16-mo study, we investigated the degree to which howling bouts varied in duration and spatial and temporal patterns across different social contexts in three groups of black howlers (Alouatta pigra) at Palenque National Park, Mexico, to assess how howling functions in intergroup spacing. We recorded 359 howling bouts, of which 42% were spontaneous with no apparent cause, 28% followed nearby howling without visual contact with the caller(s), 25% were during intergroup encounters, and 5% were during encounters with solitary individuals. The mean duration of howling bouts was 14.8 +/- SE 0.6 min. During encounters with neighboring groups or solitary individuals howling bouts lasted significantly longer than spontaneous and reply calls. Spontaneous and reply howling showed a bimodal pattern with a marked concentration around dawn and a second increase of howling in the afternoon. In contrast, howling during encounters with neighboring groups or solitary individuals occurred randomly throughout the day. In addition, howling, irrespective of the context in which it was given, occurred throughout the groups' home ranges without concentrations in the heavily used core areas or borders of the home ranges. Our findings suggest that loud calling in black howlers functions principally in regularly announcing the occupancy of an area, and is concentrated in the morning when sound propagation is optimal.