Age-Related Differences in the Use of the "Moo" Call in Black Howlers (Alouatta caraya)

Many group-living mammals and birds give both contact and distress calls. Contact calls are thought to operate in maintaining group stability and cohesion; distress calls are thought to operate in soliciting help, often from relatives. Here we propose that a single call of black-and-gold howlers (Alouatta caraya) serves both needs, but differently so during ontogeny. We collected data on the contexts of moo calling: on group diameter before, during, and after calling; and on calling rates before and after potentially stressful events or situations. Several relationships suggested a link between moo calling and group cohesion. The highest calling rates were found during travel, when separation is most likely, whereas rates were significantly lower during feeding, a sedentary activity. During periods of moo calling, group spread was significantly greater than normal, and spread decreased significantly after calling began. Moo calls were also given in situations of potential distress. When reluctant to cross an arboreal gap or refused the option of travel on an adult, infants gave moo calls more frequently than just after these predicaments were ended; infants also called more frequently immediately after a behavioral conflict. The moo call apparently has a dual role, in contact and distress, with the signal's use changing with caller age: both uses may reflect a common underlying motivation of insecurity, as different contexts come to trigger insecurity during development.