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 exists regarding its main function. During a 28-month study of five groups of black howler monkeys (Alouatta pigra) at Palenque National Park, Mexico, we examined whether howling mainly functioned in the defense of food resources, mates, or infants vulnerable to infanticide. We recorded 602 howling bouts. Howling occurred more frequently when monkeys were feeding, particularly on fruits, and less frequently when they were resting than would be expected by chance. Furthermore, howling was concentrated in areas of the home range in which major feeding sites were located. Howling did not occur more frequently when vulnerable infants or potentially fertile females were present versus absent, nor did the howling rate increase with an increasing number of vulnerable infants or potentially fertile females in the group. Howling bouts lasted on average 14.4 ± SE 0.5 min, and call duration was not influenced by the presence of vulnerable infants or potentially fertile females. The duration of spontaneous calls, however, was positively correlated to the percentage of feeding time in the vicinity of howling locations. In addition, vocal displays lasted longer when neighboring groups and extragroup males were within visual contact compared with spontaneous calls and calls in response to nearby calls in which there was no visual contact between callers. Our findings suggest that loud calls in black howler monkeys are multifunctional, but most frequently occur in the defense of major feeding sites. These calls also may function in the defense of infants and mates during encounters with extragroup males. Am. J. Primatol. 76:1196–1206, 2014. © 2014 Wiley Periodicals, Inc.