The roaring of southern brown howler monkeys (Alouatta guariba clamitans) as a mechanism of active defence of borders.

Our study aimed to understand the function(s) of roars of southern brown howler monkeys (Alouatta guariba clamitans). The study group called almost exclusively on its range borders and preferentially on early mornings, but with no dawn peak. Intergroup encounters were associated with 88% of all sessions. Predation deterrence and regulation of access to mating partners do not seem compatible with our findings, but more work is needed to reject the latter hypothesis. Their roars seem to be related to intergroup spacing, but through an active defence of borders, instead of mutual avoidance or regular advertisement of occupancy, as proposed for other howler monkey species. Copyright 2007 S. Karger AG, Basel.