Howler monkeys (genus Alouatta) are large folivorous primates living in South America. We tested for the application of both Rensch’s rule and Bergmann’s rule to body size variation in Alouatta. We found that Rensch’s rule does apply in howlers. In Alouatta, males exploit dominance rank competition, and take advantage from seasonal abundance of high nutritious fruit supply in their diet. This mating system and dietary charateristics suggest positive male selection for body size is responsible for Rensch’s rule. However, since folivory favors large body size in primates (to lower mass specific metabolic rate) and it is the primary dietary habitus in howlers, larger species do occur in the Amazon basin, originating a reversed Bergmann’s rule pattern for both males and females at the interspecific level. The spatial and phylogenetic components of such body size patterns of variation are both important, implying Alouatta ecomorphological differences to occur above the species level, justifying their non-overlapping geographic distribution.