In foregut-fermenting mammals (e.g., colobine monkeys, artiodactyl ruminants) the enzymes pancreatic ribonuclease (RNASE1) and lysozyme C (LYZ), originally involved in immune defense, have evolved new digestive functions. Howler monkeys are folivorous non-colobine primates that lack the multi-chambered stomachs of colobines and instead digest leaves using fermentation in the caeco-colic region. We present data on the RNASE1 and LYZ genes of four species of howler monkey (Alouatta spp.). We find that howler monkey LYZ is conserved and does not share the substitutions found in colobine and cow sequences, whereas RNASE1 was duplicated in the common ancestor of A. palliata, A. seniculus, A. sara, and A. pigra. While the parent gene (RNASE1) is conserved, the daughter gene (RNASE1B) has multiple amino acid substitutions that are parallel to those found in RNASE1B genes of colobines. The duplicated RNase in Alouatta has biochemical changes similar to those in colobines, suggesting a novel, possibly digestive function. These findings suggest that pancreatic ribonuclease has, in parallel, evolved a new role for digesting the products of microbial fermentation in both foregut- and hindgut-fermenting folivorous primates. This may be a vital digestive enzyme adaptation allowing howler monkeys to survive on leaves during periods of low fruit availability.