Pancreatic ribonuclease (RNASE1) is a digestive enzyme that has been recognized to be one of the most attractive model systems for molecular evolutionary studies. The contribution of RNASE1 gene duplication to the functional adaptation of digestive physiology in foregut-fermenting herbivores, mostly in ruminants, has been well documented. However, no one has ever done a comprehensive study on the carnivores, which are sister to the artiodactyls. Here, we sequenced this gene from 15 species of the superfamily Caniformia in order Carnivora, which all have a relatively simple digestive system and lack the microbial digestion in rumen or cecum typical of most herbivores. In contrast to our initial expectation that only a single RNASE1 gene is present in these carnivores, we observed a "birth (gene duplication)-and-death (gene deactivation)" process for the evolution of RNASE1 genes in all 3 species of Mustelidae family examined here, adding the growing diversity of RNASE1 gene family evolution. In addition, bursts of positive selection have been shown to contribute the enigmatic diversification of these RNASE1 genes in Mustelidae. The finding of the adaptive expansion of RNASE1 in animals without foregut fermentation provides another opportunity for further studies of the structure, function, and evolution of this gene, raising the possibility that new tissue specificity or other functions of RNASE1 genes might have developed in these species.