 <unk> may have developed as an evolutionary advantage as a signalling mechanism of reassurance between mother cats and nursing kittens . Post @-@ nursing cats often purr as a sign of contentment : when being petted , becoming relaxed , or eating . The mechanism by which cats purr is elusive . The cat has no unique anatomical feature that is clearly responsible for the sound . It was , until recent times , believed that only the cats of the Felis genus could purr . However , felids of the Panthera genus ( tiger , lion , jaguar , and leopard ) also produce sounds similar to purring , but only when exhaling . 
