Quakkas

The quokka - one of the most smiling animals in the world, is a pint-sized mammal endemic to Australia's offshore islands and distantly related to kangaroos and wallabies, is almost always happy. To see these cute beasts in action, simply scroll through the amusing photographs collected in the viral hashtag "#quokkaselfie." But why are they smiling? According to Arizona State University researchers, the quokka's joyful appearance is a result of evolution — sort of the inverse of the "resting brat face."

Quokkas are herbivores, which means they eat flora and roughage to keep their pearly whites sparkling - a tactic that also helps to keep human teeth cleaner and looking better. And, quokkas enjoy junk food, prompting officials to warn visitors to Rottnest Island not to give the quokkas parts of sandwiches. Bread becomes lodged between their teeth and, because quokkas cannot floss, feeds germs that can cause a severe or even fatal illness.