The quokka's face is short-snouted with a wide gape line and small rounded cheeks, so the resting position of the mouth reads to human eyes as an upturned smile. Panting to shed heat opens the mouth further, which strengthens the effect in the hot, exposed conditions where tourists usually photograph them.
Marsupial facial musculature is limited compared to that of primates, and quokkas do not have the fine control of the lips and cheeks needed to produce expressions in the human sense. Reading contentment into the shape is anthropomorphism, and it can mislead: an animal panting heavily in full sun on a summer afternoon on Rottnest is thermally stressed, not pleased.
Rottnest Island holds roughly ten to twelve thousand quokkas, while the mainland southwest populations are far smaller and more vulnerable to fire and fox predation. The species is listed as Vulnerable, a status the island's tame and photogenic animals conceal rather than reflect.