Drop bears are commonly said to be unusually large, vicious, carnivorous koalas that inhabit treetops and attack their prey by dropping onto their heads from above. They are an example of local lore intended to frighten and confuse outsiders, and amuse locals, similar to the jackalope, hoop snake, wild haggis or snipe.
Stories of drop bears are often told to unsuspecting foreign visitors to illustrate Australian deadpan humour. It is often suggested that doing ridiculous things like having forks in the hair or Vegemite or toothpaste spread behind the ears will deter the creatures.
There are several possible origins of the drop bear myth.
Some suggest that it is designed to discourage children from straying needlessly below eucalyptus trees, protecting them from the very real danger of getting hit by a falling branch. Arbitrary detachment of old branches is common with certain species of the eucalyptus, which are known as 'widow-makers' for this very reason. Similar theories are attached to the cone from the bunya tree.
Another possibility is that the myth is based on a real animal. It appears to have first appeared during the latter half of the 20th century, and may have its origins with Phascolarctos stirtoni or perhaps Thylacoleo carnifex, which belong to a group of extinct animals known as Australian megafauna. The prehistoric creatures were approximately twice the size of modern koalas. T. carnifex is thought to have been an arboreal (tree-dwelling) predator that may well have ambushed prey by dropping on it from overhead branches, similar to how cougars often hunt.
However, despite the merit or otherwise of these seemingly possible explanations it seems far more likely that drop bears started out as a joke. This often is expressed by scaring newcomers or foreigners with alarming stories of the alleged dangers of the bush that are almost too incredible to believe. Nevertheless the delivery is so deadpan that it convinces the listener of its veracity, unless he or she is uncommonly perceptive or is familiar with this type of joke known as a furphy. This approach can be a test of the new person and he or she will be well regarded if they see through the joke, even if tentatively at first. Obtaining affirmation of the joke from other locals is often difficult, as almost all Australians are aware of the drop bear story and will readily attest to their existence.