Australia hosts a dizzying array of venomous creatures — including spiders, snakes, jellyfish, octopuses, ants, bees and even platypuses. But why do so many Australian animals wield this bioweapon?

These slow-moving, eucalyptus-eating marsupials native to Australia are beloved for their sweet demeanors and distinctly non-human adorableness. However, koalas do share one feature in common with ...