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?

In the desolate plains of southern Australia scampers a small creature with pointy ears, a silky tail, and a cream-colored belly that ecologist Steve Morton describes as “wonderfully beautiful.” Its ...