Acacia
This species of acacia grows two kinds of thorns. The main defense is provided by pairs 5cm long arranged nearly at a right angle. Smaller stipular spines grow between the pairs of big thorns. These emerge from hollow galls, bulbous swellings 2 to 3cm across. One of four different species of ant lives in each of these igloos, which they open up by cutting holes into them. A dying bush whistles as the wind blows over these entrances. Most acacias make toxins that rush to leaves that are under attack by browsers. The whistling thorn does not. It is infested with stinging ants that swarm out and prepare to bite anything they can when the branch is disturbed. Most browsers seem to avoid infested bushes, perhaps because the ants stink of formic acid.