Weird and wonderful members of the plant kingdom - Acacia Towns

Most acacias have an alkaloid that tastes bitter, which prevents them being eaten by sheep or goats, or camels - depending on where they are in the world. However, the bullhorn acacia has developed a different strategy that allows it to grow in more moist, lush areas than its alkaloid bearing cousins, because alkaloid production slows down a tree's growth and means it is more at risk of fungal and other infections.

How do they do it? Their enlarged, hollowed-out spines, which are often used for jewellery by local people, are occupied by fiercely biting-stinging ants that protect them from browsing herbivores. The 'guests' are Pseudomyrmex ferruginea, very aggressive ants with a painful sting. Disturbed ants release an alarm pheromone and rush out of their thorn 'barracks' in great numbers. Livestock can apparently smell the pheromone and avoid these acacias by day, when they see the enlarged thorns and by night when the pheromone alerts them to the risk they are running. In addition, the fierce ants protect their home from leaf-cutting ants and run patrols that clear away invasive seedlings around the base of the tree that might overgrow it and block out vital sunlight. The acacias reward their ant helpmates with thorn tower-blocks to live in, and a carbohydrate-rich nectar from glands on the leaf stalks. Finally, they provide nourishing, protein-lipid morsels called Beltian bodies on the leaflet tips - it is believed these are specially evolved to make the ants patrol to the very edges of the leaves, and kill off any aphids or other pests they might find there.

Prior to settling on a thorn acacia, the winged virgin queen ant goes on a mating flight to the highest treetop or nearby hill. Here she gets inseminated by a winged male and then hunts for an acacia in which to lay her eggs. She cuts an entrance hole in a green thorn, hollows it out, and then deposits her eggs. Subsequent entrance holes are cut by the new generations of worker ants to create a 'city' of ants.

Acacia photograph by wayfaring stranger, used under a creative commons attribution licence