Steve Garland's photos with the keyword: Whistling Thorn

This species of Acacia forms hollow galls which are used by ants to nest in. The ants get to hunt on the tree and get a safe nest. They then sting any animals that try to eat the tree. Mutual benefits! It is called the whistling thorn because the wind makes a whistling noise when it blows across the holes in the galls/nests.