The Whistling Thorn acacia uses hollow thorns to whistle a warning to hungry animals.

The Whistling Thorn (Vachellia drepanolobium) is a dominant tree species in the East African savannas, particularly in Kenya and Tanzania. It has evolved a unique symbiotic relationship with four species of stinging ants in the genus Crematogaster. The tree produces modified thorns called domatia, which are hollow, bulbous structures that can reach up to 4 centimeters in diameter.The whistling sound occurs when wind passes over the entrance holes bored into these domatia by the resident ants. Research published in journals like 'Ecology' indicates that these holes act as resonators, similar to a flute or a whistle. This acoustic signal serves as an aposematic warning, a biological signal that alerts herbivores to a potential threat before they start feeding.Large mammals like giraffes and elephants have learned to associate this distinct whistling sound with the presence of aggressive ants. When a herbivore disturbs the branches, the ants emerge in a swarm to bite and sting the sensitive tissues of the animal's mouth and nose. This defense mechanism is so effective that it significantly reduces the amount of foliage lost to browsers compared to other acacia species.The relationship is a classic example of mutualism. The tree provides the ants with shelter in the thorns and food in the form of nectar from extrafloral nectaries. In exchange, the ants provide a mobile defense force and an acoustic warning system that protects the tree's structural integrity from massive herbivores.