The Whistling thorn is commonly found growing over much of the African savannah regions.
The whistling thorn got its name because when wind passes through the holes of the thorns made by ants it makes a whistling sound.
This tree has some unique characteristics. As its name suggests this tree is full of thorns, and this is the main way it protects itself. These thorns are larch, growing up to about 4 cms long.
Along with these thorns there are other modified thorns, which are called ‘stipular spines’ and these are joined at the branches by little bulb shaped swellings about an inch in diameter.
These stipular spines have four different kinds of ants living in them that pierce the thorns to get in. These holes give out a whistling sound when wind passes through them.
The whistling thorn is unlike other acacia trees because it does not have toxic chemicals that keep insects and birds away. Some botanists are not sure if the relationship with the ants is a symbiotic one (where both the tree and the ant benefit), or a parasitic one (where only one benefits).
One of the animals that can cause the most damage to the Whistling thorn is the elephant. The ants get together when an elephant tries to eat the leaves, and attacks by climbing up the inside of the elephant’s trunk and biting it. This causes them tremendous pain.
The stipular spines are a great value to the tree because these stinging ants would attack at even the slightest movement, and so these ants making their homes in it protect the tree.
The reason botanists are not sure of the relationship with the ants is because there are other times that some ant species would snip (prune) the branches and flowers of their whistling thorn so other enemy ant colonies cannot come to the tree.
This pruning makes the whistling thorn produce a sugary juice at the ends
One of the ways that the tree saves moisture during the dry hot climate is its leaves and evolved into tiny leaflets (pinnae), which can turn to sunlight for photosynthesis, or turn away to avoid transpiration.
There are many leaflets on the tree, so the tree has enough leaflets for photosynthesis, even after animals have grazed on them.
During very dry seasons, this tree sheds its leaves. This allows it to conserve water and nutrients that would otherwise have been lost during transpiration.
The fragrant creamy white flowers grow just before leaves grow back in the rainy season.
Like most trees in the savannah, which are fire adapted, the whistling tree too, is well used to fires, and can easily bounce back after a bush fire has ruined it to the ground.
Botanists have also observed that the tree responds to the presence of plant eaters. When herbivores are around, the plant slowly lengthens its spines. When they are not around, these spines very slowly reduce in lengths.
Every part of the whistling thorn tree serves an important part in its survival. The combination of protection the tree gets from the ants, and from its own adaptations ensures this tree’s survival in the dry savannahs.