Vachellia drepanolobium

Family / Families: Fabaceae
Scientific Name(s): Vachellia drepanolobium (Harms ex Sjöstedt) P.J.H. Hurter
Location(s): Hell's Gate National Park, Kenya
Entry Author(s): Dominic Janus
Copyright Holder(s): zimbart@Flickr aka Bart Wursten
Image Permissions: Please contact the copyright holder for permission to use.
Photographer(s) Appreciation: Thank you to Bart Wursten aka zimbart@Flickr for this great photograph of whistling thorn. Much appreciated!
Tags: Fabaceae, Fabales, Kenya, via Flickr

The gall-like spine base of Vachellia drepanolobium

To say it’s an interesting species is an understatement. Vachellia drepanolobium, or whistling thorn, is an acacia tree native to eastern Africa that grows to 6m (18 ft.) in height. The whistling from the bulbous bases of its thorns is a result of holes made by ants, but there is more to the story than that.

Whistling thorns are fire-adapted species in their range throughout East Africa. They play a foundational ecological role, in part because Vachellia drepanolobium and related species are myrmecophytes:

Ant-plants, or myrmecophytes, are plants (-phytes) that maintain a symbiotic or mutualistic relationship with various species of ants (myrme(x)-). In most cases, neither the ants nor the plants are wholly dependent upon one another for survival, but instead gain a competitive advantage from the symbiosis therefore fostering the relationship between the species.

One researched consequence of the ant-plant relationship is the suggestion that the whistling noise may be a possible anti-predatory mechanism, known as auditory aposematism.

The holes in the Vachellia drepanolobium thorn bases are also the doorways into the ant homes. Various ant species occupy these, though a single tree typically only harbours one species of ant. Each ant species has its own distinct mutualistic behavior. As noted in Wikipedia,

Crematogaster mimosae, has the strongest mutualistic relationship, aggressively defending trees from [large] herbivores while relying heavily on swollen-thorns for shelter and feeding from nectar produced by glands near the base of leaves

Another species of ant uses the tactic of destroying the nectar glands in an attempt to discourage invasions from other ants. The existence of such complex myrmecophytic (ant-tree) relationships has been linked to landscape-scale health of these trees in their interactions with large mammals. Wikipedia summarizes the article in the link:

The symbiotic relationship between the trees and the ants appears to be maintained by the effects of browsing by large herbivores. At a site in Kenya, when large herbivores were experimentally excluded, trees reduced the number of nectar glands and swollen thorns they provided to ants. In response, the usually dominant Crematogaster mimosae increased their tending of parasitic sap-sucking insects as a replacement food source. In addition, the number of Crematogaster mimosae-occupied trees declined while twice as many become occupied by Crematogaster sjostedti, a much less aggressive defender of trees. Because Crematogaster sjostedti benefits from the holes created by boring beetle larvae, this species facilitates parasitism of trees by the beetles. As a result, the mutualistic relationship between whistling thorn trees and resident ants breaks down in the absence of large herbivores, and trees become paradoxically less healthy as a result.

On top of its substantial ecological role, parts of the tree have been known to be used to make kindling or glue (Wikipedia). Additionally, they can be easily harvested due to their ability to coppice after being harvested or killed by fire, but charcoal production from the wood has proven so far to be inefficient as a source of sustainable energy. Returning to the ecological theme, this regenerative ability presumably helps whistling thorn exist as a dominant tree species in dry habitats with frequent fire regimes.