Acacia trees, now largely recognized as Vachellia and Senegalia in Africa, are a globally significant plant group. These trees and shrubs dominate arid and semi-arid regions, from the African savanna to the Australian outback. Their ability to thrive in harsh environments makes them foundational species, providing shelter and sustenance. Acacia species are a rich source of forage, offering protein-rich leaves, shoots, and pods that support a variety of animal life. The consumption of this foliage drives a complex evolutionary arms race between the plants and the animals that seek to eat them.
Large Herbivores and the Acacia Diet
The African savanna’s largest herbivores depend heavily on the Acacia as a primary food source, developing specialized physical adaptations to bypass the tree’s formidable defenses. Giraffes are particularly adept browsers, using their great height to reach the freshest, often less-defended foliage up to six meters high. Their prehensile, purplish tongues can be nearly half a meter long and are equipped with a tough, protective coating, allowing them to carefully strip leaves from thorny branches.
Kudu and Impala, which are smaller browsers, feed on accessible lower branches and fallen pods. They often rely on speed to avoid the plant’s chemical defenses from fully engaging. Elephants, however, employ sheer power, stripping bark and consuming large branches and woody stems, especially during the dry season when other forage is scarce. The destructive feeding of elephants can significantly alter the structure of Acacia woodlands.
Heavy browsing by large animals, such as giraffes, stimulates the trees to grow denser thorns and produce more nectar. This increased investment in defense and rewards helps maintain the tree’s protective symbiotic insect relationships.
Highly Specialized Insect Relationships
Acacia trees engage in highly specialized, co-dependent relationships with insects, most famously with certain species of ants in East Africa. The whistling thorn Acacia (Vachellia drepanolobium), for example, provides aggressive ant species, such as Crematogaster mimosae, with both shelter and food. The tree produces swollen, hollow thorns, known as domatia, that the ants use as nesting sites and rearing chambers for their young.
The trees further reward their protectors by producing sugary secretions from specialized glands on their leaves called extra-floral nectaries. In return for this food and housing, the ant colonies act as highly effective security forces for the tree. They aggressively defend the Acacia by swarming, biting, and spraying formic acid at any insect or herbivore—including giraffes—that attempts to feed on the foliage.
Some ants, like Crematogaster nigriceps, aggressively prune the tree’s flower buds, effectively sterilizing it to prevent contact with rival plants. This behavior suggests a trade-off where the tree sacrifices reproduction in the short term to gain increased protection and vigor against browsing mammals.
How Acacia Protects Itself
The Acacia genus has evolved a dual defense system combining physical barriers with sophisticated chemical warfare to survive heavy predation. The most obvious defense is the physical presence of thorns or spines, which vary widely in length and density across different species. Some species, like the umbrella thorn (Vachellia tortilis), exhibit both straight, stiletto-like thorns and recurved, claw-like thorns, earning the Afrikaans name “haak-en-steek,” meaning “grab and stab.”
The plant’s second line of defense involves chemical compounds, primarily high concentrations of tannins and, in some species, cyanogenic glycosides. Tannins are bitter, water-soluble compounds that bind to proteins, making the leaves unpalatable and interfering with the herbivore’s ability to digest the protein.
Acacia trees can also employ an “induced defense,” rapidly increasing the concentration of these toxins in response to being browsed. When a herbivore tears a leaf, the tree releases volatile organic compounds into the air. These compounds act as airborne chemical signals to neighboring Acacias up to 50 yards away. This warning prompts nearby trees to preemptively increase their own tannin and cyanogenic production, forcing the browsing animal to quickly move on.