Acacias comprise woody shrubs and trees native to warmer regions of the world, including African savannas. Among them, the iconic umbrella-shaped acacias dominate the landscapes of southern and eastern Africa, taking center stage in numerous sunset photographs. They are also the favourite food of giraffes
An unyielding law prevails in nature: species that fail to adapt to the ever-shifting natural world face ruthless elimination. For example, plants have developed several defensive mechanisms to deter herbivores (plant-eaters). In response, these animals are compelled to constantly evolve strategies to overcome plants’ defences.
The acacia and the giraffe are no exception. To avoid indiscriminate grazing by antelopes, buffalos, and giraffes, the acacia trees developed a clever adaptation. They now feature tall, bare trunks with leaves concentrated solely at the upper reaches, ensuring their continued existence.
However, over time the giraffes developed long necks capable of reaching higher foliage. Each giraffe consumes a whopping 29 kilograms of foliage daily. Left unchecked, this could potentially wreak havoc on the savanna woodlands.
So in the relentless struggle for survival, acacias grew thick thorns on their branches to deter the giraffes from plundering their foliage. This evolutionary adaptation earned them their name acacia, derived from the Greek word Akis, meaning thorns.
The thorny defence of acacias, however, was not fully effective, because the giraffes developed long, tough prehensile tongues stretching to nearly 45 centimetres. This, combined with their sturdy lips and palate, helped the giraffes browse the leaves and twigs without injury. It would appear the giraffes have overcome the challenge. But species continue to evolve to stay competitive or they will become extinct.
Thus, the acacias developed the next line of defence—the biosynthesis of tannins (natural chemicals) when their leaves are torn. Tannins have an unpleasant taste. They also disrupt the digestion—enough reasons to keep the giraffes at bay.
Taking their chemical defence to the next level, the acacias release ethylene gas, a pheromone, into the air when grazed by the giraffes. The neighbouring acacias downwind pick up this signal and mount a coordinated defence by increasing the production of the bitter tannins, thereby deterring the herbivore.
Yet the giraffes have learned to move on to unaffected trees located in the direction of the headwind. This strategic move is effective because pheromones are transmitted through air. It means that the acacia tree situated in the headwind direction may not receive timely warning from the neighbouring acacias.
The acacias have developed yet another defence mechanism by forming a symbiotic alliance (mutually beneficial relationship) with stinging ants—the trees offer these ants nectar and a sanctuary within their thorns, and the ants protect the trees by attacking the marauding browsers. However, this strategy appears to be less effective against the giraffes. The time lapse between the ants being alerted by browsing-induced vibrations and launching their attack allows the giraffes to retreat after a few bites of leaves before the ants’ onslaught.
The battle between the acacias and giraffes in their quest for survival is a testament to the fascinating workings of nature. It exemplifies co-evolution that occurs over millions of years.
Too good 😍😘