The Evolutionary Arms Race of The Giraffe and Acacia Tree

For millennia, giraffes and acacia trees had an evolutionary arms race against each other, until mankind intervened.

Evolution is not usually thought of as an arms race. But the term, evolutionary arms race, is a prominent facet of evolution theory. In the case of the Giraffe and the Acacia tree, it’s a story of defences, counter-defences and counter-counter-defences.

Giraffes first appeared on this planet around 5-23 million years ago (mya), in the early to middle phase of the Miocene. The Giraffidae family has two extant species – the long-necked Giraffe, and its shorter-necked cousin, the Okapi.

To understand why the giraffe is the way it is, we must turn to one of its favourite foods – the leaves and twigs of the Acacia Tree. In particular, the Southern African species of Vachellia Erioloba – also known as the “Giraffe Thorn.” Its branches are dotted with razor sharp thorns, resembling barbed wire.

Why would a plant need such thorns? To keep herbivores at bay, one would imagine. But this is simply the first line of defence – one that giraffes breached early on. The giraffe’s tongue is a blackish, purple whip – 50 centimetres (cm) in length. It’s capable of weaving through and past Acacia’s spiky thorns, to grasp the leaves it so desires.

The Acacia tree can grow up to 16 metres in height. Giraffes can eat up to 30 kilogrammes (kgs) of food each day, and the camel thorn is by far among its favourite foods. This led to the belief that the giraffe evolved its long neck in order to better feast from this delicacy.

For example, the giraffe’s cousin, the Okapi, has no such long neck and doesn’t tend to eat from the Acacia. But this theory has been heavily debated as of late. One debate regards the ‘how’.

Early 19th century biologists like John Baptise-Lamarck believed that the giraffe’s neck grew because the animal kept stretching its neck to reach hard-to-get food. His counterpart, Charles Darwin, proposed that a genetic mutation resulted in giraffes with longer necks, who were more successful at eating such food. Natural selection then tended to favour these specimens. Ultimately, it is Darwin’s theory that is accepted today – although Lamarck’s work was an important precursor to Darwin’s own theory of evolution.

In 1996, zoologist Robert Simmons and Lue Scheepers put forth the “Competing browsers” theory. After observing that giraffes tended to eat lower shrubs during the summer, they looked at other factors that could result in a long neck. The “necks for sex” hypothesis is based on the way giraffes fight over a mate – by whacking each other with their necks. Sexual selection would thus favour giraffes with longer necks.

The problem with this theory is that males don’t seem to have stronger necks than females. It thus remains contentious.

The Giraffe Acacia tree might have grown out of the reach of smaller predators, but the pesky giraffes remained a problem. To deal with them, the plants then called in Ants as reinforcement.

The ‘Acacia Drepanolobium’or ‘Whistling Thorn is a distinct case in plant-insect symbiosis. The plant’s thorns are shorter than those of the Giraffe Acacia, but often feature swellings near their base – that attract aggressive species of ant. The trees also started secreting nectar that these ants favoured. A 1992 study observed how branches with more ants attracted fewer foraging giraffes – for few things hurt more than an ant sting on the tongue. But the Acacia was not done yet. To further strengthen its defences, it set up a community alarm system, with automated defences. The Acacia’s leaves emit a warning pheromone on being chewed, which can travel up to 50 yards (450 metres). This warning activates the trees that are downwind, who start to emit tannin from their leaves. This is toxic to antelopes and can cause a Giraffe a bad case of indigestion.

But while antelopes may have learned to avoid these trees, giraffes simply started approaching them from upwind, being careful not to trigger the alarm.

Perhaps in recognition of the Giraffe’s skills at evolutionary war, some plants decided to opt for the olive branch over the thorny one. During the wet seasons, the knobthorn is an important food source for giraffes, constituting up to 40 percent of its diet. The tree attracts the giraffe by simply having no defences. In exchange for being open for business, the plant has its pollen circulated by its predators – namely Giraffes and insects.

However, while some studies suggest that the giraffe isn’t an effective pollinator, the tree’s continuing lack of anti-giraffe defences suggest that the relationship is mutually beneficial.

In the modern era, as fewer giraffes now roam the Africa plains, the Whistling Thorn may be missing its old adversary. Minus Giraffes, the Whistling Thorn stopped investing in defence; no more producing the ant houses and nectar that attracted its ant ally. But this led to a different species of ant invading it – one that also called in a friend to aid its invasion. The Crematogaster Sjostedti relies on a stem-boring beetle to build holes for its nests. The result is an insect tag-team that has doubled the rate at which the Whistling Thorn is killed off – compared to when the tree only had giraffes to deal with.

The war of the Acacia trees and the giraffes may end forever if humanity continues to chase the long-necked mammal out of its old habitats. Giraffes are classified as a Vulnerable species by the International Union for the Conservation of Nature (IUCN). Fewer than 100,000 remain as of 2015.

The topics of convergent evolution and an evolutionary arms race are fascinating portals into Natural history. It serves as Nature’s own historical record of the battles and barters between species. Understanding how plants evolved, fighting against animals and vice versa may well serve as a warning for how Nature could react to a human species – that is increasingly the cause of natural genocide.

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