Thorny Diets, Part II: Acacias & Giraffes in South Africa

By Bruce G. Marcot, Ecology Picture of the Week:

Giraffes browse.  They are near-perfect browsing machines.  They can reach parts of trees that no other land-based herbivore can reach.  They can reach tips of branches that most arboreal herbivores cannot reach.  And sometimes they browse on the most thorny trees, such as this Acacia in South Africa.

Giraffes pull twigs into their mouths by their lips and by their long prehensile tongues which can reach a good 45 cm (nearly 18 inches)!  Giraffes spend up to half of a 24-hour day browsing.   

As with thorned plants elsewhere, this acacia likely evolved thorns as a defense against excessive browsing.  But it didn't stop this giraffe from enjoying mouthfuls of leaves and spikes, alike.  Giraffes can crush the thorns with their molars apparently with little ill effect.

In fact, one study (Bond and Loffell 2001) revealed that the introduction of giraffes in a game reserve in South Africa can significantly alter the structure and composition of the plant communities.  The introduced giraffes browsed some Acacia species to local extinction and killed individual trees, although the thorned species pictured here was largely unaffected.  

This acacia tree produces very nutritious leaves which are browsed by various other game animals, as well.  After they fall to the ground, the seed pods are eaten by wild game.  The bark is also eaten by elephants and can be used for medicine.  And the heartwood is used for firewood.  But few animals (or people!) can tolerate consuming its thorns.

The thorns deter at least some browsing.  Often, thorns evolve to protect otherwise highly-palatable foliage or fruits, instead of the tree diverting its energy into producing toxins or other chemical deterrents.

  --Bruce G. Marcot

Image and text © Bruce G. Marcot, Ph.D. Research Wildlife Ecologist,
who produces the Ecology Picture of the Week website.