Mutualism of the Month: A relationship between a tree and an ant

The whistling-thorn tree Acacia drepanolobium  employs ant guards to protect the tree from the largest land mammal, the African elephant.  The swollen thorns provide housing for the guard ants.  Photo courtesy of wikimedia.org.

Every month we showcase a relationship between two or more species or groups of species that can be considered a mutualism: a relationship where both members benefit. This month’s mutualism is the ant-Acacia mutualism. 

Acacia are a genus of shrubs and trees which characteristically have thorns (the name is derived from the Greek work akis which translates to thorn).  Several species of Acacia host ant species which provide the trees with several beneficial services.

One example of the ant-Acacia mutualism takes place in the African savanna.  Within these savannas, elephants inflict extensive damage to woody plants.  Trees have evolved multiple defenses to this catastrophic herbivory (the whole tree can be consumed).  Acacia trees employ both chemical deterrents to reduce palatability as well as spines to deter elephants from grazing on them.  Despite such defenses, trees frequently suffer severe damage, resulting in the death of mature trees!  One species of Acacia, namely the whistling-thorn A. drepanolobium, employs ant body guards.  These trees seldom experience elephant herbivory.  If elephants attempt to consume branches of the whistling-thorn, ants swarm the trunk of the elephant biting the sensitive internal areas.  This defense is extremely effective.

In return, the trees pay back their body-guards with housing in the form of swollen thorns called domatia.  Many different species of plants (not just Acacia) provide domatia for ants, mites, and other arthropods (things with jointed legs such as insects and spiders) which provide the plant with some service in return.  For a video of what the inside of a typical domatia looks like (not A. drepanolobium), check out the video below (prepare for some epic music).  On top of domatia, A. drepanolobium also provides sugar-rich nectar from glands called extrafloral nectaries located near the base of the leaves. This nectar provides some species of guard-ants with all the food they need.

The mutualistic ant Crematogaster nigiceps feeding on a extrafloral nectaries on Acacia drepanolobium .  Photo courtesy of Todd Palmer.

The mutualistic ant Crematogaster nigiceps feeding on a extrafloral nectaries on Acacia drepanolobium .  Photo courtesy of Todd Palmer.

This relationship is very costly for the Acacia trees.  In an experiment where A. drepanolobium had their ants removed, the tree grew much faster than those with ants.  The trees provide all the food and shelter for a colony of ants for a seemingly small return (they may never get bothered by elephants!).  While the cost may be heavy, the benefits are enormous over the lifetime of the tree.  Without the ants, the trees are consumed completely by herbivores, removing them from the gene pool.  It is better to invest in an expensive safety net that you may never use, than to not have the safety net when you need it.  The devastating consequences of herbivory make the costs of protection a good investment.  It’s analogous to paying for a fire department.  You almost never need the service, but when your Thanksgiving turkey is on fire you’re happy they are there to put it out.

The mutualistic relationship between the trees and ants is maintained by the browsing of herbivores and breaks down in its absence.  When herbivores are experimentally excluded, trees reduce the number of nectaries and domatia.  In response, ants begin to tend parasitic sap-sucking insects as a replacement food source for themselves, as well as facilitate the parasitism of trees by wood boring beetle larvae which provide the ants with shelter.

For a more detailed explanation of the relationship check out this video by Todd Palmer of the University of Florida.

Don’t forget to check out the previous installments of mutualism of the month below and stay hungry!

More Mutualisms of the Month