On my recent trip to Tanzania in East Africa, I was entranced to discover a new tree of which I had not heard before, or if I had, I had forgotten. It’s the whistling thorn-tree, Acacia drepanolobium, and it gets its name from how its swollen thorn-bases can whistle in the wind, where ants or other insects have bored holes in the bulbous nodes. Apparently these swellings are caused by the tree itself, or in other words they are not true galls. They are naturally hollow, and several species of ants use these hollow nodules as homes to nest in. Furthermore, some of these species of ants defend the thorn-tree from grazing herbivores like elephants and giraffes. At left is a photo I made of such a swollen thorn-base harbouring nesting ants (the eggs are white and can be seen there). Those swollen bases are termed domatia.

The Wikipedia entry on the whistling thorn depicts this particular species of thorn-tree as being a myrmecophyte, that is, a plant that has formed a mutualistic relartionship with certain species of ants. The thorn-tree is described as providing shelter to the ants, and necter as food for the ants. In return, the ants will swarm out to defend the thorn-tree against grazing giraffes and elephants. They in fact swarm out on any large threat; when I picked a branch, my hand was quickly covered in ants, but they are quite small and do not bite hard at all, so it wasn’t too much of a bother. The way that Wikipedia and others paint this ant/tree relationship seems fairly straightforward – it seems like a symbiosis where both partners get something positive out of it – but that is misleading; the whole interaction is much more complex than usually shown. In fact, there are four different species of ants which so co-exist with the whistling thorn: Crematogaster mimosae, Crematogaster sjostedti, Crematogaster nigriceps, and Tetraponera penzigi. What happens is that the four species behave in different ways; research has shown that C. mimosae, the most common species found on the thorn-trees, will start over-farming sap-sucking and wood-boring insects if not kept in check by grazing elephants and/or giraffes. The resultant population growth of such other insects is bad for the tree, so one could describe elephants and giraffes as defending the tree against the ants by keeping their numbers down; the grazing animals eat a certain number of ants by accident when they graze on the tree, and it takes a while for the ants to persuade the animal to move on, which the ants do by being a nuisance by running into the mouth and nose of the grazing animal .

This whole relationship will take several blog posts to explore, so for the moment I will leave it there and come back to this soon. I include here citations of the research papers, the photos I myself took of the trees (in the Serengeti, Tanzania), and a video I found on YouTube showing the ants defending the tree against a grazing giraffe.

The swollen thorn base on a branch; several ants are visible. I assume these particular ants are Crematogaster mimosae.

A dried-out swollen thorn base, showing the holes in it made by insects, these holes being reponsible for the whistling sounds produced when  the wind blows.

A photo I made of such a whistling thorn-tree (again in the Serengeti):

Another view:

A view of the area:

 A video I found on YouTube showing a giraffe grazing on a whistling-thorn acacia tree, and showing how the ants are eventually successful in driving the giraffe off:

The literature I’ve sofar found on the whistling-thorn, ants and others in the whole larger relationship is:

Palmer, T.M., Stanton, M.L., Young, T.P., Goheen, J.R., Pringle, R.M., and Karban, R. (2008)
Breakdown of an Ant-Plant Mutualism Follows the Loss of Large Herbivores from an African Savanna
Science, 11 January 2008, Vol. 319 no. 5860 pp. 192-195
DOI: 10.1126/science.1151579, Abstract, and full text

Palmer, T.M., Doak, D.F., Stanton M.L.,  Bronstein, J.L., Kiers, E.T., Young T.P., Goheen, J.R., and Pringle, R.M.  (2010)
Synergy of multiple partners, including freeloaders, increases host fitness in a multispecies mutualism
September 20, 2010, doi: 10.1073/pnas.1006872107
PNAS October 5, 2010 vol. 107 no. 40 17234-17239, Abstract, and full text

Palmer, Todd M., and Brody, Alison K. (in press)
In press. Enough is enough: the effects of symbiotic ant abundance on herbivory, growth and reproduction in an African acacia.
http://dx.doi.org/10.1890/12-1413.1, and PDF

Palmer, Todd M., and Brody, Alison K. (2007)
Mutualism as reciprocal exploitation: African plant-ants defend foliar but not reproductive structures
Ecology. 2007 Dec;88(12):3004-11.

Cochard, Roland, and Agosti, Donat (2008)
Putting Ant-Acacia Mutualisms to the Fire
Science, 28 March 2008, Vol. 319 no. 5871 pp. 1759-1761,
DOI: 10.1126/science.319.6871.1759d, Full text

Heil, M., González-Teuber, M., Clement, L.W., Kautz, S., Verhaagh, M., and Bueno, J.C.S.
Divergent investment strategies of Acacia Myrmecophytes and the coexistence of mutualists and exploiters
Proceedings of the National Academy of Science of the United States of America 2009;106:18091-18096, Link

Field Talk: Termites enrich the soil in East Africa

Brody, A., Palmer, T., Fox-Dobbs, K., & Doak, D. (2010)
Termites, vertebrate herbivores, and the fruiting success of Acacia drepanolobium
Ecology, 91 (2), 399-407 DOI: 10.1890/09-0004.1

Pringle RM, Doak DF, Brody AK, Jocqué R, Palmer TM (2010)
Spatial Pattern Enhances Ecosystem Functioning in an African Savanna.
PLoS Biol 8(5): e1000377. doi:10.1371/journal.pbio.1000377, Link

I want to greatly thank @bug_girl and @Myrmecos for giving me information on references for papers on the whole whistling-thorn scene. I also made the following graphic for this all, but I probably will need to update it a bit to reflect the termites too soon as I blog in bits on this all.

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