The Secret Loves of Trees

November 8, 2010

Torah Kachur

Falling in love is so romantic, so blissful, so cherished in our lives.  Most people will fall in love more than once, first with the 'wait until we're married' sterilizer, then with the 'jealous defender' and finally you hit an age where want to settle down and find the 'practical answer'.  And then, after imminent divorce you find yourself with some gold digger who just can't wait for you to die and leave him or her everything.


That darling of a fairy tale also applies to trees. Trees don't have life partners, lovers, spouses or mistresses, but they do have important partnerships with ants throughout their lives.  This is what evolutionary biologists call mutualism - the cooperation of two species for mutual benefit.  New research conducted by the Pringle group at Stanford University found that lifelong monogamy for plants, just like for humans, is not the best idea.  The common African acacia tree is a bit of a slut as it partners with up to 4 different species of ants through its lifetime.  In fact, the slutty trees that had different ant partners were much better off than those that stayed loyal to only one ant.


          Torah Kachur Science in Seconds Secret Loves of Trees Crematogaster sjostedti mimosae nigriceps Tetraponera penzigi


Just like humans, young plants were found to partner with the chaste Crematogaster nigriceps species of ant first.  These ants were found mostly on small colonies of trees and also sterilized the trees.  Alas, the virginal ant made the tree wait until they were older and mature enough to consummate their partnership.  So, what's in it for the tree?  While the ant is keeping the tree honest, the acacia tree has time to mature because C. nigriceps aggressively defends the trees from herbivores.  But, like all horny (or thorny) young trees, being sterile sucks and so the tree changes partners to an aggressive non-sterilizer ant - C. mimosae.  Despite constantly being drunk, C. mimosae defends the tree and finally allows it to reproduce. 


But the 'jealous defender' stage is only cute for a while.  Depending on the size of the tree colony, some trees then shift to Tetraponera penzigi - the most 'suitable' partner for the tree.  T. penzigi is loyal and treats the tree with respect while tree puts a roof over the ants' head - a match made in heaven.  This phase of the trees life is the "any ant is better than no ant" stage.  The tree is very fecund at this stage producing many progeny, but - like all vanilla relationships - the tree gets bored.


Yes, just like humans, the most successful of the species later in life find themselves surrounded by parasitic partners.  In the trees' case, these gold-digging ants are called C. sjostedti.  These ants aren't satisfied with a roof over their heads, instead they allow their beetle buddies to move in and mooch off the tree.  Not surprisingly, these parasites and their beetle friends cause an increase in death rates for trees.  This partnership should not occur in nature because evolution should select against such a negative partnership.  Except, just like Anna Nicole Smith, the ants get one last contribution to the world from their partner - a final spreading of their wealth and seed before they die.  So, the opportunisitc ants actually do benefit the ecosystem and provide an evolutionary purpose, at least in trees.


After all of these attempts at love, the poor plant never found the one ant that could complete it.  sigh



Palmer TM, Doak DF, Stanton ML, Bronstein JL, Kiers ET, Young TP, Goheen JR, & Pringle RM (2010). Synergy of multiple partners, including freeloaders, increases host fitness in a multispecies mutualism. Proceedings of the National Academy of Sciences of the United States of America, 107 (40), 17234-9 PMID: 20855614



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