The not-so-simple relationship between ants and the whistling thorn…

Uhusiano usiorahisi tunavyofikiria kati ya sisimisi na miti aina ya Mbalimbali (Whistling thorn)

(Scroll down for Swahili)

If you read my previous post on the mutual relationship between ox-peckers and large mammals, you’ll know that a lot of times these simple relationships are actually more complicated than they seem.

Acacia/Vachelia drepanolobium in Serengeti.
Photo: Author.

A very obvious mutual relationship exists between the Whistling thorn (Acacia/Vachelia drepanolobium) and cocktail ants of the Crematogaster genus. The Acacia produces a swelling at the base of the thorns whose only purpose is to provide a home for the ants. These are called domatia. The queen ants lay their eggs in these domatia and the ant colony rears the young in the nice hollow space. The tree also produces nectar from glands purely for the benefit of the ant, that have nothing to do with the flowers. These are called extra-floral nectaries. In return for food and shelter, the ants defend the tree against browsers and insect predators by biting them and spraying them with formic acid. They’ll even prune vines and competing plants to maintain a healthy tree.

But as I mentioned, there’s always more to the story. As it turns out, there are at least three species of ant that use the domatia, and not all of them are working hard help the tree. Each of the three species uses the tree in different ways. Crematogaster mimosae is a very aggressive ant and is a nectar feeder, but it also “farms” scale insects inside the domatia that tap into the sap of the tree and produce honeydew for the ants. This ant significantly reduces herbivory from large mammals including elephants. (This is the species that is parasitized by the Lycenid butterfly. Read about it here).

Another aggressive species of cocktail ant- Crematogaster nigriceps or Black-headed cocktail ant, is a predator and attacks and eats any insect that dares to come near the Acacia. Here’s another twist in the story. Crematogaster nigriceps prunes the growing parts of the Acacia including the flowers and effectively sterilizes the whole tree. You can actually tell if this ant species is living in the Acacia by looking at the shape of the tree.

The safari guide’s impression of the difference between A. drepanolobium depending on the ant that is host.

Interestingly, these two ants also know the chemical “help” signal that Acacias produce called Methyl Jasminum, and react to it.

The third species that uses the domatia might be the biggest cheat of the mutualism system. Tetraponera penzigi doesn’t even visit the extrafloral nectaries and instead cuts them off. This ant is an agriculturalist and grows fungus in the domatia using the leaves from the Acacia as the medium. This ant doesn’t respond to mammalian predators and is of no help to the Acacia. Again there’s more to the story. There is intense competition by the different ant species for trees. The Crematogaster ants dominate the Tetraponera and it turns out that the Tetraponera seem to be destroying the nectaries to make the tree less attractive to the Crematogaster ants who would easily take over the tree. Again we’ve swung towards this relationship being parasitic instead of mutual. The ants seem to be taking advantage of the Acacia.

But of course there’s another beautiful twist in the story. If you read this blog article, you’ll know that Acacias respond to being browsed on by mammals by growing longer thorns and that they don’t waste energy on growing thorns once their branches get out of reach of browsers. The Whistling thorn takes this to another level. In addition to investing less energy growing shorter thorns if the tree is protected from being eaten by herbivores, the plant stops producing as much nectar to feed the ants. When this happens, the health of the protective ant colony declines and a fourth ant comes into play. Crematogaster sjostedi shows up and takes over the tree. C. sjostedi doesn’t take advantage of the domatia, and instead actively attracts long-horned beetles who bore into the stems and create the homes for the ants. This ant also tends to scale insects that cause further harm to the tree. So the circle of cooperation, competition, alliances, parasitism and predation between giraffes, trees, ants and beetles ends here, in a beautiful balance. But as we know too well, this is never the end of the story.

Kama ulisoma blog yangu iliopita kuhusu uhusiano kati ya ndege na mammalia wakubwa, utajua kuwa mara nyingi uhusiano huu rahisi kumbe ni tata.

Kuna uhusiano wa kutegemezana kati ya Mbalimbali (‘Whistling thorn’ Acacia/Vachelia drepanolobium) na sisimisi wa jenasi ya Crematogaster. Mgunga inatengeneza uvimbe kwenye shina la miiba inaachoitwa ‘domatia’. Kazi ya hiyo uvimbe ni kwa ajili ya hao sismisi peke yao. Malkia wa sisimisi anataga mayai kwenye hiyo uvimbe na koloni ya sisimisi inalea na kuhudumia watoto kwenye eneo hiyo nzuri. Mti pia unatengeneza nekta (maji matamu) kutoka kwenye tezi kwa ajili ya manufaa ya sisimisi, na ambayo hayahusiki na maua ya mti. Tezi hizi zinaitwa ‘extra-floral nectary’. Malipo ya mti kwa kupata chakula na malazi, sisimisi wanaulinda mti kwa maadui kwa kuwang’ata na kuwapulizia asidi fomi (tindi kali). Huwa sisimisi wanapogoa hata mimea inayoshindana ili mti uote na kuwa na afya nzuri.

Lakini nilivyo taja, hadithi haishi hapo. Inaonekana kuna spishi tatu ya sisimisi wanaotumia ‘domatia’, lakini siyo wote wanafanya kazi kuusaidia mti. Kila mmoja ya sisimisi anamatumizi tofauti ya mti. Crematogaster mimosae ni sisimisi mkali na ni mla nekta, lakini pia wanafuga wadudu wanaoitwa ‘scale insects’ ndani ya hizo ‘domatia’. Hawa wadudu wanatoboa mti na kunyonya maji kwenye mti ili kutengeneza ‘honeydew’ (maji ya asali) kwa ajili ya sisimisi. Sisimisi huyu anapunguza ulaji wa mti kwa mammalia wakubwa hata tembo. (Spishi hii ya sisimisi ndiyo wanaomelewa na kipepeo aina ya Lycenid)

Spishi nyingine ya sisimisi mkali ni Crematogaster nigriceps au “Black-headed cocktail ant” na ni mwindaji na anashambulia na kula wadudu wanaokaribia mti. Lakini sisimisi huyu siyo mwema. Crematogaster nigriceps anapogoa vipukizi vinaoota pamoja na maua, tendo linachotasisha mti. Kwa kweli, unaweza kutambua kama yeye anaishi kwenye mti kwa kuangalia sura ya mti ulivyosimama.

Cha kushangaza zaidi ni kwamba spishi hawa wawili wanafahamu ishara za kemikali za onyo zinazotengenezwa na Migunga zinazooitwa ‘methyl jasminum’ na wanazitikia.

Spishi ya tatu ya sisimisi wanachotumia domatia ndiyo mwerevu wa mfumo wa kutegemezana. Tetraponera penzigi hatembelei tezi za nekta na badala yake anazipogoa. Sisimisi huyu ni ‘mkulima’ na anaotesha kuvu ndani ya ‘domatia’ kwa kutumia majani ya mti kwa ajili ya mbolea. Sisimisi huyu haitiki mammalia wakishambulia mti na hatoi msaada wowote kwa mti. Lakini hadithi haishii hapa. Kuna ushindani mkubwa wa kupata miti kati ya spishi hawa wa sisimisi. Jamii ya Crematogaster ndiyo washindi na kumbe Tetraponera wanavyo haribu tezi za nekta ni kwa ajili ya kupunguza kivutio cha mti huu kwa spishi za Crematogaster. Hapa tena tunaona kwamba uhusiano umeenda kwenye upande wa kimelea badala ya kutegemezana. Inaonekana sisimisi wanajinufaisha na mti.

Lakini lazima kuna mengine. Kama ulisoma makala hii, utafahamu kwamba Migunga huwa inaitika kwa kuliwa na walamimea kwa kuota miiba mirefu zaidi na vilevile kupunguza matumizi ya nishati kutengeneza miiba mirefu ikiota matawi marefu kuliko kiwango wanachofikia walamimea. Mti una nyongeza zaidi. Pamoja na kupunguza uwekezaji wa nishati kwenye kuotesha miiba mirefu, miti huu ukipata ulinzi mzuri unaacha kutengeneza nekta kwenye tezi kwa ajili ya kuwalisha sisimisi. Hii ikitokea, afya ya koloni ya sisimisi inapungua na ndipo spishi ya nne ya sisimisi hutokea. Crematogaster sjostedi anakuja na kutawala mti. C. sjostedi hatumii domatia, lakini badala yake anavutia kipukusa (‘wood-boring beetle’) ambao wanatoboa shina la mti na kutengeneza makazi ya sisimisi huyu. Sisimisi huyu naye anachunga ‘scale insects’ ambao wanadhuru mti zaidi.

Hapa ndipo mzunguko wa kusaidiana, kushindana, kuungana, kulemeana na kuvamiana kati ya twiga, miti, sisimisi, na vipukusu iniaishia, kwenye usawa unaopendeza. Lakini tunavyofahamu, hii siyo mwisho wa hadithi yetu.

Sources

Visitacao, J. M. 2011. Multi-species Interactions in African Ant-Acacias. Harvard University, ProQuest Dissertations.

Stanton, M. Palmer, T. et al. 1999. Sterilization and Canopty Modification of a Swollen Thorn Acacia Tree by Plant-ant. Nature.

Palmer, T. Young, T & Stanton, M. 2002. Burning bridges: priority effects and the persistence
of a competitively subordinate acacia-ant in Laikipia, Kenya
. Oecologia.

Palmer, T. Young. T, & Stanton, M. 2008. Breakdown of an Ant-Plant Mutualism follows the loss of Large Herbivores from an African Savanna. Science.  

Truman P. Young & Bell D. Okello. 1998. Relaxation of an induced defense after exclusion of herbivores:
spines on Acacia drepanolobium
. Springer.

2 replies

  1. Ethan!

    Hope you are well. Loving these updates.

    I thought I knew this story well but had no idea about the Crematogaster sjostedi behaviour!

    Mark

    Mark Boyd Safaris 85 Ndorobo Road Nairobi +254719779132

    >

    Like

  2. I like the whole explanation about the life of the tree and the insects. I would like to know how different trees communicate to save each other against the herbivores.
    Thank you so much 😊

    Like

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