The complex mutualism between ants and the whistling thorn

On the savannas of Kenya there is a tree which is exclusively inhabited by four species of stinging ants. They are: Crematogaster nigriceps, Crematogaster mimosae, Crematogaster sjostedti, and Tetraponera penzigi. The tree is called Whistling thorn acacia (Vachellia drepanolobium) and you will see a lot of them during your safari in Kenya. Usually one ant species inhabits any given individual tree to the exclusion of other ants.

We have already talked about the mutualism between oxpeckers and large mammals in another post. Between these species of ants and the Whistling thorn acacia there is another symbiothic relationship… or it seems so..

But let’s see how.

04

SEPTEMBER 2020

Mutualism
Ants
Acacia

Mating lions among whistling thorn acacias – Naboisho Conservancy – 2020

Photo by Sara Gastaldi

Besides having regular thorns, this acacia has modified thorn pairs which are joined at the base by a hollow, bulbous swelling up to 3cm in diameter, which is called domatia and its only purpose is to provide a nest site for the ants. But why the ants should need a house in a tree? These ants have adapted to living in trees because a special type of soil, known as “black cotton”, gets waterlogged and spongy during the rainy season, and dries out and cracks during the dry season, making it difficult for ants to build nest underground.  Therefore, the queen ants lay their eggs in these domatia and the ant colony rears the young in the nice hollow space.

The acacia has also developed special glands called “nectaries” at the tips of their leaves that produce a sweet secretion just for the benefit of the ants, and they have nothing to do with 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.

Cocktail ants and scale insects

Whistling thorn acacia (Vachellia drepanolobium) with flowers

As you can imagine, savage competition for the whistling thorn exists between the four species of ants and  when branches of the trees meet the ants will invade their neighbor and battle violently for control of the tree, until one colony wins. The nigriceps ant, or Black-headed cocktail ant, comes out very badly in these battles, losing more than any of the other three species, being the least war-like, while mimosae is the most aggressive.

The nigriceps is a predator and attacks and eats any insect that dares to come near the Acacia, protecting the tree, but to defend its home against the invasion of the other ants , they actively prune their trees. They chew off all axillary (horizontal) shoots , causing the tree to grow tall and skinny. In this way the ants avoid contact with other trees which may hold enemy colonies. Pruning also causes the tree to allocate more energy to new shoots, healthier leaves and larger nectaries, which is also good for the nigriceps ants.

Whistling thorn acacia and lilac breasted roller (Coracias caudatus)

But as I mentioned in the other post about expeckers and ungulates, it is not as simple and innocent as it seems.

The mutualism between the tree and C. nigriceps turns out to be something like a parasitic relationship. In fact the ants also prune off all flower buds so the tree can’t make fruit and reproduce, and is therefore sterile. Scientists hypothesize that the whistling thorn acacia has made a trade-off between reproduction and increased vigor plus protection from browsing animals. Whistling thorn acacias bearing colonies of less destructive ants are still able to produce seeds and continue the species.

C. mimosae is the most common and aggressive ant, protecting the acacia against browsers very well, including elephants, and it is a nectar feeder. But here’s another twist in the story. Mimosae also “farms” scale insects, inside the domatia, that tap into the sap of the tree and produce additional honeydew for the ants. (Even if so aggressive, mimosae is parasitized by a species of butterfly.. we will write an article about that).

Interestingly, these two ants also know the chemical warning signal that Acacias use to communicate and defense themselves, which we talked about in another article.

Close up of domatia (swollen modified thorn)

Whistling thorn flowering

The third species, Tetraponera penzigi is not even interested in the extrafloral nectaries and instead it cuts them off. This ant is a farmer and grows fungus inside the domatia using the leaves from the Acacia as the medium. This ant doesn’t react to the mammals’ attacks and there’s actually no benefit for the acacia to host them. But, at least, there is a reason why Tetraponera p. cuts off the nectaries. Crematogaster ants dominate the Tetraponera and this latter seems to be destroying the nectaries to make the tree less attractive to the competitor ants would easily take over the tree. Once again it seems to be a parasitic relationship, instead of mutual.

As we said in another article of our blog, Acacias react to being browsed on by mammals by growing longer thorns but they don’t waste energy on that process if there’s no need to, like when their branches get out of reach of herbivores. The Whistling thorn acacia takes this to another level. In addition to investing less energy growing swollen thorns , if the tree is protected from being eaten by herbivores, the plant stops also producing nectar to feed the ants. When this happens, first mimosae change their feeding behavior and start feeding on the sap provided by scale insects they farm. This is a remarkable ecological plasticity. But suddenly the health of the protective ant colony declines and a fourth ant comes into play: Crematogaster sjostedi which will take over the tree. This species of ants might be the biggest cheat of the mutualism system. In contrast to the other species of acacia-specializing ants, sjostedi infact feeds mainly on small invertebrates and doesn’t nest in the swollen thorns but rather in stem cavities that have been made by the larvae of long-horned beetles. While other specialist ants prevent beetle infestation, this one in some unknown way seems to attract or invite the beetles. In fact when these ants were experimentally removed from whistling thorn trees, attacks by long-horned beetles also decreased significantly. Since trees inhabited by C. sjostedti were the ones that grew most slowly and showed highest death rates, the researchers believe that the long-horn beetles may be a significant factor in reducing the vitality of the trees.

C. sjostedi  also tends to scale insects that cause further harm to the tree.

Whistling thorn acacia (Vachellia drepanolobium) inhabited by cocktail ants

Whistling thorn acacia (Vachellia drepanolobium) seed pods

So,  what is the role of large herbivores that normally feed on whistling thorns? One may think this is the end of the story for that tree.. but, as we have seen, the tree survives and other species get benefits and live even better. By the contrary, the tree is more likely to die if the browsing mammals are removed from the scene.

Evidently, the browsing stimulates directly or indirectly the development of swollen thorns and the production of leaf nectar, which provide the ideal environment for C. mimosae as the primary ant specialist colonizing whistling thorn. When this stimulation or interaction between mammalian browser and tree is lacking or reduces, everything changes. C. mimosae recedes and a new constellation of organisms and interactions develops. The ant species C. sjostedti increases, as does infestation by long-horned beetles, and the trees grow more slowly and the probabilities for it to die become higher.

All this provides a wonderfully concrete picture of how things in life are interconnected. The lives of tree, ant, beetle, and browsing mammal are all tightly interwoven. When one feature changes – like the removal or reduction of browsing – the dynamics of the previously-existing relations change and a new constellation of relations develops.

What’s additionally interesting is that it is not a matter of a clear-cut chain of causal connections. You can’t simply say that lack of browsing causes the trees to form less sap and make fewer swollen thorns, which in turn causes the one species of ants to leave, which in turn causes another species of ants to inhabit the tree, which in turn causes the long-horned beetle larvae to infest the tree, which infestation, finally, causes trees to grow slowly or die.

This is our overriding habit to think in terms of causal chains. We just focus on the links between individual species and individual facts. Or, better, we isolate such links out of the totality of the phenomena and concentrate on them. An ecological phenomenon such as this one doesn’t permit us to isolate any simple, one-directional relations. Any given aspect, like the ants or the trees, shows itself to be related to an array of others. In fact, if we find something “simple,” we should be suspicious. Simplicity is more likely to be an artifact of our narrow view than an expression of an actual cause-effect relation.

Craig Holdrege, who explained us very well what happens here, suggests that they should teach the maxim: if you find a simple explanation, don’t trust it, but search for complexity. Especially when we talk about biology and ecology.

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