Mutualism with a darker side

You should know what symbiosis is. One species helping another and vice versa. But that’s not quite it. Only one of the two species needs to be helped for it to be symbiosis. There are 3 main types:

  1. Mutualism: This is where both help the other. One species helps the other, and the other helps back in return. Many people think of this as symbiosis, but it’s really only one of the forms.
  2. Commensalism: This is where one species is helped, the other doesn’t benefit, nor is it harmed. A bird building a nest in a tree; the bird is helped, the tree isn’t helped nor harmed.
  3. Parasitism: One species is helped, the other is hurt. A tick sucking blood from a mouse. Yes, this is symbiosis.

One of the really cool examples of mutualism is the acacia tree, and the ants that live in them and protect them. The tree provides a place to live and food (nectar). The ants provide protection by attacking anything that comes near, from elephants to weeds.

But researchers have recently discovered that there is a dark twist to this mutualistic relationship. The acacia tree actually enslaves the ants. Once an ant eats the nectar, it can’t get sugar from any source other than an acacia tree.

The nectar contains sucrose, a sugar. To digest the sugar, ants use the enzyme invertase to break sucrose into simpler sugars. But the acacia ants don’t make invertase properly, and can’t digest sucrose. It turns out that the acacia nectar includes invertase in it. Kind of partially digested. So the ants can eat the nectar and digest it.

But why can’t the adult ants make invertase? As larvae they can. So where does it all go wrong for the ants?

The nectar contains another enzyme that blocks invertase production. Once an ant eats the nectar, they can’t produce invertase anymore. And to be able to digest sucrose, they need to get it. From now on, they need to eat nectar that is laced with invertase: the nectar from the acacia tree.

The first time a new adult ant eats the nectar, it essentially becomes a slave. It has to protect the acacia tree, because the tree is the only source of food it can eat.

2 thoughts on “Mutualism with a darker side

  1. Whoah, hadn’t heard of that wrinkle in mutualism before. They both benefit, but also at a cost of opportunity, for the ant. But if the tree didn’t trap the ant that way, it would just eat the nectar and go, so I can see why it might be an evolutionary advantage for this species of tree.

  2. Yes. If you’re an ant, you can get free room and board. But once you have some food, you can’t leave. I expect that an ant can go to another acacia tree that’s nearby. But it can’t leave the acacia trees. While the ants are certainly getting something good out of this, it does violate my moral instincts because the ants aren’t given a choice about it. If they’re born into a colony, they eat right away, and then they’re enslaved for life. Well, no one said that nature was fair.

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