Predatory Signals

The kudu antelope explains how and when to eat.

Have you ever wondered why you can’t eat a large amount of acacia leaves without falling sick? It’s not that your body can’t handle so much acacia. It’s because, as you browse, the leaves actually make themselves harder to eat.

What’s more, it doesn’t happen just in that trees leaves. It happens to all the neighbouring trees as well.

Acacia trees are quite all right if you eat just a few leaves from them. But if you eat too many, like when there’s a drought and there’s no other food to eat, then the trees get worried. They start generating tannins in their leaves to keep you away. Tannins taste bad, and they also stick to plant proteins, making them harder to digest. If you have too much of it, you can get poisoned.

If your tree starts making tannin, there’s no point in moving on to the next tree. That one would have become poisoned too. To find an edible tree, you’ll need to move at least forty-five metres away. Or, you can travel upwind.

Tannin is not easy to make. It takes a lot of effort. And it uses a lot of energy. That’s why acacia trees don’t make it in their leaves all the time. They only do it when they actually need to: when they themselves are in danger, or when their neighbours are.

How do acacias know when their neighbours are getting over-grazed? Because they send out chemical signals into the air to warn each other. The chemical, ethylene, that the acacias send out can travel about 45 metres — which is how far you need to go to find a tannin-free tree.

Or, you can travel upwind. The ethylene signal travels in the air — so it won’t go far if the wind is blowing in the opposite direction.

Are you surprised that acacia trees can detect chemicals in the air? Actually, you can do it too! That’s what you’re doing all the time, when you smell something. Of course, you normally use a different method to send out warning signals: movement.

If a lion comes to attack, you’ll probably start running, without even thinking about it.

Actually, it’s not quite true that you don’t think about it. Or rather, it depends on what you mean by “thinking”. When you see a lion stalking you, or see other people running from one, your brain processes it and sends signals to your legs to get them moving. You don’t consciously think about it, but it happens in your brain all right.

A similar thing happens in acacia trees, except that there’s no central ‘brain’ to process everything. Instead, when you bite off a leaf, an electric signal gets sent down directly to the other leaves, which begin making tannin to protect themselves.

Acacia signals are very slow. They travel only about one inch in a second. That means, the faster you browse, the more you’ll get to eat before the leaves realise and starts making tannin.

Like you and the lion, most animals are adapted to run away from those who eat them. But those are just instructions in their brain. Instructions that can be altered, if only you know how.

The Toxoplasma gondii begin their life in tiny packets, spread out across the land. They can be in the soil, water, or anywhere else they land up. And there they lie, waiting to get eaten.

The Toxoplasma can live in any warm-blooded animal, whether bird or beast. Once the packets get eaten, their walls are dissolved by the host’s digestive system, releasing thousands of sporozoites out into the open. These sporozoites quickly grow and spread, hitch-hiking on the bloodstream to reach all parts of the body. They infect muscle and brain tissues, to avoid getting caught and thrown out by the immune system.

But while Toxoplasma can live anywhere, there’s only one specific kind of place where they can complete their life cycle: inside the intestine of a cat.

Like other animals, most mice and rats would be wary of their predators. If they smelt cat, they would be careful and avoid venturing out. When infected by Toxoplasma, however, they begin to act a bit…differently. They seem to lose their fear of cats. Sometimes, they even get slightly attracted.

The effects are very specific. Infected mice act just like normal mice. You can’t tell the difference — until, that is, when a cat comes hunting.

Toxoplasma is said to have specific effects on other animals too. Female humans, for example, seem to be more intelligent, affectionate, social, and likely to stick to rules when infected by the parasite. Males get less intelligent, and more loyal, frugal and mild-tempered. And all humans — both male and female — become more prone to guilt, self-doubt, and anxiety.

While it’s not clear how much of that is true, what is true is that Toxoplasma makes mice and rats more susceptible to getting eaten. And it does that by tweaking their brains.

People are not sure how exactly Toxoplasma works. But research has found that infected cells send out a chemical messenger called GABA. While GABA has many jobs, one of the things it does is to suppress feelings of fear and anxiety.

In other words, Toxoplasma tells you not to panic — even when you actually should.

While Toxoplasma makes creatures get eaten by predators, there’s one fungus that actually is the predator. The zombie-ant fungus gets into the bodies of ants, and, bit by bit, takes control of its body. It then makes the ant do a very specific task: climb up a plant, to a height of exactly twenty-fine centimetres, and hold on tight.

Twenty-five centimetres is the best height for the zombie fungus to live. It can then finish eating the ant, from inside out. Finally, through the ants head, it sends out a long stalk, filled with spores that drop down to find another victim.

The fungus does all this without taking direct control of the brain. Unlike Toxoplasma, it doesn’t even try to confuse the brain with wrong signals. Instead, it comes in between to directly control the central nervous system. It sends out signals that the ant would have once sent. And the ant’s real brain is helpless, effectively trapped inside its own body.

Compared to signals modifying behaviour and predators modifying prey, the acacia’s warning signals don’t seem so surprising any more. Next time you start browsing on one, you can be glad it won’t turn around and start eating you!

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