Sequestering toxins: the pros and cons
A toxin is a poison made by a plant or animal’s body. And just as evolution can help these plants or animals develop toxins, it can also help whatever’s eating them to become resistant. But not only that, there’s a biological concept for what happens when an animal eats a toxin, and doesn’t avoid getting hurt: they actually become poisonous themself as a result. It’s called sequestration. Hannah Rowland from the Max Planck Institute for Chemical Ecology took Phil Sansom through the idea by looking at some of the animals who do it…
Hannah - For example, a caterpillar that feeds upon a poisonous plant takes in the chemical defences of the plant, into its stomach, into its gut. And instead of excreting those, they actually move them from their gut into special parts of their body.
Phil - So you don't just survive the poison. You sort of make it your own poison.
Hannah - Exactly that yes. Sequestration is in lots of different animal species. So it's in insects, toads, there are snakes that feed on toads, and then there are even birds called pitohuis that live in Papua New Guinea. Very famously are the poison dart frogs - they also have sequestered chemical defences from the ants that they feed on.
Phil - Are they the tiny brightly colored ones that live in the rainforest?
Hannah - They are. They're in Australia, Costa Rica, Panama, some of which are so poisonous that they are used on poison arrows to kill prey and to even be used maybe in warfare.
Phil - Are all these animals that you're talking about - are they all doing it with the same poison?
Hannah - Some of them they share. So for example, the toads, snakes that live in Japan, and the Monarch butterfly, they have one thing in common - they are sharing the same kind of chemical defence. But they're very diverse. And also most animals don't just have a single chemical defence, but they have a whole suite. There's this battle going on that the plant doesn't want to be eaten, and the caterpillar doesn't want to be eaten by a bird - with a decision that's not really a conscious decision. This really tight specialist relationship means that you get this matching of chemical defences.
Phil - Let's use the caterpillars as a bit more of an example - what's going on inside them? Are there little cells or molecules that are, you know, taking away the poison before it gets into the stomach or what?
Hannah - In some caterpillars that can feed on these plants, but don't sequester the compounds, these chemicals can't pass through the lining of the stomach, into the body of the caterpillar. But in the Monarch, and in other milkweed butterflies that do store these chemical defences, the compounds pass through the lining of the stomach and into the body cavity. This can sometimes happen in a passive way, so it just passes through the lining of the stomach, or it can be actively transported. And when it's actively transported, we sometimes also see that there are special storage units in these insects, special chambers and compartments, where these chemical defences get concentrated and stored, ready to deploy.
Phil - So you've got to not get poisoned. You've got to make the special compartments. You've got to get the poison in there. Is this not quite a lot of effort for the caterpillars or quite a lot of energy that they need to use up?
Hannah - That's a really good question because we always think as scientists taking in chemical defences must be costly to the animal. The costs we would look for are things like, how well do they grow? How quickly do they develop? Maybe, how does their immune system work? We can also think of other costs, like how are they attacked by their predators or by parasites? So we find that lots of animals that sequester pay a really big cost through being parasitised - so by some animals that lay their eggs in a caterpillar, and those eggs then develop within the caterpillar and eat the caterpillar from the inside and then kill the caterpillar and emerge as a parasite. So that's one of the costs that we see. We sometimes see reductions in immune systems. There's also great benefits to sequestering. So some caterpillars grow better because there's sugars in these toxins and they use them to grow. And they certainly defend them against lots of different types of animals that want to eat them.
Phil - Do you mean you can actually see stuff like this if you do comparisons between different butterflies or different plants?
Hannah - Yes. So some research that we've been doing is to look at the costs to Monarch butterflies of feeding on milkweed plants that vary in how much chemical defence is available to the butterfly. We've hypothesised that maybe this is physiologically costly to them. And what we found is that the more that they store as caterpillars and take through into adulthood, the less able they are to create these bright and conspicuous warning signals. So they’re less red and they're less conspicuous. So the really toxic ones just aren't as bright, but they're really nasty.