Poison Garden: The Toxic Twins

As promised, we are going to look at two other poisonous beauties that are hosted by my Poison Garden. To an untrained eye these two butterflies look very much alike, but their caterpillars are utterly different. Why is that so? Is it mere coincidence?

Plain tiger butterfly (Danaus chryssipus chryssipus).

Plain tiger butterfly, side view.

Tawny coster butterfly (Acraea violae)

Turns out nobody really knows the answer, but I think these two vibrant visitors tell a great story. Butterflies have been flying around since the dawn of flowering plants, and that's about 140,000,000 years ago! Butterflies have waged a war with plants for longer than all of humanity put together (about 7 million years). Given the aeons, many plants evolved deadly toxins to prevent caterpillars from eating them, so much so that if we humans are to eat these plants, we will die of poisoning. But not for the caterpillars because some of them evolved toxin resistance. They either make antidote to the plant toxins, or acquired special genetic mutations to prevent the toxins from reaching their targets. What's more, some caterpillars can steal plant toxins and use them as protection against their own predators. Toxin versus resistance, outwit or be outwitted, an arms race for millions of years!

And these two, the plain tiger (Danaus chryssipus), and the tawny coster (Acreae violae) emerged as evolutionary winners. Both of them feed exclusively on deadly poisonous plants, and steal the plant toxins to deter their enemies. Their beauty is a big show-off of toxicity because birds, spiders and even wasps (vision hunters) generally avoid eating the butterflies. Biologists call this kind of warning colour aposematism. But of all the possible warning colours, why do these two look so similar?

Caterpillars of the plain tiger butterfly feeding on Calotropis gigantea.

Here, I'm making a very bold hypothesis. Of course it needs to be proven, but here's what I think is happening between the two. Previously in my common rose article, we learnt that harmless butterflies sometimes mimic poisonous ones to gain protection from predators (common rose vs. common mormon). The mimic can be so convincing, even I have difficulty distinguishing them from afar. But mind you, they don't become perfect mimics overnight. Even a slight resemblance can offer a little more chances of survival, and over millions of years, one ends up with a perfect mimic. When a harmless organism mimics a harmful one, we call it Batesian mimicry. But how about the possibility that two harmful organisms mimic one another?

Tawny coster caterpillars feeding on Passiflora Lady Margret (hort.).

That's what I think is happening between the plain tiger and the tawny coster! Although they are totally unrelated and feed on different host plants (one contains a heart toxin, another contains cyanide), they are equally distasteful to most predators. Hence, it is possible that by chance, one of them (their ancestor) started looking like another, and gains extra survival advantage to the point where it becomes a model to the one it's trying to mimic and vice versa. Over aeons, you can get two harmful species looking pretty much like one another. We call this Müllerian mimicry and it can be proven by complex mathematical models. The precise mechanism of Mullerian mimicry remains debatable, but it is a well-recognised phenomenon in biology. Regardless, imagine if you are a bird who eats butterflies. Red and black, with white spots mean poison. It doesn't matter if you need to go closer to ID a tawny coster or plain tiger, you'll just avoid them all together. That's the idea!

Newly eclosed tawny coster butterfly.

There we go, two poisonous butterflies, two different stories that end up reading similar. Is that convincing to you? See, Nature and evolution are full of deceit and complicated relationships we humans are barely beginning to understand. To learn to appreciate them is to ultimately preserve Nature because in the end, we are all connected. If butterflies and pollinators go extinct, we would follow soon. But for those who think that all these creatures are created in a snap, and that they are here for a grand purpose, think again, read more and widen your horizons. It's not flat on the surface.

We are leaving the realm of toxic butterflies for now. Next time, we will be expanding the definition of poison and look at creatures that I often encounter, which uses a complex protein poison scientists call venom.