CREATURE FEATURE. Painted Grasshopper

Those of us who value nature probably have some biases about which organisms we find most intriguing. For many people, brighter and more colourful creatures warrant more attention. I realised that I too harbour this bias when I stopped in amazement at the sight of a particular grasshopper. This grasshopper is the sort of animal that looks odder and odder the longer you look at it — anywhere your eyes may rest on its body, you will see unexpected colour and outrageous pattern. The wings are finely veined, yellow lines running through a delicate surface that fades from green to reddish-pink. Under the wings, its abdomen is striped with blue and yellow; at the other end of the grasshopper, the antennae are similarly striped. In between are six blue, orange, yellow and green spiny legs, a radially striped head and chest and beady blue eyes. Why might anything be this crazily coloured?

And this is when I checked myself, embarrassed by my easy fascination with the bright and colourful, because the reason for this grasshopper’s colours lay right in front of me, within the unremarkable plant that it was sitting on — a milkweed. Milkweed sap teems with toxins that discourage all kinds of animals from feeding on the plant. When eaten in large quantities, these toxins cause many herbivores to vomit violently. Herbivores learn to associate this unpleasant nausea with milkweed, and avoid eating the plant in the future. The avoidance is well-founded, for if an animal manages to eat about twice as much milkweed as caused it to vomit, it will die.

But imagine you’re a plant-feeding insect searching for something to nibble away at without needing to be wary of the jaws of a hungry goat. Wouldn’t you do well to pick a plant that the larger herbivore won’t touch? This was the strategy adopted by the grasshopper, who single-mindedly chomped away at the margin of a milkweed leaf, oblivious to the toxin-laced sap that oozed from the leaf’s veins. Clearly, it had evolved to tolerate milkweed toxins.

But the grasshopper’s colouration indicates it does more than just ingest milkweed toxins — it likely holds on to them and deploys them in its own body, making it toxic too. A bird, for example, that eats the grasshopper will share the same fate as the herbivores that ate the milkweed, and learn to avoid eating grasshoppers that look like the nausea-inducing kind.

Being toxic doesn’t help that first grasshopper — it must get eaten for the bird to learn to avoid others of its species. But birds must eat, and if toxic grasshoppers looked similar to non-toxic, edible grasshoppers, the bird would simply have to eat the next grasshopper it came across. The risk of a violent vomiting episode would be outweighed by the bird’s need to feed. And so it’s in the best interests of both the bird (or any other predator) and the grasshopper that the toxic grasshoppers look as distinctive as possible. Sporting more colours than your average fruit basket offers all subsequent grasshoppers a chance to live another day. It also saves the predator a second harrowing, near-death experience. Meanwhile, the milkweed, its leaves only slightly chewed, finds itself at the centre of an ecological and evolutionary drama.

( Ambika Kamath studies organismic and evolutionary biology at Harvard University )

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