Speaking of Science: Why are chillies hot?

WHY ARE fruits pleasant to smell and nice to taste? It is an evolutionary survival strategy. The smell entices birds and animals to come and eat the fruit. The seeds within are properly packed so that they are not all eaten but dispersed through the plumbing line of the eater. In so passing through the animal, the seed gets ever-so-lightly processed by mechanical and enzymatic means, so that when it is dropped on the soil, it is ready for germinating and generating a new plant. The sweetness of the fruit pulp and its attractive smell are the "come hither" signals in the self-propagation game played by the "selfish gene" of the plant.

Yet there is a seeming paradox that we encounter in the case of some fruits. The jackfruit has a forbiddingly thick skin, which needs to be taken care of first. The fibre netting covering the fruit needs to be pried open next, and the pleasantly pungent smell of the pulp enjoyed (or tolerated) before the pulp is eaten. The seed that you dig out of all this labour is hard; more a nut than a seed so that it can be roasted, or dried and powdered, and consumed. Having been so endowed by nature, the jackfruit has to adopt a propagation strategy very different from the grape. And it does. The case of the resinous fruit of Ramphal or the Vilva tree, with fruits hard and round as cricket balls, is equally paradoxical. Sir Isaac Newton was lucky not to have sat under one of these trees, and to have chosen an apple tree. The gravity of the consequences would have led to a delay in our understanding of the phenomenon of gravity, and even other aspects of physics such as the laws of motion and the nature of light.

There is a remarkable tree called Ginkgo biloba, also called the Chinese Yew or the maidenhair tree. Botanists believe that it might be the oldest living species of trees in the world. It is so sturdy and resilient that apparently it was the first vegetation to return to the soil of Hiroshima and Nagasaki after the atom bomb devastated the land there. The fan-like leaves of the ginkgo tree are a veritable pharmacopea of substances that are beneficial to human health. But take care not to go too near a female ginkgo tree when she fruits! They fall on the ground below, crack open and emit an odour that is nauseatingly reminiscent of human excrement! The landscape gardeners of the city of Washington DC were keenly aware of this, and took care to plant only male ginkgo trees to line the avenues making up Lafayette Square near the White House. The avenues are majestic, the trees are dark and deep, and the only stink you experience around there comes from the politics of the neighbourhood.

It is clear that the plant scientists have an interesting puzzle on their hands. The fruit of a tree is meant to facilitate seed dispersal by the consumer (The use of this word here is technical. Scientists also classify seed predators and dispersers among the consumers). Yet the fruit in several cases seems not to it this easy, and throws a googly. The chilli plant is a case in point. The fruit looks nicely coloured, has a mildly attractive scent but take a bite {frac12} it bites you back with its spicy "heat"! How is an animal or bird supposed to disperse its seeds, if the fruit intimidates and repels the consumer with its heat? Drs. Joshua Tewksbury of the University of Florida and Gary Nabhan of the Northern Arizona University in the US got together to crack this paradox, and report their findings in Nature (26 July, 2001).

The substances responsible for the spicy hotness flavour of the chilli is called capsaicin. Four years ago, Dr. David Julius and associates at the University of California at San Francisco showed that capsaicin goes and sits at one of the taste receptor protein molecules inside our mouth, and generates a sensation of intense heat and some pain. Laboratory studies by Drs. J. R. Mason, L. Clark and associates had shown that capsaicin has the property of repelling or poisoning mammals but not birds. (On an aside, we humans are so quirky. Capsaicin is meant to repel us. Yet we desire it and find vicarious pleasure in eating it, and spicing our food with it. Such acts of living dangerously, or machismo, is a mild form of showing off, or advertising to the opposite sex that you are so strong and healthy that you can withstand such punishing acts. The conclusion you would like drawn out of this is that your genes are superior and that it is worth breeding with you). This selectivity in toxicity has been explained by ecologists as a form of survival strategy termed "directed deterrence". Capsaicin deters animals that tend to eat off the whole fruit-pulp, seed and all. On the other hand, birds eat the fruit and drop off or disperse the seeds. This is precisely what the chilli plant would like {frac12} disperse its seeds so that propagation can occur. If the seed were not dispersed but predated, it is the end of the line for the plant. Directed deterrence operates in a manner that selectively discourages seed predators, and lets beneficial seed dispersers do their job! After all, the function of a fruit is to facilitate seed dispersal. The role of capsaicin may be understood then as a selective deterrent: Mammals no, Birds welcome!

Drs. Tewksburry and Nabhan planned and conducted a series of experiments to verify the features of the directed deterrence hypothesis. First, they investigated a grove of about 150 chilli plants (for those who are finicky about which plant, the chillies they used were the ones that grow in the wild. The pods are very small and very spicy; they have lots of capsaicin), using video cameras. During the day, it was only birds that came over to feed on the plant. Of the birds, it was the Curve-billed Thrasher bird that polished off most of the chillies. No animals, bingo! But then, the animals that feed on such plants in the Arizona desert where the experiments were conducted, are nocturnal ones such as the cactus mice and the packrat. A truly complete experiment would be one that looks at these night- raiders as well. In order to do so, the scientists offered fruits of both chilli and desert hackberry (the most common fruiting plant of the area) on the experimental sites. They found that during the day, both fruit types were removed in equal amounts. At night, only hackberry fruits were eaten and the chillies left alone. What do these observations mean? During the day, the mice and rats do not come out and the birds eat both the berries and the chillies, with no special preference or deterrence. At night, the birds are gone and the mammals feast of the berries and avoid the chillies.

Is it the capsaicin that deters the animals? Will they eat capsaicin-free chillies? Happily enough, such a toothless tiger is available. One particular variety of chilli, called capsicum chacoense, is known which is similar in size, shape, colour and nutritive content to the more common C. annuum, but is nonpungent because it is a capsaicin-free mutant. (See how useful mutants can be in studies of this kind? Also, as one who likes the smell, taste and flavour of chillies but cannot handle their pungency or `heat', I shall write to Dr Tewksbury and get hold of C. chacoense to grow in our backyard, after obtaining my fire-eater wife's permission). When the sites included all three fruits- the hot, non-hot chillies and blackberries, birds polished all the three with no special preference At nights, rats and mice went for berries and the non-hot chillies; but once they tasted the pungent variety (could not tell ahead of time, since they all look alike) they stopped eating all chillies- hot or not! The capsaicin that they bit on stimulated their sensory receptor and caused them to quit and cool off. Thus, capsaicin selectively influences the feeding preferences of consumers.

There is a further twist to the story. The directed deterrence idea holds that the rats and mice (deterred species) should be seed predators. They should destroy the seed and make it unfit for germination, whereas the undeterred species (the birds) should be seed dispersers. They would be expected to drop the seed unhurt, or even aid it by chemically preparing it and priming it better for germination, and also deposit it in a spot favourable for germination. The scientist{frac12} duo tested this point by looking at the germination profiles of the pungent and the harmless chillies and the berries. What they found was that the chacoense pepper seeds that came thorough the rats did not germinate at all, but the ones that came through the thrashers did as well as control seeds (those planted directly from fruit). Also, even the pungent chilli seeds that came through the gut of the thrasher were seen to be able to germinate just as well.

Thrasher birds seem to do one other quid-pro-quo to the chillies. They drop them often under the shades of other shrubs. Such an environment increases the survival of the seedling. We thus have a case here of not only directed deterrence by the plant but also directed dispersal by the consumer. This is an example of the "made for each other" type mutual help between animals and plants. To be sure, neither of them is deliberately altruistic. Each has evolved to benefit itself in the game of life. Selective deterrence of the capsaicin in the chilli keeps the predator away, but not the bird. The bird tends to benefit from this since it can eat more chillies without being bothered by the capsaicin. The dispersal strategy of the bird happens to benefit the plant; the birds fly to the shrubs looking for food there and happen to drop the chilli seeds in the germination- helpful soil there. Interaction between the two species ends up benefiting both. And what do we humans do? First of all, we no longer disperse seeds in the wild or the open. This is a distinct disadvantage to the plant. Secondly, we select and breed seedless fruits such as grapes and oranges {frac12} "directed consumption". (I wonder what the anti-GM-food and nature-food people have to say about this practice).

Talking of evolution and co evolution, different mutants of the chilli and so on, it occurs to me that the ancestor chili plant, the mother of them all, would not have had capsaicin in it. This molecule and its associated pungency would have been the result of one or more mutations that let the mutant plant deter predators and gain survival advantage. In time, mutants of this kind would have flourished while the ancestral and capsaicin-free variety would have been sidelined. Dr. Tewksbury tells me that the molecular ancestry of the chilli is being worked out using DNA analysis. It appears that capsaicin-like molecules may have evolved twice, and that they have been lost at least that many times, all within the familiy of plants loosely called capsicum peppers. There is a closey allied species that is also not pungent, and it may be very close to the most ancestral chilli, without the heat. He is going over this winter to Bolivia to seek it out.

D. Balasubramanian

L. V. Prasad Eye Institute

Hyderabad 500 034

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