Why do plants with berries often have thorns? Aren鈥檛 such plants sending mixed messages? Why would they want to repel animals that would eat their berries and spread their seeds?
鈥� I have found that larger animals, such as horses, prefer to eat the stems of plants that produce berries, thus damaging the structure of the plant. Thorns are a helpful deterrent. Creatures that prefer the berries and do not damage the rest of the plant, such as birds and insects, are able to bypass the thorns to eat the berries and help spread the seeds.
Cordelia Moore, Ashurst Wood, West Sussex, UK
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鈥� The messages aren鈥檛 mixed: the plants are saying 鈥渆at my fruit, not my leaves鈥�. Most animals that benefit the plant by eating fruit and dispersing seeds are dexterous enough not to hurt themselves. A bigger, clumsier herbivore might not bother separating fruit from foliage, but if the latter came with a mouthful of thorns, the animal might think again.
A related issue is why some tasty fruits have highly distasteful skins 鈥� oranges, bananas and mangoes are good examples. To enjoy these, an animal has to be big and powerful enough to peel the fruit, and thus probably capable of eating the peeled fruit whole. This means swallowing the contained seeds and depositing them many miles away, rather than just nibbling the juicy bits and leaving the seeds unhelpfully close to the parent plant.
This is similar to the reason that unripe fruits are small, inconspicuously coloured, sour and rich in unpleasantly astringent tannins. The situation changes when the seeds are fully developed and could benefit from being dispersed.
Stephen C. Fry, Professor of Plant Biochemistry, School of Biological Sciences, University of Edinburgh, UK
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