Nature Watch: July 2018
As the first drops of summer rain hit the hot, dry ground a heady perfume is released.
Animals raise their noses and sniff the air, people do too. It is a distinctive earthy, metallic smell but always welcome; it only comes after a dry spell. It is called petrichor and is a mixture of organic compounds produced by plants and soil bacteria. These oily substances accumulate during dry weather and are released as the first drops of water liberate the oils from the dry earth and leaves. I wonder if the plants can detect this scent in the air – do their weary roots, exhausted from trying to extract moisture from the parched soil, relax at last?
As the rain hits the ground it must wet the surface before it can begin to seep in and do some good. A light rain will do nothing for the plants, it will evaporate again before it can reach down to the roots. A sudden heavy rain might simply run off into ditches and gutters. But in steady rain you can almost feel the plants sighing with pleasure, like a cricketer draining a pint after a hot innings.
Earth worms take the chance to explore their patch when it rains – they need a moist skin in order to breathe so they can’t come out in dry weather. Their burrows help to direct the rain-water down into the ground – just one of the ways in which they engineer the soil to be more productive for life. But they are vulnerable to predators because birds have learned their habits. (A whole column could be devoted to earthworms – there are around 30 species in Britain!)
Herring gulls play a rather mean trick – they don’t even wait for the weather to change. They dance a rain-dance, pattering their feet rapidly on the grass, with a penetrating look in their eye. They are not calling up the rain – they are mimicking it. The worms rise innocently to the surface expecting to find newly washed grassland, only to encounter the mighty beak of a hungry gull. It is a sophisticated technique – it would appear that the gulls’ feeding behaviour is shaped by evidence-based science!
We once watched a blackbird foraging in our garden during hot weather. At last he found a slug though he didn’t seem to relish the prospect. He eyed it suspiciously and then spent a long time trying to wipe off the sticky slime, pausing now and again to wipe his gummed-up beak on the patio steps. Worms are evidently preferable but in dry weather they are hard to extract from the baked soil. When it rains the canny blackbird appears on the lawn, shaking drops of water off her beak, keen to collect some easy food for her hungry brood. If the alternative is a sticky slug we can imagine her relief at the first whiff of petrichor.