Tag Archives: Nature Communications

Petrichor

“Scent of earth, sweet with the evening rain”, wrote Edith and Saretta Nesbit in All Round The Year (1888). A spell of fine, dry weather was suddenly punctuated by a short, sharp burst of rain. As I walked in the garden, I was conscious of an intensely earthy, fresh, almost sweet aroma, as if the shower had woken the earth and the plants from their slumbers and they were rejoicing by giving off this distinctive fragrance. It is one of the most evocative and invigorating smells of summer and so alluring is it that you can even buy it in a bottle.

For several generations enterprising perfumiers in Kannauj in India’s Uttar Pradesh have captured and absorbed the scent in sandalwood oil in a process which takes around fifteen days to complete. Having baked clay in a kiln, they immerse it in water held in copper cauldrons called degs, sealed with earth. A cow dung fire is lit under the cauldron and the resultant vapour travels through bamboo pipes to condense in receivers, over a base of oil, to form what they call matti ka attar or “earth perfume”, an essence released by the interaction between earth and water. It is used as a perfume, in air fresheners and, because of its soothing properties, in aromatherapy.

Although James Joyce did not seem the sort of chap who would splash a bit of perfume behind his ears, he too recognised that the fresh smell after a shower of rain was due to some reaction with the earth. In 1916, he wrote in A Portrait of the Artist as a Young Man, how “the trees in Stephen’s Green were fragrant of rain and the rain-sodden earth gave forth its mortal odour, a faint incense rising upward through the mould from many hearts”. Curiously though, it took scientists until 1964 to understand quite what was going on.

That many dry clays and soils gave off a peculiar and characteristic odour when moistened with water was a phenomenon recognised in all standard mineralogy textbooks at the time, but Joy Bear and Richard Thomas, working for the CSIRO Division of Mineral Chemistry in Melbourne, were intrigued to understand why and how. They set about steam distilling rocks that had been exposed to warm, dry conditions.

What they found, and documented in their ground-breaking paper, Nature of Argillaceous Odour (Nature, March 7, 1964)[1], was a yellowish oil trapped in the rocks. It took an interaction with moisture to release it. For want of a better word, they called the oil petrichor, a compound word made from two Greek words, petra, meaning rock, and ichor, which, in mythology, was used to describe the ethereal fluid which flowed through the veins of the gods instead of blood. 

Even a modest increase in humidity is sufficient to fill the pores in rocks and soil with tiny amounts of water, which flush out the oil and release the petrichor into the air. When it begins to rain, the process is accelerated, and the wind helps to disperse the aroma.

Bear and Thomas may have explained why petrichor is produced, but it not until 2015 that two scientists from the Massachusetts Institute of Technology, Young Soo Joung and Cullen Buie, explained the mechanics of the process in a paper published in Nature Communications[2] . Using high-speed cameras to film what happened when raindrops hit the ground, they discovered that on impact they started to flatten, trapping tiny air bubbles. These bubbles then shot upwards, rather like in a glass of champagne, pushing through the surface of the droplet, before bursting out into the air in a fizz of aerosols.        

The number of aerosol droplets generated was dependent upon not only the speed at which the droplets hit the surface and on the properties of the surface itself but also the intensity of the rainfall. Perhaps counter-intuitively, they found that light and moderate rain showers generated more aerosol droplets than did prolonged, heavy downpours.

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[1][1] https://www.nature.com/articles/201993a0

[2] https://www.nature.com/articles/ncomms7083

Why Do Zebras Have Stripes?

Quite why zebras have stripes is a question that has long fascinated scientists. Theories abound including the obvious one that they provide the animal some form of camouflage against their natural predators. However, the fly in this particular ointment is that it isn’t particularly effective. Zebras are the prey of choice of lions and other large predators and they seem to get through their fair share of the ungulates, camouflage or no.

It was in the 1930s that the idea of stripes having something to do with controlling the number of flies that land on and bite zebras gained some traction. After all, they live in parts of the world where flies carry deadly disease and where the constant draining of blood can affect the animals’ general well-being. A big step forward was made in 2014 when Tim Caro of the University of California and his colleagues published the results of their research in the journal, Nature Communications in a paper entitled The function of zebra stripes. I will shut my eyes to the fact it was published on April 1st.

They looked at various species of zebras, horses and asses, and sub-species, trying to establish whether the ecology of the area in which they lived had any influence on their propensity to have stripes. The scientists found that where there was a preponderance of inimical insects, particularly the tsetse flies, the equids were more likely to have stripes. Moreover, those areas of the body most vulnerable to attack generally had a greater density of stripes.        

These findings lent some credence to the theory that the zebra’s stripes may have evolved to ward off biting flies. But how? After all, an insect wouldn’t be able to distinguish between a striped animal and a plain one until it was close by. Some further field research was needed to put some flesh on these theoretical bones.

The indefatigable Tim Caro and his team took up the challenge by examining the behaviour of tabanids, horse flies to you and me, on striped zebras and plain horses on a farm in Britain. Their results, published in the journal, Plos One, on February 20, 2019 in an article entitled Benefits of zebra stripes: Behaviour of tabanid flies around zebras and horses, revealed that fewer flies landed on the zebras than the horses although there was no noticeable difference in the number buzzing around them. Interestingly, flies seemed to be surprised to encounter stripes, they were British, after all, and either veered off or were so discombobulated that they were unable to land on the animal’s skin.  

Their research was followed up by a series of ingenious experiments conducted by a team of Japanese scientists, led by Tomoki Kojima. In a paper intriguingly entitled Cows painted with zebra-like striping can avoid biting fly attack, published in Plos One on October 3, 2019, they revealed that they took six pregnant Japanese black cows, painting on two of them white stripes some 4 to 5 centimetres wide. Two they painted black and the other two were left as nature intended them.

The process was repeated twice more so that each cow spent three days striped, painted black or unpainted. The scientists also took high-resolution photographs of the cows at different times of the day, the more easily to count the number of insects that had landed on them. They were also interested to see whether the animals exhibited any behavioural traits suggestive of being bothered by insects, such as flicking their tails or stamping their feet.

When the results were analysed, the scientists found that only 55 flies visited the cows with white stripes, compared with 111 on the black-painted cows and 128 on the plain ones. Behaviours designed to repel flies were deployed less frequently by the white-striped cows than the others, 40 times every thirty minutes compared with 53 and 54 respectively.  

The stripes were not completely fool proof as the ersatz zebras were bitten, perhaps because the flies use other senses such as smell to locate their victims, but the implication is that stripes do have some deterrent effect on flies. Clearly, more research is needed and an efficient way of striping cattle has to be found. It took the scientists around five minutes to paint the stripes on each cow, so it may be some time before we see farmers deploy this technique to give their animals a modicum of respite when flies are at their most active.  

But I will tell you one thing. Since I bought a zebra onesie, there have been no flies on me!