Nature 2016 August

Poetry in monochrome – the nature of grey versus black

I was awoken in the early hours as the dawn broke a few days ago by a gang of grey squirrels engaged in a noisy exchange. No doubt if one could interpret the staccato chatter the language employed would be no less profane than that of the proverbial ‘Billingsgate fishwives’

At best, grey squirrels are never more than a loose association of family members but squabbles and violence are never far below the surface. This time of year there is tumult within the squirrel population. They breed twice a year, summer and winter and the noisy argy-bargy denotes the period during which mature female squirrels kick their adolescent offspring, or kits, out of the drey (aka. nest). Meanwhile male squirrels are making moves to secure the favours of the most desirable females and that accounts for the noisy dawn chorus. Being crepuscular – more active at dawn and dusk – chatter can explode at both ends of the day.

Grey squirrels communicate through a complex repertoire of sounds. Rather than speech, consider it a means of expressing emotions, threats or desires. A kind of ‘squirrel poetry’ comprising squeaks, low- pitched barks, and high-pitched chatter. Above all the most frequent repost is a coarse – “mehr mehr mehr”. It’s not just vocalization that’s central to the interactions but also body posturing, such as tail-flicking. This is deployed to warn other squirrels about predators. Male squirrels also make an affectionate coo- purring sound used when courting.

The use of vocal and visual communication has been shown to vary by location, based on elements such as noise pollution and the amount of open space. For instance, populations living in large cities generally rely more on the visual signals: in rural areas, those ‘poetic rhymes’ predominate.

I have mentioned the so-called ‘black squirrel’ variant in a previous Nature Notes. When I last wrote about them there were probably only 25,000 black squirrels, representing no more than one per cent of the total UK population of over two million. Some of these are ruddy/ brown/black while others are a sleek jet black. A black squirrel was first sighted in Letchworth, Hertfordshire in 1912, and is now the town’s mascot. Since then they have slowly but steadily spread east and west, reaching just a short leap from here at Whipsnade. Recent research has confirmed there is a dominant gene that has been ‘switched on’ in the black form. It is linked to higher concentrations of testosterone suggesting both males and females with the gene are more aggressive and will out-compete the pale form. So numbers of the black variety are on the increase.

The peppered moth – a sonnet

Arising from our ashen pit of toil,
As forge and mill did shape this unkempt land; The blackness of the trees from coal and oil, Contrasted with the skin nature had planned. A single, fragile pearl encased in jet,
Your pallor marked you out for all to see;
In contrast to our progress, blood and sweat, Your population had no industry.
And then from deep within you came a switch, We came across your shadow in the sky;
Your alabaster pelt had turned to pitch, Forced to adapt so that you would not die.
I wonder if we ever get it right,
Will you turn back from darkness into light?

By Samuel Illingworth

This modern sonnet is inspired by the recent natural history of the peppered moth. Feeding on birch, it is found in rural woodlands and urban gardens and land fringing industrial areas. Like all lepidoptera when at rest during daytime it is at risk of providing a tasty meal for birds. As the name implies, the moth’s wings and body display a speckling graduating from black to white. When seen at a distance it gives the moth a pale grey hue.

Over millions of years, through the process of natural selection, its colouration has evolved to resemble the pale bark of silver birch and the light-coloured lichen that favours the bark. Consequently, the closer its shading and camou age came to resemble those of bark and lichen, the greater its success in countering predation. Even the caterpillar displays adaptive qualities by closely resembling thin twigs of birch. Occasionally, genetic mutation will produce all-black or all-white moths. But this is a disaster for these moths as they would stand out against the background and the vast majority would be quickly picked off by grateful birds.

The early part of the 19th century saw rapid industrialisation of Britain. This was particularly the case in the large towns and cities of northern and midland England. Soot, the waste product of the coal fuelling factories, iron furnaces and steam railway engines was acidic. Across industrialised areas and the countryside immediately surrounding these towns it progressively killed off delicate lichens highly sensitive to airborne pollution. Soot also blackened all but the smoothest surfaces it attached to, including the rough bark that clad trees. Rapidly the paler form of peppered moth resting on darkened bark was exposed to predation by birds. Though research into these observations has been challenged over the years, it is believed that the darker (melanic) moth variety stood an increasingly better chance of survival as it merged into the sooty background. Over less than 50 years the most frequently found variety had switched from pale grey to black. By the beginning of 20th century more than 95% of moths trapped displayed a sooty-black appearance.

Some of you may remember the Clean Air Act which came into effect in 1964. Very gradually, measures were put in place to prevent sooty deposits being discharged into the atmosphere. Now, the pale grey peppered moth has become the predominant variety again. Even in the heartlands of heavy industrial activity, natural selection has reversed the trend. However, today and sadly in common with most lepidoptera, the overall numbers of peppered moths is as little as a fifth compared to 200 years ago. If Tennyson were alive today he may well have composed an ode to the peppered moth.

That’s all ‘in grey and black’ for this time.