V E R Y   G O O D   I N D E E D !

If you get the chance to see it, you really should. The two highlights of the exhibition for me were getting to see Darwin’s original Red Notebook in the flesh, so to speak, and being shown the original Galápagos mockingbird specimens which first set Darwin to wondering about evolution by the Beagle Project‘s Karen James, who has recently been working with these very specimens.

The potentially embarrassing moment of the evening came when I seized the opportunity (over mulled wine) to ask a snail expert working for the International Commission on Zoological Nomenclature a question which has been bothering me for over six years now: why are there are so few snails in my garden? It hadn’t occurred to me that there are an awful lot of snails in the world, and that this particular expert might specialise in those from another part of it (like the African rift lakes, for example).

Still, I think I got away with it.

It isn’t every Thursday morning that one makes a paradigm-destroying observation while rushing, umbrella-in-hand, to one’s car. But that is exactly what happened to me last Thursday. It was pouring with rain, I was running six minutes late for work, I was wet and cross about it, when suddenly, there it was, as bold as brass and as bright wet as day, in front of my very eyes: a common (but, until that point, not at all garden) snail, Helix aspersa, sliming its way across my driveway.

You have no idea how ridiculously happy this made me feel. It is six years this month since I moved to my home in the Yorkshire Pennines, and, in all that time (with one very minor exception), I have seen (if you’ll forgive the inappropriate cliché) neither hide nor hair of a snail. And, believe me, I’ve looked.

I first wrote about the lack of snails in my garden in 2002, in an essay entitled …So Let’s All Be Scientists! It was my contribution to Darwin Day Collection One: The Single Best Idea Ever [ISBN: 0972384405, Amazon.com], a collection of articles, reviews and cartoons in celebration of Darwin and science. In the essay, I suggested a number of hypotheses—some of them more serious than others—why there were no snails in my garden.

Until 06:16 last Thursday, the acidic soil hypothesis (i.e. the acidity of the soil preventing snails from forming shells) was my favourite explanation for the dearth of snails, but that has had to go by the wayside. I am now beginning to favour the out-competed-by-slugs hypothesis. For the first four years that I lived in this house, the garden was literally plagued by slugs: thousands and thousands of slugs. But, for the last two years, the number of slugs has dropped considerably. I’m not sure why this should be—a combination of an unusually dry summer last year, and more dilligent weeding by yours truly is my best guess—but maybe the marked drop in slugs has let the snails get a (literally) single foot in the door. A Darwinian mollusc war in my own garden: who’d have thought it?

I will continue to monitor the situation with renewed interest.

One of my unsent letters was on a subject which Gould said was in his top-three for reader feedback. In his essay Left Snails and Right Minds (published in his book Dinosaur in a Haystack), Gould wondered why old engravings of snails often show their shells spiralling the wrong way (the vast majority of snail shells have right-handed spirals; the old engravings often showed them left-handed). Some years after first reading the essay, I came across the following letter in Volume 5 of The Correspondence of Charles Darwin, which caused me to start drafting another unsent letter to Gould:

Charles Darwin to James de Carle Sowerby, 21st January, 1851

My dear Sir

I am much pleased with the Plates.— […] Only one figure will require a weighty alteration, viz P. rigidus. Nevertheless, I hope that you will loook [sic] over your Figures carefully for I saw a good many little blemishes; & the Plate is not very clean.—

The few reversed figures are unfortunate.— […]

Your’s Sincerely
C. Darwin

Sowerby was artist to the Palaeontographical Society and was preparing the engravings for one of Darwin’s barnacle books, Fossil Cirripedia (1851). From earlier letters, it is quite clear that Darwin was extremely frustrated with Sowerby’s slow progress. In contrast, a few reversed figures seemed relatively unimportant—although I thought Gould would have been amused that his great hero also suffered from reversed engravings.

By a strange co-incidence, shortly after reading Gould’s essay, I read a chapter in Richard Dawkins’s (then) latest book, Climbing Mount Improbable, which also dealt with shells. In it, Dawkins explained how the shapes of all shells can be recreated on a computer using just three parameters (or ‘genes’), which he named flare, verm, and spire. This time, I actually did write to the author:

Richard Carter to Richard Dawkins, 8th August, 1996

Dear Prof. Dawkins,

I greatly enjoyed reading your recent, excellent book, Climbing Mount Improbable and, in particular, the chapter on shells. However, I think I may have spotted a small mistake: on page 153, when talking about what you have called the spire parameter, you state:

Spire has no limits: negative values trivially indicate an upside down shell.

Although it is hard to visualise, I reckon that, if you turned this “upside down” shell the right way up, it would actually be spiralling in the opposite direction to a shell with a positive spire (anti-clockwise, as opposed to clockwise). Although this is hardly earth-shattering, I would argue that it is not trivial – after all, your favourite sparring partner, Stephen Jay Gould, dedicated an entire chapter to anti-clockwise snail shells in his recent, equally excellent, Dinosaur in a Haystack. […]

Yours sincerely,
Richard Carter

To my great delight (and my dad’s: he has boasted about it ever since), Dawkins emailed me straight back, confirming the error:

Richard Dawkins to Richard Carter, 15th August, 1996

Dear Mr Carter

Thank you for your letter of 8th August.

Yes, you are correct that the computer shell with a negative spire would beanticlockwise, and it is certainly not trivial. If I had thought of this,it would have saved me the trouble of building in a separate gene forhandedness. Damn! […]

With best wishes

Yours sincerely
Richard Dawkins

(Let it not be said that Richard Dawkins never admitted to making a mistake.)

Then, yesterday, I spotted another interesting example of reversed images, which, bearing in mind the subject matter, I’m sure would also have amused Gould. While re-reading my recent Red Notebook post The Expression of Emotions in Darwin, I suddenly realised that the photograph of Darwin that was the subject of the piece also appears on the covers of two books I have recently read—but in mirror image:

Original image of Darwin

Books showing reversed image

A quick check of the buttons on Darwin’s waistcoat on a higher resolution version of the image confirmed that the photograph of Darwin on the left is shown the correct way round, whereas the photographs appearing on the covers of the two books are shown in mirror image.

Plus ça change, as we say in Yorkshire.

Guardian: Slicker snails stick to the trail

Snails that follow the slime trails left by others do it to save their strength, according to scientists. By using trails already on the ground, they can save two-thirds of the energy they use in making fresh trails of their own.

Snails use a third of the energy from the food they eat on making the mucus needed to move around. “It’s ridiculous – if we spent a third of our energy just walking around, we wouldn’t get very far,” said Mark Davies, of the natural and social sciences department at the University of Sunderland…

The long trails of mucus left behind by snails have multiple roles. “Most snails are pretty blind – in complex habitats where visual tracking might be difficult, following a mucus trail might be a handy thing to do,” said Dr Davies. “In the marine environment, the trails can act as a trap for food the snail might like to eat, because the mucus is sticky.”

Snails also know if a mucus trail is their own and can detect the sex of snails that have laid other trails. “It’s handy if you want to follow a trail to mate with another snail,” said Dr Davies.

If snails expend so much energy making mucus, I wonder if they might also eat some of the mucus from the trails they follow, in the same way that some spiders eat their own silk, some snakes their own shedded skin, and some mammals their own afterbirth. It would seem to be an efficient way of getting the right sort of nutrients they would need to make their mucus.

(Not that I know anything about snails, you understand.)

Back in 2002, I wrote an essay entitled (in tribute to Huxley) …So Let’s All Be Scientists! It was my contribution to Darwin Day Collection One: The Single Best Idea Ever [ISBN: 0972384405, Amazon.com], a collection of articles, reviews and cartoons in celebration of Darwin and science. In the essay, I observed:

…for all the thousands of slugs I have found, I have never come across a single snail in my garden. Why is that? Is it too cold (I live in the Pennines)? Do the slugs eat them […], or out-compete them? Am I just not looking hard enough? Or is there simply not enough calcium in the area to allow snails to make shells (possibly because the soil is too acidic)?

Well, it would appear that the not looking hard enough hypothesis had some merit. The other weekend, I was moving a rockery in my garden (don’t ask), when I came across this:

A very small snail!

OK, as snails go, it wasn’t exactly the biggest (or, indeed, the alivest), but a snail is a snail, and an ugly fact (no matter how small) is an ugly fact: my no snails in my garden hypothesis is well and truly falsified. So now I have had to modify it slightly:

There are no big snails in my garden.

Actually, I have been keeping an active look-out for snails ever since I wrote my essay, and I can confidently say that I believe my modified hypothesis is correct. Which is odd, because I have observed large snails a couple of hundred yards away from my garden, albeit at a significantly lower altitude (I live on a very steep hill).

But, despite this stark evidence to the contrary, I still like my acidic soil explanation for the (near) absence of snails in my garden. In fact, until last week, I was growing increasingly confident that it was the correct explanation, having first confirmed that the soil in my garden is indeed acidic, and having recently come across the following in the New Scientist subscribers’ archive:

Acid attack

Acid rain has progressively thinned the shells of eggs laid by British thrushes over the past 150 years, a new study suggests. Ornithologists fear that the trend could make thrush eggs less likely to hatch…

[Rhys Green of the Royal Society for the Protection of Birds] thinks that acid rain, caused by sulphur emissions from the burning of fossil fuels, is the most likely cause. This would reduce both the calcium content of leaf litter consumed by worms and the abundance of snails, which together make up a large part of the birds’ diets.

So maybe there is still enough calcium in my garden for some very small snails, but not enough for any snails bigger than a couple of millimetres across.

I will get to the bottom of this one! Eventually.

Postscript [18-Jun-2007]: Hypothesis well and truly falsified!