So You Want to Be A Scientist 2010 Experiments - Ruth Brooks: Homing Snails

Original Idea

Ruth Brooks

What is the homing distance of the Garden Snail (Helix aspersa) that decimates my plants? How far away do I have to dump them before they find their way back to my garden?

Introduction

There is much anecdotal evidence that, like homing pigeons, snails are able to find their way back to their own gardens, if ejected.

Distances of up to a mile or even more have been reported. Yet this has not been substantiated with scientific evidence. So I wanted to find out for myself if these accounts of epic journeys by snails were true, or not. But before establishing distance, it was necessary to find out whether snails actually had a homing instinct.

Method

To begin, I set up two pilot experiments in my own garden. For each experiment, I collected separate samples of snails from two different areas of my garden. I called these areas 'Home' and 'Away' bases. The snails were marked, counted and numbered. The distance between bases was measured.

They were then released midway in a neutral - i.e. unattractive to aspersas - space. The release point was the centre of a tin tray on which a compass circle had been drawn, with the compass points marked. As the snails left the circle, their exit points were marked, and recorded, to establish their direction of travel. The snails were monitored over several weeks to check their progress.

The next step was to establish distance. A national experiment was set up on-line: The Great Garden Snail Swap. Neighbours collected snails from a particular patch in their gardens, marked and counted them, and swapped them over. On the on-line questionnaire, they recorded when the first snail arrived back, and also the distance between the two bases. Also recorded: barriers, and weather..

Results

The results showed startling evidence of homing instinct over 8 metres and 10 metres, with Fisher's Exact Test showing P. values of 0.00004 and 0.0014.

Findings in this experiment were complicated by a spell of exceptionally dry weather, during which many snails disappeared - presumably in shade and sealed up in their epiphragms. But in those instances where snails were recovered over short distances (up to 10 metres), there was again strong evidence of homing instinct. Over longer distances, particularly over 30 metres, results were inconclusive.

Conclusions

From the evidence so far, it seems clear that Helix aspersa has a homing instinct, and can 'home' to distances up to 10 metres. Over 10 metres, conclusions are difficult due to the variables mentioned above.

But implications for frustrated gardeners, who want to know exactly how far away they can dump their garden snails, are that it would be safe to take them 100 - 200m.

Therefore, there is no need to kill them. The implications of not using pesticides are: healthier microbacteria in the soil, and less danger of poisoning to pets and birds. And the feel-good factor for humans

Scientific Mentor

David Hodgson

Dr Dave Hodgson, Senior Lecturer School of Biosciences, University of Exeter
I study the mechanisms that help to maintain biodiversity. I am interested in the population dynamics and behaviour of all organisms, but especially invertebrates.

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