Title: Research into Homing Distance of Helix aspersa (Garden Snail)
Snails are important pests of gardens/horticulture. Many desire humane means to control them. But, anecdotal evidence suggests an 'ejected' snail will return to its 'home'.
Is this really homing behaviour, or just one-way movement towards food or safety? We ask: can snails home?
Can we use snails to engage children and the general public in biodiversity research?
Null Hypothesis: Snails are no more likely to travel home than to move away.
Alternative Hypothesis: Snails tend to move towards home.
1. Homing Instinct Trials.
– snails collected from two locations.
– marked and released at a central point.
– snails monitored for several days.
– returned home or moved away?
2. The Great Snail Swap.
– public asked to swap snails with friend/neighbour.
– monitor the number that returned or stayed away.
– submitted results online.
Animal Behaviour: Powerful evidence that snails have homing instinct. Confirms gardeners' lore, using objective research. Future work: how do they do it?
Snail Ecology: Snails are faithful to their resting and feeding sites, but many snails disappear and others replace them. Future work: What determines population size and abundance?
Snail Control: We have a choice:
1. Use chemical control. But, what indirect effects are we causing to our environment and to beneficial species? Humane methods may be better for all.
2. Eject our snails (but suffer their homing instinct).
– be careful to eject snails at least 50-100m away.
– consider placing them in a non-hostile habitat that could become their new 'home'.
Science in our Gardens: Snails are a wonderful study organism for gardeners, schoolchildren, families, amateur scientists everywhere!
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.
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 results from these two experiments 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. Two more Instinct Experiments were carried out on the University Campus by Dave Hodgson. Again, the results showed powerful evidence of homing instinct.
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 (divided into 'bands'). between the two bases. Also recorded: barriers, and weather.
Key: H,A = number of home and away snails; m = distance between bases; Fisher's test gives probability of getting our results by chance alone. Small p-values confirm homing instinct.
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. This could have been due to the many variables: terrain, e.g. a wood; the type of barrier: e.g. road, building; the hot weather; or the actual distance itself.
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, on the evidence so far, it would be safe to take and place them elsewhere at a distance of, say, 100 to 200 metres. 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!
For distances over 30 metres, I would like to devise further experiments in more controlled conditions, working with only one variable at a time. Although not replicating natural conditions, it would hopefully move us forward in our quest to find out exactly how far we must take our snails before they home back to chomp on our precious lettuces. It would also provide valuable information on snails' behaviour such as adaptation to disturbance, and to different conditions. And also how snails remember their journeys!
I would like to do much more research on Homing and behaviour, and memory.
Ruth Brooks and Dr Dave Hodgson, 2010
Acknowledgments: This work was made possible by the BBC, Michelle Martin, Sophia Ratcliffe, Cath Hill, neighbours Sue, Carol, Bruce, Peter, Sandra and Dave, my daugher Sarah for help and encouragement, Professor David Dunstan, the estates team at Sparsholt College, teachers and pupils at St John's School Totnes, everyone who posted valuable (or sarcastic) comments on Facebook, and our national team of snail swappers!
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