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Snails in your garden? Warm up your throwing arm, scientists suggest

Charles DarwinScience
A statistical analysis of hundreds of snails finds support for the idea that they have a homing instinct
Gardeners can overcome snails' homing instinct by relocating the pests at least 20 meters away, study finds
Optimal snail control can "be achieved with a stronger throwing arm or mechanically assisted lobbing"

Sea turtles have it. So do salmon and Giant Burmese pythons. Pigeons are really famous for it. Now scientists report that some garden snails have a homing instinct too – but gardeners can overcome it with a simple heave-ho.

In a two-year experiment involving a quantitative ecologist, an experimental physicist and hundreds of snails, the data demonstrated that relocating the mollusks at least 20 meters away was just as effective a pest-control strategy as using poisons. The results were published Friday in the journal Physica Scripta.

“Gardeners who dislike unnecessary slaughter will wish to know if it is in fact necessary or indeed useful to kill the snails they find,” wrote the study authors, David Dunstan of Queen Mary University of London (the physicist) and David Hodgson of the University of Exeter in England (the ecologist). “This study is primarily aimed at answering that question.”  

No less a scientist than Charles Darwin had noted the possibility that snails might have a homing instinct, the study authors wrote. Writing in “The Descent of Man,” Darwin relayed the story of a Roman snail (Helix pomantia) that appeared to have found its way home after being moved into “a small and ill-provided garden.” The snail climbed over a wall to reach a more lush garden, then returned to guide a fellow snail to the better digs.

In 1950, researchers in Sweden tested the homing ability of Roman snails by plucking them from their homes in Stockholm and releasing them dozens of feet to several miles away. About half of the snails returned, according to their report in the ecological journal Oikos.  

The new study was prompted by the refurbishment of a “small suburban garden” in the spring of 2001. About 120 new plants were installed, but it took only a few days for Cornu aspersum snails to ruin them.

“Reluctance to follow the advice of more experienced gardeners to kill any/all snails found led to the decision instead to mark them and throw them over the garden wall into some convenient wasteland,” the researchers wrote. The researchers estimated that each toss deposited a snail about 5 meters (16 feet) from the center of the garden, causing the creatures to land with an impact speed of nine meters per second (roughly 20 mph).

Before flinging a snail out of the yard, Dunstan marked it with Tipp-Ex correction fluid so it could be recognized if it returned. Each time a snail returned over the next six months, it got an additional dab of white-out.

Altogether, 416 snails were plucked and tossed from the garden 1,385 times. Clearly, many were repeat visitors, including two that returned 13 times.

The following year, Dunstan modified the experiment to see if he could nail down whether some snails indeed had a homing instinct or whether they might be returning simply out of convenience. So he enlisted the help of government statistician Malcolm Sorrell, who lived four houses away. Sorrell agreed to let Dunstan deposit half of the snails in his yard, which was 20 meters away. The rest of the snails were chucked over the wall as before.

In 2002, the experiment lasted only four months and resulted in 558 captures of 301 individual snails. (Those numbers were way down from the comparable period in 2001, which included 1,048 captures of 383 snails.)  Only three of the snails taken to Sorrell’s house returned to Dunstan’s garden – too few for the scientists to conduct a statistical analysis of a homing instinct, they wrote.

With the data in hand, Dunstan and Hodgson created many mathematical models – some of them rather sophisticated – to describe the snails’ behavior. Taking into account births, deaths and the time needed for a snail to creep back over the wall and into the garden did not improve the models’ predictive value. Accuracy improved when they accounted for deaths only of snails that returned to the garden at least once. Perhaps this was because some snails were smart enough to stay away after being manhandled by Dunstan, the researchers theorized.

But the models became much better when they allowed for the possibility of a homing instinct. One effect of this decision was to divide the snails into two groups – those who were “residents” of Dunstan’s garden and those who were merely visitors. The data implied that perhaps 200 of the snails were residents with a homing instinct, and that they recruited younger snails to join the community as the year progressed.

“The data presented here are strongly suggestive of a homing instinct,” the researchers concluded.

Even so, killing the snails “showed little advantage” compared to simply removing them, since “snails casually found and killed are only a small sample of a much larger population,” they wrote. “Even if systematic searching and molluscicide is carried out on a grand scale, from our data it would take months to reduce the total population significantly.”

The fact that only three snails made the 20-meter trek from Sorrell’s garden to Dunstan’s garden prompted the researchers to conclude that death is too good for these slimy pests: “Even better cultural control could be achieved with a stronger throwing arm or mechanically-assisted lobbing.”

Let me be the first to nominate this research for an Ig Nobel Prize.

Though the study may seem silly, it should rightly be seen as a demonstration of “the practical power of statistical analyses,” according to an editorial by Physica Scripta Editor Suzy Lindstrom. By demonstrating that gardeners can lay off the poisons without sacrificing their gardens, Dunstan and Hodgson have performed a real service. 

“When they see their fragile plants being devoured, gardeners often resort to desperate measures, such as spreading poisonous pellets to rid themselves of this menace,” she wrote. “Eradication in this way has inevitable consequences on other forms of wildlife further down the food chain.”

Copyright © 2014, Los Angeles Times
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