You can’t miss it even from a distance
I seem to particularly like animals that have few friends. Some in fact, which I find fascinating, are despised by the general public. Perhaps I feel sorry for creatures whose only handicap is that they are not cuddly or cute – and this includes slugs; naked “slime balls” as they are called by some and “detested garden pests” by others. What all those slug haters are unaware of, I suppose, is that folk medicinally therapeutic uses of slugs and their slime include treatments for skin problems like acne and dermatitis, inflammation, calluses and warts and that the cosmetics industry of today also still has some applications for them.
Anyway, in New Zealand in our garden I studied the “homing behaviour” of these lettuce lovers and found that they crawled very precisely every night (after having destroyed more of our vegetables) to their resting places under nearby stones. There they would rest and digest until ready for another outing the next night. Capture and release experiments by others (not myself) have shown that also the common garden snail exhibits homing behaviour and that it returns to sites favourable for overwintering with an angular error of less than 30%. Such sites could easily be as far away as 40 metres from the feeding areas.
Garden slugs have been shown by time-lapse photography to forage over an area extending at least 4.5 metres from the same hole in the soil or resting place under a log or boulder from which they emerge early in the night and return to early in the morning. How do they do it? A Japanese scientist believed infra-red vision could be involved; others thought it more likely that a slime-trail was laid, but neither of these theories could be supported experimentally. Observations, on the other hand, that slugs on their homeward locomotion frequently exhibited a characteristic “head-waving” behaviour and that a slight wind from the direction of the home base reduced the slug’s error, suggested that smell could be involved. In fact, the removal of the long tentacles or surgical disconnection of the olfactory nerve has been reported to eliminate homing in the slug and moreover reduced the distance over which it could detect the smell of the stinkhorn fungus (Phallus impudicus) from 120 to 20 cm.
However, a beautiful and almost 15 cm long mottled brown Japanese leatherleaf slug, which I kept in a terrarium and which consistently rested in one corner of the cage after its nocturnal forays never experienced the slightest wind in the cage. But of course smell as well as slime trails could still have provided the essential clues. Although I did work with Russian scientists on regenerating tentacles of slugs (they do grow back), I could not possibly snip off one (or both) of the tentacles of my pet slug (especially since it had produced approximately 60 beautiful babies), just to test whether it would then still find its home. That odours, however, can indeed be a powerful attractant (and a repellent too),we humans know only too well; that seabirds locate cliffs and islands from afar not only visually, but may also smell them over great distances has, however, only more recently received support and that slugs not only detect, but actually like the smell of their home does seem very likely to me.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2017.
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