Pretty simple question, really, but I've often wondered how slugs survive: from the perspective of a bird or a frog or pretty much any carnivorous animal, a slug would seem to be the perfect prey: no nasty crunch bits to deal with, just a tasty chunk of meat, no defences, and -- crucially -- no ability to run away.  How is it that every slug I see on a pavement or patio isn't instantly gobbled up by a passing bird?

Well Mike I'd wait for more from the other biologists, but I do know that many of them have toxic mucus on their bodies which limits their edibility.

Still that's a good point as I can't imagine they are *all* toxic. Surely then they would tend towards a warning colouration whereas many are simply brown, black or even have a speckled camoflage.

I think, as Dave said, the slime deters predators.

I've seen a nature documentary where a colony of ants was attacking a huge slug. At first, the slug oozes out lots of slime, which the ants get caught up in. But in time, the slug dried out of slime and the ants overwhelmed it. Not only that, but the ants then started to rescue those that were still caught in the mucus.

The French also hang snails in baskets and wait for the snails to dry out of mucus before preparing escargot...

I'd also add that most slugs tend to live in fairly well hidden places. You may see the ones on the pavement or patio, but you don't see the thousands under the paving slabs, in the drains, under rotting wood, etc.

Slime is a great deterent against small predators and it can work on big ones too, but most of all it's best to stay hidden if you're a squishy morsel.

I would agree with most of the previous answers. Slugs are cryptic and (mostly) nocturnal to begin with, but they are not as much fun to eat as you might imagine (hmm). If, say, you're a bird and you grab a slug and try to eat it, it immediately produces lots of extra sticky slime that can coat your bill and feathers. Slug slime is remarkable stuff, with loads of qualities that make it viscous and adhesive, and even water resistant. It basically isn't worth the bother, especially if you have feathers to worry about (recall that feathers need to be kept clean and in good shape to serve in insulation and aerodynamics). Having said that, slugs are of course eaten in their millions by assorted predators, including small mammals, reptiles and amphibians, and many birds. But there are an awful lot of slugs out there.

PS - it's not true that slugs have no crunchy bits. They retain an internal shell: they are highly modified snails don't forget (slugs evolved from snails, not the other way round).

Well, they don't crunch when I step on them.

But perhaps they scream!

Maybe slugs are racing snails? I think potholing snails might be more accurate though.

Slugs are a polyphyletic "group" that has arisen from convergence on a shell-less (or shell reduced) snail-form. They must be doing something right for shell-loss to have evolved several times, and I think the pressure for this adaptation has come from accessing shelter where a shell would prevent access.

Last edited by Paolo Viscardi (21st Jun 2007 10:21:55)

Also, aren't (shelled) snails constrained in where they can give: they require environments where there is a reasonable amount of calcium available. Slugs are not constrained in this way and hence are far more flexible than (shelled) snails in habitat choice. I forget where I read this.. it might be nonsense.

I remember something about that from many, many years ago. Probably when I was an undergrad.