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Thursday, June 9, 2011

Bananas in the woods

It is finally getting warm here in western Oregon, but it is still very humid so the banana slugs (Ariolimax columbianus) are still active.  I have a soft spot for banana slugs.  The idea of a six-inch long monster yellow slug that looked like a piece of overripe fruit seemed too far-fetched to believe when I knew them only by reputation.  I was delighted to find them just as large and spectacular in life as legend made them out to be.  My interest in them from a biological point of view began while I was doing research for my Master’s degree, on the seed dispersal dynamics of salmonberry in the Oregon Coast Range.  One rainy afternoon in early June, I looked down and realized there were a dozen massive slugs slowly working their way through the tangled vegetation within the bounds of my one-meter-square quadrat.

Photo: National Park Service

That’s a lot of animal when you look at it from a biomass standpoint, even if the animal possesses a very slow metabolism.  Slugs eat everything, from fungus and fruit to their dead kin and other animals’ feces.  Mostly they are herbivores.  Work in the 1970s demonstrated that slugs can affect plant community composition in the forest understory.  They also act as a host to a number of generalist parasites (another good reason not to eat one, even if the slime didn’t put you off), and they likely disperse the spores of fungi as well.  In other words, slugs do stuff to their environment. 

I found salmonberry seeds in their droppings and asked the obvious question: are they seed dispersers, or seed predators?  I set up ten Tupperware slug houses, raided the nearby forest for volunteers for science, and kept ten slugs going on potato, carrot, and fresh lettuce in addition to the salmonberries and other wild fruits in my living room.  You could actually hear the slugs chewing.  Fortunately, my roommate thought this was intriguing if a bit weird.  After the animals obligingly ate a series of fruits from the local forest, I let them go, perhaps a bit fatter than they were before.

Salmonberry (Rubus spectabilis) and its two color morphs. Photo: J.A. Gervais

I planted the seeds, and discovered that banana slugs do disperse viable seeds, although their gut kills some of them.  Interestingly, red salmonberry seeds were more likely to die during gut passage than orange ones.  All of the other seeds from native plants producing fleshy fruits also were capable of germination.  I tried to use the word “molluschrory” to indicate this novel dispersal agent of seeds, but my co-author on the paper, ecologist Mary Willson, shot that down.  It is hard to have your tongue in your cheek in a scientific paper, apparently, and have others get the joke.

Slugs aren’t the most spectacular seed dispersers, because in the twenty four hours that it takes them to pass a seed, they don’t go very far.  In fact, slugs have fairly small home ranges with a number of shelters that they will return to repeatedly, following their own slime trails around their neighborhood.  The slime they secrete under the muscular foot is astonishing in itself.  Slugs change the chemistry of the slime according to their needs, secreting a more watery version when traveling along the ground, a tough cord they use to rappel down from the heights of a shrub, and thick viscous goo to deter predators.  They can change the composition of the slime to fit the circumstances, and the chemistry can be altered in a matter of minutes.

Predators can track the slugs from their trail of slime, so their trails may work against them.  One slug, the jumping slug (several species in the genus Hemphillia) actually twists off its slime trail, breaking the chain of evidence a predator might use to track it down.  Another slug, the tail-dropper of the genus Prophysaon, drops a bit of its tail to distract a predator while it makes its getaway.  I am not making this up.  The banana slug has no such defense, and must either outrun or hide from its enemies, which include predatory ground beetles, snakes, Pacific giant salamanders, and even other slugs.  You can blink more than once while watching a predatory slug hot on the trail of a fleeing banana, but it is still a race of life and death for the participants.

The magnificent blue-gray tail-dropper (Prophysaon coerleum), photo: USDA Forest Service

Scientific literature is not often very amusing, but one of the best papers in my collection describes how to individually mark slugs by freeze branding them.  The publication included illustrations of miniature brands and instructions on how to use dry ice to do the deed.  The spotted slugs’ markings are unique to the individual, so photographic records work for identifying them.

Getting to know slugs as individuals revealed that banana slugs can reach at least six years of age in the wild.  They aren’t prolific egg layers, relying on their longevity to produce enough young to carry the species.  Simply put, a nice-sized banana slug may be older than the vast majority of mammals on earth.

 The study of banana slugs pretty much petered out 30 years ago, although the Northwest Forest Plan did at least pay them lip service in terms of requiring that forest managers survey for them.  It is good to remember there is a great deal we don’t know about our fellow travelers, even the ones right underfoot.  Step carefully.


  1. Fun article. Slugs that shed tails? Who knew? And, hey, maybe banana slugs would make good pets being so long-lived. The vet bills would be much lower than for our furry friends, I'd think ;)

  2. The diversity of slugs is simply amazing. Cool BLOG !