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The Native Slug of our western rainforests

Getting to know Ariolimax columbianus

By Stéphane Perron and Marie-France Dubois,

Whistler Naturalists

One of the most frequent summer wildlife sightings in Whistler has to be the banana slug. Yet, few people pay much attention to this rainforest creature. Perhaps the slow and slimy images put us off, but the fact is that banana slugs are an important member of our rainforest ecosystem and worthy of a closer look.

Banana slugs ( Ariolimax columbianus ) can’t be identified solely by colour. Although they can be bright yellow (hence, along with their banana shape, the common name), in Whistler they are anywhere from waxy white to dark black and all forms of greens and Halloween candy browns in between. They sometimes have dark spots or blotches. The best way to identify a banana slug is by the keel (ridge) that runs from the end of the tail to the mantle (the flap covering the top of the body behind the head).

Banana slugs are the longest in North America, generally 15 to 20 cm and up to 25 cm. Banana slugs prefer the damp, shady habitat of coastal rainforests from northern California to Alaska. They are rarely found in urban areas.

Banana slugs love to eat. Mushrooms are their favourite food but they eat almost anything. They will munch on both living and decaying plants, fruits, seeds, bulbs, lichen, animal droppings (very convenient around the Valley Trail…) and animal carcasses. Because they prefer the menu offered on dense forest floors, banana slugs seldom visit gardens. Banana slugs will eat garden plants if given no choice, but the reputation for raiding gardens and causing extensive plant damage belongs to non-native European slugs like the giant garden slug ( Limax maximus ) and chocolate arion ( Arion rufus ), both of which have a pebbly, usually black, skin.

If you take time to observe a banana slug, notice its breathing port on the right side of its body, behind the head, and how it opens and closes. Yes, slugs have a lung, and the breathing port is what protects it. Also notice its tentacles. There are two sets. The upper longer pair are the eyes which can’t do much more then gauge light intensity. The lower ones are sensory organs for feeling and smelling. Smell is a slug’s strongest sense to find food.

One of the more interesting aspects of slugs is their sex life. Slugs are hermaphrodites, meaning that they have male and female sexual organs and that they use both when copulating, which lasts many hours. Choosing a mate of matching genital size is important for slugs, since if either slug miscalculates it may get stuck and be unable to pull out afterwards. When this happens the unstuck partner gnaws off the stuck one’s slughood – scientist call this "apophallation." Makes one glad to be a mammal.

Another noteworthy characteristic of slugs is their slime. All parts of the body have slime-producing glands. Some produce thick, sticky slime, which protect the slug’s body from rough surfaces and prevents its dehydration. It can also be used as defense. When a slug is threatened it produces a lot of thick slime and contracts into a near-ball shape. Predators in Whistler include salamanders, newts, crows, ducks, shrews, moles, raccoons, snakes, and dogs.

Another type of slime produced in the foot is thin, and that’s the one that leaves the "slime trail" you see on the Valley Trail early in the morning. Slugs can apparently lower themselves from high places (and I would love to witness this), hanging from a thin slime "cord" produced at their tail ends.

Slugs lay clutches of up to 30 eggs in holes and crevices where the ground is moist. The eggs take three to eight weeks to hatch. The mortality rate for the eggs is high due to predation from birds and mammals. Banana slugs seldom reach old age but their natural lifespan is estimated at three to six years, based on other land slugs.

Banana slugs play an important role in cleaning up the forest floors and in dispersing seeds and spores. Their droppings are also a valuable source of nitrogen-rich fertilizer. Our forests would be a poorer place without banana slugs.

If you are interested in further reading, most of this information comes from a book written in 1988 by Alice Bryan Harper entitled "The Banana Slug" and published by Bay Leaves Press. Happy slugging.

Upcoming Events

Wednesday, May 29 Nature Walk . Meet at 6:30 p.m. in Function Junction. Turn left (east) at the lights, then another left into the parking lot. We'll assemble at this point, then drive further up the Cheakamus Road for a walk through old growth and second growth, Alice in Berryland. Good walking shoes needed. Call Mitch Sulkers (604-932-3707) for details.

Saturday, June 1 — Bird Walk . Meet at 7 a.m. at the bottom of Lorimer Road near the entrance to the Catholic Church. Novices and newcomers welcome. Call Michael Thompson (604-932-5050) for details.

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