10 Slimy Facts About Banana Slugs

spotted banana slug crawling across a fern leaf
Banana slug slime helps them to simultaneously glide across and cling to surfaces!.

Ciaran Meister / Shutterstock

Banana slugs are bright yellow and huge, reaching nearly 10 inches in length and weighing over four ounces. Three species are currently recognized by science. Banana slugs live in the moist conifer forests of the Pacific Northwest, from Central California into Alaska. They are slow and strange, and their best attribute is their slime.

And they're also so beloved that they're even a mascot for the University of California, Santa Cruz. Festivals are held throughout their range to celebrate the banana slug. Songs are written about them, and one band is even named for them.

What could make these slugs so popular? Read on for 10 facts about these endearing slugs.

1. Banana Slugs Blend In With Their Environment

yellow banana slug with brown spots near a similarly colored leaf on the forest floor
ZargonDesign / Getty Images

Despite sometimes being a shade of bright yellow, banana slugs blend in with their environment. This is because the leaves and needles on the forest floor in their range tend to be yellow when they reach the ground.

Some banana slugs are spotted, while some are more green, brown or bright banana yellow. Dark-colored slugs aren't a solid dark color. Instead, their base color is darker than average, and they are heavily spotted. Slugs with few spots and those without spots are lighter in color. Banana slugs change color depending on their age and environmental conditions.

2. Their Slime Starts as Dry Granules

It would take a vast amount of water to produce slime constantly. As a result, the banana slug has a novel adaptation that makes the slug's environment do much of the heavy lifting. Banana slugs dispense dry granules of mucus, which then absorb surrounding water. A granule can absorb several hundred times its volume in water, helping the slug create maximum lubrication with minimum exertion.

This is why banana slugs need to be in moist environments. All that water around them is critical for keeping them on the move.

3. They Are Slow

Banana slugs are extraordinarily slow. A speedy banana slug only moves 7.5 inches per minute. The same study measured some moving a mere 4.6 inches per minute. A large banana slug in another study moved only 6.5 inches in two hours. This lack of speed makes them one of the slowest animals on the planet. They even use the mucus plug extending from their tail to slow their pace when descending from trees and tall plants.

4. Their Slime Numbs Predator Tongues

The slime covering banana slugs helps deter would-be predators, and not just because of the stickiness. Along with increasing the slime production to create a sticky mouthful, the slime also contains chemicals that act as an anesthetic, numbing the tongue and throat of an animal that tries to eat it. It only takes one attempt to figure out that banana slugs aren't worth the trouble as a snack.

Meanwhile, that same slime helps provide a meal for the slug. As plant matter and debris clings to the slug, the mucus helps to slowly slide it all down to the end of its body. The snail can turn around and feast on what's been gathering at its rear end.

5. Their Slime Is Both a Lubricant and an Adhesive

Slime is simultaneously a liquid and solid, or rather, a substance somewhere in between the two. Slug slime is a liquid crystal, organizing molecules in a structured but flexible way. This makes it a non-Newtonian fluid. The banana slug uses muscle contractions to create waves in the slime to wash it in the direction it wishes to travel—the solid-state of the mucus grips, acting as a forward anchor.

Researchers are looking into how to harness this dual power for methods of locomotion.

6. Their Slime Provides Messages to Other Slugs

Slime contains many exciting properties and chemicals — and no one knows this better than the banana slugs themselves. As they travel along and leave behind a trail of slime, they're also laying down notes to one another. Other slugs can read the messages and follow the tracks. These messages summon a mate to follow during mating season when the slugs add pheromones to their slime.

7. They Have Holes on the Side of Their Head

Santa Cruz Mountains Banana Slug with large visible hole on the side of head
IRCrockett / Getty Images

Banana slugs have three openings on the right side of their head. The most visible is the pneumostome that banana slugs use to draw breath. The slug opens and closes the hole to breathe, rather like a blowhole on a whale. Open allows the air to reach the lungs; closed prevents drowning or drying out in inclement weather. The other, smaller holes on their head are the anus and the gonopore, used for reproduction.

8. Their Mating Varies Based on Species

light colored banana slugs mating curled around each other

the_luna / Flickr / CC BY 2.0

Banana slugs are hermaphrodites, meaning they have both male and female genitals. They can use these to self-fertilize in cases when necessary. Unusually, the basics of sex vary between banana slug species. A. dolichophallus engage in copulation that lasts up to four hours. These slugs join their gonopores by spiraling their penises together. A. californicus, on the other hand, only mates for 10 to 20 minutes, with a single penis involved per mating act. Both of these species attempt to eat their mate's penis after copulation.

9. They Are Vital to the Redwood Ecosystem

yellow banana slug eating a mushroom at night

btwashburn / Flickr / CC BY 2.0

The banana slug diet of leaves, fecal matter, fungi, and other dead matter converts it into rich soil. They also consume berries, excreting the seeds in the fertile humus environment of their waste. This, in turn, supports the plant's germination, particularly since the taste of the slug's excreted seeds is unpalatable to rodents. Banana slugs then serve as food for other creatures, including salamanders and snakes.

10. They Are Sometimes Dormant

Banana slugs enter a period of torpor called estivation. This is similar to hibernation but occurs during periods of heat and dryness. The banana slug buries itself in leaf litter and then covers itself in slime. Estivation lasts until the slug senses that conditions have improved. Banana slugs also hibernate during periods of extreme cold. In addition to the mucus coat, they bury themselves deeply to insulate themselves from the weather.