How Do Leopard Slugs Mate? Hanging From a Rope of Slime

Step by step: the leopard slug mating ritual

The mating of the leopard slug (Limax maximus) is one of the most unusual in the animal world. It usually takes place at night in summer and early autumn, in mild, damp weather.

1. Following. One slug finds another, probably by following the chemical signals in its slime trail, and tracks it closely.
2. Circling. The two slugs circle each other, licking and nibbling each other’s bodies and producing large amounts of mucus. This courtship often lasts an hour or more and can go on for several hours.
3. Climbing. The pair climbs a wall, fence, tree branch or plant stem, still circling and touching.
4. Hanging. Twisted together in a spiral, the slugs slide off the edge and lower themselves on a thick cord of mucus, which can stretch tens of centimeters. They hang tail-up, gently rotating.
5. Exchanging sperm. Each slug pushes out its long, pale bluish-white reproductive organ from an opening on the right side of the head. The two organs wrap around each other and spread into a fan- or flower-like shape below the pair, and sperm passes in both directions.
6. Separating. The organs are pulled back in, and the slugs part. Usually one drops to the ground, while the other climbs back up the rope, often eating the mucus as it goes.

Why are both slugs male and female?

Like nearly all land slugs and snails, the leopard slug is a simultaneous hermaphrodite: each adult has working male and female organs at the same time. A single organ called the ovotestis makes both eggs and sperm, which then travel along separate ducts.

During mating, sperm is exchanged in both directions, so each partner fertilizes the other. For a slow animal that lives alone and is active only on damp nights, this is a big advantage. Any adult of the same species it meets is a potential mate, and each encounter produces two batches of fertilized eggs instead of one.

Snails do the same, though some add extras such as love darts. That story is told in are snails male or female?

What is the slime rope made of?

The rope is made of the same kind of mucus that slugs produce for crawling, a watery gel built from large proteins and sugars. During courtship, leopard slugs release extra mucus, and it forms a thick, elastic cord as the pair lowers itself.

The rope is strong enough to hold the weight of two large slugs, which together can be well over 30 cm (12 in) long when stretched out. Because the mucus is rich in water and nutrients, slugs often eat it afterward rather than waste it.

Why do they hang in mid-air?

Scientists are not sure why leopard slugs mate hanging from a thread. One idea is that the reproductive organs are so long that they need open space to unfold and entwine properly, and hanging lets gravity help. Another is that it lifts the pair away from ground predators such as beetles and shrews.

Several related keeled slugs also have long courtships and large, entwining reproductive organs, but the hanging display is best known from the leopard slug. Within this group, the details of the courtship and the shape of the organs differ between species, which may help each slug find a partner of its own kind.

What happens after mating?

Sperm received during mating is stored and used to fertilize eggs. Some days to weeks later, both slugs lay eggs in damp soil, under logs or stones, or in leaf litter. The eggs are transparent, slightly yellowish and about 6 mm (1/4 in) long, laid in clusters. A single slug can lay hundreds of eggs over its life.

There is no parental care. The eggs hatch in about a month, depending on temperature, and the young slugs look like small, pale versions of their parents. They grow slowly and usually need about two years to reach adulthood. Leopard slugs live about 2.5–3 years.

How do other slugs mate?

Slug mating is surprisingly varied:

• Banana slugs (Ariolimax) of North America’s Pacific coast mate on the ground. After mating, one partner sometimes chews off the other’s male organ, a behavior called apophallation; the injured slug survives and can still mate as a female.
• Roundback slugs (Arion), such as the large black slugs of Europe, mate on the ground in a ring or tight huddle.
• Sea slugs, including sea hares and nudibranchs, are also hermaphrodites. Some sea hares mate in chains, with each animal acting as male to the one in front and female to the one behind.

Among all of these, the hanging, glowing display of the leopard slug is the most famous, and it has been filmed for several nature documentaries.

Myths and facts

Myth: The slime rope is spun like spider silk. Fact: Slugs have no silk glands. The rope is ordinary mucus, thickened and stretched by the weight of the hanging pair.

Myth: One slug is the male and the other the female. Fact: Both play both roles at once and both lay eggs.

Myth: The blue organs glow in the dark. Fact: They do not produce light. Their pale, translucent color simply reflects light strongly, so they seem to shine in photos taken with a flash.