Leopard Slug (Limax maximus): Facts, Diet and Mating

What does a leopard slug look like?

The leopard slug is pale gray to gray-brown or yellowish, with dark spots and broken stripes. The spots on the mantle, the shield-like hump behind the head, are usually rounded like a leopard’s, while the body behind it often carries two or three dark stripes along each side. Colors and patterns vary a lot, and many named color forms have been described, but none is a separate species.

Like most land slugs, it has two pairs of tentacles: a long upper pair with eyes at the tips that sense light, and a shorter lower pair used for smell and taste. Both pairs can be pulled back into the head. The rear part of the body has a short ridge, or keel, near the tail tip, a feature of the keeled slugs (family Limacidae). The sole of the foot is pale and uniform, and the slime is colorless and only slightly sticky.

On the right side of the mantle is a round breathing pore called the pneumostome, which opens into a simple lung. In the leopard slug, it lies in the rear half of the mantle, a useful way to tell keeled slugs from the roundback slugs (Arion), in which it sits in the front half.

A shell hidden inside

Slugs evolved from snails, and many have kept a remnant of the shell. In the leopard slug, a thin, oval, whitish plate about 13 mm (1/2 in) long lies hidden under the skin of the mantle. It stores calcium and protects some internal organs, but it is far too small to hide in.

This internal shell was known in ancient times; the Roman writer Pliny the Elder mentioned it. Shells of different keeled slug species look so alike that they cannot be used to tell the species apart. For more on the difference between slugs and their shelled relatives, see slug vs snail.

How big are leopard slugs?

Adult leopard slugs usually measure 10–20 cm (4–8 in) long when stretched out and crawling. At rest, the body contracts and looks much shorter and plumper. The species name maximus means “largest”, and it is indeed one of the biggest slugs in Europe. Its close relative, the ash-black slug (Limax cinereoniger), can be even longer, and North America’s banana slugs are similar in size. The record holders are compared in what is the biggest slug in the world?

Habitat and range

The leopard slug is native to western, central and southern Europe, including Britain and Ireland, and to parts of North Africa. It lives in deciduous woodland, hedgerows, parks and gardens, and it often moves into damp cellars, sheds, greenhouses and outbuildings. By day it hides under logs, stones, loose bark or in wall crevices, coming out at night and after rain.

People have carried it around the world in soil, plants and produce. It was first recorded in North America in Philadelphia in 1867 and is now found in most US states and in southern Canada. It has also been introduced to South America, southern Africa, parts of Asia, Australia and New Zealand.

What do leopard slugs eat?

Leopard slugs are omnivores and scavengers. Their main foods are fungi, including mushrooms and molds, and decaying plant material such as rotting leaves. They also eat dead animals and pet food left outdoors, and they are known to chase and eat other slugs, reaching a top speed of about 15 cm (6 in) per minute when pursuing prey.

They feed by scraping food with the radula, a tongue-like ribbon covered in thousands of tiny teeth. Because they prefer fungi and decaying matter over healthy green plants, leopard slugs do relatively little damage in gardens compared with smaller, plant-eating slugs.

Behavior

Leopard slugs are nocturnal and solitary. They are most active on mild, damp nights and retreat to shelter during dry or cold weather. When disturbed, a leopard slug pulls its head under the mantle but does not curl up tightly like some other slugs.

They have a strong homing instinct. After a night of foraging, an individual often returns to the same crevice or hiding place, following its own slime trail by smell. This behavior was studied in detail by the neuroscientist Alan Gelperin in the 1970s, and the species has since become a model animal for research on learning and memory. Leopard slugs can learn to avoid foods that have made them ill, a form of learning called conditioned taste aversion.

How do leopard slugs move?

A leopard slug glides on its single muscular foot. Waves of muscle contraction ripple forward along the sole, pushing the body ahead over a thin layer of mucus. The slime does two jobs at once: it acts as a glue that lets the slug grip vertical walls and the underside of leaves, and as a lubricant that lets it slide forward. Producing all that mucus costs a lot of water and energy, which is one reason slugs move slowly and stay in damp places.

Normal crawling speed is only a few centimeters per minute, but a hunting leopard slug can reach about 15 cm (6 in) per minute. That is still slow by most standards, which is why slugs and snails feature among the slowest animals.

The body is mostly water and has no waterproof covering, so a leopard slug can dry out quickly in sun or wind. It avoids this by being active at night and after rain, and by hiding in cool, humid shelters by day. In hard frost or long droughts it retreats deeper into soil, leaf litter or buildings.

How to tell a leopard slug from other slugs

In Europe and North America, the leopard slug is one of the easiest large slugs to identify:

Young leopard slugs can be harder to recognize, because their spots and stripes are faint at first.

Mating and reproduction

Like other land slugs, leopard slugs are hermaphrodites: each one has both male and female organs. Their mating ritual is one of the most remarkable in the animal world. Two slugs follow each other and circle for a long time, sometimes for hours, licking each other’s bodies. They then climb a wall, tree or plant stem, twist around each other and slowly lower themselves on a thick rope of mucus.

Hanging in mid-air, both slugs push out long, pale bluish-white reproductive organs from openings on the right side of the head. The organs entwine to form a fan- or flower-like shape and the slugs exchange sperm. Afterward they separate; one usually climbs back up the rope, often eating the mucus as it goes. The full sequence is described in how do leopard slugs mate?

Both partners later lay eggs, and each can produce hundreds over its life. The eggs are transparent, slightly yellowish, about 6 mm (1/4 in) long, and are laid in clusters in damp soil or under logs. They hatch in about a month.

Lifespan and life cycle

Young leopard slugs look like small, paler adults. They grow slowly and usually need about two years to reach maturity, depending on temperature and food. The usual lifespan is about 2.5–3 years, which is long for a slug; many smaller garden slugs live only about a year.

Predators and threats

Leopard slugs are eaten by hedgehogs, badgers, shrews, toads, slow worms, thrushes, blackbirds and ground beetles. They are also hosts to parasites, including tiny white slug mites that swarm over the body and into the lung, and several kinds of nematode worms.

The species is assessed as Least Concern by the IUCN. It is widespread and common in its native range and has spread widely elsewhere.

Leopard slugs and people

Because they eat fungi, rotting plants and other slugs, leopard slugs are often seen as a gardener’s friend rather than a pest, although they can sometimes damage soft seedlings or stored vegetables. In North America, where the species is introduced, it is part of a wider group of European slugs that have settled in gardens and cities.

Their unusual mating has made them favorites of nature filmmakers. A sequence of leopard slugs mating on a slime rope was filmed for David Attenborough’s BBC series Life in the Undergrowth (2005), and photographs of the pale blue, entwined organs are some of the most striking images of slug behavior.

Myths and facts

Myth: Slugs are snails that have lost their homes. Fact: Slugs do not leave their shells. In the leopard slug, the shell has become a small internal plate over millions of years of evolution.

Myth: All slugs are garden pests. Fact: Leopard slugs feed mainly on fungi and decaying matter and also eat other slugs.

Myth: Leopard slugs are poisonous to touch. Fact: They are harmless to handle, although their slime is hard to wash off. Wash your hands afterward, as with any wild animal.