When the Common earthworm comes out at night – to search for food and find a mate – it will try to cling on to the top of its burrow with the tip of its tail. This not only helps the earthworm make a speedy exit should it get attacked by a predator but it also prevents the earthworm from losing its way home. Common earthworms do, however, sometimes venture further afield, leaving the safety of their burrows. Tests have shown that they have a homing instinct – in one study, Common earthworms found their way home after more than three hours on the soil surface and from as far away as nearly a metre. To return to base, the earthworm carefully backed along its own outward trail until its tail reached the top of the burrow.28
It’s thought that Common earthworms also leave a chemical trail that contains pheromones. Serving as a guidance system back to the burrow, the trail acts as an ‘attractant’, encouraging the earthworm to follow its own scent until it reaches home. Other species, such as the American River worm (Diplocardia riparia), leave a ‘repellent’ pheromone trail, which the worm actively avoids trying to come into contact with again. This species of worm is a scavenger and, as such, doesn’t need to find its way home or want to follow the same trail twice looking for food.
Common earthworms may even ‘pass on’ their burrows to their offspring after death. The earthworm lays its cocoon in its burrow – usually in the topsoil layer – either partly embedded in the burrow wall or placed on a tiny ‘nursery’ side burrow, dug horizontally from the vertical shaft (cocoons can survive much colder temperatures than adult earthworms – see Do Worms like cold or warm weather? page 95). Researchers have observed juvenile earthworms taking up residence in burrows abandoned by an adult earthworm when it dies. The large amount of energy it takes to build a burrow may be one of the reasons why a juvenile could ‘inherit’ the family home, although adult earthworms don’t share a burrow and most young earthworms have to make their own way as soon as they are big enough.
WHAT ANIMALS EAT EARTHWORMS?
The earthworm is possibly the favourite thing on a mole’s menu. A typical European mole (Talpa europaea) can eat sixty earthworms in a day. Rather than ‘hunting’ through the soil, which would be an exhausting, hit-and-miss process, the mole has developed a number of canny strategies to ensure his or her fill of earthworms.
The first is that mole tunnels are effective and deadly ‘earthworm traps’. Moles are surprisingly fast movers – if they hear a worm inadvertently stumbling into one of their tunnels, they race along and grab the worm before it has chance to escape.
During summer, however, deep-burrowing earthworm activity slows down – many just head deeper down into the soil and go into a period of dormancy called aestivation (see Do earthworms like warm or cold weather? page 95). This means earthworms are less likely to come into contact with mole burrows. As a way of surviving these leaner months, moles create ‘larders’ where they keep earthworms to eat later. Mole saliva contains a toxin that paralyses the earthworm but keeps it alive; one quick chomp to the head and the worm is immobilised but can be kept fresh for weeks. Moles have been known to keep hundreds of ‘zombie’ worms in these special storage chambers, in a kind of hellish stasis – alive but unable to wriggle away.
When the time finally comes to eat the earthworm, the mole has one last trick up its sleeve. To maximise the nutrition from the worm, the mole doesn’t want to eat all the soil that is sitting in the earthworm’s gut. So, in the same way you squeeze toothpaste from a tube, the mole pushes the earthworm between its paws to remove all the earth and dirt from its body. This purging of soil from the earthworm may also help to minimise the amount of wear and tear on the mole’s teeth, which would otherwise be quickly worn down from the abrasive effects of chomping through endless gritty material.
Earthworms are the staple diet of many other mammals, including hedgehogs, badgers, shrews, weasels, otters and stoats. Many of the species that feed on earthworms are nocturnal and take advantage of the feeding habits and lengthy night-time mating rituals of certain earthworm species. Foxes and owls, for example, are well known for foraging on juicy, large Common earthworms (Lumbricus terrestris) when they surface under the cover of darkness.
Many birds eat earthworms. In parks and suburbs, common garden birds such as blackbirds, thrushes and robins feed voraciously on earthworms; on farmland, it’s starlings, rooks, lapwings and gulls that often follow the plough or harrow as it disturbs the soil. You may sometimes have wondered why it is that seagulls – whose natural habitat is by the seaside – are so good at locating farmers ploughing up their soil miles inland. The answer may be that gulls spend much of their time at very high altitudes and have super-keen eyesight.
The eastern Australian duck-billed platypus is also rather partial to earthworms and can eat 800 in a single day. They are one of the only animals who find their food by electrolocation. As the platypus pushes its bill through the mud in the bottom of streams and rivers, its electroreceptors pick up minute electrical currents generated by the earthworm’s muscular contractions.
At such great heights they can survey land for many miles around, which helps them locate feeding opportunities. Gulls also keep an eye out for lapwings; lapwings live on farmland and soon gather behind a tractor if it’s churning through soil. The presence of a group of lapwings tells the gulls, who are high above, that there’s a feast of earthworms and other invertebrates below. Gulls feeding at ground level then attract other gulls and so, in no time at all, you have a huge flock of birds swooping and diving behind a tractor, hoping for a juicy worm or two.
Different birds feed on different species of earthworm, depending on their foraging strategy; robins, which rely on visual or auditory cues, tend to eat surface dwellers or deep-burrowing earthworms when they come up to the top of their tunnels, while long-billed sandpipers, which use their beaks to probe the soil and search for earthworms by touch, tend to hunt for shallow-dwelling earthworms that live just under the surface. Certain species of birds, such as lapwings, gulls, plovers, thrushes and blackbirds, also ‘paddle’ for worms. They stamp the surface of the ground with their feet to mimic the sound of vibrations caused by burrowing moles, sending the worms underground fleeing to the surface.
A number of other insects also eat earthworms, although some prefer their meals less wriggly than others. It’s not a subject well studied and the few examples we have often come from amateur sightings or surprise moments captured in the process of filming wildlife documentaries. In a lovely letter published in the Monthly Review in the early part of the nineteenth century, the editor notes: ‘A letter to the Secretary from Mr. Power of Market Bosworth, Leicestershire, was read, describing the manner in which the common garden snail… and the slug… feed on the common dew worm or earth worm Lumbricus terrestris, when dead or dying. This is performed in the night; and as Mr Power observed these animals would not attack a living worm, he attributes it to the prickles on its surface, which the worm, when in health and vigour, has a power of erecting, as well probably for defence against snails, as for the purpose of drawing straws, &c. into its retreat.’29
One remarkable animal that preys on earthworms looks rather like a giant earthworm itself. The caecilian is a limbless, snake-like amphibian that can be found in the tropical regions of South and Central America, Africa and southern Asia. It has incredibly powerful jaws and scientists long wondered why an animal that preferred its meals in the form of soft and juicy earthworms needed such a strong bite. Researchers watched how caecilians ate their worms and, to their astonishment, observed them spinning vigorously like a crocodile’s ‘death roll’ in an attempt to rip the earthworm into tiny pieces. When the bits of digested earthworm were recovered from a caecilian, each fragment looked like twisted rope. This amazing strategy explains the bite strength – only by having such powerful jaws can the caecilian grip on to the earthworm as it corkscrews. By tearing the earthworm into small chunks, the caecilian can devour its prey without any grasping limbs.30
How to he
lp earthworms #6
WATER THE WORMS
● Worms thrive in a cool, damp environment. Adding a decent layer of organic material to your soil (see How to help earthworms #1 page 28) will help keep moisture trapped in the soil but it’s also important to water your garden or allotment over the warmer months if the soil gets too dry.
● Always water in the early morning, when the weather is still cool, or the early evening. This not only stops too much of the water being evaporated by the heat of the day but also prevents pollinating insects such as bees and butterflies from feeling the full force of a hosepipe.
CAN EARTHWORMS DEFEND THEMSELVES?
I sometimes think we consider too much the good luck of the early bird and not enough the bad luck of the early worm.
Franklin D. Roosevelt to Henry M. Heymann (1919)
Despite their soft, vulnerable bodies, earthworms have a few strategies to give themselves a fighting chance when predators attack. Many species, including the Common earthworm (Lumbricus terrestris), will, if grabbed, thrash about wildly in an effort to free themselves. If the earthworm’s tail is ripped off in the fight, it may grow another one back (see Can an earthworm regrow if it’s cut in half? page 80). As we have seen, some species of earthworms, including the Grey worm (Aporrectodea caliginosa), deliberately drop their tails as a defence mechanism, a practice known as autotomy (see page 81).
Some species of earthworm secrete a noxious fluid if they are under threat. Australia has at least two ‘shooters’ – the Giant Gippsland worm (Megascolides australis) can fire fluid up to 10 centimetres away from its body, while Didymogaster sylvaticus can reach three times that distance, earning itself the rather brilliant nickname ‘Squirter worm’. Even the well-known Tiger worm, found in many a compost bin, emits a disgusting, rotted-garlic-like liquid if it’s distressed or roughly handled – the second part of its Latin name, Eisenia fetida, literally means ‘foul-smelling’.
Some scientists also suspect that the earthworms’ own bristles, or setae, make them unpalatable to some other insects. We know, for example, that earthworms have setae sharp enough to pierce another earthworm’s skin during mating (see How do earthworms have sex? page 113), so perhaps these bristles are also enough to make certain insects think twice before attacking.
HOW DO EARTHWORMS HAVE SEX?
There are no male or female earthworms – they’re hermaphrodites, which means each earthworm has both male and female sexual parts.
For an earthworm to reproduce, it has to be sexually mature. This only occurs once that characteristic ‘saddle’ or clitellum appears on the earthworm’s body; on the Common earthworm (Lumbricus terrestris), the clitellum appears at about six weeks old and sits about a third of the way down its body, nearest the head end.
Earthworms also select similar-sized partners, probably because it would make for tricky sex to be wildly different in size. Age doesn’t seem to come into it, however, and studies have shown that old earthworms can successfully inseminate young ones. In an interesting study of Tiger worms (Eisenia fetida),31 it was found that sperm production didn’t really decrease with age, even when the worm was six years old. The same worm’s female reproductive functions, however, dropped off radically after three years. An old earthworm, therefore, can get another earthworm pregnant, but may struggle to be itself impregnated.
The courtship ritual of the Common earthworm is a tender affair, with an initial ‘getting to know you’ session that involves prospective partners visiting each other’s burrows above ground. Under the cover of darkness, an earthworm will stretch out and attempt to poke its head into a neighbouring burrow, keeping the tip of its own tail anchored in its own burrow should it need to make a hasty retreat. The number of times an earthworm visits a mate’s burrow varies – sometimes only once or twice, at other times more than a dozen. The visits are often brief, 30–60-second affairs, but longer ‘deep burrow’ visits sometimes occur, lasting several minutes.
The object of the earthworm’s desire will then reciprocate the visit, each earthworm moving back and forth between each other’s burrow openings like giddy teenagers.32 While surface-dwelling earthworms don’t have burrows to visit, they also seem to have a courtship ritual – Tiger worms’ prospective partners have been observed repeatedly caressing each other, with short, gentle touches, before mating.33 The pre-sex courtship ritual can last anything from a few minutes to an hour, before they get down to business. When they’re ready to mate, the two earthworms lie next to each other facing in opposite directions – head to tail – glued together in a tight embrace with a sticky mucus produced by the clitellum. There, they’ll stay stuck together in a sexual marathon that can last anything from one to three hours. Other earthworms will sometimes come up and touch the couple mid-act, an unwelcome interruption that can shorten the length of mating but doesn’t stop the process. Charles Darwin was struck by the ardour of earthworm lovemaking; their passions, he wrote, were ‘strong enough to overcome… their dread of light’.
Passionate they may be, but monogamous they are not; for most species of earthworm, multiple partners are common. And there’s a darker side to these creatures’ sex lives. To improve an earthworm’s chance of success, it has an alarming secret weapon. Scientists have noticed that the Common earthworm has special setae (see also pages 91 and 94) on its body that it stabs into its mate’s body during sex. These dagger-like bristles pierce the mate, damaging its skin, and inject a hormone that seems to improve the chance of the earthworm’s sperm being accepted by the mate and lengthen the time before the mate searches for another lover.34
The clitellum is absolutely crucial to the whole mating process. The sticky mucus it produces, which initially glues the worms together so they can mate, hardens into a collar that slips forwards along each earthworm, collecting the other worm’s sperm and their own eggs along the way. This mucus-collar eventually slides off the worm’s head, both ends of the collar seal up, and it dries into a tiny lemon-shaped cocoon, which can nourish and protect the baby earthworms as they grow inside.
Observations from commercial worm breeders have even noted earthworm threesomes among Eisenia andrei (a close relative of the Tiger worm) and interbreeding between different species such as E. fetida, E. andrei and E. hortensis (the European night crawler), although this rare cross-species sex seems to result in infertile cocoons.
Depending on the species, earthworms repeatedly mate throughout the year. The Common earthworm, which is deep burrowing, will only produce about ten or so cocoons a year but smaller, surface-dwelling earthworms mate vigorously and frequently, producing as many as a hundred cocoons in one year. This might be because worms living nearer the surface are more prone to drought and predators. In general, however, earthworms in temperate countries tend to mate more frequently in spring (March/April) and autumn (September/October) when the weather is neither scorching hot nor freezing cold, a habit observed as early as the 1780s when the magazine The Monthly Review, Or Literary Journal, Enlarged noted: ‘…the common earth-worm propagates its species above ground, when the weather is mild and moist or the earth dewy.’35
Some species of earthworms don’t even need a mate. Some are parthenogenetic, which means that they just have female reproductive organs but don’t need a male to fertilise their eggs. Some earthworms, being hermaphroditic, choose to self-fertilise, and bend in half to impregnate themselves, a strategy that can help if an earthworm struggles to find a mate. Tiger worms self-fertilise about 10 per cent of the time.
CAN YOU TRAIN A WORM?
I would not enter on my list of friends,
(Tho’ grac’d with polish’d manners and fine sense,
Yet wanting sensibility) the man
Who needlessly sets foot upon a worm.
William Cowper, The Task (1785)
Earthworms have been exposed to some fairly unpleasant experiments in the name of research. One, conducted back in the 1960s, tried to determine whether Common earthworms (Lumbricus terres
tris) showed any capacity for learning.36 The experiment involved a maze. The earthworms had to try and navigate the maze, while being exposed to light, heat and electric shocks as stimuli. Electric shocks were dished out as punishment for the worm making ‘incorrect choices’, i.e. going the wrong way, while the reward was being returned to the safety of the home container.
The experiment was designed to see whether the earthworms would learn how to get through the maze based on good or bad ‘memories’ of their choices. And whether they could repeat the puzzle time after time.
Incredibly, the experiment showed that the earthworms were indeed learning how to navigate their way through the maze. Not only that, but the frequency at which they made correct choices actually increased with each subsequent test. So, the earthworms got better and better at remembering their way through the maze with each run. After a two-week break, however, the earthworms ‘forgot’ their routes and, as the researcher noted: ‘All changes in the earthworm which are produced in these experiments are transient by mammalian standards. After 15 days of rest, the performance of a highly trained [earthworm] is indistinguishable from an experimentally naïve one.’
DO EARTHWORMS SLEEP?
The answer to this question depends on how you define sleep. Some people describe it as a physiological state, which includes things such as altered consciousness, brainwave patterns consistent with sleep, reduction in sensory activity, sporadic eye movements or relaxed muscles. It’s a definition that works well when applied to mammals, but for creatures who don’t have such complex bodies or brains, a simpler behavioural definition is more useful.
The Book of the Earthworm Page 6
