What does an earthworm do all day? Darwin was one of the first scientists to document, with any rigour, the idiosyncrasies of earthworms. Since then, subsequent studies and experiments have revealed some extraordinary behaviours. From ‘herding’ instincts to weather preferences, courtship rituals to defensive mechanisms, there’s much more to the average earthworm than meets the eye.
Other questions, such as do earthworms sleep, can they feel pain, or how do they find a mate, are still being explored – it seems there’s still plenty we don’t fully understand about life underground.
The next time you go out on your morning or evening ramble, if you chance to see a worm in your path, do not kick it aside, not step over it; but take it from the ground, and lay it in the palm of your hand…
Have you ever noticed that earthworms tend to head to the surface after a rainstorm? Scientists used to think that worms headed upwards through the soil to prevent them drowning in their water-filled burrows. We now suspect this isn’t true, as earthworms prefer moist soil and many species can actually survive submerged in oxygen-rich water for weeks (see Can earthworms survive underwater? page 74).
We still don’t know for certain why earthworms head upwards during a rainstorm, but one theory is that it might be something to do with migration and that earthworms find it easier to move bigger distances across the wet soil surface than going through the soil, especially if it’s dry.
It’s a risky strategy, however, as exposure to strong UV light can be lethal for an earthworm, so another more convincing theory is that the drumming of the rain mimics the vibrations of moles digging through the soil. In order to avoid being eaten, on hearing these vibrations earthworms move up through the soil to get out of the way. Certain species of birds, including seagulls, blackbirds and thrushes, actually mimic this vibration by tapping the ground with their feet to lure unsuspecting earthworms to the surface, where they are gobbled up. Worm charmers use the same trick.
Scientists recently tested this hypothesis. At the Annual Sopchoppy Worm Gruntin’ Festival in Florida, worm charmers use a special trick to entice earthworms to the surface. They hammer wooden stakes about 30 centimetres into the soil and then scrape the stake with a long piece of steel called a ‘rooping iron’. The rhythmic, rasping noises that are created by steel scraping on the wood sound like low-pitched grunts (hence the name for the festival) and without fail seem to bring hundreds of earthworms to the surface.
The researchers wanted to explain why the ‘rooping’ technique worked and set up seismic recording equipment to measure what was happening underground. It turned out that scraping with a rooping iron caused vibrations that registered about 100 hertz, an almost identical frequency to the sound of moles digging. The flight response of earthworms to the sound of moles is thought to be an evolutionary one, and deep-seated. In countries such as New Zealand, which don’t even have moles, earthworms seem to have a ‘memory’ of this predator and still behave in the same way if they hear vibrations.
On 14 September 1972, Cleveland Airport was brought to a standstill when hundreds of thousands of earthworms appeared on the runway. Heavy rain had brought the worms to the surface of the ground and they’d inadvertently crawled on to the runway, causing aircraft to skid dangerously on landing.
It’s surprisingly difficult to pull an earthworm back out of the soil if it’s burrowing. Writing in the middle of the nineteenth century, the nature writer James Samuelson noted: ‘It has perhaps never occurred to you to enquire how it is, when you endeavour to draw a worm forth from the earth, that it can offer such resistance to your efforts, as almost to necessitate your tearing it in two before you can extract it.’23 Darwin also noticed that they ‘can seldom be dragged out of the ground without being torn to pieces’ and we can often observe garden birds struggling to winkle them out of the soil.
We already know that the earthworm’s body is actually covered in tiny stiff bristles called setae (see How do earthworms move? page 66). These setae face slightly backwards, helping the earthworm anchor itself into the ground as it pushes itself forwards through the soil. Without them, the earthworm would just slide around on the same spot. These little spikes also make it difficult to pull an earthworm out of its burrow, especially tail first, as the bristles dig in and create resistance. If a garden bird grabs a worm by its front end, it probably has a better chance of pulling it out of its burrow. In the struggle, the earthworm’s setae can fall off. If the worm manages to escape and live to tell the tale, its bristles will grow back, just like fingernails.
GROW GREEN 'MANURES'
Research has shown that using cover crops is an easy way to introduce organic material to the soil and boost earthworm numbers by as much as 300 per cent.25
● Plant a cover crop in a vegetable garden at the end of summer. Also known as ‘green manures’, cover crops protect the otherwise bare soil over autumn and winter, reducing the amount of weeds and preventing the soil from becoming too cold. At the end of the winter season, cover crops are dug back into the soil, organically nourishing the earth and helping feed the plants that follow – for winter cover crops, look for plants such as grazing rye or winter vetch.
● You can also use cover crops during the growing season to ‘fill in’ spaces where you have lifted out your other vegetables and to attract pollinating insects – mustard, crimson clover and phacelia are lightning quick to germinate and will grow in just a few weeks.
Experiments also suggest that garden birds, such as robins, carefully listen for cues that an earthworm is close by. Once the sound of an earthworm is located, some garden birds cock their heads to one side. We used to think this was so they could listen even more closely, but recent research suggests garden birds are head-cocking to locate the earthworm by sight, before they make their final, deadly strike.24
We do not avoid evil by fleeing before it, but by rising above or diving below its plane; as the worm escapes drought and frost by boring a few inches deeper.
Earthworms are at the whim of the weather. Unlike mammals, which can regulate their own body temperature, the earthworm is a cold-blooded creature and its temperature is directly affected by its surroundings.
Different species of earthworms, however, prefer different temperatures. Surface dwellers, such as the composter’s favourite – the Tiger worm (Eisenia fetida) – can tolerate temperatures between 0°C and 35°C but seem happiest at a balmy 25°C. The Common earthworm (Lumbricus terrestris), on the other hand, prefers life deep in the soil, where it’s a cooler and more consistent temperature throughout the year.
Deep soil temperature is remarkably stable. At the surface and just under, the soil temperature rises and falls in line with changes in the surrounding air temperature. The further down into the soil you go, however, the less the soil is affected by the temperature of the air. At 4 metres underground, the soil temperature is a fairly steady 10°C, at 1.5 metres underground it varies between about 5°C and 15°C, and at 50 centimetres underground, the temperature can fluctuate between just above freezing and just under 20°C.
The full moon that appears during March is also known as the ‘Worm Moon’, named after the earthworms that re-emerge at the beginning of spring when the ground starts to thaw.
Given that the Common earthworm’s optimum temperature is between 7°C and 12°C, it makes sense for the earthworm to move around in the soil, searching for the most comfortable temperatures. In winter, this means spending much of its time in the deeper layers of soil. The Common earthworm can’t tolerate temperatures below freezing but their egg cocoons can survive for weeks in soil as cold as –5°C. Each cocoon has a protective dehydration mechanism; in response to sub-zero temperatures, the cocoon begins to lose water, effectively freeze-drying itself until conditions improve and the weather warms up.
Other species of earthworm, including Nordenskiold’s worm (Eisenia nordenskioldi), found across large parts of Russia, and the Octagonal-tailed worm (Dendrobaena octaedra), native to Eastern Europe and Western Siberia, have to cope with long, very harsh winters. They have evolved a neat trick whereby they rapidly increase the amount of glucose in their body fluids, which acts like a natural antifreeze and stops their bodies being destroyed by ice crystals.
Back in temperate climates, if soil becomes too hot or too dry, some species – including the Common earthworm – will go into a period of dormancy called aestivation. The main aim of aestivation is to stay moist, so the earthworm coils itself into a tight knot to reduce its surface area, seals itself in a chamber lined with mucus to keep the humidity high, and lowers its metabolism so it doesn’t lose too much water. Then, in this ‘summer stasis’, the earthworm can sit out the scorching temperatures and dry soil for as long as three weeks, until things return to normal.
It depends on the species of earthworm. Surface dwellers eat decaying organic matter – such as dead leaves, berries, rotting wood and flowers. Shallow-dwelling earthworms eat soil, which is high in organic matter. And deep-burrowing earthworms drag larger pieces of decaying plant material, such as leaves, into their burrows. Some species of earthworm also eat fungus and rotting animals.
People sometimes call earthworms ‘omnivores’, but a more accurate term is a ‘detritivore’ or animal that feeds on dead organic material.
One of the mysteries of earthworm eating habits was how they coped with toxic plants. Some plants are spectacularly poisonous and contain chemicals – called polyphenols – designed to stop them being stripped by herbivores. These toxins still carry on being potent even after the plant or its leaves are dead.
Scientists at Imperial College, however, worked out how earthworms seem to be able to stomach almost anything, even if it’s poisonous. It turns out that earthworms’ guts contain molecules – called drilodefensins – that neutralise certain chemicals. No other animal on the planet has these toxin-busting molecules – drilodefensins are found only in the guts of earthworms.26
What’s really interesting about earthworms, however, is what comes out of their back ends. Worm poo or ‘worm casts’ are wonderfully rich in nutrients and minerals, which are in a form readily available for plants to use. Compared to the surrounding soil, worm casts contain on average five times more nitrogen, seven times more phosphorus and eleven times more potassium.27
The constant eating and pooing of earthworms moves an extraordinary amount of soil. Worm casts deposited on the soil surface slowly accumulate, burying whatever is lying on the surface – this is often how archaeological artefacts and ancient monuments get slowly submerged over centuries. It’s estimated that in just one single acre of land, earthworms add a fresh, 5-centimetre layer of topsoil every year. That’s about 8 tons of soil.
The very name Earthworm, so distinctive as it is of the habitat of these animals, seems to have been expressly invented in order to crystallise into one word the remarkable distributions of these creatures.
Most earthworms don’t have homes. Both surface dwellers and shallow-dwelling species move through organic matter, eating and pooing it out behind them in the form of crumbly worm casts.
Surface dwellers don’t create burrows at all – they don’t live in the soil – and prefer life under layers of moist, warm leaf litter. Shallow-dwelling species do create tunnels as they eat their way through the soil but quickly fill them up again with excreted worm casts as they move along.
Deep-burrowing species, however, such as the Common earthworm (Lumbricus terrestris) create permanent burrows, which they keep clear of debris. To do this, they leave their poo on the surface of the soil, at the entrance to their burrow, in piles known as ‘middens’. Unless it’s disturbed, the earthworm will stay in the same burrow its whole life and keep adding to the midden, creating an ever-increasing pile of poo. These permanent burrows are so well constructed they can remain stable for decades.
Darwin noted that his Common earthworms sometimes even built little roofs over their burrows: ‘Where fallen leaves are abundant, many more are sometimes collected over the mouth of a burrow than can be used, so that a small pile of unused leaves is left like a roof over those which have been partly dragged in… They often or generally fill up the interstices between the drawn-in leaves with moist viscid earth ejected from their bodies, and thus the mouths of the burrows are securely plugged.’ Why they did this, Darwin wasn’t sure, but he suspected the leaves provided a means of regulating temperature and humidity within the burrows, acted as rain covers, and offered a level of concealment from hungry predators.
While surface-dwellers and shallow-burrowing species of earthworm don’t have permanent burrows, there’s no place like home for the Common earthworm (Lumbricus terrestris).
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.
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
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.
I sometimes think we consider too much the good luck of the early bird and not enough the bad luck of the early worm.
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.
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.
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.
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 terrestris) 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.’
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.
If we talk about sleep as a behaviour, then we look for signs such as lack of movement, slowing down of normal function or non-responsiveness to external stimuli over the period of a twenty-four-hour day (longer periods of ‘sleep’ than twenty-four hours tend to be classified as hibernation or dormancy). Using the behavioural definition of sleep, earthworms do seem to have a period in the day when they rest. Research with the Common earthworm (Lumbricus terrestris), for example, has shown that they’re super busy from dusk until dawn but that during daylight hours their oxygen consumption drops, indicating a period of slowdown.
DON'T USE CHEMICALS
● Earthworms have tolerated decades of pesticide use but their ability to detoxify themselves of harmful chemicals comes at a cost – soil that is regularly sprayed with pesticides seems to produce earthworms that are smaller in size and less able to reproduce (see Are Earthworms in trouble? page 22).
● Aim for a pesticide-free garden: healthy soil makes for healthy plants, which in turn are more able to resist disease; attract more beneficial insects (which eat garden pests); and look into natural pest solutions such as companion planting, organic sprays or biological control (such as nematodes).
Tucked away in the back pages of the New York Times for 5 August 1925 is an extraordinary, tiny snippet of news. The headline hails the news ‘SAYS EARTH WORMS SING’ and reads as follows:
‘German Professor Finds They Emit Sounds of a Soprano Pitch. FREIBERG, Germany, Aug. 4 —Professor Mangold, a German teacher of zoology, says that he has discovered accidentally that earthworms can sing. He placed a dozen earthworms under a glass cover in carrying out certain experiments and to his surprise rhythmical [sic] sounds of soprano pitch emanated from the container. The savant insists that an investigation showed the music came from the earthworms.’
Mangold was the first scientist to suggest that earthworms make sounds. Two years later, in 1927, another academic, Rudolf Ruedemann – fascinated by Mangold’s observations – asserted in Science that, based on his own personal experiences, ‘American earthworms also produce sound’. He noted ‘…on a sultry May evening […] earthworms in our garden back of the house could be distinctly heard. Being incredulous at first, I sat quietly on a chair until I also heard an exceedingly fine rasping noise all around me. It was a chorus of almost unbelievably small voices in the dark […] We have since heard the singing every year, always on warm spring evenings about and after dusk.’37
Where these two eminent men differed, however, was in their theories as to how the earthworms made their sounds. Mangold believed the sound to come from their mouths, while Ruedemann suggested it was the earthworms drawing their bristly setae over some hard object near the entrance to their burrows, possibly as a way of attracting a mate.
The mystery of the ‘singing’ earthworms has yet to be solved and little research has been done into the sounds earthworms make, if any. Farmers who breed worms for vermiculture have been known to record ‘popping’ sounds coming from their compost bins, while the Nage people of the eastern Indonesian island of Flores have long maintained that a species of earthworm in their local environment makes a ‘croaking’ noise.38
Of the sounds that have been recorded, however, one of the most likely explanations for earthworm sounds is, at least in part, their movement through soil. Sound waves are produced when small grains of organic matter move or rub against each other, or when small cracks are created in the soil. Depending on the size and number of earthworms, the noise may be considerable. David Attenborough, in his memoir Life on Air, remembers the extraordinary underground noises made when he was filming the Giant Gippsland earthworm in Australia. He compares the loud squelching noise, caused by the earthworm dragging its body through the wet soil, to the noise of a toilet: ‘as you walk through the South Australian meadows you may hear, immediately behind you, what sounds like someone flushing a lavatory.’39
They perhaps have a trace of social feeling, for they are not disturbed by crawling over each other’s bodies, and sometimes lie in contact.
Are earthworms sociable? It seems an extraordinary question to ask of an animal that is effectively just a tube with a mouth and bum, and yet a recent scientific experiment suggests that the lowly worm has a much more sophisticated communal life than was ever imagined.
At a Belgian university, researchers discovered that Tiger worms (Eisenia fetida) used touch to communicate with each other and influence each other’s behaviour. By using touch, the earthworms formed ‘herds’, which then all travelled together as a group in the same direction.
In one experiment, a cluster of forty earthworms were placed in a chamber of soil that had two identical chambers branching off it. Scientists expected that the earthworms would evenly distribute themselves among the soil, but instead they moved as a group, all ending up in one chamber together. Repeated tests showed the same ‘herding’ pattern.
But how did the earthworms decide as a group which chamber to move to? To test whether the earthworms were using chemical signals or touch to communicate, researchers created a maze, which the earthworms had to navigate. If individual earthworms travelled by themselves, they went via different routes through the maze. The fact that they didn’t follow each other suggested that the worms weren’t navigating by any kind of chemical trail left by the worm in front.
However, when two worms travelled together, they stayed together on the journey and ended up at the same destination. By crossing bodies or rubbing next to each other, the earthworms seemed to communicate through social cues and end up in the same place. Researchers also noticed that the earthworms often clustered together into compact groups when they were out of the soil, and suspect that many other species of earthworm may follow similar patterns of behaviour.
Quite why earthworms seem to form ‘herds’ is a question yet to be answered. One theory is that grouping into clusters – as is the case with many different animals – offers a greater level of protection from predators such as flatworms.
Earthworms also like to socialise in the evenings. Under the cover of darkness, they come to the top of their own burrows and investigate their neighbours. Gripping on to its own burrow with its tail, the earthworm will poke its head into the entrance of a neighbouring burrow, seeking a potential mate (see How do Earthworms have sex? page 113).
It’s easy to imagine that the life of an earthworm is fairly sedate. And yet, like many species, they are known to take risks. For most of the time, creatures survive by avoiding dangerous situations. Under certain circumstances, however, such as lack of food, risk-taking may be the only option to give an animal a fighting chance.
While this is an easy behaviour to observe in large mammals, no one had thought that an animal as simple as an earthworm would make calculated decisions in the face of difficulty. One recent study,40 however, proved that the Common earthworm (Lumbricus terrestris) took risks in the face of starvation. In the experiment, scientists took three groups of earthworms (non-starved, half-starved, and fully starved) and presented them with two foraging choices. The high-risk choice had lots of food but was in bright light – which is harmful to earthworms; the low-risk choice had little food but was nice and dark. The fully starved and extremely hungry group of earthworms not only selected the high-risk food more often than the other two groups, but also made their choices quicker than the others. Earthworms, it seems, are prepared to risk the threat of predators and desiccation from daylight if food sources dwindle.
While the angels, all pallid, and wan,
Uprising, unveiling, affirm
That the play is the tragedy, ‘Man,’
And its hero, the Conqueror Worm.
There does seem to be some correlation between how deep an earthworm lives in the soil and how long it’ll last. Surface dwellers are relatively short-lived – one to three years – but reproduce frequently and easily, so populations quickly recover. Shallow burrowers have a greater life expectancy – up to five years – but it’s the deep burrowers who can reach the impressive age of ten years or more. They only lay a few cocoons a year, so the downside is that populations are quickly decimated if the soil is disturbed. Left to their own devices, however, and with plenty to eat, deep-burrowing earthworms can live well into their teens.
No creature is too bulky or formidable for man’s destructive energies—none too minute and insignificant for his keen detection and skill of capture. It was ordained from the beginning that we should be the masters and subduers of all inferior animals. Let us remember, however, that we ourselves, like the creatures we slay, subjugate, and modify, are … temporary sojourners here, and co-tenants with the worm and the whale of one small planet.
This is a tricky one. It can be difficult to tell if an animal is experiencing pain, especially if there is no verbal communication. Also, scientists don’t agree on what pain is. When a worm is wriggling on a hook, for example, is this just an unconscious reflex or something more akin to pain as we experience it?
All animals have something called nociception. This is a capacity to react to harmful things. Through their neural systems, all animals react to dangerous or noxious stimuli, in order to avoid being either damaged or killed. This capacity is partly based on unconscious reflexes, rather than necessarily a conscious experience.
So, when a worm wriggles at the end of a hook, is it just the creature automatically reacting in some way but not actually experiencing pain in the same way we do? The debate rages on. Some scientists insist that earthworms don’t feel pain but instead are simply responding, mechanically, to a harmful stimulus (in the same way we might close our eyes to a bright source of light). Others aren’t so sure.
Researchers who challenge this notion look for other signs that would suggest an animal is in distress. Earthworms have been shown to produce two kinds of chemical – enkephalins and beta endorphins – which are believed to help them endure pain.41 If a creature produces painkilling chemicals, so the theory goes, it must be doing so in response to pain. Whether this pain is like the sensation we feel, we don’t know, but it’s an interesting avenue to explore.
ADD MORE WORMS
● Often in new-build properties or ‘instant gardens’, if soil has been imported or moved from elsewhere on site, the earthworm population can be dramatically reduced or even completely eliminated. In this instance, it can be useful to buy in earthworms and introduce them to the soil. While it can be tempting to buy a bag of earthworms and hope for the best, unless you first improve the soil conditions the new residents won’t survive.
● Most commercial worm farms recommend that you improve the soil before you add new earthworms to a plot, ideally leaving the manure, compost, leaf mould or other organic material to ‘season’ and begin to decay for a year before introducing earthworms (see How to help earthworms #1 page 28).
● When the plot is ready for its new arrivals, how you introduce the earthworms will depend on the species. Deep- and shallow-burrowing earthworms, for example, will need ‘digging in’ – scoop out the soil to a trowel’s depth, pop them in and then cover them over again.
● Or even better, you can buy colonies of mixed worms (deep and shallow burrowers but not surface dwellers/compost worms) in biodegradable boxes; you simply dig a hole and bury the entire box, water it and cover it with soil, and the earthworms will slowly work their way out of the box and into the surrounding soil, getting used to the new conditions as they go. Within no time, the earthworms should be settling in, mating and going on to repopulate your garden soil and bring it back to health.
The idea that earthworms might experience pain isn’t a new one, however; nor is the notion that we should not willingly inflict harm on them. In an article written by a Reverend Ljunggren for the New York Times in 1931, he recalls the experiments of a Swedish professor who wanted to establish whether earthworms felt pain. His tests, which involved electrical shocks, convinced the professor that earthworms were, indeed, capable of feeling pain and he spent the rest of his career trying to persuade anglers not to use earthworms as bait. ‘The good professor,’ wrote Rev. Ljunggren, ‘filled with compassion for the thousands of poor worms that were impaled annually upon fishermen’s hooks, went on a lecture tour to try and persuade [them] to discard the worm as bait. But they merely smiled and continued to halt their hooks as their fathers had done, and as no doubt, their descendants will continue to do.’42
