The Book of the Earthworm

by Sally Coulthard

  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.

  How to help earthworms #7

  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).

  DO EARTHWORMS MAKE ANY NOISES?

  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

  DO EARTHWORMS MAKE FRIENDS?

  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.

  Charles Darwin, The Formation of Vegetable Mould through the Action of Worms, with Observations on their Habits (1881)

  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).

  DO EARTHWORMS TAKE RISKS?

  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.

  HOW LONG DO EARTHWORMS LIVE?

  While the angels, all pallid, and wan,

  Uprising, unveiling, affirm

  That the play is the tragedy, ‘Man,’

  And its hero, the C
onqueror Worm.

  Edgar Allen Poe, ‘The Conqueror Worm’ (1843)

  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.

  DO EARTHWORMS EXPERIENCE PAIN?

  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.

  Sir Richard Owen, Lecture to the London Society of Arts, The Raw Materials of the Animal Kingdom (1852)

  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.

  How to help earthworms #8

  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

  ENDNOTES

  1. Natural England Commissioned Report NECR145: ‘Earthworms in England: distribution, abundance and habitats’ (2014): http://publications.naturalengland.org.uk/publication/5174957155811328.

  2. Lowe, C.N., ‘Interactions within earthworm communities: A laboratory-based approach with potential applications for soil restoration’, University Of Central Lancashire, Faculty Of Science (April 2000): core.ac.uk/download/pdf/9632799.pdf.

  3. Butt, K.R. et al., ‘An oasis of fertility on a barren island: Earthworms at Papadil, Isle of Rum’, The Glasgow Naturalist (2016) Volume 26, Part 2, pp. 13–20.

  4. Meentemeyer, V. et al., ‘World patterns and amounts of terrestrial plant litter production’, BioScience 32(2), (1982), pp. 125–128.

  5. Xiao, Z. et al., ‘Earthworms affect plant growth and resistance against herbivores: A meta-analysis’, Functional Ecology (18 August 2017).

  6. Zaller, J.G. et al., ‘Herbivory of an invasive slug is affected by earthworms and the composition of plant communities’, BMC Ecology 13, 20 (2013): https://doi.org/10.1186/1472-6785-13-20.

  7. Decaëns, T. et al., ‘Seed dispersion by surface casting activities of earthworms in Colombian grasslands’, Acta Oecologica 24(4) (2003), pp. 175–185.

  8. Dann, L., ‘Major survey finds worms are rare or absent in 40% of fields’, Farmers Weekly (22 February 2019): www.fwi.co.uk/arable/land-preparation/soils/major-survey-finds-worms-are-rare-or-absent-in-20-of-fields.

  9. Kanianska, R. et al., ‘Assessment of Relationships between Earthworms and Soil Abiotic and Biotic Factors as a Tool in Sustainable Agricultural’, Sustainability 8 (9): 906 (7 September 2016).

  10. Scheer, R. and Moss, D., ‘Dirt Poor: Have Fruits and Vegetables Become Less Nutritious?’, Scientific American (27 April 2011).

  11. Givaudan, N. et al., ‘Acclimation of earthworms to chemicals in anthropogenic landscapes, physiological mechanisms and soil ecological implications’, Soil Biology and Biochemistry 73 (2014), pp. 49–58: DOI: 10.1016/j.soilbio.2014.01.032

  12. The National Severe Storms Laboratory, Severe Weather 101: Flood Basics: www.nssl.noaa.gov/education/svrwx101/floods

  13. Xiaofeng Jiang et al., ‘Toxicological effects of polystyrene microplastics on earthworm (Eisenia fetida)’, Environmental Pollution 259 (April 2020).

  14. Paoletti, M.G. et al., ‘Nutrient content of earthworms consumed byYe’kuana Amerindians of the Alto Orinoco of Venezuela’, Proceedings of the Royal Society of London. Series B: Biological SciencesVolume 270, Issue 1512 (07 February 2003)

  15. Cianferoni, A., et al., ‘Visceral Larva Migrans Associated With Earthworm Ingestion: Clinical Evolution in an Adolescent Patient’, Pediatrics 117(2): e336–e339 (1 August 2006).

  16. Reynolds, J.W. and Reynolds, W.M., ‘Earthworms in Medicine’, American Journal of Nursing 72(7):1273 (August 1972).

  17. Mira Grdisa, M., ‘Therapeutic Properties of Earthworms’ in Bioremediation, Biodiversity and Bioavailability, Global Science Books (2
013): http://www.globalsciencebooks.info/Online/GSBOnline/images/2013/BBB_7(1)/BBB_7(1)1-5o.pdf

  18. Chuang, S. et al., ‘Influence of ultraviolet radiation on selected physiological responses of earthworms’, Journal of Experimental Biology 209 (2006), pp. 4304–4312: doi: 10.1242/jeb.02521.

  19. Seymour, M.K., ‘Locomotion and Coelomic Pressure in Lumbricus terrestris L’, Journal of Experimental Biology 51 (1969), pp. 47–58.

  20. Quillin, K.J., ‘Kinematic scaling of locomotion by hydrostatic animals: ontogeny of peristaltic crawling by the earthworm lumbricus terrestris’, Journal of Experimental Biology 202 (1999), pp. 661–674.

  21. Zhang, D. et al., ‘Earthworm epidermal mucus: Rheological behavior reveals drag-reducing characteristics in soil’, Soil and Tillage Research 158 (May 2016), pp. 57–66.

  22. Verdes, A. & Gruber, D.F., ‘Glowing Worms: Biological, Chemical, and Functional Diversity of Bioluminescent Annelids’, Integrative and Comparative Biology 57(1), (July 2017), pp. 18–32.

  23. Samuelson, J., Humble Creatures ( John Van Voorst, London, 1858).

  24. Montgomerie, R. and Weatherhead, P.J., ‘How robins find worms’, Animal Behaviour 54 (1997), pp. 143–151.

  25. Cranfield University, ‘Earthworm population triples with use of cover crops’ (25 September 2019): https://phys.org/news/2019-09-earthworm-population-triples-crops.html.

  26. Liebeke, M. et al., ‘Unique metabolites protect earthworms against plant polyphenols’, Nature Communications 6:7869 (2015): doi: 10.1038/ncomms8869.