Call of Nature: The Secret Life of Dung

IT’S NOW TIME to get the trowel out, or the stout knife, and to consider a more formal dissection of the pat. When it emerges, fresh, fragrant, glistening, extruded into the world, mammalian dung is a more or less uniform homogeneous mass. But as soon as it hits the ground it starts to change. Even runny cow dung, if deposited on a hot sunny day, starts to dry out. This has important implications for many dung-feeding and dung-breeding flies which rely on its soft moistness to feed and lay eggs.

There is a useful analogy to be made in comparing a pile of dung to a pond. The surface has its own specialist fauna. Water, by virtue of the complex physics of hydrogen bonds, has a surface tension, forming a meniscus layer, a film on which water skaters can skate and whirligig beetles whirl. The pond surface fauna has its own name, the neuston. Slightly more prosaically, dung has a rind or crust, but the division between external and internal dung insects is a good one.

Flies are the major inhabitants of the outer dung surface. The yellow dung flies, Scathophaga, with all their posturing and mate-guarding bravado, take up vantage points across the surface. They do not have powerful or stout egg-laying equipment, so they can only penetrate the freshest skin to lay their eggs. They are joined, albeit cautiously, by the wing-waving ant flies, Sepsidae, and the brash hubbub of green- and bluebottles. To the onlooker this is just as much a mad scramble as the thousands of dung beetles descending on an elephant dropping. It is chaos, and where there is the distraction of chaos, there is always someone waiting to take advantage.

This is where the pride of Kent, Emus hirtus, wades in, along with the other predatory rove beetles. Wade isn’t, perhaps, the best description for the movement of these insects. These carnivorous rove beetles are agile and active, and those that hunt on the dung surface are elegant and graceful beyond the down-to-earth situation in which they find themselves. They flit about at top speed, first skulking down the side of the dung, then scampering pell-mell across the top, scattering flies much like an overenthusiastic puppy might scatter pigeons on its first walk in the park. Inevitably the first few flies in its path are too quick; they have good eyesight and rapid reactions, but the hurly-burly obscures their attack, and they soon haul off a victim. It is rapidly dispatched by the sharp scimitar-like jaws. Many years ago, when I wanted to photograph the prettily mottled greenish bronze Ontholestes murinus, I had to stick it in a glass tube in the fridge for half an hour, to slow it down enough to get a few shots. I resisted the temptation to spread some fresh dung on the kitchen table, and made do with a bit of garden turf for a backdrop instead.

Most of the regular rove beetle predators are just plain black, or slightly metallic bronze, but their quick movements and flighty behaviour makes them obvious and eye-catching. There runs a tale, embellished I’m sure, but more than merely apocryphal, of the late Roger Dumbrell, Sussex antiques dealer, naturalist and a bit of a character, waiting at a bus stop, when his eye was caught by a large shiny Philonthus rove beetle landing on some dog dung in the street. It quickly disappearing under the fresh excrement, but without hesitation he bent down, flicked over the dropping, and quickly tubed the beetle, much to the amazement, and possibly disgust, of the others in the queue. I knew Roger well enough to believe that he was quite capable of this self-assured behaviour; it was he who was partly responsible for introducing me to beetles, and the fascinating delights of dunging.

Emus and Ontholestes are formidable beasts, and although some of the larger Philonthus (P. spinipes at 17 mm, and P. splendens at 14 mm) might make a meal of a blow fly, it will be some of the smaller game they tackle most often. Cow pats are rather one-dimensional, they’re really just thick pancakes, but horse droppings have a labyrinthine convolutedness offering a much more complex surface. Without needing to push or burrow, tiny flies can crawl into the natural crevices. Here they might avoid the angry scathophagids, but they must constantly be on the lookout for roving attackers.

Pulling back the upper boluses of horse dung, the inner surfaces are sometimes seething with small insects. Here are moth flies (sometimes called owl midges, family Psychodidae), lesser dung flies (Sphaeroceridae), dung midges (Scatopsidae) and fungus midges (Sciaridae). These are all tiny surface dwellers, living and feeding on the outside of the dung, but depositing their eggs on or just under the surface.

Maggots are rather more streamlined than adult flies, and with an evolutionary history of wriggling in putrid decaying matter, they are quite at home in the soft mushy interior of the dung. They do not have it to themselves, though.

My pond analogy is tenuous, I agree, but I’m going to stretch it even further here, by suggesting that it is possible to swim through dung. Called ‘swimming’ dung beetles, the smooth lines of Sphaeridium scarabaeoides allow it to push easily through the soft molten interior of fresh cow dung. Like dung flies and rove beetles, these are rapid colonisers of fresh dung and when they land on the surface they quickly fold away their membranous flight wings and quite literally dive in. Swimming is exactly what they do, and they have broadened legs, some fringed with paddles of stiff hairs which turn their nominally running and walking limbs into oars.

Plenty of their close relatives, in the water-loving family Hydrophilidae, remain water beetles, swimming through water, crawling over submerged vegetation or clawing their way through pond-side mud. Here they feed on whatever they can find in the way of rotting detritus; some of the larger species are partially or wholly predatory. Sphaeridium actually swims in its own food. What a luxury. As a cow pat dries, the outer rind becomes a crust, but Sphaeridium is a tough insect, with powerful leg muscles and it can still push into the drying skin, long after the myriad dung flies have given up and moved on. A pat a day or so old, dry to the touch, can be distinctively pocked with small oval holes, where the beetles have continued to push their way into the still-soft interior, leaving characteristic entrance craters, as if the dung has been blasted with a small-bore shotgun.

Fig. 33 Closely related to water beetles, Sphaeridium swims in the dung, using its legs like oars.

Joining them in the viscid mire are the clown beetles (Histeridae). These smooth-lined, capsule-shaped beetles are also perfectly adapted for pushing through moist dung. Shining black, sometimes marked with vague red or orange blotches, they are sleek enough to push through the soft mire using their hugely flattened and toothed legs as paddles. Both adults and larvae are predators, mostly feeding on the fly maggots which live inside the dung mass, but happy enough to take whatever comes into their jaws.

Fig. 34 Smooth lines and powerful flattened legs make histerids excellent dung burrowers.

The obvious dung beetles living inside the main body of the dung are the dwellers. In most temperate zones this means the ovately cylindrical Aphodius, which are not so much swimming as burrowing into the dropping. Wood-boring beetles are, likewise, mainly cylindrical. With smooth heads, clean lines and broad legs, they can easily push into the dung, even when it starts to dry out. Unlike the swimmers, Aphodius and the other large dung beetles tend not to dive in on top of the dung, but rather push their way underneath, using the many air spaces and crevices where the dung meets the grass thatch or leaf litter. Some of the larger species such as A. rufipes, A. fossor and A. erraticus (the biggest UK species at 12, 10 and 8 mm, respectively) spend most of their time on the underside of the pat, whilst the smaller A. pusillus, A. consputus and A. granarius (all around 3–4 mm) are more able to find gaps and weaknesses in the fibrous material and push up into it.

Fig. 35 Aphodius fossor, a sleek black monster in the dung.

It is inside the dung that the grubs of these beetles, and the maggots of the various flies, will develop. This makes the dung attractive to a whole new series of creatures. Rooks, crows, magpies and other corvids can sometimes be seen picking apart cow pats and eating the insides. They are not, of course, eating the dung itself, but are feasting on the fat larvae inside. During autumn and winter, when food is thin on the ground, two-thirds of cow pats can be pecked open by birds. Earlier on in the season, a scattered dropping is more likely a sign that entomologists have been at work. Badgers are also attracted to grub-laden cow pats; they lift off the upper portion of the pat, as if they are prising off a lid, and scoff down the tasty morsels they uncover.

The region where the dung meets the soil is not quite so crisply defined as the rind developing on top of the pat. The activities of dung beetles, dwellers living just under the pat, and tunnellers actively excavating the soil serve to blur this boundary. The shallower burrows, some little more than scrapes in the soil, may contain particles of dung which although they have been shovelled there by beetles, may still be joined, more or less remaining part of the main dung mass. Others go deeper and the fragments of dung carried off to form brood balls become satellite dung morsels. If rollers are present, the dung from a single dropping may end up being scattered across scores of square metres, at depths varying from a few centimetres to over 2 m. It very quickly gets to the point where it is difficult to know where the dung ends and the soil begins.

Despite lack of clarity, the dung–soil interface remains an important ecological zone. Here is a natural fissure along which creatures can creep. The soil delivers moisture to prevent the dung drying out too much, and the fermentation of the gently mouldering dung raises the temperature very slightly. This may only be a fraction of a degree in a single pat, but in large manure heaps the internal temperature may reach near scalding, up to 75°C. This is one of the principles on which composting works for gardeners and farmers, the heat reaching a point at which fungal spores, microbial diseases and unwanted annual seeds are killed. During my brief teenage dung beetle survey of South Heighton I was able to find beetles, even in the depths of winter, by searching in the sheltered root thatch at the centre of the gently fermenting dung. Here they remained active, or at least not completely dormant through December to February, when almost every other living thing had shut up shop to hibernate.

Just as the tunnellers are removing small portions of dung and relocating these down into the soil, so too soil inhabitants start moving out of the humus layer into what may or may no longer be part of the pat. The archetypal soil organism is the worm, and since these are ubiquitous wherever there is a rich soil humus component, they start to feed on the dung, from the underside, almost as soon as it appears. The common earthworm (also called the lobworm), Lumbricus terrestris, is a familiar garden species, and many others occur, but this is replaced by the distinctive reddish barred brandling worm, Eisenia fetida, in areas rich in organic matter such as compost bins, manure heaps and areas of the paddock where animals are regularly corralled, allowing a build-up of dung material in the soil. The lobworm remains in its mucus-lined burrow, emerging at night to grasp whatever dead plant matter it can, pulling it down into the ground and devouring it in safety. The difference between a bit of dead grass, or a bit of grass cut and chewed and passed through a herbivore digestive system, is minimal as far as the worm is concerned.

In Britain, at least, earthworms do not just play cameo roles, they are major movers. Something in the region of 30–60% of pat material is removed by them, working unseen and all but unnoticed in the hypogean dark.

Slugs and snails also move in. On holiday in France again, some years ago, I was fascinated to watch the giant edible snails, the escargots Helix pomatia, regularly feeding on fox droppings around the edges of the fields. The foxes had been feasting on the windfall wild plums and greengages, so their runny excrement was far more fruity-fibrous (rather than meaty-feathery) than normal for a carnivore. It was still pretty offensively aromatic, but the snails were wolfing it down. This was a warning, if one were needed, about the dangers of harvesting wild snails for the pot, unless you know what you’re doing and can safely purge their digestive systems in a salad-filled vivarium for several days. I was not tempted.

Slugs, in particular, are important soil/dung invertebrates, more so than snails, which find it difficult to manoeuvre their shell encumbrances. Slugs, by virtue of being able to insinuate themselves into the narrow spaces in humus and root thatch, are very much soil critters. They are also more adventurous in their food choices; many are predatory, attacking snails and each other, or scavenging riper decay products than snails’ mainly plant-based diet. Slugs are some of the main dung-feeding animals in my own back garden, where they readily attack fragrant fox and cat droppings. The high-meat diet of those animals, no doubt, produces dung more redolent of decaying flesh – to the taste of the often predatory or carrion-scavenging slugs hereabouts.

At some point, the boundary between dung and soil is completely blurred. Humus is arguably the constant churning remains of decaying organic matter and earthworm casts anyway. Charles Darwin wrote an authoritative monograph on earthworms (Darwin 1881), and came to the conclusion that in 10–20 years, they could turn over the top 15 cm of soil, eating, digesting and casting it up again. In a well-grazed (and therefore well-manured) field the gradient from dung to soil is soon imperceptible. The soil subsumes the dung. The soil is the dung.