Wednesday, January 1, 2014
"What falls from a cow and rings like a bell? …
For such repellent subject matter, ordure is surprisingly interesting stuff. Perhaps this should in fact come as no surprise; it is after all a major phase in the biological cycle of energy and material, and is a vital resource to many animals and plants. That sounds simple, but all sorts of questions arise. For one thing, why should the stuff occur in so many forms: uncreative slugs and jobbies, such as those of dogs, baboons and porcupines; sloppy pads like those of cattle; hard, neat pellets like those of rats; clustered pills like those of horses or goats; or elaborately sculpted like those of large caterpillars? Some are smooth in texture, some granular, some fibrous, and in some you can see seeds or visible parasites. Most of us don’t give the matter a thought; it is just the way things are, and not things we care to investigate intimately.
But those shapes and textures are not random. They reflect the foods, ecologies and gut shapes of the animals. The usual scheme of digestion is: break up the food to expose its nourishing bits; wet it thoroughly so that the enzymes can work on well separated substrate molecules; and treat it to a series of digestive juices that give special treatment to different components in turn. Then extract what nutrients you can from the result, and finally extract water so that you can void the residue without waste or fouling.
Of course there are exceptions and elaborations almost beyond imagining. For instance, beasts that fly have little selective pressure to develop house training. To them the concept of crying gardyloo would be alien. Pigeons and seagulls are famous spotters and bats are hardly more considerate, passing what they no longer want without considering where they may be, or what may be passing beneath.
Not surprisingly, such facts are exploited by plants that disperse seeds by the oral-anal route. Mistletoes are important examples, because that way some of their sticky seeds land on twigs. Some plants depend absolutely on the creatures that eat their fruit and disperse the seeds in the manure. Various plants need the dispersal, the preparation of the seed case for germination, and the nutrition that accompanies the seed when it is voided. Such softening of the seed or its case can make the manure of large animals such as elephants very attractive to rodents or birds that eagerly pick out the tasty bits whenever their benefactors have been eating the right stuff.
Fliers with no shortage of water are likely to void their wastes in dismissive squirts. Many birds of prey do so and the only care they take is not to get the stuff on themselves. Eagles have been known to electrocute themselves by sitting on a high voltage wire and closing the circuit when they hit another wire with such a squirt.
However, not many animals can afford to waste water, so the trend is to retain the excreta in the last part of the gut before unloading, and absorb the moisture till the wastes become firmer and drier. A common sign of ill health is for an animal to foul itself because its droppings pass through while still too runny to cope with cleanly. For most animals it is important to get as clean away from their excreta as possible, if not for the sake of hygiene, then to avoid attracting predators.
For mobile animals such as savannah-grazing antelope, this is no problem: they drop their souvenirs and move on with no more concept of house training than birds. Creatures tied to a stationary territory or a nest however, either must put up with their own dung or apply sanitary measures in the interests of hygiene or safety. Those that pig it often make a virtue of being so revolting that competitors and predators stay away; hoopoe nests are notoriously foul and many a moth larva has survived in a bitten apple flung away by a human too finicky to profit by a tiny windfall of rich protein! Other creatures, such as fly larvae that live in flesh or fruit, use their own secretions and excretions to kill and pre-digest host tissue.
To survive in filth, animals must develop a high degree of resistance to infection. One strategy is to secrete substances that discourage many microbes, but to develop a relationship with crowded populations of yeasts and the like that outcompete most harmful germs. The resistance of such mucky species to bad husbandry may make them convenient to keep in captivity, while clean, mobile species can be a challenge to keep healthy. But there are counter examples. For instance, capybara and a surprising number of other animals will insist on defecating in their water and unless there is a good supply of unfouled water for drinking, trouble simply must follow sooner or later.
When animals are tied down to a small area and still need to keep clean for safety and for health, they have to develop suitable housekeeping practices. Some birds remove their nestlings’ droppings, some rely on ants and other scavengers to char for them, some animals bury their manure, and I have known a charming sedentary Pamphagid grasshopper who would extrude a longish, hard dropping most of the way out, then raise one hind leg and neatly flick it several metres away. The kick had sufficient force to send the projectile ricocheting about the laboratory (much to my puzzlement before I caught her at it). In a remarkable parallel, at least one species of Hesperid caterpillar which lives in a rolled up leaf, constructs a dropping inside its hind gut. Its anus has a shutter with a catch. Over a period of some twenty minutes blood pressure builds up till the shutter pops and the pellet shoots out to land somewhere safely remote. Actual experiment has demonstrated that many parasitoids actively seek out droppings of caterpillars and similar hosts or prey, as part of their strategy of finding the hosts themselves.
Such a build-up of pressure may sound like the constipatee’s nightmare, but there is worse. The larvae of many Hymenoptera that live on concentrated food with little waste, such as bees and many wasps, have a sealed anus and they simply concentrate their wastes in a visible dark blob in the hind gut, till they are about to pupate. Then they let the lot go at once. Some species then roll about in their cells and form part of the muck into a neat, parchmenty brown cocoon in which they shed their skin and pupate. The pupa is a delicate structure that at first looks like sculpted crystal, and on seeing it in its elegant casket, it may be difficult to associate it with such sordid origins.
By and large, animals with a rich, digestible, or at least plentiful diet, tend to be the ones with the prosaically shaped turds. I cannot think of a single carnivore that voids anything but lumps or splashes. This reflects the simple, tubular shape of the large gut when it has little to do but extract water from a stew of unwanted meat residues. Simple-shaped turds from herbivores tend to be full of undigested bits that more a sophisticated digestion for a more demanding diet would have reduced to anonymous slush.
Paradoxically, many poorly digested turds are particularly rich in pre-digested food and often they are very popular snacks. When fresh they do not usually attract the original owners, which makes sense; the very reason they were voided, was that they contained more unwanted material than reward, and probably a batch of poisons and pathogens into the bargain. Therefore, the manure of one’s own species is best treated as revolting, or at most as a social signal, to be sniffed, but not to be eaten. In contrast the manure of other species does not always give the right chemical signals and animals may react more strongly to the attractants than to the repellents. For instance, hyenas seem to feed avidly on the fresh scats of jackals and of hunting dogs, and hunting dogs on at least those of jackals; and many domestic dogs, if they are kept on a healthily austere diet (which implies that they are always hungry) will eagerly snap up human, cat or rabbit droppings, sometimes even dog droppings that have weathered into something like anonymity.
If there are not too many parasites in circulation, such coprophagy does no harm and in fact can supplement an unbalanced diet. I have read that when the rabbit population in Britain was first reduced to a fraction by myxomatosis, there were many deaths among sheep dogs that no doubt had been fed poorly and hitherto had supplemented their diet with bunny pills.
Those of us with a taste for pongy cheese such as Limburger, are in no position to raise our noses.
Rabbits in turn do not have a rich diet in the wild, and unlike cattle they do not have a rumen in which to digest plant fibre efficiently. Their solution is an interesting demonstration of the versatility of evolution. Like some rodents (which by the way rabbits are not) that eat a lot of fibrous vegetable matter, rabbits void some of their faeces prematurely as special soft pellets in which microbes have free rein to grow and digest stubborn fibres. They then eat such pellets as they emerge. From these pre-digested portions they pick up newly-released nutrients and digest most of the microbes into the bargain. It is an important source of vitamins as well as harnessing extra digestive power. If human vegans took their principles to the logical conclusion and adopted this rabbit habit, they could do without vitamin B12 supplements. But Vegans are not rabid about logic or consistency of logic.
This is the closest Lagomorphine approach to the hare chewing the cud, as referred to in the bible. (Hares, like rabbits, are not rodents but members of the Lagomorpha.) But it seems that the behaviour depends on a taste for certain faecal components, so rabbits will also eat the scats of some other species. Keep rabbits in a grassy enclosure with cats and you will have very little cat manure to clean up. The rabbits scoff it as a supplement to the grass. It seems to be a good supplement too, rich in phosphates, nitrogen and other minerals, and energy-rich material to boot. This is hardly surprising, considering the price of cat food nowadays!
Unlike dogs, the cats don’t return the compliment. Cats are specialist carnivores, and accordingly leave that sort of thing to dogs and similarly inferior species.
Nor are these instances of coprophagy unusual; clean chicken manure is an excellent supplement to coarse, low quality fodder for cattle. Rumen flora love it and so do the cows. Suitably sterilised and powdered, poultry manure actually gets incorporated into some cattle rations nowadays. Given the opportunity, free range cows are not so fussy; they don't demand treated droppings and they wolf down whatever they can get, generally without obvious ill effects. In the wild, starving elephants will eat weathered elephant droppings and baby elephants eat such droppings freely; it is their way of picking up the gut microbes that they need for digesting the coarse plant food that elephants eat so much of.
Weathered carnivore droppings usually don’t last long in the wild either; herbivores, including tortoises, know better than to waste a good source of calcium and other minerals. Farmers have long offered the equivalent of such resources plus salt licks, by forming mineral and energy supplements into blocks for cattle to lick or nibble. Commercial production of such blocks is big business.
Another example of where droppings are valued sources of minerals is in some forms of "puddling" by butterflies. They seem so fairy-pure that many people are shocked to find that many species of butterflies not only will suck at juices and fermenting fluids, but also at mud and — wait for it — dung. This habit is known as puddling. It is very important especially in regions where there is a deficiency of certain mineral salts, in particular sodium, which is practically absent from most plant foods.
So far we have largely discussed the manure of animals with simply shaped guts, or with a blind gut or caecum where the small gut meets the large bowel or colon. Up to that point most things that happen to the food do not show in the manure, though there are special exceptions. For instance the way fibre has been chewed or components have been dissolved will show in the manure of eaters of coarse plant foods. Black rhino for example are browsers and they have teeth specially adapted to the shearing of the twigs that they eat. The sheared twigs are obvious in the dung.
What comes next after digestion is the process that shapes the output in preparing the dung for disposal. Dissect an animal and you see that only the large gut gathers the mush into lumps, where its colour darkens as the microbes work on the bile pigments and as the slush becomes concentrated and solidifies as the water gets extracted. Light-coloured stools may have many causes, but they usually mean that the stuff has passed through too fast for bacterial darkening, perhaps because of diarrhoea or a sudden glut of food.
In this process too, the shapes of the turds begin to make sense. Guts like humans’ or dogs’ are more or less tubular and any scats that harden in there take up that shape, or get shaped that way during extrusion. On the other hand, food that is rich in fibre, especially if the animal has no rumen, or if the fibre is too challenging for the rumen, has to be elaborately extracted in the colon. Perissodactyla and Lagomorpha, the orders of horses and of rabbits, are examples. They digest and absorb what they can in the stomach and small gut, then pass the rest, not into the colon, but into the relatively huge caecum. Humans are adapted to a diet far lower in fibre so our caecum is hardly more than a junction box with the appendix attached. Our appendix is mainly a structure for exposing lymph nodes to gut microbes, and storing populations of our own personal gut microbes (our "gut flora", as it is imaginatively known) so that our immune system is prepared to deal with anything our alimentary lifestyle exposes us to.
Non-ruminant eaters of fibrous plant food however, are heavily reliant on the caecum for digestion of the refractory bits. It is a good system, and most ruminants have well-developed and busy caeca too, but the most imposing specimens where all the serious work gets done, are found in the likes of horses and rhino. Ruminants like cattle, goats, and antelope deal with fibre the other way round, and though they also have large caeca, for fermentation you can’t beat the rumen, so their caeca are relatively smaller than those of horses. The rumen is derived from the oesophagus, where the stuff is stored till it is pre-digested. It then comes back up to be chewed properly and goes down again to be filtered and passed through to the stomach and small gut for normal processing.
Heavier reliance on caecal digestion means that the colon has to absorb many more nutrients as well as water and salts and it cannot do this as effectively as the small gut. Still, needs must, so the more important the caecum, the more elaborate the colon. Humans have lightly segmented colons, but plant eaters, especially those that rely on caecal digestion, have the colon segments sacculated, extended sideways into fancy compartments in which the food is collected and squeezed and sucked dry by the increased absorptive area of the gut walls. After due exhaustion, the food forms not just pellets, but clusters which reflect the shape of the final chambers as seen in dissection. You can see this clearly in the shape of say, fresh horse manure or the manure of the smaller antelope.
One also can see a great deal in the form of smaller animals’ droppings. Sucking bugs such as aphids live on diluted plant juices and have to get rid of large quantities of water and sugar. This they turn to good account by excreting it as the so-called honeydew that attracts ants. The ants protect the aphids and in some cases care for them, even building shelters. The care of an ant however, especially some species, is an insecure thing, and I have seen ants carrying away cut-up aphids where I had relied on them to protect my aphid cultures.
Caterpillars usually feed on fibrous foods and cannot afford the long, leisurely digestion process of the cow, so they eat fast, absorb what they can, and compress the rest into dry pellets for excretion. The absorption is vital for such small animals that cannot afford to lose much water, and many species have far more elaborately sculpted hind-gut linings than any horse. The droppings are practically works of art, and they may be more spectacular than their creators. Large caterpillars of many species of emperor moths or hawk moths are very impressively camouflaged and often I have found them only when I first noticed their droppings under the food plant.
Don’t talk **** people say? But **** is like evolution; it pervades biology. Ignore it and you eviscerate the subject. You don’t have to like the stuff to find that following it up leads you into everything from nutrition, through behaviour, economy, conflict, competition, and epidemiology to anatomy. Biology is unbalanced without images of the neatly packaged droppings of nestlings in the beaks of mother birds, or of the enthusiasm of huge dung beetles circling in to mounds of fresh elephant dung, or indeed the vile-smelling ordure that some snakes will smear on you if you handle them. It may not be an obvious conversational opening for a non-politician, but start talking **** and some of us find it hard to stop.