With a little help from microbes

Animals cannot produce the enzyme cellulase, so they are unable to digest the cellulose which, as we have seen, makes up the bulk of the tissues of plants. Nor, consequently, can they readily gain access to the nutritious contents of cellulose-walled cells of plants. So it is not surprising to find that many herbivores have evolved a variety of associations with micro-organisms like bacteria, fungi and protozoans, which are able to digest cellulose. Such associations markedly increase the proportion of a plant that a herbivore can use as food, and, in addition, converts the totally indigestible parts of the plants to protein-rich bodies of microbes which the herbivore can digest.

Dung-eaters

The eating of faeces - one's own or that of any other animal - is something that people consider to be quite disgusting, even harmful. In the animal world carnivores - and most omnivores - similarly will have nothing to do with their own faeces, usually burying them or depositing them well away from places where they eat and sleep. Not so for herbivores, however. Among a great many animals that eat plants, eating dung - their own or that of others - is a widespread and common practice with a perfectly sensible and adaptive function in this harsh and inhospitable world. It is technically called 'coprophagy'; literally 'dung-eating', from the Greek kopros = dung,phagein = to eat. Before I explain what its function is in nature, let me give you an example.

On tropical coral reefs at Palau in the Caroline Islands one can observe what I call 'cascading coprophagy'. Carnivorous fish, hunting on the reef, are followed by schools of herbivorous fish that catch and eat the carnivores' faeces as they fall through the water. Deeper down, other herbivorous species are harvesting the faeces of the herbivores above them. And so it goes, the high-protein droppings of the top carnivores passing down through a range of herbivorous species, providing a progressively poorer diet as it does so. Little if any, however, ever goes to waste on the floor of the sea.

Furthermore this is no casual event, but a repeatable and predictable practice. The herbivores actively seek out the carnivores. Some can consume 25 per cent of their own fresh weight in a two-hour feeding bout. Over 45 species may be involved. But none eats the faeces of their own species; only those of fish that eat a higher protein diet than themselves.

So, here is a behaviour that enables herbivorous fish to supplement their diet with a significant amount of extra protein. Without it their food of seagrasses or seaweed would barely support reproduction, let alone growth of their young. It ensures that more of the scarce protein in the habitat contributes to the next generation.

But this example is a little atypical. Most coprophagic animals eat their own faeces, or, when very young, that of their parents. Many adult herbivores actively feed their faeces to their young. Nevertheless, the ecological function remains clear - to conserve rare protein for the production and growth of young in a world in which most animals suffer a chronic shortage of protein. However, as I said, it is a practice nearly entirely confined to herbivores, and largely to those herbivores which are hind-gut fermenters. These are animals that have specialised micro-organisms living in the last part of their alimentary canal; the colon and its outpocketings, the caeca. There the microorganisms multiply and are eventually passed out with the faeces.

These micro-organisms - usually bacteria, but also protozoans - digest the cellulose in the herbivores' diet; something the herbivores themselves cannot do. Many of these micro-organisms also use the unavoidably wasted metabolic nitrogen of their hosts: as urea in the saliva, or absorbed through the wall of the rumen of mammals; as urine recycled in the gut of birds and reptiles, or as uric acid in those of insects. In this way it is incorporated into the bodies of the microbes instead of being excreted in the urine or faeces. And some gut bacteria can fix atmospheric nitrogen. Finally, they all produce amino acids which the herbivore can absorb through the wall of the colon.

But the micro-organisms' bodies are themselves a concentrated source of digestible protein. However, as they live beyond the small intestine where the products of digestion are absorbed, this valuable source of protein would be lost when passed out in the herbivore's faeces. Hence dung-eating.

There is another large group of herbivorous animals which are fore-gut fermenters. Grazers mostly, like cows and kangaroos, which have microorganisms in specialised outgrowths of their stomachs rather than their colons; at the beginning rather than the end of their alimentary tract. There the microbes do the same as those in hind-gut fermenters: digest otherwise indigestible cellulose and produce essential amino acids. But these sorts of herbivores will only eat their faeces if they are suffering acute protein malnutrition. Normally they do not need to do so because they digest their microorganisms and absorb their manufactured amino acids when these pass back from the fore-gut into the small intestine. So a cow is not really a herbivore, but a 'cryptic carnivore'! She eats microbes that have first fed on grass taken into her rumen.

A particularly interesting fore-gut fermenter is the South American hoatzin. It is in many ways a most peculiar bird, and a strict herbivore. Like all strict herbivores it is very particular about what it will eat, feeding selectively on high protein buds, shoots and new leaves. In the breeding season females further restrict their diet to new growth of just four species of high-nitrogen legumes. Its crop and oesophagus have become greatly enlarged as the site of fermentation, but the crop remains highly muscular and has interior cornified ridges which finely grind the food. So the bird is essentially a ruminant, like the cow. Except that, unlike the cow, which must regurgitate its cud to chew it up, the hoatzin achieves both 'chewing' and fermentation at the same site! And an extract of its crop contents will digest cell wall material equally as efficiently as that from the rumen of a cow. However, it pays a price for enlisting microbes to supplement its low protein diet. Its sternum is much reduced to make way for the enlarged crop and oesophagus, reducing the area for attachment of flight muscles. As a consequence of this and the weight of its voluminous fore-gut, it is a very poor flyer, spending most of its time crawling through the branches of trees.

The hoatzin was thought to be the only bird that is a known fore-gut fermenter. But there is possibly at least one other, the kakapo, or night parrot of far-away New Zealand. Kakapo are very large (the world's heaviest parrot), flightless and solitary birds. Currently the species is staring extinction in the face but for the strenuous efforts being made to preserve and breed from the approximately 80 birds left alive.

They, too, are strict herbivores, even as chicks, living on leaves, grass seeds and fruit. They exhibit many of the special behaviours I have already described in other herbivores seeking to improve their access to soluble nitrogen. They select soft new tissues of leaves and chew up and extract just the juice from more fibrous tissues. They preferentially feed on pollen and unripe seeds. This latter preference extends to the same predilection for the unripe seeds of introduced walnut and pine trees as their distant Australian cousins. In addition kakapo have a specially ridged palate for grinding their (often tough and fibrous) food and the keel of their breastbone is very small, reducing attachment for wing muscles, and making way for an enlarged fore-gut.

Even with these special adaptations they mostly lack sufficient protein in their diet to be able to breed. The female raises her single chick on her own, feeding it on a concentrated mix of fruit and seed. This involves a tremendous and prolonged task of gathering food and bringing it back to the nest for the chick which rapidly grows bigger than her; while she wastes away to skin and bone. As a consequence, females attempt to breed only in 'mast' years -

Imagens Kakapo
Figure 3.1 The New Zealand kakapo is one of possibly only two birds that are fore-gut fermenters. Like the other one - the South American hoatzin - its enlarged fore-gut has resulted in reduction of attachments for wing muscles to the point where it can no longer fly. Photo courtesy of Don Merton.

seasons when their food plants set a great abundance of fruit and seed. Then they feed heavily on partially developed green fruit and seeds in the weeks leading up to breeding.

But to return to hind-gut fermenters and the eating of dung. When we look we find a wide range of plant-eating animals, from crustaceans to mammals, have this behaviour. And in all it serves the same function -supplementing a low-nitrogen diet of plant material with high quality protein.

Termites, or white ants, are a prime example. They live on a diet of wood. But not really. Like the cow, they are not the pure herbivores they appear to be. You will be hard-pressed to find food with less protein in it than wood; it is mostly cellulose and lignin which the termites are unable to digest. So they cannot survive on wood alone. They must enlist microbes to digest it, and then digest the microbes.

Termites harbour a large population of microbes, mostly protozoans, in their enlarged hind-gut. But these do more than digest the wood the termites eat; they also fix atmospheric nitrogen and recycle metabolic waste nitrogen. Experiments have shown that the rate at which they fix nitrogen can vary

200-fold, depending on how much nitrogen is already in the diet. But it can contribute up to half of all the nitrogen that a colony of termites needs.

As with all insects the nitrogenous waste of white ants is uric acid. It is excreted directly into the hind-gut where it is recycled by anaerobic microorganisms which metabolise it and synthesise amino acids. This recycling can provide enough nitrogen to support up to 30 per cent of the biomass of a colony.

Termites probably can't digest the bodies of their microbes in the hind-gut to any extent, but they achieve this by actively eating their own faeces.

The indications are, however, that in spite of gut microbes providing a much enhanced and concentrated supply of protein food, this is often barely enough to get by. Whenever an opportunity offers, termites prefer to feed on wood that has already been attacked by fungi. And the more it is decayed -the more the wood has been converted to fungus - the more they thrive. Some white ants have taken this strategy much further. They farm a special sort of fungus, cultivating it in their nests on beds of their own faeces. These faeces are still largely plant material only slightly digested by gut microbes after one quick pass through their gut. After some six to eight weeks the termites re-ingest both faeces and fungi from the beds. But they also browse on the fungus as it grows on the beds. In particular they harvest special protein-rich spores which grow from the mat of fungus. As a final touch the winged adults, before they leave to establish new colonies, swallow several of these spores so that they can establish their own new farms.

One highly specialised example of the benefit of eating faeces loaded with micro-organisms is found in a species of Californian damp-wood termite. A reproductive pair starts a colony. They produce two sorts of faeces; ordinary dry ones and special ones made up of the bodies of the protozoans in the their hind-guts. They eat these special ones that contain 1400 times more protein than their ordinary faeces, which they do not eat. The females eat more of these special pellets than do the males. In fact a male gives most of his to his mate once she starts producing eggs. And when the eggs hatch both parents feed most of their protein-rich pellets to the young. A nice example not only of concentrating available protein to the production of eggs and the growth of young, but with father contributing to child-rearing to boot!

There is an American species of primitive cockroach, closely related to termites, which also lives entirely on wood, and carries microbes in special hind-gut pouches. They have taken this channelling of microbial protein towards the raising of their young a step further. The young of a pair of these cockroaches stay with their parents after they hatch, and feed on special anal fluid produced by the adults. This is a rich soup of concentrated microbes. The young quickly acquire micro-organisms of their own from this fluid, so that they could, at least in theory, then survive away from their parents. Yet they stay with them, continuing to grow while eating this high-protein diet, for more than three years! Perhaps they do this because life for them is still a knife-edge existence. Half of them die very soon after hatching, and only one-third survive their first year.

Studies with another cockroach more familiar to most, the ubiquitous German cockroach, have shown just how significant it can be for very young animals to gain access to protein via their parents' faeces. These insects are not, of course, herbivores. On the contrary they will eat anything organic. Yet their young, when first they hatch, are often restricted in their ability to forage for food. They can, however, survive and moult to the second stage (when they are much more able to forage) when prevented from eating anything other than the faeces of adult cockroaches. And they do even better if they are given only the faeces of female cockroaches.

Returning for a moment to fish, it was long thought that no herbivorous fish had evolved this sort of association with gut microbes, possibly because their juveniles, like those of all fish, are carnivores. But quite recently two species of buffalo bream, which are abundant in temperate and tropical waters of Australia, have been found to be true hind-gut fermenters. They both have greatly enlarged, thin-walled, caecum-like pouches which house many bacteria and protozoans. These fish eat red and brown algae, biting off large pieces and swallowing them whole. Unlike most herbivorous fish they do not chew their food up, nor do they have grinding mechanisms in the gut, to break up the cellulose walls of the algae. Instead they retain the food in the gut for a very long time (some 21 hours compared to the few hours for most herbivorous fish) allowing plenty of time for the micro-organisms to do the job for them. They usually feed in groups so dense that the water is cloudy with their faeces. Interestingly, when their juveniles first settle on the reef from their free-swimming life as carnivores, they swim among the adults. At this time they have fully developed caecal pouches, but with no microbes in them, and still eat small amounts of invertebrates. But they soon acquire microbes by eating the faeces of the adults, and quickly change to eating nothing but algae.

Many herbivorous mammals are hind-gut fermenters and are coprophagic, at least when they are young. The domestic horse is a good example. Like all domestic animals (and very few wild animals) its nutrition, and especially its protein nutrition, has been intensively studied. Foals from soon after birth start to eat their mother's faeces. This behaviour ensures that they acquire the necessary micro-organisms for their hind-gut. But they continue to be coprophagic, albeit at a decreasing rate, for six weeks. That this is a response to a need for extra protein in their diet is revealed by studies with adult horses, which are not usually coprophagic. If, however, they are kept on a diet which has just enough protein to maintain their body weight, they quickly revert to this behaviour, consuming all of their own faeces immediately after each defecation. Supplementing their diet with urea to raise its crude protein content stops the behaviour in as little as a week. Returned to the original diet the animals are again eating their own faeces within seven to ten days.

Herbivorous rodents like the voles, hares and rabbits, are hind-gut fermenters, carrying bacteria in their highly modified colons and specialised caeca. Their colons can separate liquid and bacteria from coarse particulate matter, and recycle it back through the caecum. And they all, like those dry-wood termites, produce two kinds of faecal pellets; soft and hard. The soft ones are pure caecal contents - concentrated microbial protein. The hard

Figure 3.2 The Australian ringtail possum is an animal which produces both soft faecal pellets of concentrated microbial protein and dry fibrous ones. Being nocturnal it eats the soft microbial pellets while resting during the day, and discards its hard dry pellets when moving about during the night. Photo courtesy of Bob Baldock.

Figure 3.2 The Australian ringtail possum is an animal which produces both soft faecal pellets of concentrated microbial protein and dry fibrous ones. Being nocturnal it eats the soft microbial pellets while resting during the day, and discards its hard dry pellets when moving about during the night. Photo courtesy of Bob Baldock.

ones are dry fibrous material containing little nitrogen. The animals selectively eat the soft caecal pellets, taking them directly from the anus when they are resting. They void their hard pellets when they are actively moving about.

Australian marsupials that eat foliage, like the ringtail possum and the koala, are also hind-gut fermenters. The ringtail exhibits all the gut specialisations of the rodents, and produces separate soft faecal pellets which it ingests while resting during the day. The koala on the other hand feeds these special pellets only to its young. As soon as the young koala puts its head out of the mother's pouch she starts feeding it on soft caecal faeces - called 'pap' -which she takes directly from her anus. She continues this supplementation of her milk for some six weeks during which time the youngster is growing exponentially.

The green iguanid lizards in Panama provide another example of the value of young animals eating the dung of their elders. They are the only known herbivorous lizards whose young are not carnivores. The young of all others start life eating invertebrates. Young iguanids are, however, from the moment they hatch, far more selective than older iguanids. They feed exclusively on the new growth of a legume that contains particularly high levels of soluble protein. And they pass this food through their gut very quickly, 'creaming off' the readily accessible nitrogen. Yet in spite of these adaptations, and the benefit of a reserve of yolk carried over from the egg, they must have the help of microbes, housed in their hind-gut, to succeed. Immediately the young iguanids hatch from the egg they climb into the foliage, where they seek out and closely follow adult iguanids. To obtain an inoculation of the specialised gut protozoans they need they must eat the faeces of the adults. One pellet would be sufficient for this, but they persist with this behaviour for three weeks or more, growing rapidly in the process. Artificially deprived of this continuing diet of microbial protein they hardly grow at all.

The various species of birds known as grouse that live in the northern climes of Britain, Europe and North America, are all herbivorous hind-gut fermenters. That is except when they are very young. Like the chicks of the domestic fowl, from the moment they hatch grouse chicks run free and fend for themselves, albeit aided and abetted by mum. And for the first weeks of their lives they eat mostly insects, gradually increasing the proportion of vegetable matter in their diet as they grow. And, like all herbivores, as adults they are very selective in what they will eat, concentrating on the buds of new soft growth.

The most famous of these birds is probably the Scottish red grouse, managed for centuries as a 'sporting' bird - i.e. for shooting. Successful management relies on their selective feeding. On the moors the heather (their sole food plant) is repeatedly burnt in a mosaic of small patches, maintaining a constant supply of new flush growth for them.

Microbes Hindgut Fermentation

Figure 3.3 The Scottish red grouse is another hind-gut fermenter which produces both soft pellets containing concentrated bacteria from its caeca and hard, fibrous, low-protein pellets. Yet, unlike other hind-gut fermenters, there is no record of the adult birds or their young eating their highly nutritious caecal pellets. Photo courtesy of Andrew MacColl.

Figure 3.3 The Scottish red grouse is another hind-gut fermenter which produces both soft pellets containing concentrated bacteria from its caeca and hard, fibrous, low-protein pellets. Yet, unlike other hind-gut fermenters, there is no record of the adult birds or their young eating their highly nutritious caecal pellets. Photo courtesy of Andrew MacColl.

Like other hind-gut fermenters grouse all have a pair of large caeca containing bacteria. These bacteria break down cellulose cell walls to allow access to the cell contents, synthesise essential amino acids and recycle nitrogen from the breakdown of metabolic uric acid. The latter is transported as urine from the cloacal area (the common site of urination, defecation and egg-laying in birds and lizards) to the caeca. To do this the birds have evolved a retrograde flow in the intestine so that its contents move forward against the normal peristaltic flow back towards the rectum and cloaca.

Their caeca and intestine are much better developed and longer than those of their close omnivorous relatives, the pheasants, quail, partridges and turkeys, reflecting their much greater dependence on a diet of relatively low quality bulk food. And the smaller females have longer small intestines and caeca than the heavier males, better equipping them to cope with this poor food when developing their eggs.

Grouse, like their mammalian and marsupial counterparts, can separate small particles of food in the colon and concentrate them in the caeca, leaving the coarser material to pass out as faeces. However, they, like their mammalian counterparts, produce two types of faecal pellets, one made up of these coarse particles and the other of soft material derived directly from the caeca.

Strangely, however, unlike all other hind-gut fermenters they have never been observed to eat these caecal pellets. And I say 'observed' purposely.

These pellets contain more than twice as much nitrogen as the woody pellets. Even if the birds could absorb considerable amounts of nutrients directly from the caeca, eating these pellets would constitute a significant boost of bacterial protein to their diet. Especially for the newly hatched chicks, which, presumably, must anyhow eat some of their mother's faeces to become inoculated with the caecal bacteria. So, too, for the breeding hens, improving the quality of their eggs. It seems strange that such a rich source of readily digested protein should go to waste.

Furthermore, there are very few caecal pellets among many woody ones on the mounds of droppings under red grouse night perches; caecal faeces make up only 12 per cent of the birds' output of dry matter. Do the birds indeed eat much of their caecal droppings at night, but nobody has thought to look and see?

A final example. One showing a further curious twist to this adaptation of eating faeces; but without involving micro-organisms. The breeding hen of the European goldfinch eats only milk-ripe seeds, and feeds her nestlings on a regurgitated and partly digested paste of these called 'chyme'. At the same time, until the nestlings are about 10 days old, she eats their droppings and incorporates them into the chyme. Thereafter she discards the young's pellets over the side of the nest. In these first few days after hatching the young birds' digestive efficiency is minimal, so their faeces contain much unabsorbed protein as well as their metabolic nitrogen. So, feeding their droppings back to them during that time makes good sense: scarce protein, otherwise wasted, is recycled and used. The value of recycling this otherwise unused nitrogen to young birds until such time as they are able to fully digest it, is illustrated by being quite common behaviour in many species of birds, including those that are mandatory carnivores. The European swift, which feeds exclusively on insects, is one such. Like the herbivorous goldfinch, the hen eats all of the nestlings' faecal pellets for the first three weeks of their life.

All these examples are but a few of many, and all are variations on the same theme. Clearly, the eating of dung is common in nature. And in all cases it fulfils the same ecological function. It allows herbivorous animals to eat high-protein micro-organisms that have first digested the herbivores' diet of low-nitrogen plant material. In this way they gain the necessary minimum amount of protein needed for the production and growth of their young, and which they could not get from plant food alone. Occasionally we see that it can also provide a necessary protein supplement for the fast-growing young of carnivores.

Detritus-feeders

There are other sorts of animals that do not permanently harbour microorganisms in their guts, yet still rely on them as an essential source of protein food. These are animals that have become specialised to eat dead plant material - detritus. And their mode of feeding indicates a way in which the more specialised use of internal microbes could have evolved. Some have internal microbes which break down detritus in their gut, but many rely upon external ones to digest the plant material. They may then eat both the detritus and the microbes growing on it, digesting the latter and passing the former out as faeces. Or they may graze just the microbes, leaving the detritus to grow another crop of micro-organisms. Similarly when the residual plant material is eaten it may be recycled a number of times, via coprophagy, each time being first enriched anew with fresh microbes. In fact 'detritus-feeder' is really a misnomer for these animals. They are actually living on a diet of microbes that have first grown on the dead plant material. It is not hard to imagine how ancient creatures like these could have evolved into the animals with highly specialised guts wherein equally highly specialised microbes live permanently.

The common Mediterranean woodlouse, or slater, has become widespread throughout the world, and is a familiar feature of most people's gardens. They mostly eat dead leaves, but also take live plant material and animal matter when available. If confined to a diet of pure detritus, however, they must become coprophagic, relying on eating their own faeces along with the detritus to subsist. But studies in England have shown that not just any old faeces will do. They have little interest in one-day-old ones, but avidly eat them when they are about three weeks old. At this age the faeces contain their greatest concentration of micro-organisms - mostly fungi - and 10 to 100 times more than uneaten leaf litter that has been aged for the same time under identical conditions! The scientists who studied this behaviour rather aptly named the process an 'external rumen'; the cultivation and then eating of micro-organisms is essentially the same as that achieved in the rumen of a cow, but before the microbes are ingested.

Another species of woodlouse found in Yorkshire has evolved a step ahead of the external rumen. These animals have a gut that functions in a way analogous to that of the mammalian rumen, but they do not have any obligate internal microbes. They depend entirely on ingesting those which grow freely on detritus in the field. Their hind-gut is nearly as long as their body, and is divided into distinct specialised regions. The first of these is a large sac in which the detritus is held for 24 hours while the microbes continue the digestion they had started before the woodlouse ate the detritus. The products of this digestion of cellulose are absorbed through the walls of the sac. The residual detritus, plus the micro-organisms, are then passed back to the next region. Here the microbes are digested by the woodlouses' enzymes and the products of this digestion absorbed through its highly papillated surface. The residual detritus is then passed back to the rectal region and ejected.

Not all detritus-feeders eat their faeces, however. Some, in fact, very specifically eat only the microbes growing on the detritus, as we saw earlier with some termites which cultivate specialised fungi growing on their faeces which consist of partially digested wood. There is another woodlouse that feeds on decaying sycamore leaves in Sweden which does a similar thing. A fungus grows on these leaves, gradually breaking down and digesting their cellulose. It grows into large black spots on the leaves - hence its name, tar spot fungus - and these colonies contain the highest levels of protein: pure, concentrated fungal tissue. The woodlice eat these colonies first, leaving large holes in the leaves. They will then gradually eat the tissues surrounding the spots, which contain much less fungus and are less nutritious. Only as a last resort will they eat the veins of the leaves which contain little nitrogen.

Turning to quite different animals in a different environment, there are small marine crustaceans called copepods that produce faecal pellets of fine particles of algae encased in a membrane secreted in their mid-guts. In the sea this skin is rapidly colonised by bacteria as they penetrate to the algal material within. The crustaceans then remove and eat just the membrane and its attached bacteria. The membrane is of little nutritive value but the bacteria amount to a significant supplement of high-protein food. An indirect benefit of ingesting just the membrane is that it causes the pellet to crumble and disintegrate in the surface waters where the copepods feed. Here bacteria again attack the algal particles, further increasing the supply of bacterial food for the copepods. And on the broader front this behaviour serves, as it does in other cases of marine herbivores eating dung, to salvage as much nitrogen as possible before it sinks beyond reach.

Many marine and freshwater snails are detritus-feeders. But, again, many of them are not, not really. They are actually grazers, eating nothing but the micro-organisms - diatoms, micro algae, bacteria - growing on the surface of living or dead plants. Others, however, are true coprophages. Two species of snail which live in the Thames estuary are a good example. They feed on the bottom deposits of organic debris, but are not evenly distributed through it. They are far more concentrated in places where the debris is fine-grained, and sparse where the deposits are coarse. There are many more bacteria in the fine deposits. The snails repeatedly ingest this fine debris and deposit their faeces back into it. In their guts they extract only the bacterial protein, returning the residue in their faeces to the bottom deposits where new bacteria grow on them.

A fairly extreme case of a herbivore gaining nutritional benefit from micro-organisms is that of marine shipworms. Anybody who has had anything to do with boats and the sea will know about these creatures and the damage they can do to any wood left in the sea for long. They are actually not a worm, but a very modified type of bivalve clam that bores into the wood and lines the tunnel with its shell. They are adapted to a way of life in many ways akin to that of termites on the land. As we saw, wood is a food with perhaps the least nitrogen of any.

The young worms are free-swimming and live by filtering plankton out of the water with their gills. They continue to do this after they have settled and built their tunnels. However, they have been experimentally grown to maturity in seawater filtered free of plankton, so can subsist without this source of animal protein. Probably they can get some of their protein by digesting the marine fungi which quickly infest any wood in the sea. Mostly, however, they are dependent on nitrogen supplementation by very specialised bacteria living in special glands in their gills.

These bacteria can digest cellulose, synthesise essential amino acids and fix nitrogen. In a species of shipworm which lives in the Sargasso Sea (where the density of plankton is notoriously low) these bacteria can fix nitrogen at a rate which doubles a worm's cellular nitrogen in one and a half days. This is more than 20 times faster than they do in several other species of worm living in coastal waters where there is an abundance of plankton. The bacteria in young worms can fix nitrogen even faster than those in adults. These young ones have been known to increase their length more than 30 times in a month. They could not filter plankton fast enough to sustain such a rate of growth.

Yet even with the aid of fungi and bacteria, on a diet of wood alone these worms still live on a hairline between sufficiency and deficiency of nitrogen. With plankton excluded from their diet many species of shipworm can grow but are unable to reproduce.

Finally there is a group of herbivorous animals that has really fine-tuned the business of enlisting micro-organisms to improve their nitrogen nutrition. These are insects, mostly aphids and psyllids, which feed on an exclusive diet of the phloem sap of plants. This sap is rich in sugars, but low in nitrogen. In fact it often completely lacks some amino acids that are essential for the animals to survive, let alone grow and reproduce. All these insects have a special organ outpocketing from the gut called a mycetome. Within this live a variety of specialised micro-organisms - bacteria, usually, but also in some species fungi or virus-like organisms. So specialised are these microbes that they have become physically simplified, can live only in these special myce-tomes, and are passed from generation to generation via the insects' eggs. However, unlike the microbes in the cow's rumen, they are not digested by their host. Instead they produce essential amino acids which the host insect absorbs as a vital addition to its plant food.

Continue reading here: Meateating vegetarians and cannibals

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