As I have already alluded to several times, this propensity of plant-eaters to eat animal tissue can include eating one's own kind - cannibalism. A fine line can be drawn, of course, between being nourished by your mother's body (e.g. placenta, milk, trophic eggs, mucus, empty eggshells) on the one hand, and on the other eating your siblings, or (possibly your own) young. However, this latter form of more 'conventional' cannibalism is something that is, contrary to the belief of many, also widespread and common in nature. It is not a rare event seen only in animals under extreme stress such as over-crowding. It can be found in all forms of animal life, from single-celled microbes to mammals. And it takes many forms. Mate eats mate, parents eat their young, the young eat their parents, or more commonly their siblings.
Nor is it what many ecologists would have you believe, a so-called 'self-regulating' device to control the size of a population below the maximum size it could achieve by consuming all its available food. It is not a device to reduce numbers. On the contrary - it serves to increase the numbers that survive. It does this by using more of the food in the environment than would otherwise be used, and concentrating it into fewer, but successful animals. When food is short, without cannibalism some might survive; with cannibalism more will survive.
But it is not just any old food that is thus used more effectively. Ecologically cannibalism is just a special form of predation, functioning to increase access to animal food - to help alleviate the chronic shortage of protein in the natural world, and convert more of it to the next generation than could be the case without cannibalism. So it is not too surprising to find that cannibalism is most common among herbivores. Nor that it is largely restricted to breeding females and their fast-growing young. These are two points in the life cycle of any animal, herbivore or carnivore, when access to an enriched supply of protein is vital.
For example, it is fairly well known that a female praying mantis will eat her mate, often while he is still copulating with her. But she is much more draconian than this. Having been successfully impregnated, she continues to produce sex pheromone, so that more males are attracted to her. As fast as they arrive she promptly catches and eats them before they can mate with her! Research has established that this additional protein food increases both the number and viability of her eggs. And, incidentally, recycles to the next generation protein that would otherwise be lost in now-redundant males!
As we have seen termites are herbivorous, albeit with the aid of microbes. Yet their workers will kill and eat many of their own kind when nitrogen in their food is very low, and especially when the nest is producing the winged reproductives which must leave the nest to attempt to establish new colonies. And all cast skins, the injured and the dead in the nest are quickly devoured.
Many female mammals will eat their young (or resorb them as embryos) if food is short and they themselves are at risk of starving. Ecologically it is better to abandon the attempt to breed, recycle the young that were anyway doomed, and try again when conditions are favourable.
However, cannibalism by the young is even more ubiquitous. They may eat their mother entirely (some spiders do this) or part of her (as milk or egg-yolk for example). On the other hand they more commonly eat their siblings. Caterpillars reared together in small containers will eat their smaller fellows -and their larger ones once they start to spin a cocoon. At this stage they are unable to walk away or defend themselves, and their still-active brethren will quickly pounce on them. I have watched in horror as this happened in small cultures of hard-to-replace insects I had been trying to rear.
I related how garden snails which hatch first will eat unhatched eggs, and how evolution has refined this form of cannibalism in other species (both vertebrate and invertebrate) which produce unfertilised 'trophic' eggs, which are duly eaten by the young that hatch from the fertilised eggs. There is a more bizarre variant of this behaviour. Some sharks and salamanders have uteri in which their young grow. Here just a few of their embryos develop large jaws with teeth and proceed to eat their less well-endowed siblings! In cases like these it is possible to argue that this is just an extension of the mother's body nurturing her young. Nevertheless, if we define cannibalism as eating some or all of the body of another member of your own species, then all such nurturing, placental and milk feeding included, falls under this heading.
In general two broad cannibalistic 'strategies' can be recognised - the 'Lifeboat' strategy and the 'Grazer' strategy. The first strategy is fairly obvious. As the supply of food declines, or if it is scarce to begin with, the strongest
members in a population eat the weaker. In this way the available food is concentrated to fewer individuals, but they each get enough to mature and produce a new generation. This is clearly better than all getting an equal, but inadequate share, with the result that possibly none survive to reproduce.
The codling moth provides a good example. Normally there are not enough developing seeds in an apple to support the growth of more than one caterpillar. So if a moth lays more than one egg in an apple the larger (and usually first-hatched) caterpillar eats its sibling.
The same story holds for the young of many raptor birds. Usually two or more eggs are laid each breeding season, but only in very good years can the parents catch enough prey to fledge more than one nestling. The oldest and largest will appropriate the lion's share of the food brought to the nest, and eventually kill and eat or eject from the nest its smaller and starving siblings.
An indication that this behaviour has been around for a long time is the recently described case of a cannibalistic species of Bacillus. When bacteria run short of nutrients they stop dividing and begin to form spores - a condition in which they can survive, often for many years, in a state of suspended animation. It has been found in one species that once an individual bacterium lacks adequate nutrients and enters the stage that would lead to sporulation, it releases a chemical killing factor into its surroundings. This chemical stops other nearby bacteria from forming spores and causes them instead to disintegrate, releasing their contents. These provide additional nutrients for the killer bacterium, enabling it to postpone sporulation and continue to grow and replicate.
The grazer strategy, on the other hand, is more subtle. Most commonly this strategy is seen where the young eat food that is not available to the adults - and then they are later eaten by the adults - often their own parents. Again, this concentrates otherwise inaccessible protein to fewer but successful individuals. For example, very small scorpions can catch animals that are too small for their mother to catch. She then eats most of them, thus 'grazing' food which would otherwise not have been available for her to convert to the next generation. Similarly, the free-swimming larvae of land crabs eat small planktonic animals in the ocean; food that is not accessible to the adults on land. Then when these larvae transform to miniature crabs and come ashore, many of them are eaten by the protein-hungry adults.
There are many more examples of cannibalism in nature, all of which achieve the same end - recycle and concentrate limited protein to fewer individuals so that more of them survive to pass their genes on to the next generation. But let us finish by taking a closer look at a more sophisticated form of the grazer strategy.
If you went walking in a European forest early on a spring morning you could come upon a line of marching soldiers. Follow, and you would see them fan out onto a battlefield and begin to fight with soldiers swarming on from the other side. The fighting is deadly - but silent, for these soldiers are all female workers of the European wood ant. And their foes are other workers of the same species from another nearby nest.
Such warfare starts when the nests have become active again after winter, and workers begin sallying forth in search of food. Battles rage all day, workers returning each morning to the same battlefield to resume hostilities. War may last for a month, with casualties commonly exceeding 75 000 workers from just one nest on just one battlefield. And a nest may be fighting battles with several different nests at once on different battlefields. So it is an expensive business in terms of the loss of food-gathering workers from a nest.
But this is not just war for the sake of it, nor to defend territorial limits. All slain enemies are taken back to the nest and fed to the growing young grubs there. Warfare topped by cannibalism!
Why would neighbours of the same species do such things to each other? Surely it is counterproductive behaviour? And 'unnatural'?
Not so. The key lies in the need for a nest to get enough protein to raise the next generation. Battles start when a nest's demand for protein is at a peak
- when it is raising its annual sexual brood of winged males and females that will later depart to establish new nests. This happens in the spring when the usual prey of the ants (other, mostly plant-eating insects) are not abundant enough to meet this demand. If workers did not feed other workers to their young, their next generation would either fail, or be damagingly reduced in size. And all nests have the same problem. So warfare is inevitable!
However, as soon as other insects become numerous, peace is restored. In summer, when insect prey is abundant, the ants convert this plentiful food to large numbers of new workers. Next spring, when prey is again scarce, these now-aging workers constitute a live store of food to be fed - by way of these wars - to the next sexual generation.
But let us not overlook cannibalism among humans. It is generally thought by most people - including some biologists who should know better
- to be an abhorrent and 'unnatural' aberration brought on only in times of extreme stress or deprivation, such as shipwreck or the famous example of the survivors of a plane crash in Chile. On the contrary, for early people living as hunter-gatherers it was a common practice (albeit disguised by various religious or cultural justifications). Modern archaeological investigations are producing mounting evidence (such as finding human myoglobin from heart and skeletal muscle absorbed inside pottery cooking containers, and in fossilised human faeces) of cannibalism having long been a common part of human activity. Indeed, it is not so long ago that this was so - perhaps it still occurs today! A study in 1974 of'pay-back' warfare and cannibalism among small isolated groups of Papua New Guineans showed that this behaviour contributed 10 per cent of the protein to the diet of these people who were living where game was in chronically short supply. The only difference between this and the story of the ants is that 'surplus' young male humans were being recycled rather than sterile female worker ants!
So, a general picture emerges; vegetarians are not really vegetarians - at least not when they are growing youngsters.
Why, then, is this access to animal tissue (or, as we saw in the previous chapter, nitrogen-rich micro-organisms) apparently so vital and universal for herbivores? And if not for grown adults, at least for females generating and nurturing their young; and for neonates? Simply because plant food - a vegetarian diet - just does not provide sufficient protein for the rapid and exponential growth of a young animal's body. And this carnivory is still necessary even though, as we have seen in earlier chapters, all herbivores have evolved a wide variety of behavioural, anatomical and physiological adaptations to maximise their access to what digestible nitrogen there is in their plant food.
There is, however, one unequivocal exception to this apparent rule. That is the caterpillars of plant-eating moths and butterflies (and, possibly, the immature stages of some grasshoppers and locusts). All live on an exclusive diet of the leaves of plants. Many are known to be fierce, cannibalistic carnivores when given the opportunity. Anybody, like me, who has tried to rear such animals in captivity soon learns this. But they can be (and commonly are) raised on nothing but a diet of their plant host. The only possible animal protein they can get is the shell of the egg from which they hatch. Most hatching insects routinely do this, and as we saw earlier, may depend upon such behaviour to survive or breed. But even if denied this source of protein, these caterpillars can survive, grow and reproduce through repeated generations eating nothing but plant tissues.
How can they do this? There are two possible clues, both of which await careful investigation by insect physiologists. The first is the very high pH of the gut of these larvae - often pH 10 or more, as high as pH 12.5 in one species of termite; a very caustic brew! But such an alkaline environment is extremely efficient at extracting virtually every last trace of protein from the ingested plant tissues; much more so than the acidic gut secretions of other animals. This may increase the efficiency with which they can extract nitrogen from their food just sufficiently to tip the balance.
The second clue - and possibly linked to the first - is the presence in leaf proteins of'Rubisco' (Ribulose biphosphate carboxylase/oxygenase). This is an enzyme found in cell chloroplasts, and chloroplasts originated as ancient micro-organisms that became permanently incorporated into the cells of early plants. Rubisco is a protein made of amino acids still much the same as those found in present day free-living microbes. Maybe the capacity to gain access to this animal-like protein in leaves, combined with the super-efficiency of extracting the last remnant of it from the cells of the leaves, frees these caterpillars from the need to be carnivores when they are very young?
Nevertheless, the list of vegetarians which are not truly so is diverse, and growing. Why has it taken so long to discover this universal nitrogen-hunger of plant-eaters? Because, I suspect, in Science, as in everyday life, so often we do not see something until we go looking for it. I am reminded of the (possibly apocryphal) story about the anthropologist investigating the diet and feeding habits of a Polynesian tribe in the South Pacific. He could not understand why the children were so fat and healthy. From his detailed recording of what they ate at meal times once they were finally weaned (usually not until they were three to five years old), he found that they ate practically nothing but very starchy vegetables: their elders ate what little meat was available! As a consequence they should have been suffering from quite severe protein malnutrition. However, what he had not observed was what the children ate between meals - insect grubs, shellfish, crabs and other small marine invertebrates, and seeds.
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