IV Microbes as the Cause of Senility
Relations between longevity and the intestinal flora—Ruminants—The Horse—Intestinal flora of birds—Intestinal flora of cursorial birds—Duration of life in cursorial birds—Flying mammals—Intestinal flora and longevity of bats—Some exceptions to the rule—Resistance of the lower vertebrates to certain intestinal microbes
In the actual state of our knowledge it is impossible to make a final examination of my hypothesis, as there are many factors about which we are incompletely informed. Nevertheless, it is possible to confront the hypothesis with a large number of accurately established facts.
Although the life of most mammals is relatively short, there are to be found in the group some which live relatively long, as well as others whose life is short. The elephant is an example of the long-lived mammals, whilst ruminants are short-lived forms. In the last chapter, I stated that sheep and cattle became senile at an early age, and did not live long. They are striking exceptions to the rule according to which the duration of life is in direct relation with the size and length of the period of growth. The cow, which is much larger than a woman, and the time of gestation of which is about the same, or a little longer, acquires its teeth at four years old, and becomes senile at an early age; it is quite old at between sixteen and seventeen, an age when a woman is hardly adult; at the age of thirty, practically the extreme limit for bovine animals, a woman is in full vigour.
The precocious old age of ruminants, the constitution of which is well understood, and which are carefully tended, coincides with an extraordinary richness of the intestinal flora. Food remains for a long time in the complicated stomach of these animals, and afterwards the digested masses remain still longer in the large intestine. According to Stohmann and Weiske,57 in the case of sheep it is a week until the remains of a particular meal have finally left the body of the animal. The excreta of sheep, normally solid, do not betray any special putrefaction in the intestine, but if the body is opened there is abundant evidence of the process. The intestinal contents are richly charged with microbes and give off a strong odour of putrefaction. It is not surprising that under these conditions, the life of sheep should be short.
Another large herbivorous animal, the horse, also dies young, after a premature old age. Although it does not ruminate and possesses a simple stomach, the process of digestion is slow, and enormous masses of nutritive material accumulate in the huge large intestine. Ellenberger and Hofmeister58 have shown that food remains in the alimentary canal for nearly four days. It remains in the stomach and the small intestine only 24 hours, but about three times as long in the large intestine. This is remarkably different from what happens in the case of birds, in which there is no stagnation during the passage of food through the digestive canal.
The structure of birds is adapted for flight, the body being as light as possible, many of the bones and the cavities of the body containing air-sacs. The absence of a bladder and of a true large intestine prevents the accumulation of excreta, these being ejected almost as rapidly as they are formed. The process of ejection, which takes place often in birds, is not so inconvenient as in mammals. The hind limbs are not used in flight, so that they offer no obstacle to evacuation. Thus birds may discharge their droppings while flying.
Such structure and habits make it not surprising that the alimentary canal of many birds contains only a scanty intestinal flora. Parrots, for instance, which are remarkably long-lived birds, harbour very few microbes in the intestine. The small intestine contains almost none, the rectum so few that the fæcal matter appears to be formed of mucus, the waste of the food, and only a very few microbes. M. Michel Cohendy, who has examined the intestinal flora at the Pasteur Institute, was unable to isolate more than five different species of microbes living in the alimentary canal of parrots.
Even in birds of prey which feed upon putrid flesh, the number of microbes in the intestine is remarkably limited. I have investigated the case of ravens which I fed on flesh which was putrid and swarming with microbes. The droppings contained very few bacteria, and it was specially remarkable that the intestines had not the slightest smell of putrefaction. Although the opened body of a herbivorous mammal, such as a rabbit, gives off a strong smell of putrefaction, the body of a raven with the digestive tube exposed has no unpleasant smell. This absence of putrefaction in the intestine is probably the reason of the great longevity of such birds as parrots, ravens, and their allies.
It might be said, however, that the long duration of life in birds is due to the organisation of these animals, rather than to the scantiness of their intestinal flora. To meet this objection, it is necessary to turn to the case of cursorial birds.
There are some birds incapable of flight, the wings of which are badly developed, but which have strong limbs, and can run with great rapidity. Ostriches, cassowaries, rheas, and tinamous, are well known examples of cursorial birds. They live on the surface of the ground, and their habits resemble those of mammals. When they are attacked by enemies, they escape by running so quickly that some of them (ostriches and rheas) outstrip even a horse. However, like mammals, they cannot discharge their secretions when they are running quickly. Tinamous (Rhynchotus rufescens), which I have observed in captivity, however quickly they may be running, stop abruptly to discharge their excretions. M. Debreuil, at my request, made observations on this matter, and assured me that the tinamous and rheas (Rhea americana) in his park always stood still for this purpose. He has noticed that the droppings, however abundant, were always deposited in heaps. With regard to ostriches, M. Rivière, director of the experimental Gardens at Hamma, Algeria, has been kind enough to give me the following information. “The discharge of excreta,” he said in a letter in January, 1901, “is less frequent than in other birds, but the comparatively small size of the enclosures here makes it impossible for me to assert that the animal could discharge its droppings if it were running for a length of time; a priori I should think that this did not happen. Normally the bird stands still for defæcation, the tuft of feathers on the tail is lifted up, and there is a violent contraction of the abdominal muscles before the sphincters of the cloaca are suddenly opened to discharge the excrement with violence.”
I believe that the remarkable development of the large intestine in these running birds has been acquired to obviate the danger which is caused by the animal having to stop for defæcation. Although the huge cæca of these birds have a digestive function, particularly on plants rich in cellulose, I cannot think that the cæca of cursorial birds have been developed for digestion. As a matter of fact, some birds which are not cursorial live on the same kind of food (herbage, seeds, and insects) and have much smaller cæca, the cæca indeed, in some, for instance, the pigeons, being quite rudimentary.
It is not surprising that the accumulation of food material in the large intestine of running birds is associated with the presence of an extremely rich intestinal flora. Microscopic examination of the excrement of such birds shows this at once. Although the intestinal contents and excrement of many other birds show the presence of very few microbes, belonging to a small number of species, the same materials taken from running birds show enormous quantities of microbes, belonging to a large number of species. In the cæcum of the rhea (Fig. 14) there are bacterial threads, spirilla, bacilli, vibrios, and many kinds of cocci. In the tinamous, the intestinal flora is if possible even richer. According to the statistical investigations of M. Michel Cohendy, the quantity of intestinal microbes in cursorial birds is not less than that found in mammals, even in man.
If I am correct in the view that I have been explaining, cursorial birds, on account of their rich intestinal flora, ought to have a shorter duration of life than that of flying birds. I will now turn to this side of the question. Amongst cursorial forms, there are some of the largest living birds, ostriches being actually the largest living birds, whilst an extinct running bird, the Aepyornis of Madagascar, was the largest known bird. According to the rule that large animals live longer than small animals, ostriches should be able to reach a great age. The facts, however, are against this. M. Rivière, who rears ostriches in Algeria, and has a great experience of them, writes to me as follows: “I have no confidence in the stories about the longevity of the ostrich which were told me in the Sahara; they rest on no facts. My personal observation is not very large, but it is quite exact. Some of the ostriches which have been hatched here have lived for 26 years. I do not estimate the duration of life of this bird at more than 35 years, and only one case of this age have I seen myself in 20 years. The bird was a female, a good layer and sitter; she died of old age, showing all the signs of decrepitude, the skin excoriated and lumpy, the feathers degenerate and dry. The bird laid eggs until nearly the end of her life, but at irregular intervals, and the shells were granular instead of being smooth and polished.”
In a farm near Nice, where ostriches are reared, there was recently an old male called “Kruger,” which was supposed to be 50 years old.59 Countess Stackelberg has been good enough to try to get information for me about this, and informs me that although they have not exact knowledge at the farm, they believe that it must be 50 years old. M. Rivière thinks this statement very surprising, and has nothing in his own long experience to confirm it.
The facts which I have been able to get together do not attribute a long life to other running birds. Gurney mentions that a cassowary (Casuarius westermanni) lived 26 years in the Zoological Gardens of Rotterdam, and that three Australian emus (Dromaeus novae-hollandiae) had lived in the same Gardens for 28, 22, and 20 years. M. Oustalet (Ornis, 1899, vol. x, p. 62) mentions another emu of the same species which died in London at the age of over 23 years. The rhea (Rhea americana), another large running bird, does not live so long. “Boecking thinks that its duration of life should be set down at from 14 to 15 years. According to him, many of these birds die of old age.” (Brehm, Oiseaux, vol. ii, p. 517).
It is striking to compare the short life of cursorial birds, which nevertheless thrive and reproduce in captivity, with the remarkable longevity of so many other birds (parrots, birds of prey) which, although they are much smaller, have been kept alive for from 80 to 100 years. It would be difficult to find a more striking argument in favour of the view that richness of the intestinal flora shortens life. When birds become adapted to terrestrial life and acquire a huge large intestine in which microbes can abound, their duration of life is diminished.
Just as some birds, losing the aerial mode of life, have come to resemble mammals, so also some mammals have become flying animals, provided with wings and in some respects resembling birds. Bats are the most familiar instance. The large intestine, which is extremely useful to running animals, not only ceases to be an advantage but is harmful to flying creatures, insomuch as it increases the weight of the body uselessly. Bats, accordingly, have no cæcum whilst the large intestine is changed in structure and function. Instead of being a capacious tube, serving as a reservoir for the refuse of the food, the large intestine of bats has the same diameter as the small intestine. Its structure is nearly identical. It is provided with glands, and as I have already mentioned in the last chapter, it digests the food in the same way as the small intestine. In fact, the large intestine has become simply a part of the small intestine, the total length of the gut being reduced. Bats, therefore, can no longer retain their secretions but have to empty the intestine almost as often as most birds. I find that Indian fruit bats (Pteropus medius) discharge their excreta very often. Microscopic examination shows that there is an absence of microbes quite unusual in the case of a mammal. The alimentary canal of bats is nearly aseptic, containing only a few single bacteria. I have fed these fruit bats with the same food (carrots) which I have given to rabbits, guinea pigs, and mice; whilst the bats accomplished the process of digestion in 1-1/2 hours, and deposited excreta containing fragments of carrot, the rodents took very much longer for digestion and large quantities of waste matter accumulated in the cæca. The intestinal flora too, although the food in each case was the same, showed remarkable differences in these animals. It was almost absent in the bats, whilst in the rabbits, guinea-pigs and mice it consisted of a mass of microbes of different species. The excrement of the bats had no unpleasant odour, and the digestive canal of these bird-like mammals was free from putrefaction. Fruit bats fed upon fruit discharged excreta with a pleasant odour of apples and bananas. We have seen that birds which live a life similar to that of mammals acquire a rich intestinal flora and do not live so long as aerial birds. It would be extremely interesting to ascertain the duration of life of bats, mammals which live like birds and have a very scanty intestinal flora. I have been unable to get any exact information as to the duration of life of the true bats, that is to say, the insectivorous bats, as all the requests that I have addressed to specialists have proved fruitless. It appears, however, that it is a popular belief that bats live long. There is a Flemish phrase: “as long-lived as a bat,” and a similar phrase is common in Little Russia.
As for the fruit-eating bats, I have been able to ascertain that even in captivity, where the conditions are unfavourable to them, the duration of life is relatively long. I have had in my own possession a fruit bat (Pteropus medius) which was bought in Marseilles 14 years ago. It showed no signs of old age, and the teeth were in perfect condition. It died of some acute disease accidentally contracted. I know of another bat of the same species which lived in captivity for more than 15 years, and I have been informed that60 in the London Zoological Gardens, a fruit bat has lived for 17 years. If these bats were adult when caught, it would be necessary to add something to the known figures.
Although I do not know the exact duration of the life of bats, it is clearly relatively long for mammals no bigger than guinea-pigs. The difference is remarkable if we compare it with the life of sheep, dogs and rabbits, mammals very much larger in size, but possessed of a rich intestinal flora.
The series of facts that I have been discussing strengthens my conviction that the intestinal flora is an extremely important factor in the causation of senility. It must not be supposed, however, that all the known facts can be explained equally easily on this hypothesis. The harm done by microbes cannot always be measured by their abundance in the alimentary canal. In the first place, it must be remembered that some microbes are useful; moreover, microbes, even although their products are very dangerous, may exist in quantities in an organism, and yet do no harm if the organism has the power of resisting bacterial poisons. Thus, for instance, the bacillus of tetanus, which thrives in the alimentary canal, and which can endanger life if the wall of the gut is wounded, does not harm a crocodile or a tortoise, as these animals are extremely resistant to the poison of tetanus. Dr. Favorsky, by experiments at the Pasteur Institute, has shown that the poison of botulism can be absorbed with impunity by some birds, and by tortoises, although death follows if a very small quantity of it be introduced into the alimentary canal of a mammal.
The bodies of man and of higher animals are possessed of a complex mechanism which resists the harmful action of bacteria and their poisons. The various parts of this mechanism may act differently, with the result that there is great variation in the power of resistance. Thus, however abundant microbes may be in the intestine, they may bring little harm to an organism that has a high power of destruction or neutralisation of the toxins, or when these harmful products are unable to pass through the intestinal wall. It is in this way that I explain some exceptions to the general rule, which are exceptions only in appearance. Such a case is that of the nocturnal birds of prey. Although the diurnal birds of prey (eagles, vultures, etc.) have very short cæca, in which the food is never found, owls have very large cæca, which may be as long as 10 cm. (Eagle-Owl, Bubo maximus). These long cæca, however, contain debris of the food only in the enlarged terminal portion, and the food masses contain a very small number of microbes. Notwithstanding a great difference in the length of the cæca between the owls and the eagles, these two groups of birds do not differ greatly in longevity. But the difference in the cæca does not imply a corresponding difference in the intestinal flora which appears to be very scanty in both cases.
It is possible that the elephant is a more real exception to the rule. Here is a case of a mammal with an enormous large intestine and a capacious cæcum, and which none the less is capable of surviving for a century. I have had no opportunity of investigating the elephant from this point of view, and have no explanation to suggest.
Monkeys and man differ from most mammals in so far as they possess a long duration of life, although their large intestines are very capacious. I have been unable to get exact information as to the longevity of monkeys, but I understand that these animals live longer than domesticated mammals, such as the ox, sheep, dog, and cat. Anthropoid apes are supposed to be able to reach the age of 50 years. The only other mammal with a longevity similar to that of the elephant is man.