PART II

LONGEVITY IN THE ANIMAL KINGDOM

I

THEORIES OF LONGEVITY

Relation between longevity and size—Longevity and the period of growth—Longevity and the doubling in weight after birth—Longevity and rate of reproduction—Probable relation between longevity and the nature of the food

The duration of the life of animals varies within very wide limits. Some, as for instance, the males of certain wheel animalculæ (Rotifera) complete their cycle of life from birth to death in 50 or 60 hours, whilst others, like some reptiles, live more than 100 years, and quite possibly may live for two or three centuries.

Enquiry has been made for many years as to whether there are laws governing these different durations of life. Even the most casual observation of domesticated animals has shown that, as a general rule, small animals do not live so long as large ones; mice, guinea pigs, and rabbits for instance, have shorter lives than geese, ducks, and sheep, whilst these again are survived by horses, deer, and camels. Of all the mammals which have lived under the protection of man, the elephant is at once the largest, and the most long-lived.

However, it is not difficult to show that there is no absolute relation between size and longevity, since parrots, ravens, and geese live much longer than many mammals, and than some much larger birds.

As a general rule it may be said that a large animal takes more time than a small one to reach maturity, and it has been inferred from this that the length of the periods of gestation and of growth were in proportion to the longevity. Buffon[28] long ago stated his opinion that the “total duration of life bore some definite relation to the length of the period of growth.” Therefore, as the period of growth is, so to say, inherent in the species, longevity would have to be regarded as a very stable phenomenon. Just as any species has acquired a fixed and practically invariable size, so it would have acquired a definite longevity. Buffon, therefore, thought that the duration of life did not depend on habits or mode of life, or on the nature of food, that, in fact, nothing could change its rigid laws, except an excess of nourishment.

Taking as his standard the total period of development of the body, Buffon came to the conclusion that the duration of life is six or seven times that of the period of growth. Man, for instance, he said, who takes 14 years to grow, can live 6 or 7 times that period, that is to say, 90 or 100 years. The horse, which reaches its full size in 4 years, can live 6 or 7 times that length of time, that is to say from 25 to 30 years. The stag takes 5 or 6 years to grow, and reckoned in the same way, its longevity should be 35 to 40 years.

Flourens[29] although supporting his principle, thought that Buffon had been inexact in calculating the period of growth. In his opinion a better result can be obtained by taking the limit of growth as that age at which the epiphyses of the long bones unite with the bones themselves. Using such a mode of computation, Flourens laid down that an animal lived 5 times the length of its period of growth. Man, for instance, takes 20 years to grow, and he can live for 5 times that space, that is to say, 100 years; the camel takes 8 to grow, and lives 5 times as long, _i.e._, 40 years; the horse, 5 to grow, and lives 25 years.

However, even if we consider only the mammalia, it is impossible to accept Flourens’ law, without considerable reserve. Weismann[30] has referred to the case of the horse, which is completely adult at 4, but lives not merely 5 times that period, but 10 or even 12 times. Mice grow extremely quickly, so that they are able to reproduce at the age of 4 months. Even if we take 6 months as their period of growth, their longevity of 5 years is twice as long as it would be according to the rule of Flourens. Amongst domesticated animals, the sheep is slow in reaching maturity; it does not acquire its adult set of teeth until it is 5 years old, and cannot be regarded as adult until then. None the less, at the age of 8 or 10 years, it loses its teeth and begins to grow old, whilst by 14 it is quite senile.[31] The longevity of the sheep, therefore, is not quite three times its period of growth.

If we turn to other vertebrates, the variations in the relation of growth and the duration of life are still greater. Parrots, for instance, the longevity of which is extremely great, grow very quickly. At the age of 2 years, they have acquired the adult plumage and are able to reproduce, whilst the smaller species are in the same condition at the age of one. Incubation, moreover, is very short, not more than 25 days, and in some species not three weeks. None the less, parrots are birds which enjoy a quite remarkable longevity. The incubation period of domestic geese is 30 days, and their period of growth is also short. However, they may reach a great age, cases of 80 years and of 100 years being on record. In contrast with these, ostriches, the incubation period of which is 42 to 49 days, and which take 3 years to become adult, have a relatively short life.

H. Milne-Edwards[32] many years ago contended that there was no importance in the supposed law of relation between gestation and longevity. He sums up his criticism as follows: “Although the period of uterine life is longer in the horse, that animal does not live so long as a human being; and some birds, the incubation of which only lasts a few weeks, can live more than a century.”

Bunge[33] has recently taken up the study of the relations between the duration of growth and longevity, and has suggested a new means of investigation. He has observed that the period in which the new-born mammal doubles its weight is a good index of the rapidity of its growth. He has shown that whilst a human child requires 180 days to reach double its weight at birth, the horse, the longevity of which is very much less, doubles its weight in 60 days; a calf takes only 47 days for this; a kid 15 days; a pig 14 days; a cat 9-1/2; and a dog only 9 days. Although these facts are very interesting, the exceptions are too great to make it possible to base a law of longevity upon them. The period of weight-doubling in the horse is nearly 7 times longer than that in the dog, and yet the longevity of the horse is not more than 3 times that of the dog. The goat, which takes much longer than the dog to double its weight, has a shorter total life.

I observed myself that new-born mice quadruple their weight in the first 24 hours. The doubling of weight in their case requires a time 36 times less long than that of the cat, and yet the cat lives only 5 times as long as the mouse.

It is fair to say, however, that Bunge himself does not draw a definite conclusion from these figures and has published them only to stimulate interest in the subject. He is against the view of Flourens, and points out that although the multiple 5 is valid for man, it is not so in the case of the horse which finishes its growth in 4 years and yet reaches the age of 40 much less often than human beings attain that of 100 years.

Although it is impossible to admit the existence of exact relations between size and the period of growth on the one side, and longevity on the other, in the mode which Buffon and Flourens have followed, it is none the less true that there is something intrinsic in each kind of animal which sets a definite limit to the length of years it can attain. The purely physiological conditions which determine this limit leave room for a considerable amount of variation in longevity. Duration of life therefore, is a character which can be influenced by the environment. Weismann in his well-known essay on the duration of life, has laid stress on this side of the problem. Longevity, according to him, although in the last resort depending on the physiological properties of the cells of which the organism is composed, can be adapted to the conditions of existence and influenced by natural selection, like other characters useful for the existence of the species.

If a species is to remain in existence, its members must be able to reproduce and the progeny must be able to reach adult life so that they in their turn may reproduce. Now, it happens that there are some animals the fecundity of which is extremely limited. Most birds which are adapted to aerial life, and the weight of which is therefore to be kept down, lay very few eggs. This happens in the case of birds of prey, such as eagles and vultures. These birds nest only once a year, and generally rear two or frequently only a single nestling. In such circumstances the duration of life becomes a factor in the preservation of the species, more important since eggs and chicks are subject to many dangers. Eggs are devoured by many kinds of animals, whilst unseasonable cold may kill the chicks. If the members of such a species were incapable of living long, the unfavourable conditions of life would soon lead to extinction. Those animals which reproduce rapidly generally have a relatively brief duration of life. Mice, rats, rabbits, and many other rodents seldom live more than 5 or 10 years, but reproduce with enormous rapidity. It is almost possible to imagine that there is some sort of intimate link, possibly physiological, between longevity and low fertility. It is a current opinion that reproduction wastes the maternal organism and that mothers of many children grow old prematurely and seldom reach an advanced age. This would seem to mean that fecundity was the cause of the short duration of life. However, we must guard ourselves against such a theory. Longevity, at least in the case of vertebrate animals, differs extremely little in the two sexes, although the cost of the new generation to the adult organism is very much greater in the case of the female than of the male parent. None the less, females frequently reach a great age, especially in the human race where women reach 100 years, or live beyond that time, much more often than men.

Low fertility, however, cannot itself be regarded as a cause of longevity, as there are some very fertile animals which none the less attain great ages. There are parrots which lay two or three times a year, producing six to nine eggs in each clutch. The ducks (Anatidæ) are distinguished for considerable longevity and very high fertility, each nest containing rarely less than six and sometimes as many as sixteen eggs. The common Sheldrake lays from twenty to thirty eggs. Tame ducks, in some parts of the tropics, lay an egg daily throughout the season. Wild ducks lay from seven to fourteen eggs in one nest. Ducks and geese, none the less, frequently attain considerable ages, ducks having been known to live for 29 years. Even the common fowl, which is a notoriously prolific bird, may reach an age of twenty to thirty years.

It will be said, however, that these birds are exposed to many enemies during youth. Chickens, ducklings, and goslings are ready prey for hawks, foxes and small carnivora. The longevity is possibly to be explained as an adaptation for the preservation of the species by compensating for the great destruction of the young. Weismann explains in this way the longevity of many aquatic birds and other creatures that are much preyed on. It must be noted, however, that the longevity cannot depend on the risks run by the young birds, but must have arisen independently. If this had not occurred, creatures, the young of which are destroyed in great numbers, would have ceased to exist, as many species have disappeared in geological time. The longevity of prolific animals, the young of which are destroyed in numbers, must be due to some cause which is neither fertility nor the destruction of their offspring. This cause must be sought in the physiological processes of the organism and can be attributed neither to the length of the period of growth nor to the size attained by the adults.

After having discussed various theories of the cause of the duration of life, M. Oustalet,[34] in a most interesting essay on the longevity of vertebrates, came to the conclusion that diet was the chief factor. He thinks that there is a “definite relation between diet and longevity. For the most part herbivorous animals live longer than carnivorous forms, probably because the former find their food with ease and regularity, whilst the latter alternate between semi-starvation and repletion.” There are certainly many instances which give support to the view. Elephants and parrots, for instance, are vegetarian and reach very great ages. On the other hand, there exist long-living carnivorous animals. Many observations have made it certain that owls and eagles reach great ages, and these birds live on animal food. Ravens, which live on carrion, are also notorious for the duration of their lives. There is no exact knowledge as to the ages reached by crocodiles, but although these live on flesh, it is certain that their longevity is great.

We must seek elsewhere for the real factors that control duration of life. Before stating my conclusion, I will review what is known as to the duration of life of different animals.

II.

LONGEVITY IN THE ANIMAL KINGDOM

Longevity in the lower animals—Instances of long life in sea-anemones and other invertebrates—Duration of life of insects—Duration of life of “cold-blooded” vertebrates—Duration of life of birds—Duration of life of mammals—Inequality of the duration of life in males and females—Relations between longevity and fertility of the organism

It is wonderful to what an extent the duration of life varies amongst animals, the slightest examination of the facts showing that very many factors must be involved.

As the higher animals are nearly always larger than invertebrates, if there be a definite relation between longevity and size, one would expect to find that vertebrates live longer than invertebrates. However, this is not the case. Amongst animals of extremely simple organisation, there are some which reach a great age. A striking example of this is found in sea-anemones. These animals have a very simple structure, without a separate digestive canal, and with a badly developed, diffused nervous system, and yet have lived very long in captivity. More than forty years ago, I remember having seen in the possession of M. Lloyd, the Director of the Aquarium at Hamburg, an anemone that he had kept alive for several dozen years in a glass bowl. Another sea-anemone, belonging to the species _Actinia mesembryanthemum_, is known to have lived 66 years. It was captured in 1828 by Dalyell, a Scottish zoologist, and was then quite adult, and probably about 7 years old. It survived its owner for 36 years, and died in Edinburgh in 1887, the cause of death being unknown. Although they are thus capable of living so long, the rate of growth of members of this species is rapid, and their fertility is very high. According to Dalyell, these anemones reach the adult condition in 15 months. The specimen in his possession, in the 20 years from 1828 to 1848 produced 334 larvæ, then after a period of sterility it gave birth, in one night (1857) to 230 young anemones. This extraordinary prolificness decreased with age, but even when it was 58 years old it used to produce from 5 to 20 at a time. In the seven years from 1872 onwards, it gave birth to 150 young anemones.[35] This animal, which certainly was not more than the fortieth or the fiftieth of the weight of an adult rabbit, lived six or seven times as long.

Ashworth and Nelson Annandale have published their observations on another sea-anemone, of the species _Sagartia troglodytes_, which was 50 years old. It differed from younger examples only in being less prolific.

There are other polyps, such as _Flabellum_, which do not live more than 24 years, although we have no knowledge as to the cause of the different duration of life.

The variation in the length of the life of molluscs and insects is extremely great. Some species of gasteropods (_Vitrina_, _Succinea_) live only a very few years, whilst others (_Natica heros_) can reach thirty years. Some of the marine bivalves, as for instance, _Tridacna gigas_, can live to sixty or a hundred years.[36]

Insects are animals as variable in their duration of life as they are in other respects. Some live only a few weeks; some of the plant-lice, for instance, die in a month. In the same order of Insects, however, (Hemiptera) there are species of cicada which live thirteen to seventeen years, that is to say, much longer than such little Rodents as rats, mice, and guinea-pigs. The larva of an American species spends seventeen years buried in the ground in orchards, where it feeds on the roots of apple trees, and the species is known as _Cicada septemdecim_, because of this duration of life. In the adult stage the insect lives little more than a month, just time enough to lay the eggs, and bring into the world the new generation, which in its turn will not appear above ground until after another period of seventeen years.

Between these extremes of long and short life, there is to be found amongst insects almost every gradation of longevity. Science, in its present state, has failed to find any law governing these facts. Rules which hold good up to a certain point in the case of the higher animals break down in their application to insects. The large grasshoppers and locusts, for instance, live a much shorter time than many minute beetles. Queen bees, the fertility of which is very great, live two or three years and may reach a fifth year, whilst worker bees, which are infertile, die in the first year of their existence. Female ants, although these are small and extremely prolific, reach the age of seven years.[37]

We know so little about the physiological processes of insects, that we cannot as yet make even a guess at the cause of this great variation in their longevity. It is more probable that we shall find some explanation in the case of vertebrates concerning which we know much more.

Analysis of the facts shows that whilst in the evolution from fish to mammal there has been a great increase in complexity of organisation, there has at the same time been a reduction in the duration of life. As a general rule, it may be laid down that the lower vertebrates live longer than mammals.

The facts about the longevity of fish are not very numerous, but it seems clear that these animals reach a great age. The ancient Romans, who used to keep eels in aquaria, have noted that these fish would live for more than sixty years. There is reason to believe that salmon can live for a century, whilst pike live much longer. There is, for instance, the much quoted instance of the pike stated by Gessner to have been captured in 1230 and to have lived for 267 years afterwards. Carps are regarded as equally long lived, Buffon setting down their period of life as 150 years. There is a popular idea that the carp in the lakes at Fontainebleau and Chantilly are several centuries old, but E. Blanchard throws doubt on the accuracy of this estimate, inasmuch as during revolutionary times most of the carp were eaten when the palaces were overrun by the populace. There is no doubt, however, that the life of carp may be very long indeed. Not very much is known about the duration of life in batrachians, but it is certain at least that some small frogs may live twelve or sixteen years, and toads as many as thirty-six years.

More is known about the life of reptiles. Crocodiles and caymans, which are large and which grow very slowly, attain great ages. In the Paris Museum of Natural History there are crocodiles which have been kept for more than forty years without showing signs of senescence. Turtles, although they are smaller than crocodiles, live still longer. A tortoise has lived for eighty years in the garden of the Governor of Cape Town, and is believed to have reached the age of two hundred years. Another tortoise, a native of the Galapagos Islands, is known to be 175 years old, whilst a specimen in the London Zoological Gardens is 150 years old. A land tortoise (_Testudo marginata_) has been kept in Norfolk, England, for a century. I am informed that in the Archbishop’s palace at Canterbury, there is to be seen the carapace of a tortoise which was brought to the Palace in 1623 and which lived there for 107 years.[38] Another tortoise, brought to Fulham by Archbishop Laud, lived in the Palace for 128 years. I have already referred to a specimen of _Testudo mauritanica_, the history of which is known for 86 years, but which is probably much older.

Very little is known as to the longevity of lizards and serpents, but it may be inferred from what I have said about other reptiles that reptiles as a class are able to reach great ages.

It is an easy inference that the great duration of life in cold-blooded animals is associated with the slowness of the physiological processes in these creatures. The circulation, for instance, is so slow, that the heart of a tortoise beats only 20 to 25 times in a minute. Weismann has suggested that one of the factors influencing the duration of life is the rapidity or slowness of the vital activities, the times taken by the processes of absorption and nutrition.

On the other hand, the blood is hot and the vital activities are rapid in birds, and yet birds may attain great ages. Although in the last