Popular Science Monthly/Volume 16/April 1880/Size of Brain and Size of Body


By H. W. B.[1]

IT may be stated generally that the larger the animal the smaller is the proportionate size of the brain. As an example of this we may take the case of two of the largest animals now living, viz., the whale and the African elephant. The whale possesses one of the largest brains that is found in any animal, but, if we compare the size of its brain to that of any of our domestic animals, such as the dog, we find that it has a very small brain in proportion to the size of its body. The same is the case with the brain of the elephant, which is certainly the largest brain of any land animal, but which, compared to the size of the body, is very small. Another set of animals in which the brain is comparatively small is the reptiles. This group includes a number of animals which are not included in the popular sense of the word, such as the crocodiles, the turtles and lizards, as well as the snakes. In these the brain is small comparatively to the size of the body, as it is also in the amphibia. The small size of the brain in these two classes of vertebrates is peculiar, as it runs through all the various groups, although most marked in the larger members of each one. The birds also follow the rule that the largest of them have smaller brains compared to their bodies than the smaller ones have. A good example of this may be seen in the case of the ostrich, which has the largest body of any member of the group, but a much smaller brain in proportion to its body than many of the smaller birds. As a class, however, the birds have large brains in proportion to their body, when compared to the other vertebrates, and so present a contrast to the reptiles, in which, as already stated, the brain is small throughout the whole group. In the next class of vertebrates—the Monotremata—the brain is large in proportion to their body, but it must be taken into account that this group contains very few large animals, being composed almost entirely of small ones. The size of the brain in the echidna, and the ornithorhynchus, both belonging to this group, is especially large in proportion to the small bodies which the animals possess. This is more remarkable when we consider the low position they occupy in the vertebrate series. In this manner we might go on through the whole series of vertebrates, showing how the larger animals have relatively smaller brains, and also the reverse, that the smaller animals have larger brains for their size.

We must now consider, however, the relative size of the brain in animals about the same size, as it is only in that way that we can gain information on the subject. Owing to the difficulties which attend the investigation of this subject, comparatively little is accurately known about it. It may be stated generally that the brain of domestic animals is larger than that of wild animals of a corresponding size of body. As an example of this we may take the case of the dog and the wolf. If the brains of those animals are compared, it will be found (if the animals compared are of the same size) that the brain of the dog is the larger. Again, if we compare the brains of a dog, a badger, and a musk-deer about the same size, we find that the brain of the dog is the largest, those of the other two animals being about the same size. It will be observed that, in classifying the size of an animal, we do not go by the height the animal stands, because many animals whose bodies are about the same size differ in height only on account of their legs being longer. In making comparisons, therefore, we compare animals whose bulk is the same, irrespective of the actual height they may stand from the ground. Among the wild animals of a similar size, we also find considerable difference between the comparative size of the brain and that of the body. As an example of this, we may take the case of the lion and tiger. The brain of the lion is much larger than that of the tiger; however, that might be expected from the lion being a larger animal than the tiger; but the brain is much larger in proportion than the difference of size of the two animals would account for: therefore the brain of the lion is larger, in proportion to the size of his body, than that of the tiger is in proportion to his body.

But we must now compare the size of the brains of domestic animals of the same size. Although the horse stands higher than the ox, yet both those animals may be classed together for the comparisons of their brain. We find that the brain of the horse is very much larger than that of the ox. The camel also may be included in this class, as being about the same size. Although many camels are larger, still the bulk of the body is not very much greater than that of the horse and ox. Its brain is very similar in size to that of the ox, but smaller than the horse's brain. The brain of the sheep is a good deal larger than that of the goat, although their bodies are similar in size. The sheep and the pig are also animals which we might classify if we do not take extreme sizes, but compare animals similar in age or bulk. We find that the pig's brain is larger than the sheep's, and corresponds in size very nearly to that of the dog. In the cat the size of the brain, in proportion to the body, is much larger than that of the domestic rabbit, although the size of the two animals is very much the same.

In these examples given we have not taken into consideration the order to which the animal belongs in the vertebrate series, but only compared similar sized animals, and In all cases we have compared the brains of adult animals. This is a very important point, as it is found that in all animals, including man himself, the size of the brain, in proportion to the size of the body, is much greater in young animals than it is in the adult. In some animals the head is found to grow enormously in size as the animal reaches adult age, but the brain does not increase to the same extent. There is generally some reason to be found for this in those animals where it takes place. For example, in the elephant the head of the young animal is by no means out of proportion to the size of its brain; but if we bisect the head of an adult animal we find that the brain only occupies a small cavity, and the rest of the skull is composed of plates of bone with air-cells between them. In the young elephant we find none of those plates and air-cells between the outer and inner layers of cranium, but simply the two layers of bone close to one another; but we will also find that at this stage the young elephant has no large tusks to carry, and its trunk is light, so that its head is comparatively light. The case is, however, quite different in the adult, when there are two large tusks and a large trunk to carry. In order to support this great weight he requires strong muscles. The great increase in the size of his head, therefore, is to afford a requisite extent of surface for the attachment of the muscles. In order to get this, combined with lightness, the skull is composed of those plates and air-spaces mentioned.

A very interesting question, but one which it is very difficult to answer, is whether the intelligence of the animal corresponds to the size of its brain. It is very difficult to make comparisons in many animals, as one animal shows his intelligence in one way, and another in another way. However, going over some of the animals whose brains we have compared, we may take as an example the horse and the ox. The horse has the larger brain, and he has undoubtedly the greater amount of intelligence. We find that horses can be trained to a great extent, as may be seen daily in a circus, but the ox has never been so trained. Again, a dog is the most intelligent animal there is, and he has a large brain in comparison to the size of his body. On the other hand, however, if we examine some of the Monotremata, we find, as already stated, that the brain is very large in proportion to the size of the body; but the animals of this group would by no means be taken as a standard of animal intelligence. It appears very probable, therefore, that in young animals and in the lower classes of the vertebrates the size of the brain has comparatively little to do with the intelligence the animal possesses, but that in the higher vertebrates there is some relation between the amount of intelligence and the quantity of brain matter. The question may be asked. Why is it that the elephant, since it has the largest brain of any land animal, is not the most intelligent animal there is—more intelligent even than man, if the intelligence of an animal depends on the amount of its brain-matter? The answer to this question is easy. This animal being so large requires proportionally larger nerves and larger nerve-centers, to supply the muscles and sensory organs of his body, in the same manner that a larger magneto-electric machine is required when twenty electric lights have to be supplied by it than when it has to supply only ten. The elephant has, in reality, a smaller quantity of brain material available for his intelligence than the dog, because the dog has a much smaller body, and requires smaller nerves. Moreover, the intelligence, it has been proved, is situated in the upper part of the brain, or cerebrum, as it is called. Now, the dog's cerebrum is very much larger, in proportion to the size of his body, than that of the elephant, after allowing for the general law that larger animals have smaller brains in proportion to their body than smaller ones have. The number of smooth and tortuous eminences called convolutions, separated by grooves, which cover the whole surface of the upper brain or cerebrum has also been proved to have something to do with the amount of intellect of the animal. The brains of those animals which possess superior intellect are generally more highly convoluted and more deeply divided by the grooves than those of lower intellect. This may be very well seen by comparing the brains of the horse and the ox. It will at once be seen on looking at the brains of those two animals that the horse's brain is the more convoluted and altogether the more complex structure of the two. The same thing may be seen in the brain of the pig on comparing it with that of the sheep. As an example of brains where the convolutions are few in number and the grooves between them very shallow, we may take those of the echidna and ornithorhynchus, already mentioned as being very large in proportion to their bodies, while they themselves are of a low type. The intellect of those animals evidently does not correspond to the size of their brain. The probability is, therefore, that their brain-matter is of a low type and consequently a larger quantity of it is required. Besides these examples cited there are many more that could be brought forward. It may be stated generally, then, that the intelligence of an animal depends principally upon the size of the brain in proportion to the size of its body, the size of the cerebrum, and also upon the number of convolutions and the complexity of its structure, although there are many exceptions to this rule which we are still unable to account for.

Another interesting point in connection with this subject is the great increase in the size of the brain that has taken place within the last few hundreds of years, without a corresponding increase in the size of the body in all animals. This very interesting fact we learn from fossil zoölogy. The brain of animals at the present day is much more developed than it was in former times. This may be owing to the struggle for existence which there is, the animals which are weaker in body and intellect gradually being extinguished by the stronger, so that only the latter remain and are allowed to propagate the species. We know that exercise and training strengthen the brain and increase its weight in man, so the probability is the same thing takes place among the lower animals. There is every likelihood, therefore, that the brain will still go on developing as time advances.—Land and Water.

  1. Henry Ward Beecher (Wikisource contributor note)