Many animals are able to take note of movements so small that they are hardly or not at all observable by men. The so-called "thinking" and "calculating" horses and dogs owe their fame to this ability. Naturally they really know neither how to calculate nor how to read. Often unknown to their owners they have trained themselves to very slight signs given by the trainer on the accomplishment of the right solution of the task, without his own knowledge. The animals have learnt by experience that they receive a tasty morsel when they cease to tap with paw or hoof as soon as the sign in question appears. Their master, however, thinks he is rewarding them for having correctly solved their problem. But the real signs consist of very slight movements of the body, of the head or even only of the features of the questioner. These signs are the expression of the conclusion of a state of tension on the part of the questioner, which occurs as soon as the correct number of beats has been reached.
The highest type of action is represented by intelligent action, that is, conduct which depends upon an insight into the connection of things and events and upon their causal relationships. In telligent actions, above all, distinguish men from animals. For merly it was said that animals act on instinct, man on reason. And in truth this insight is lacking in most animals. It is only recently that it has been shown that in a small group of animals there is a mode of conduct which recalls reasoned actions of men and is certainly a precursor of such. These animals are the anthro poid apes, man's nearest relatives. From observations on anthro poids in zoological gardens the conclusion had long been reached that as regards their conduct these creatures are to be placed far above all other animals. The way they handle objects at their disposition, and employ the most diverse means of attaining their end, whether it be a fruit or other desired object, makes their whole conduct appear highly human. Our knowledge has been very considerably advanced by the carefully planned experiments made by W. Kohler (1917), with chimpanzees in the German Anthropoid Station on Teneriffe, under conditions which were as natural as possible and in a climate similar to that of their home. The work of Yerkes, S. J. Holmes and Miss Kohts is also of great value.
Kohler's experiments were of quite a simple nature. They resembled those now employed with quite small children. For the experimental animal it was always a question of attaining some object, such as a fruit, which could not be reached directly by grasping it with the hand. A banana with a string attached to it, which lay out of reach of the arm, was at once pulled in by the chimpanzees with the aid of the string. Hungry dogs, cats or horses in a similar situation would simply have starved. Ex periments have demonstrated that dogs will not of their own accord drag meat from a table, even if they have been trained to gain possession of the meat on a word of command by pulling at a string attached to the meat and hanging down from the table.
In other words, although they are physically capable of accom plishing the task, it is too difficult for them. If a loose fruit lay out of reach of the chimpanzees, in front of the bars of their cage, they soon succeeded in pulling it in with the aid of some object such as a stick, a wire, a loop of cord or a stone. Thus they interposed between themselves and the thing desired a material intermediary object which was not a portion of their own body. We can therefore speak here of the use of tools. Probably something similar was involved when the animals fetched pack ing cases which were placed at their disposal and climbed on to them in order to reach fruit hanging from the roof of the room in which they were confined. Or again when, in order to attain the same object, they placed a bamboo stem vertically, quickly climbed up it and seized the banana whilst jumping off. One of the apes used pieces of bamboo four metres long for this pur pose. The chimpanzees accustomed themselves to use the sticks for all possible other purposes, such as for levers to force open packing cases, tools to dig up roots, for touching a mouse or lizard held out to them, and finally even as a weapon, although not for hitting but for throwing. The animals learnt all this on their own account without its having been shown to them by men. A curious use of straws became the fashion with them for a time. They used these for catching ants, which they waylaid and licked up whenever they could gain possession of them, doubtless on account of the taste of formic acid in the insects. In the present instance an ant pathway led over the outside of a cross-beam of the wire-netting enclosure. Kohler describes how first one of the chimpanzees, then another, and finally the whole company, held out straws and spikes through the netting on the beam so that in a short time the straws were covered with ants. Then the prey was quickly drawn in and licked off in the apes' mouths. All the animals in the station could be seen squat ting next to one another along the ant pathway, each with a straw, looking like a row of anglers along a stream. Such a use of tools, which is evidently intentional, is found in no other animals. It is true that certain sand-wasps pound the sand at the entrance to their nests with a small stone held between the jaws. This, however, is a purely instinctive action. The blood-red weaver ants, Oecophylla smaragdinea, found especially in Ceylon, stick together the leaves out of which they form their nests with spun thread. In the preparation of this they use their larvae, somewhat like living distaffs. The worker ants take hold of the larvae with their jaws and force them by a gentle pressure to extrude the secretion from the large spinning glands. In so doing they move the larvae to and fro, pressing them now against one now against the other of the two leaves which are gradually being stuck together, so that eventually a dense and strong fabric results. This behaviour, too, in contrast to the real use of tools, is an instinctive action.