THE COLORS THAT ANIMALS CAN SEE

What colors can animals see? Is the world more brightly colored or duller to animals than it is to us? To find out the answers to these questions, scientists have used a method of training the animals to come to different colors, which is similar in principle to the method used in studying the sense of hearing in animals.

Let us take bees first of all, partly because more exact scientific research has been done on the color sense of bees than of almost any other animal. It is especially interesting to know what colors bees can see, because these insects visit flowers to get sweet nectar from them to make honey, and in so doing the bees incidentally carry pollen from flower to flower. On the fact of it, it would seem very likely that bees are attracted to flowers by their bright colors. But possibly it is the scents that attract the bees, or perhaps it is both color and scent. So, among other things, we want to know whether bees can really see the colors of flowers, and if so, what colors, and if so, what colors they can see. Exactly how is found out?

A table is put in a garden, and on the table a piece of blue cardboard is placed, on which there is a watch glass containing a drop of syrup. After a short while bees come to the syrup and suck up some of it. The bees then fly to their hive and give the syrup to other bees in the hive to make honey. Then they return to the feeding place which they have discovered. We let the bees go on doing this for a while, after we take away the blue cardboard with the syrup on it. Instead of this card we now put on the table a blue card on the left side of the first feeding place and a red card to the right of the first feeding place. These cards have no syrup on them but only an empty watch glass lying on each. Thus the blue card is on the left, the red card on the right, and there is nothing where the first blue feeding used to be. After we have arranged these new cards, we have not long to wait. Very soon bees arrive again, and it can be seen that they fly straight on to the blue card; none go to the red card.

This behavior of the bees seems to indicate two things. The first is that the bees remember that blue means syrup and so they fly to the blue. Since they did not go to the place on the table where the syrup used to be, but flew to the blue card which had been placed on the left, it really was the blue card that attracted them, not the place where the syrup had previously been. We have trained the bees to come to the blue card. And the second thing our experiment seems to mean is that bees can tell blue from red. But can they? This is not yet quite certain. The reason for our doubt is as follows. It is well known that there are a few people in the world, very few, who cannot see colors at all. These people are totally colorblind. To them all colors look like different shades of gray. They may be able to tell red from blue, because red will perhaps look darker and blue lighter in shade, but the colors are not red or blue. It might be, then, that bees are really colorblind, and that in the experiment they came to the blue card not because they saw it as blue, but just because it appeared lighter in shade than the red card. Perhaps they had really been trained to come not to blue, but to the lighter of two shades. We can find out quite simply if this is so by another training experiment.

On our table in the garden we put a blue card, and all around this blue card we put a number of different gray cards. These gray cards are of all possible shades of gray, from the extremes of white to black. On each card a watch glass is placed. The watch glass on the blue card has some syrup in it; all the others are empty. After a short time bees find the syrup as before, and they come for it again and again. Then, after some hours, we take away the watch glass of syrup which was on the blue card and put an empty one in its place. Now what do the bees do? They still go straight to the blue card, although there is no syrup there. They do not go to any of the gray cards, in spite of the fact that one of the gray cards is of exactly the same brightness as the blue card. Thus the bees do not mistake any shade of gray for blue. In this way we have proved that they really do see blue as a color.

We can find out in just the same way other colors bees can see. It turns out that bees see various colors, but these insects differ from us as regards their color sense in two very interesting ways. Suppose we train bees to come to a red card, and having done so we put the red card on the table in the garden among the set of different gray cards. This time we find that the bees mistake red for dark gray or black. They cannot distinguish between them. Thus it appears that red is not a color at all for bees; for them it is just dark gray or black. In reality, further experiments have shown that bees can see red as a color but only when it is very brilliantly illuminated: They are relatively insensitive to red.

That is one strange fact: here is the other. A rainbow is red on one edge, violet on the other. Outside the violet of the rainbow there is another color which we cannot see at all. The color beyond the violet, invisible to us, is called the ultraviolet. Although invisible, we know that the ultraviolet is there because it affects a photographic plate. Now, although we are unable to see ultraviolet light, bees can do so; for them ultraviolet is a color. Thus bees see a color which we cannot even imagine. This has been found out by training bees to come for syrup to various different parts of a spectrum, or artificial rainbow, throw by a quartz prism on a table in a dark room. In such an experiment the insects can be taught to fly to the ultraviolet, which for us is just darkness.

We will leave the bees now and turn to birds. Cocks have striking colors in their plumage - striking to us, at any rate - while hens possess only dull tints. But can hens see the colors of the cock as we can see them? Can the peahen, for instance, see the wonderful colors of the peacock? To answer this question we must know what colors a bird can actually see. This has been studied in the following manner. A lamp and prism are set up to throw a spectrum of rainbow colors on the floor of a dark room. On the different colors of the spectrum, grains of corn are sprinkled, and then a hen is brought in. She pecks at the grains of corn and gobbles up all she can see. After a time we remove the hen and take note of what grains are left untouched by her. We find that the hen has eaten nearly all the gains which were in the red, in the yellow, and in the green regions of the spectrum. We find that she has taken a few of the grains in the blue light, but the hen leaves the grains in the violet untouched. This means that she cannot see the grains which are in the violet light, and she is not able to see those in the blue very well either. For she did not pick up many of them. So, violet is just like black to the hen, and blue is not a very bright color.

This has been confirmed with homing pigeons on which colored spectacles were fitted; with red and yellow specs the birds flew home normally, but with green, and especially blue, they were unable to do so. A human being could see clearly through the blue celluloid of which the spectacles were made, but evidently blue is like a blackout to the bird, and it is well known that homing pigeons cannot find their way in dim light or darkness.

Other birds are like this, too, which seems strange at first, because some birds are themselves blue. The kingfisher, for instance, is blue. Are we to conclude that the kingfisher is unable to see the beautiful color of its mate? This does not follow; the kingfisher can probably see his mate’s blue plumage, for our experiments do not show that birds are unable to see blue at all. Birds just do not see this color very well; for them to see blue, the blue must be intense. And, indeed, the color of the kingfisher is very bright. Yet it is not all birds that have such difficulty in seeing blue; owls, on the contrary, are more sensitive than we ourselves to the blue end of the spectrum.

And what can dogs see? The answer to this question is disappointing: dogs apparently see no color. The answer is disappointing because many owners of dogs will naturally be sorry that their dogs cannot see colors which to them are beautiful. But then, they may reflect that dogs have an extraordinarily keen sense of smell. The dog’s world is rich in enjoyable smells, even if it may be colorless.

How do we know if dogs are colorblind? This has been tested in the same way that it has been discovered what dogs can hear. The attempt has been made to train dogs to salivate when they are shown certain different colors, just as they were trained so that their mouths watered when definite musical notes were sounded. Such experiments have turned out failures; it has been found impossible to make dogs distinguish colors from one another as signals for their dinner. This question requires further testing with other techniques, but so far as the available scientific evidence goes, dogs seem to be colorblind. Many dog owners will disagree with this, being convinced that their dogs know, for instance, the color of a dress. But the evidence given for this has never been sufficiently rigid for a scientist, who is not certain that the dog did not really respond to some other clue or sign than the color - to a smell, for instance, or to the particular behavior of the wearer of the dress.

Experiments have been made, too, to test the color sense of cats; although these experiments may not yet be conclusive, they have indicated, so far, that cats are colorblind. Different cats were trained to come for their food in response to signals of each of six different colors. But the cats always confused their particular color with one of a number of shades of gray, when these were offered at the same time as the color.

Monkeys, on the other hand, are able to distinguish colors. They have been trained successfully to go their meal to a cupboard, the door of which was painted in a certain color, and to ignore other available cupboards with differently colored doors, in which there was no food. Apart from monkeys and apes, however, most mammals seem to be colorblind, at any rate those which have been scientifically tested. Even bulls have been shown not to see red as a color. In spite of popular belief they are not excited by red, and they cannot distinguish red from dark gray. No doubt any bright waving cloth excites a high-spirited bull.

Color blindness in mammals, other than monkeys, is comprehensible when one considers the lives of the animals in a wild state. For nearly all wild mammals are nocturnal or crepuscular. Wolves and lions hunt mostly at night, while antelopes and wild cattle graze at night, or in the evening when colors are dim. But monkeys, in the forests where they live, are awake and about in the daytime, and there are abundant colors for them to see in the bright tropical light.

Moreover, the color blindness of mammals other than monkeys accords with the fact that the animals themselves are more or less dull-colored; their coats are brown or yellow, black or white. Only in monkeys are greens, bright reds, and blues found. These are colors which recall the brilliant tints of birds and of fish, animals which also possess color vision.

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