Eyes and noses are important keys to the bright colours of birds and comparative sombreness of hairy-coated creatures. Take a dog and an oriole as good examples of the two extremes. When a dog has lost his master, he first looks about; then he strains his eyes with the intense look of a near-sighted person, and after a few moments of this he usually yelps with disappointment, drops his nose to the ground, and with unfailing accuracy follows the track of his master. When the freshness of the trail tells him that he is near its end he again resorts to his eyes, and is soon near enough to recognise the face he seeks. A fox when running before a hound may double back, and make a close reconnaissance near his trail, sometimes passing in full view without the hound’s seeing him or stopping in following out the full curve of the trail, so completely does the wonderful power of smell absorb the entire attention of the dog.
Let us now turn to the oriole. As we might infer, the nostrils incased in horn render the sense of smell of but slight account. It is hard to tell how much a bird can distinguish in this way—probably only the odour of food near at hand. However, when we examine the eye of our bird, we see a sense organ of a very high order. Bright, intelligent, full-circled, of great size compared to the bulk of the skull, protected by three complete eyelids; we realise that this must play an important part in the life of the bird. There are, of course, many exceptions to such a generalisation as this. For instance, many species of sparrows are dull-coloured. We must remember that the voice—the calls and songs of birds—is developed to a high degree, and in many instances renders bright colouring needless in attracting a mate or in locating a young bird.
As we have seen, the sense of smell is very highly developed among four-footed animals, but to make this efficient there must be something for it to act upon; and in this connection we find some interesting facts of which, outside of scientific books, little has been written. On the entire body, birds have only one gland—the oil gland above the base of the tail, which supplies an unctuous dressing for the feathers. Birds, therefore, have not the power of perspiring, but compensate for this by very rapid breathing. On the contrary, four-footed animals have glands on many portions of the body. Nature is seldom contented with the one primary function which an organ or tissue performs, but adjusts and adapts it to others in many ingenious ways. Hence, when an animal perspires, the pores of the skin allow the contained moisture to escape and moisten the surface of the body; but in addition to this, in many animals, collections of these pores in the shape of large glands secrete various odours which serve important uses. In the skunk such a gland is a practically perfect protection against attacks from his enemies. He never hurries and seems not to know what fear is—a single wave of his conspicuous danger signal is sufficient to clear his path.
In certain species of the rhinoceros there are large glands in the foot. These animals live among grass and herbage which they brush against as they walk, and thus “blaze” a plain trail for the mate or young to follow. There are few if any animals which care to face a rhinoceros, so the scent is incidentally useful to other creatures as a warning.
It is believed that the hard callosities on the legs of horses are the remains of glands which were once upon a time useful to their owners; and it is said that if a paring from one of these hard, horny structures be held to the nose of a horse, he will follow it about, hinting, perhaps, that in former days the scent from the gland was an instinctive guide which kept members of the herd together.
“Civet,” which is obtained from the civet cat, and “musk,” from the queer little hornless musk deer, are secretions of glands. It has been suggested that the defenceless musk deer escapes many of its enemies by the similarity of its secretion to the musky odour of crocodiles. In many animals which live together in herds, such as the antelope and deer, and which have neither bright colours nor far-reaching calls to aid straying members to regain the flock, there are large and active scent glands. The next time you see a live antelope in a zoological park, or even a stuffed specimen, look closely at the head, and between the eye and the nostril a large opening will be seen on each, side, which, in the living animal, closes now and then, a flap of skin shutting it tight.
Among pigs the fierce peccary is a very social animal, going in large packs; and on the back of each of these creatures is found a large gland from which a clear watery fluid is secreted. Dogs and wolves also have their odour-secreting glands on the back, and the “wolf-pack” is proverbial.
The gland of the elephant is on the temple, and secretes only when the animal is in a dangerous mood, a hint, therefore, of opposite significance to that of the herding animals, as this says, “Let me alone! stay away!” Certain low species of monkeys, the lemurs, have a remarkable bare patch on the forearm, which covers a gland serving some use.
If we marvel at the keenness of scent among animals, how incredible seems the similar sense in insects—similar in function, however different the medium of structure may be. Think of the scent from a female moth, so delicate that we cannot distinguish it, attracting a male of the same species from a distance of a mile or more. Entomologists sometimes confine a live female moth or other insect in a small wire cage and hang it outdoors in the evening, and in a short time reap a harvest of gay-winged suitors which often come in scores, instinctively following up the trail of the delicate, diffused odour. It is surely true that the greatest wonders are not always associated with mere bulk.