The Fierasfer, a little fish of the Mediterranean, installs himself in the respiratory cavity of a Holothurian; he does not live at the expense of his host’s flesh, but contents himself with levying a tax on the foods which enter the cavity. It is a case of commensalism of which there are very numerous examples. Other cases may be mentioned which are still further removed from parasitism. Among these may be mentioned the birds who relieve large mammals of their vermin.
One of them, the Red-beaked Buffalo bird (Buphaga erythrorhyncha), lives in Abyssinia. This bird is insectivorous. He has remarked that the ruminants constitute baits for flies; therefore he never leaves these animals, hops about on their backs and delivers them from annoying parasites; the buffaloes, who recognise this service, allow the bird to wander quietly over their hide. The Buphaga, who gives himself up entirely to this kind of chase, is often called the Beef-eater. He is only found in the society of flocks, of camels, buffaloes, or oxen. He settles on the back, legs, and snouts of these living baits. They remain passive even when he opens the skin in order to draw out the flies’ larva; they know the benefit of this little operation. The patience of the oxen is certainly due to custom, for it is observed that herds which are not used to this bird manifest great terror when he prepares to alight on them, so that they even take flight from this small aggressor.
Sometimes it is not easy to understand the advantages derived by the animal from the conditions in which he is usually found. Thus, for example, there is a fish, the Polyprion cernium, which accompanies driftwood on which Barnacles have fixed themselves. Yet the remains of these Crustaceans are never found in his stomach, and it is known on the contrary that he lives exclusively on other small fish. It is possible that these find their food in fragments of wood at the expense of the barnacles, and that therefore the Polyprion which hunts them is always near driftwood thus garnished.
Methods of utilising the captured game. — Frequently it is not enough for the animal to obtain possession of his prey. Before making his meal it is still necessary to find a method of making use of it, either because the eatable parts are buried in a thick shell which he is unable to break, or because he has captured a creature which rolls itself into a ball and bristles its plumes. Here are some of the more curious practices followed in such cases.
Sometimes it is a question of carrying off a round fruit which offers no prominence to take hold of. The Red-headed Melanerpes (Melanerpes erythrocephalus) of North America is very greedy with regard to apples, and feeds on them as well as on cherries. It takes him a considerable time to consume an apple, and as he is well aware of the danger he runs by prolonging his stay in an orchard, he wishes to carry away his booty to a safe and sheltered spot. He vigorously plunges his open beak into the apple; the two mandibles enter separately, and the fruit is well fixed; he detaches it and flies away to the chosen retreat. Apes are very skilful in utilising their booty. Cocoa-nuts are rather hard to open, but Apes do not lose any part of them; they first tear off the fibrous envelope with their teeth, then they enlarge the natural holes with their fingers, and drink the milk. Finally, in order to reach the kernel they strike the nut on some hard object exactly as Man would do. The Baboons (Cynocephali), whose courage is prodigious, since they will fight in a band against a pack of dogs or even against a leopard, are also very prudent and very skilful. They know that courage is no use against the sting of a venomous snake, and that the best thing is to avoid being bitten. The scorpion, whose dart is perfidious, also inspires their distrust, but as they like eating him they endeavour to catch him. This is not indeed very difficult if one carefully observes his movements, and it is possible to seize him suddenly by the tail, as I have often done, without being stung. Apes employ this method, pull out his sting, and crunch the now inoffensive Arachnid. They also like ants, but fear being bitten by them; when they wish to enjoy them, they place an open hand on an ant-hill and remain motionless until it is covered by insects. They can then absorb them at one stroke without fear.
One would not think that an animal so well defended as the Hedgehog need fear becoming the prey of the Fox. Rolled in a ball, bristling with hard prickles which cruelly wound an assailant’s mouth, nothing will induce him to unroll so long as he supposes the enemy still in the neighbourhood. It is vain to strike him or to rub him on the earth; he remains on the armed defensive. Only one circumstance disturbs him to the point of making him quit his prudent posture; it is to feel himself in the water, or even simply to be moist. The fox is acquainted with this weakness, therefore as soon as he has captured a hedgehog he rolls him in the nearest marsh to strangle him as soon as his head appears. It may happen that there is no puddle in the neighbourhood suitable for this bath; it is said that in this case the fox is not embarrassed for so small a matter, and provides from his own body the wherewithal to moisten the hedgehog.
The combination is complicated, and approaches more nearly the methods employed by Man when the animal makes use of a foreign body, as a tool or as a fulcrum, to achieve his objects. A snake is very embarrassed when he has swallowed an entire egg with the shell; he cannot digest it in that condition, and the muscles of his stomach are not strong enough to break it. The snake often finds himself in this condition, and is then accustomed either to strike his body against hard objects or to coil himself around them until he has broken the envelope of the eggs he contains.
The Snake himself is treated in this way in South America. The Sulphur Tyrant-bird picks up a young snake by the tail, and, flying to a branch or stone, uses it like a flail until its life is battered out.[27]
It would be a paradox to attribute great intelligence to Batrachians; yet certain facts are recorded which show them to be capable of reflection. Among others the case is quoted of a green frog who obtained possession of a small red frog, and who proposed to swallow him. The other was naturally opposed to the realisation of this scheme and struggled with energy. Seeing that he would not succeed, the green frog went towards the trunk of a tree and, still holding his victim, struck him many times vigorously against it. At last the red frog was stunned, and could then be swallowed at leisure.
Gasteropods are not always protected by their calcareous shells any more than tortoises are by their carapaces; for certain birds know very well how to break them. Ravens drop snails from a height, and thus get possession of the contents of the shell.