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CHAPTER XVIII

MUTUAL AID AND COMMUNAL LIFE AMONG

ANIMALS

More ancient than competition is combination. The little feeble fluttering folk of God, the spinning insects, the little mice in the meadow, the rat in the cellar, the crane in the marshes or the booming bittern, all these have learned that God's greatest word is together and not alone. He who is striving to make God's blessing and bounty possible to most is stepping into line with nature. The selfish man is the isolated man.-OSCAR CARLTON McCULLOCH.

Man is not the only social animal, nor the only animal species whose individuals live in mutually advantageous relations with each other, and in mutually advantageous relations. with individuals of other animal kinds. Just as man lives communally and mutually helpfully with other men, so do the members of a great honeybee or ant community live together: and as we find various other animals as dogs, horses, and doves living under the care and protection of man and returning to him a measure of service in work, companionship, or other helpfulness for his care and feeding, so do we know of hundreds of kinds of other insects that live commensally with ants, each party to this commensal or symbiotic life gaining something from and giving something to the other party of this arrangement. Indeed, the communal life of such insects as the social bees, wasps, and ants is developed along true communistic lines far more specialized than the communism shown by man.

Just as students of human society can trace a series of steps from a very primitive living together or communal life among men to the present highly specialized condition, so among various animals we can find a long series of gradatory conditions of social life from mere gregariousness like that of a band of wolves

or a herd of bison, to the extremely specialized, interdependent and unified community of the honeybee, or agricultural ant. Before taking up this series of stages in true social or communal development among the lower animals, however, we may profitably give some attention to the conditions of animal association commonly known as commensalism or symbiosis in which individuals of one species are associated to their mutual advantage with individuals of different species.

In the relations of parasite and host, discussed in the last chapter, all the advantages of the association lie with the parasite. The other animal involved, the host, suffers inconvenience, injury, often untimely death. But in commensalism and symbiosis both associating kinds of animals reap advantage, or

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Fig. 227.-Remora Echeneis remora, with dorsal fin modified to be the sucking plate

by which the fish attaches itself to a shark.

at least neither suffers in any serious way from the effects of the other's presence. The two kinds live together in harmony and usually to their actual mutual advantage. The term commensalism may be applied to denote a condition of loose and often not obviously equally mutual advantageous association, while symbiosis is used to refer to a more intimate and persistent association with maybe marked coöperation and mutual advantage. A few examples of each are given in the following pages. Of course, no marked line of demarcation can be really drawn between the two conditions, any more than we can establish a sharp distinction between the predatory and parasitic modes of life.

A curious example of commensalism is afforded by the different species of Remoras (Echenididæ) which attach themselves to sharks, barracudas, and other large fishes by means of a sucking disk on the top of the head (Fig. 227). This disk is made by a modification of the dorsal fin. The Remora thus attached to a shark may be carried about for weeks, leaving its host only to secure food. This is done by a sudden dash

through the water. The Remora injures the shark in no way save, perhaps, by the slight check its presence gives to the shark's speed in swimming. In the mouth of the menhaden (Brevoortia tyrannus) a small crustacean (Cymotha pragustator) is almost always present, always resting in the front of the lower jaw. This arrangement is of advantage to the crustacean, but is a matter of indifference to the fish. Latrobe, who first described this fish, compares the crustacean to the prægustator or foretaster of the Roman tyrants—a slave used in prevention of poisoning. Whales, similarly, often carry barnacles about with them. These barnacles are permanently attached to the skin of the whale just as they would be to a stone or wooden pile. Many small crustaceans, annelids, mollusks, and other invertebrates burrow into the substance of living sponges, not for the purpose of feeding on them, but for shelter. On the other hand, the little boring sponge (Clioma) burrows in the shells of oysters and other bivalves for protection. These are hardly true cases of even that lesser degree of mutually advantageous association which we are calling commensalism. But some species of sponge “are never found growing except on the backs or legs of certain crabs.” In these cases the sponge, with its many plant like branches, protects the crab by concealing it from its enemies, while the sponge is benefited by being carried about by the crab to new food supplies. Certain sponges and polyps are always found growing in close association, though what the mutual advantage of this association is has not yet been found out. Among the coral reefs in the South Seas there lives an enormous kind of sea anemone or polyp. Individuals of this great polyp measure two feet across the disk when fully expanded. In the interior, the stomach cavity, which communicates freely with the outside by means of the large mouth opening at the free end of the polyp, there may often be found a small fish (Amphiprion percula). That this fish is purposely in the gastral cavity of the polyp is proved by the fact that when it is dislodged it invariably returns to its singular lodging place. The fish is brightly colored, being of a brilliant vermilion hue with three broad white cross bands. The discoverer of this peculiar habit suggests that there are mutual benefits to fish and polyp from this habit. “The fish being conspicuous,

is liable to attacks, which it escapes by a rapid retreat into the sea anemone; its enemies in hot pursuit blunder against the outspread tentacles of the anemone and are at once narcotized by the 'thread cells' shot out in innumerable showers from

the tentacles, and afterwards drawn into the stomach of the anemone and digested.”

Small fish of the genus Nomeus may often be found accompanying the beautiful Portuguese man-of-war (Physalia) as it sails slowly about on the ocean's surface (Fig. 228). These little fish lurk underneath the float and among the various hanging threadlike parts of the Physalia, which are provided with stinging cells. The fish are protected from their enemies by their proximity to these stinging threads. Similarly, several kinds of medusæ are known to harbor or to be accompanied by the young or small adult fishes (Caranx, Psenes).

In the nests of the various Jarywelen

species of ants and termites many different kinds of other insects have been found. Some

of these are harmful to their I'16. 229. --The Portuguese man-of- hosts, in that they feed on the war, Physalia, with men-of-war food stores

food stores gathered by the infishes, Nomeus gronovii, living in the shelter of the stinging feelers. dustrious and provident ant, but (Specimens from off Tampa, Fla.) others appear to feed only on

refuse or useless substances in the nest. Some appear to be of help to their hosts by cleaning the nests and by secreting certain fluids much liked by the ants. Over one thousand species of these myrmecophilous (ant-loving) and termitophilous (termite-loving) insects have been recorded by collectors as living habitually in the nests of ants and termites. Many of them (they are mostly small

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beetles and flies) have lost their wings and have had their bodies otherwise considerably modified, usually in such wise that they come greatly to resemble in external appearance the ants with which they live. The owls and rattiesnakes which live with the prairie dogs in their villages afford another familiar example of commensalism. Of a more intimate character, and of more obvious and certain mutual advantage, is the well-known case of the symbiotic association of some of the numerous species of hermit crabs and certain species of sea anemones. The hermit crab always takes for its habitation the shell of another animal, often that of the common whelk. All of the hind part of the crab lies inside the shell, while its head with its great claws project from

the opening of the FIG. 229.-Hermit crab within a shell on which is growing shell. On the sur- a colony of Podocoryne carnea. This colony is composed

of several different kinds of polyp individuals, the stingface of the shell ing ones being situated along the front margin of the near the opening shell. (After Weismann.)

there is often to

be found a sea anemone, or sea rose (Fig. 229). This sea anemone is fastened securely to the shell, and has its mouth opening and tentacles near the head of the crab. The sea anemone is carried from place to place by the hermit crab, and in this way is much aided in obtaining food. On the other hand, the crab is protected from its enemies by the well-armed and dangerous tentacles of the sea anemone. In the tentacles there are many thousand long, slender stinging threads, and the fish or octopus that would obtain the hermit crab for food must first deal with the stinging anemone. There is no doubt here of the mutual advantage gained by these two widely different but intimately associated companions. If the sea anemone be torn away from the shell inhabited by one of these crabs, the crab will wander about,

carefully seeking for another anemone. When it finds it, it

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