CHAPTER V

SOCIAL LIFE OF ANIMALS

1. Partnerships-2. Co-operation and Division of Labour-3. Gregarious Life and Combined Action—4. Beavers—5. Bees—6. Ants-7. Termites-8. Evolution of Social Life—9. Advantages of Social Life-10. A Note on the Social Organism—11. Conclusions

THE over-fed plant bears many leaves but its flowers are few; the animal which eats too much becomes fat; and we know that within the living body one part may grow out of proportion to the others. It seems as if organ competed with organ within the living engine, as if one tissue outgrew its neighbours in the living web, as if there were some struggle for existence between the individual units which form the city of cells in any of the higher animals. This idea of internal competition has been elaborated by a German biologist, Roux, in a work entitled The Struggle of Parts within the Organism, and it is full of suggestiveness. It can be verified from our own experience; but yet it seems strange. For we rightly think of an organism as a unity in which the parts are bound together in mutual helpfulness, being members one of another.

Now, just as a biologist would exaggerate greatly if he maintained that the struggle of parts was the most important fact about an organism, so would a naturalist if he maintained that there was in nature struggle only and no helpfulness.

Coherence and harmony and mutual helpfulness of parts — whether these be organs, tissues, or cells—are certainly facts in the life of individuals; we have now to see how far the same is true of the larger life in which the many are considered as one.

1. Partnerships.-Animals often live together in strange partnerships. The "beef-eater" birds (Buphagus) perch on cattle and extract grubs from the skin; a kind of plover (Pluvianus ægyptius) removes leeches and other parasites from the back of the crocodile, and perhaps "picks his teeth," as Herodotus alleged; the shark is attended by the pilot-fish (Naucrates ductor), who is shielded by the shark's reputation, and seems to remove parasites from his skin.

Especially among marine animals, we find many almost constant associations, the meaning of which is often obscure. Two gasteropods Rhizochilus and Magilus grow along with certain corals, some barnacles are common on whales, some sponges and polypes are always found together, without there being in any of these cases either parasitism or partnership. But when we find a little fish living contentedly inside a large sea-anemone, or the little pea-crab (Pinnotheres) within the horse-mussel, the probable explanation is that the fish and the crab are sheltered by their hosts and share their food. They are not known to do harm, while they derive much benefit. They illustrate one kind of "commensalism," or of eating at the same table.

But the association between crabs and sea-anemones affords a better illustration. One of the hermit-crabs of our coast (Pagurus prideauxii) has its borrowed shell always enveloped by a sea-anemone (Adamsia palliata), and Pagurus bernhardus may be similarly ensheathed by Adamsia rondeletii. Möbius describes two crabs from Mauritius which bear a sea-anemone on each claw, and in some other crabs a similar association occurs. It seems that in some cases the crab deliberately chooses its ally and plants it on its shell, and that it does not leave it behind at the period of shell-changing. Deprived of its polype companion, one was seen to be restlessly ill at ease until it obtained another of the same kind. The use of the sea

anemone as a mask to the crab-and also perhaps as aid in attack or defence-is obvious; on the other hand, the sea-anemone is carried about by the crab and may derive food from the crumbs of its bearer's repast.

Commensalism must be distinguished from parasitism, in which the one organism feeds upon its host, though it is quite possible that a commensal might degenerate into a parasite. Quite distinct also is that intimate partnership known as symbiosis, illustrated by the union of algoid and fungoid elements to form a lichen, or by the occurrence of minute Algæ as constant internal associates and helpful partners of Radiolarians and some Cœlenterates.

2. Co-operation and Division of Labour. -The idea of division of labour has been for a long time familiar to men, but its biological importance was first satisfactorily recognised by Milne-Edwards in 1827.

Among the Stinging-animals there are many animal colonies, aggregates of individuals, with a common life. These begin from a single individual and are formed by prolific budding, as a hive is formed by the prolific egglaying of a queen-bee. The mode of reproduction is asexual in the one case, sexual in the other; the resulting individuals are physically united in the one case, psychically united in the other; but these differences are not so great as they may at first sight appear. Many masses of coral are animal colonies, but among the members or "persons," as they are technically called, division of labour is very rare ; moreover, in the growth of coral the younger individuals often smother the older. In colonial zoophytes the arborescent mode of growth usually obviates crushing; and there is sometimes very marked division of labour. Thus in the colony of Hydractinia polypes, which is often found growing on the shells tenanted by hermit-crabs, there may be a hundred or more individuals all in organic connection. The polypes are minute tubular animals, connected at their bases, and stretching out from the surface of the shell into the still water of the pool in which the hermit-crab is resting. But among the hundred individuals there are three or four castes, the differences between which probably

result from the fact that in such a large colony perfect uniformity of nutritive and other conditions is impossible. Individuals which are fundamentally and originally like one another grow to be different, and perform different functions according to the caste to which they belong.

Many are nutritive in form like the little freshwater Hydra-tubular animals with an extensile body and with a terminal mouth wreathed round by mobile tentacles. On

these the whole nutrition of the colony depends. Beside these there are reproductive "persons," which cannot feed, being mouthless, but secure the continuance of the species and give rise to embryos which start new colonies. Then there are long, lank, sensitive members, also mouthless, which serve as the sense-organs of the colony, and are of use in detecting food or danger. When danger threatens, the polypes cower down, and there are left projecting small hard spines, which some regard as a fourth class of individuals-starved, abortive members like the thorns on the hawthorn hedge. In recognising their utility to the colony as a whole we can hardly overlook the fact that their life as individuals is practically nil. They well illustrate the dark side of division of labour.

FIG. 14.-Colony of Hydractinia echinata. a, nutritive individuals; b, reproductive individuals; abortive spines; and there are

also long mouthless individuals

specialised in sensitiveness. (From

Chambers's Encyclop.; after Allman).

Herbert Spencer and Ernst Haeckel have explained very clearly one law of progress among those animals which form colonies. The crude form of a colony is an aggregate of similar individuals, the perfected colony is an integrate in which by division of labour greater harmony of life has resulted, and in which the whole colony is more thoroughly compacted into a unity. Among the Stinging

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animals, we find some precise illustrations of such integrated colonies, especially in the Siphonophora of which the Portuguese Man-of-War (Physalia) is a good example. There is no doubt that these beautiful organisms are colonies of individuals, which in structure are all referable to a "medusoid" or jellyfish-like type. But the division of labour is so harmonious, and the compacting or organisation of the colony is so thorough, that the whole moves and lives as a single organism.

E. Perrier in his work entitled Les Colonies Animales (Paris, 1882), shows how organic association may lead from one grade of organisation and individuality to another, and explains very clearly how sedentary and passive life tends to develop mere aggregates, while free and active life tends to integrate the colony. With this may be compared A. Lang's interesting study on the influence of sedentary life and its connection with asexual reproduction-Das Einfluss des Festsitzen (Jena, 1889). Haeckel, in his Generelle Morphologie (2 vols., Berlin, 1866), was one of the first to shed a strong clear light on the difficult subject of organic individuality, its grades and its progressive complexity. To Spencer, Principles of Biology (2 vols., London, 1863-67), we owe in this connection the elucidation of the transition from aggregates to integrates, and of the lines of differentiation, i.e. the progressive complication of structure which is associated with division of labour.

3. Gregarious Life and Combined Action.- Most mammals are in some degree gregarious. The solitary kinds are in a distinct minority. The isolated are exposed to attack, the associated are saved by the wisdom of their wisest members and by that strength which union gives. Many hoofed animals, such as deer, antelopes, goats, and elephants, live in herds, which are not mere crowds, but organised bands, with definite conventions and with a power of combined resistance which often enables them to withstand the attacks of carnivores. Marmots and prairie-dogs, whose "cities" may cover vast areas, live peaceful and prosperous lives. Monkeys furnish many illustrations of successful gregarious life. As individuals most of them are comparatively defenceless, and usually avoid coming to close quarters with their adversaries; yet in a body they are formidable, and often help one another out of scrapes. Brehm tells how he encountered a troop of baboons which defied his dogs and retreated in good order up the