Collins New Naturalist Library. L. Matthews Harrison

Чтение книги онлайн.

СКАЧАТЬ 14 of which are extinct; 257 families, of which 139 or 54 per cent are extinct; and 2,864 genera, of which 1,932 or 67 per cent are extinct. Thus the living species represent only a small fraction of the total that have lived since the class Mammalia evolved. The diversity and number of species in all the orders except perhaps the rodents reached their peak in the Miocene or Pliocene epochs – some twenty-five to three million years ago – since when they have declined to their present level.

      In the British Isles, excluding the whales, we have mammals representing nine of the nineteen orders now living, but of these only seven are indigenous, the two others are either introduced and naturalised, or are derived from stocks of domestic animals. Our fauna is therefore no more than a small sample of the mammalian diversity that ornaments the fauna of the world. It has, moreover, not contained a larger number of orders since the land became generally habitable by warm-blooded animals at the end of the great glaciation of the Pleistocene epoch, about half a million years ago. In the preceeding epochs, before the ice ages, several other orders were represented by species of mammal that became extinct long before the present pattern of the fauna evolved.

      Before we discuss the problematic origin of the present fauna we should enumerate and specify the species about which we shall be speaking. In following Simpson’s arrangement of the orders the indigenous species are necessarily not separated from those that have been introduced or are extinct.

      ORDER MARSUPIALIA

      The marsupials differ so fundamentally from the other mammals that they are placed in a separate Infraclass, the Metatheria, whereas the other mammals of our fauna are included in the Infraclass Eutheria. The marsupials show many unique anatomical characters, but are popularly known merely by a single one, as the mammals that carry their young in a pouch. This is not universally true, for some of them are pouchless; but in all of them the young are born at a comparatively early stage of development and thus need to be carried attached to the mother’s nipples. The marsupials are typically the mammals of the Australasian region, but in addition many species live in South America and one, the Virginian opossum, extends into North America. The living species are divided into eight families of which one, containing the kangaroos and wallabies, is represented in our fauna by a single introduced species.

       Family Macropodidae

      Macropus rufogriseus, the red-necked wallaby or Bennett’s wallaby, is a medium sized kangaroo-like animal weighing up to about 30 pounds, sometimes nearly 50 pounds. It is native to south-eastern Australia and Tasmania; but as it is easily kept in captivity it is commonly exhibited in zoos and parks of many lands, whence it sometimes escapes. Small feral populations of that origin have become established in Sussex and Derbyshire; smaller colonies deliberately introduced on Herm in the Channel Islands, and on Lambay Island off the coast of Co. Dublin in Eire have died out. Although the English populations have been established for over thirty years they remain small because wallabies are liable to suffer heavy mortality in severe winters.

      ORDER INSECTIVORA

      The insectivores are mostly small mammals characterised by many primitive or generalised mammalian characters. They are considered to be descended with least change from the ancestral stock of the mammals, though all living species have various specialised adaptations. Insectivores live in most parts of the world except Australasia and South America; they include the tenrecs, hedgehogs, moles, desmans, and shrews.

       Family Erinaceidae

      Erinaceus europaeus, the hedgehog, our only mammal with prickles in its skin, is the largest of our insectivores. It is present throughout the mainland of Great Britain and Ireland and is common in lowland areas, particularly in the suburbs of towns. It is also found in many of the islands, but has probably been introduced into most of them by man. It lives in woods and hedgerows, coming into the open to feed as night falls. The hedgehog and the dormouse are our only mammals besides the bats that hibernate in winter.

       Family Talpidae

      Talpa europaea, the mole, is our only mammal that spends nearly all of its life underground. Its cylindrical body, some five to five and a half inches in length, is covered with black velvety fur. The fore limbs and their muscles are highly adapted for tunnelling in the earth, and the strongly clawed hands are broadened internally by an extra bone, the radial sesamoid. The hind feet are similarly but less conspicuously reinforced by an accessory sesamoid bone. The eyes are minute and hidden by the fur, and there is not an ear pinna, but the long snout is plentifully supplied with special touch organs. The sites of mole burrows are shown by the conspicuous heaps of earth pushed up to the surface as the mole digs its underground tunnels, in which it spends most of its life feeding mainly on earthworms which fall into the burrows. Moles are present throughout Great Britain but not Ireland or the Isle of Man; they are not found on many of the smaller islands except Anglesey and the Isle of Wight, Alderney and Jersey.

       Family Soricidae

      The shrews are small mouse-like animals with velvety fur, long pointed snouts, small eyes and ears. They are insectivorous and carnivorous, generally seeking their food under thick vegetation and litter below which they have runways; some species dig burrows. The first incisor teeth are large and project forwards, acting like forceps in picking up small food objects; the back teeth bear sharp pointed cusps. Shrews need comparatively large quantities of food, and are consequently active by day and night, alternating short periods of activity and rest throughout the twenty-four hours. They soon die of starvation if denied food for a few hours.

      The genus Sorex contains two British species, the common shrew, S. araneus, and the pygmy shrew S. minutus, the latter being our smallest British mammal. The enamel of the tips of the teeth is red in both species, which are distinguished by size, relative length of tail which is longer in the pygmy shrew, and colour of the fur, darker in S. araneus but lighter in S. minutus. The common shrew is found throughout England, Scotland and Wales and on many of the islands, but is absent from Ireland, Orkney, Shetland, the outer Hebrides and Man. The pygmy shrew, although less abundant, is found throughout the whole of the British Isles except Shetland, the Scilly and Channel Islands. The common shrew is peculiar in showing chromosome polymorphism, that is, the number and form of the chromosomes differs in animals from different parts of the country.⁶² Both species are annuals: young born in one summer breed in the next and die in the following autumn, so the winter population consists entirely of immature animals, and none normally lives through a second winter.

      The water shrew, N. fodiens, is the only British species of the genus Neomys, and is easily distinguished by its larger size and the black colour of the fur on the upper parts; it too has red-tipped teeth. Although it is aquatic and has fringes of stiff hairs on feet and tail that aid in swimming, it is nevertheless often found at considerable distances from water in woods and hedgerows in similar places to those inhabited by other shrews. It lives in burrows in the banks of clear streams and ponds; when it enters the water the fur traps air so that it appears silvery. The fur nevertheless soon becomes wet and is dried on landing by squeegeeing through the tight fitting burrow. Its food consists of invertebrates and even creatures as large as itself such as frogs and small fish. It is found throughout the mainland but is absent from Ireland, Isle of Man, and the western and northern islands of Scotland. The water shrew is unique among British mammals in being the only one with a poisonous bite, because the submaxillary salivary glands contain a venom that paralyses small prey.

      The other two species of British shrew belong to the genus Crocidura, at once distinguished from the rest by their white teeth. Their ears are larger than in the others, and the tails bear a number of long scattered hairs. They are found only in the Scilly and Channel Islands, where they live in habitats similar to those of the common and pygmy shrews. The lesser white-toothed shrew, C. suaveolens, is found on most of the Scilly Islands, Jersey and Sark; it was probably unintentionally СКАЧАТЬ