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William Collins

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First published 1974

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Ebook Edition © FEBRUARY 2019 ISBN: 9780007406463

Version: 2019-02-13

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EDITORS:

Margaret Davies C.B.E. M.A. Ph.D.

Sir Julian Huxley F.R.S. M.A. D.Sc.

John Gilmour M.A. V.M.H.

Kenneth Mellanby C.B.E. Sc.D.

PHOTOGRAPHIC EDITOR:

Eric Hosking F.R.P.S.

The aim of this series is to interest the general reader in the wild life of Britain by recapturing the inquiring spirit of the old naturalists. The Editors believe that the natural pride of the British public in the native fauna and flora, to which must be added concern for their conservation, is best fostered by maintaining a high standard of accuracy combined with clarity of exposition in presenting the results of modern scientific research.

CONTENTS

COVER

TITLE PAGE

COPYRIGHT

NOTE TO READERS

PLATES

FIGURES

EDITORS’ PREFACE

AUTHOR’S PREFACE

1. The Pinnipedia

2. The grey seal – introductory

3. The grey seal – age determination, longevity and maturity

4. The grey seal – reproduction

5. The grey seal – annual moult and spring behaviour

6. The grey seal – feeding and food; pre-breeding assemblages

7. The grey seal – the breeding rookery: cow and pup

8. The grey seal – the breeding rookery: bulls, territory and mating

9. The grey seal – moulters, dispersal, marking experiments and later movements

10. The grey seal – population dynamics, mortality and pregnancy rates

11. The common seal – introductory

12. The common seal – reproduction, pup, moulting, food and numbers

13. Other species of pinnipede

14. Conservation of British seals

PICTURE SECTION

APPENDICES: A Classification of Pinnipedia

B Breeding rookeries of grey seals

C Life-tables for grey seal cows and bulls

D Farne Island seal counts, 1956–1968

E Research and world conservation of seals

FOOTNOTES

BIBLIOGRAPHY

INDEX

ACKNOWLEDGEMENTS

ABOUT THE PUBLISHER

PLATES

1. Fur-seals fanning themselves (C. Bertram)

Head of grey seal

Head of Californian sea-lion

2. Porth Laoug, Ramsey Island, Pembrokeshire

Beach on Eilean nan Ron, Oronsay, Inner Hebrides

3. Shell-sand beach and herd of grey seals, Shillay, Outer Hebrides

Breeding rookery in wallows, Shillay, Outer Hebrides (R. Atkinson)

4. Breeding rookery in central Fianuis, North Rona (J. M. Boyd)

Ring-marking a moulter, North Rona (J. M. Boyd)

5. Shore rookery, Muckle Greenholm, Orkney (J. Woolston)

Breeding rookery, Staple Island (and Brownsman), Farne Islands (J. C. Coulson)

6. Aerial view of Gasker, Outer Hebrides (W. St. C. Reid in Boyd, J. M. 1957)

Bull and cow grey seals at wallow, North Rona

7. Territorial grey seal bull, Brownsman, Farne Islands

Breeding grey seal cow, Brownsman, Farne Islands

8. Longitudinal section of root of canine tooth of grey seal

Two canine teeth from 1st year grey seals

Two adult canine teeth (bull and cow) grey seals

Cement layers in tooth of 8-year-old grey seal

Cement layers in tooth of 18-year-old grey seal

9. Section of immature testis of grey seal

Section of mature but resting testis of grey seal

Sections of mature and active testes of grey seal

10. Section of immature epididymis of grey seal

Section of mature but resting epididymis of grey seal

Section of mature and active epididymis of grey seal

Abdominal testis in ‘82-day’ foetus grey seal

11. Section of ovary with developing oocytes, grey seal

Right and left ovaries in section, grey seal

12. Free blastocyst in utero, grey seal (R.Cox)

Young embryo (c. 31 days), grey seal

Early foetus (c. 55 days) in utero, grey seal

13. Early male foetus (c. 62 days), grey seal

Young male foetus (c. 72 days), grey seal

Later male foetus (c. 113 days), grey seal

14. Moulting haul-out at Aber Foel Fawr, Ramsey Island

Closer view of the moulting haul-out at Aber Foel Fawr (K. M. Backhouse)

15. Two types of moulting grey seal pups, Ramsey Island

16. Newly-landed grey seal bull, shell-sand beach, Shillay

Grey seal cow giving birth, Farne Islands (W. N. Bonner)

17. Newly-born grey seal pup, Ramsey Island

Grey seal pup with erupting teeth, Ramsey Island

18. Grey seal cow identifying her pup by smell, Ramsey Island

Grey seal cow attacking pup not her own, Shillay

19. Grey seal pup suckling, Farne Islands

14-day-old grey seal pup, Ghaoidmeal, Inner Hebrides

20. Grey seal pup in typical early moult, Scroby Sands, Norfolk (J. Woolston)

Grey seal bull moulter, Ramsey Island

21. Two grey seal bulls contesting territory, North Rona

Grey seals in copulation, Farne Islands (J. C. Coulson)

22. Branded grey seal cow, Farne Islands (J. C. Coulson)

Grey seals hauled-out on West Hoyle Bank, Cheshire (J. C Gittins)

23. Common seal cow and pup, the Wash (W. N. Bonner)

Two common seals (heads), the Wash (W. N. Bonner)

24. Common seals hauled-out on sandbank, the Wash (W. Vaughan)

Common seal bull ‘salmon-leaping’, Mousa, Shetland

FIGURES

1. External features of seal and sea-lion

2. Limb-bones of seal and fur-seal

3. Skulls of seals (grey and common) and of sea-lion

4. Side view of pinnipede body

5. Bradycardia in seal, porpoise and manatee

6. Kidneys and posterior vena cava in seals

7. World distribution of the grey seal

8. The origin of the three populations of grey seals

9. Distribution of grey seals in British waters – recorded sightings

10. Distribution of grey seals in British waters – breeding areas

11. External differences between grey and common seals and between bull and cow grey seals

12. Jaws and teeth in grey and common seals

13. Growth and maturation of the os penis

14. The os penis index

15. Distribution by age of cow and bull grey seals

16. The male genitalia of the grey seal

17. Growth and maturation of the testis in the grey seal

18. Variations during the year in adult testicular weights in the grey seal

19. The female genitalia of the grey seal (adult)

20. The female genitalia of the grey seal at birth

21. Foetal growth rate in the grey seal

22. Counts of grey seals on West Hoyle Bank and on the Farne Islands

23. Counts of grey seals on West Hoyle Bank, 1951–56 and 1966–70

24. Early stages in territory formation on Shillay

25. Daily pupping rates for Ramsey Island, North Rona, Staple Island and Brownsman

26. Daily pupping rates in neighbouring rookeries

27. Breeding sites on Ramsey Island

28. Breeding sites on Eilean nan Ron and Ghaoidmeal

29. Breeding sites on Shillay

30. Breeding sites on North Rona

31. The Outer Farne Islands

32. Breeding sites on the Farne Islands

33. Different types of social arrangements in different groups of grey seals

34. The grey seal pup in the first four weeks

35. Bull territories on Shillay

36. Types of marker used on grey seal pups

37. Recoveries of young marked grey seals

38. Growth rates in grey seal pups

39. World distribution of common seals

40. Distribution of common seals in British waters – recorded sightings

41. Distribution of common seals in British waters – breeding areas

42. Rare species of pinnipede in British waters – ringed seal, harp seal and bearded seal

43. World distribution of ringed seals

44. World distribution of harp seals

45. World distribution of bearded seals

46. Rare species of pinnipede in British waters – hooded seal and walrus

47. World distribution of hooded seals

48. World distribution of the walrus

49. Numbers of pups born on the Farne Islands, 1956–70

50. Maximum counts and numbers of pups born on the Farne Islands, 1956–68

51. The Farne Islands

52. Average monthly counts on the Farne Islands, 1956–68

53. World distribution of Phocidae

54. World distribution of Otariidae and Odobænidae

EDITORS’ PREFACE

SEALS are fascinating animals. Everyone knows what they look like, but most people, even some experienced naturalists, have only seen them alive in zoos, where the seals, and their relatives the sea lions, are such firm favourites with the crowds. It is not really difficult to see quite large numbers of both grey and common seals by visiting the correct coastal areas of Britain, but few casual visitors find themselves in the right place at the right time. The exception is the Farne Islands, where for many years great numbers of tourists have been able to see equally large numbers of grey seals, and to see the animals from boats without having to make the effort of walking long distances over sandy beaches and mud flats, or of scrambling energetically down steep and often treacherous cliffs.

Although seals are so well known, and are the subject of many fables and nursery tales, our knowledge of their life history and ecology was, until recently, very imperfect. We had little accurate knowledge of the size of their populations, and of whether they were increasing – as fishermen who believed they endangered their livelihood said – or whether they were in imminent danger of extinction – as some conservationists firmly insisted. Legislation on their protection, and efforts towards their control, were based on guesswork.

Professor Hewer would be the first to insist, as he does in several places in this book, that our knowledge is still imperfect and that much more research on these animals is still needed. But nevertheless our knowledge is now reasonably firmly based, and we are therefore glad to be able to publish this up to date and authoritative book at this time. We believe that, for the first time, it gives in one place the overall picture of these animals that many have been waiting for. It also enables the reader to understand the complex problems of seal conservation. Seals must be preserved, and this may sometimes include killing seals where (as in the Farne Islands) their numbers are too great for the habitat to support.

Professor Hewer modestly plays down his own contribution to our vastly increased knowledge of seals. He has himself, for over twenty years, been closely involved in all the major research in Britain. He has combined the careful laboratory investigations of the trained scientist with detailed studies of the animals, alive, in their natural habitats. As one fully employed as a university teacher (where his many students will testify to his conscientiousness) he has not always had the time or the opportunity to do, himself, all the experimental or observational work he would clearly have enjoyed, but all other workers in this field have gained so much from his freely given help and advice that his contribution to the whole subject has been unique. All other workers on British seals will readily acknowledge Professor Hewer as their spokesman.

Scientific knowledge of a subject is one thing, and this is the basis of this book, but its value is immensely enhanced by the fact that Professor Hewer really ‘knows’ seals. He has watched and lived alongside his subjects, and has come to know them as only one who is a skilled observer and a field naturalist can. These genuine observations on the living animals give this book a quality rarely found in works which also satisfy the most rigid canons of scientific accuracy.

AUTHOR’S PREFACE

THIS book has taken an unconscionable time being born. The usual excuses can be given but basically the subject has been a particularly active field of research over the past ten years, not only for myself but for the number of workers who have been drawn into it and whose contributions have been constantly filling in gaps in our knowledge. It has always been tempting to put off the day so that something interesting and useful might be included. The present moment is apt, not because there is a lull in research work, but because several events have combined to give research an added impetus and considerable advances can be anticipated in the near future. A stock-taking is therefore appropriate and this I have attempted to do.

Public opinion during this time has been aroused and the whole question of our seal populations has become over-heated largely in inverse ratio to the information available. While such debate is not conducive in itself to the elucidation of facts, it does create a political atmosphere in which funds become available for research. While my own work in the laboratory has been covered by the usual university finance, the collection of material and the field observations could only have been possible with the aid of official support, in this case through the Nature Conservancy. As the problems grew in number and scope, the appointment of special workers in seal biology became possible and I have not suffered the undue frustration of seeing interesting aspects neglected because they were beyond the capabilities of myself.

When I became involved in this work (1951) Mr J. L. Davies had already given some account of grey seal breeding on Ramsey Island and this had been followed up by Dr L. Harrison Matthews and others in 1950. Apart from Fraser Darling (now Sir Frank Darling) these were the only zoologists to interest themselves actively in research on British seals this century. Yet there were a number who had experience in Antarctica such as Dr Matthews himself and much is owed to their interest and encouragement over the last twenty years. It is very fitting that the head of the newly formed Seals Research Unit at Lowestoft, Nigel Bonner, served his apprenticeship in the antarctic.

For two things, and two things only, I take some credit; the first, a matter of priority, was that I decided that no further advance could be made in the understanding of the biology of the seals without having a completely reliable method of determining the age of a specimen. When in 1960 it was first possible to obtain specimens of breeding cows and bulls (in the otherwise ‘protected season’) the rapid formulation of a provisional life-table, which appeared as a breakthrough in understanding the life of the grey seal, was really due to the finding of a reliable method of age-determination some 5 years earlier.

The second, a matter of method, was that I believed that advances could best be made by keeping laboratory findings and field observations in close contact, each feeding back information or suggestion to the other for further research. For this reason I have spent much time in the field at all times of the year as well as using laboratory techniques to unravel the yearly sexual cycles.

This book is about seals and not about the people who study seals. I have therefore not introduced a lot of extraneous matter about the difficulties of field work other than as explanations as to why certain information is not available. Those of us, professional and amateur alike, who have observed seals in the ‘field’ have done so because we like the work no matter that occasionally conditions are somewhat inclement. Such do not last for long and modern types of transport and facilities really make things much easier than heretofore. When Dr Gorvett and I went to Shillay in 1954–55 no suitable portable radio equipment was available. In 1959 for the first North Rona expedition we had a receiver-transmitter capable of covering 100 miles. True it needed two people to carry the two parts and the massive batteries, while a 12 ft. aerial had to be erected (and dismantled each time in case it blew away). By 1962 the several parties working in Orkney each had two-way radio to each other (and the coastguard) in apparatus easily portable by one person. Parallel advances were made in sound apparatus for recording vocalisations. But perhaps the greatest benefit has been in the use of plastic containers and insulating material for the collection of material for later laboratory examination. By the mid-1960’s I was able to obtain specimens of tissues and blood in Shetland, keep them at the low temperature necessary and send them to Oxford, where they arrived in perfect condition, for electron-microscopy and biochemical analyses. The advent of the high-speed inflatable dinghy has also contributed much towards landing parties obtaining material just as the helicopter and small plane have aided observation and censusing. One marvels at the work of early investigators such as Prof. W. Turner of Edinburgh who contributed much to our knowledge of seal and whale anatomy by the dissection of bodies many days old cast up on the beaches of Scotland. Nowadays it is possible to live among the seals in comfort and safety thanks to the advances in camping equipment, desiccated and tinned foods and weatherproof clothing. Although as Dr Backhouse has said there comes a time when we exclaim, ‘What we suffer in the cause of science!’, it is soon over and success is an ample reward.

It remains for me to thank the very large number of people without whose co-operation both the research and this book would have been impossible. First come the field-workers: J. L. Davies who started the ‘seal movement’ after the second war; the members of the Northumberland, Durham and Newcastle-upon-Tyne Natural History Society ably inspired and led by Mrs Grace Hickling assisted by Dr J. Goulson of Durham University, A. W. Jones, I. M. Telfer and others; Prof. J. D. Craggs, N. F. Ellison and others who have recorded on the West Hoyle Bank for 15 years; U. M. and L. S. V. Venables whose work on common seals has been outstanding; Dr J. Morton Boyd who has maintained the grey seal work in the Nature Conservancy (Scotland) over the past 15 years, assisted by numerous other workers on the annual visits to North Rona. Among these must also be counted those who collected material: Dr J. D. Lockie who sent me the first from carcases in the Berwick-on-Tweed area, E. A. Smith who contributed so much from Orkney, Jack Landscail of Orkney and William Laurenson of Shetland whose marksmanship and skill in reclaiming bodies made collection as humane and as least wasteful as possible. My thanks go especially to those who have accompanied me on trips to uninhabited islands and have had to put up with my eccentricities: Dr Gorvett and the late J. W. Siddorn, both of Imperial College, Drs J. D. Boyd and J. D. Lockie and the late James MacGeogh, all of Nature Conservancy (Scotland) and most of all Dr K. M. Backhouse, who has been with me so often and to so many places and whose cheerfulness and resourcefulness have meant so much to me. I must also thank those who have encouraged me from time to time in the work: Dr L. Harrison Matthews F.R.S., Prof. E. C. Amoroso F.R.S., Prof. R. J. Harrison F.R.S., Dr R. M. Laws and other members of the Joint Committee.

Lastly I come to those who have read the manuscript and whose comments have been of great value to me: W. N. Bonner for the grey seal and W. Vaughan for the common seal. Nevertheless I must emphasise that all errors and omissions together with expressions of opinion are my sole responsibility. I do not mind sticking my neck out if it stimulates someone to find out the true facts.

CHAPTER 1
THE PINNIPEDIA, THEIR MODE OF LIFE AND RELATIONS WITH OTHER MAMMALS

THERE are only two truly British species of seal, although a number of others may occasionally be seen in our water, usually in the north. Before these are considered in detail it is necessary to see how they fit into the group (Order) to which they belong, the Pinnipedia. These comprise the hair or true seals, (Phocidae), the fur-seals and sea-lions (Otariidae) and walruses (Odobaenidae). Collectively they may be found in all the oceans of the world although they are certainly most numerous, both in species and in individuals, in the cooler waters of the arctic and antarctic regions. A systematic list of all species of Pinnipedia with their common names and rough distributions will be found in Appendix A. However they are not the only group of marine mammals and a glimpse at the other forms which have reverted to an aquatic existence is an aid in recognising the special features which are the basic adaptations to life in the sea for warm-blooded air-breathing vertebrates such as the mammals.

Two other groups have forsaken their ancestral methods of living on land and taken to a wholly marine existence. These are the Cetacea, or whales, porpoises and dolphins, and the Sirenia, or sea-cows. All three groups are of great antiquity (in terms of mammals) and it is not altogether easy to be certain of their ancestral connections in any detail, since fossil forms are scarce and fragmentary. On the whole it may be stated that the Sirenia have connections with forms which are also related to the elephants (Proboscidea) while both Cetacea and Pinnipedia are related to carnivoran stock. It is not surprising therefore to find that the Sirenia are vegetarians, feeding on seaweed, the Cetacea and Pinnipedia carnivorous, feeding on fish, squids and crustacea and other marine animals. The Cetacea broke away at a very early date long before the present carnivora became a defined Order of mammals. The Pinnipedia on the other hand have more recent connections and are directly related to the Carnivora. Indeed until recently they were always included as a Sub-order, and some systematists still so regard them.

The members of these three orders have features in common which have been evolved independently as essential adaptations to marine life. The most conspicuous of these is the streamlining of the animal by the production of a thick layer of blubber under the skin which not only smoothes out angularities but also provides an insulating layer against the low temperature of the water. In addition the limbs are reduced in all, the long bones of the fore-limbs are shortened and in the Cetacea and Sirenia the hind-limbs are lost altogether, while in the Pinnipedia these hind-limbs are much modified. The body too is elongated and roughly spindle-shaped in its proportions. These modifications deal with two problems connected with water, namely its low temperature and its greater density. There remains a third which is in some ways more serious, namely, that these animals being mammals are firmly committed to air breathing, possessing lungs, so that access to the air is essential and a means of preventing the entry of water into the trachea and lungs equally so. All therefore have nostrils which are normally closed and are opened only by voluntary muscles when the head is above the surface of the water. The provision of oxygen to maintain activities when the animal is submerged is made in different ways in the different groups. For example Cetaceans dive with full lungs, pinnipedes with empty ones, but the details of respiration are complex and will be dealt with later. Propulsion through the dense medium of water has also led to another convergent feature, the fusing of the digits by webbing, either thin or thick, thus forming a flipper or fin out of the normal mammalian hand or foot. There is also a tendency for the hair to be reduced in length although this is not universal in the groups. In Cetacea and Sirenia the body is almost naked, but the vibrissae or moustachial hairs are retained either as normal tactile organs as in the Sirenia or much reduced in the Cetacea. In neither of these groups does the hair contribute to insulation. Nor does it do so in the hair-seals and other pinnipedes where the hair is short and easily wetted so that the water comes into direct contact with the skin. Only in the fur-seals is the hair dense so that on immersion a layer of air is trapped among the hairs of the undercoat and direct water to skin contact is prevented. Here the hair is accessory to the blubber as an insulating structure.

We are now beginning to deal with features in which there are considerable and obvious divergences between the groups and these can best be described by noting them in the pinnipedes and then briefly contrasting them with what appears in the other two orders. Unlike the Cetacea and Sirenia which are entirely aquatic throughout life, the Pinnipedia have not lost all contact with the land. Some spend more time in the water than others, but all come to land (or ice) for breeding and for basking between feeding bouts and therefore have retained an ability to move on land. This is achieved by two different methods, one found in the true seals and the other in the fur-seals, sea-lions and walruses (Fig. 1). In these latter the hind-limbs are still capable of being directed forwards and acting as a foot, albeit on an extremely short leg which is buried in blubber down to the ankle. Thus with the associated flexure of the trunk in the lumbar region they can move rapidly over a land surface. The true seals, however, have hind-limbs which are directed backwards and can only trail on land. Movement is therefore much more laboured in a terrestrial habitat. A comparison between the limbs of a furseal or sea-lion and those of a true seal shows clearly the very considerable differences not only in structure and posture, but also in use.

The fore-limbs of a fur-seal are long and on land are capable of reflexion between the wrist and palmar surface. The forearm and wrist form a vertical prop as it were, while the hand, supported by the meta-carpal and digital bones, lies flat on the ground (Fig. 2a). The web between the digits is thin, in fact the whole of the distal part is much thinner and longer than in the true seals. This is associated with the much greater use which the fur-seals make of their fore-limbs in swimming. The claws are quite rudimentary and in some almost missing altogether. In the true seals the fore-arm is buried in blubber, and only the wrist and palmar surface with short digits protrude as short flaps (Fig. 1). The webbing cannot be distinguished as such for the digits are united by thick tissue so that the separate digital lines are not visible externally. The claws are strong and used for grooming the surface of the body. In the water these limbs are used principally for changing direction or for slow paddling, never for rapid movement, when they are held tightly pressed against the flanks. The digits still retain, as Backhouse has shown, an ability to flex with considerable power so that the animal can haul itself up over boulders and rocks and to some extent compensates for the loss of power of the hind-limbs. At any rate this is true for the northern true seals. In the antarctic species this ability if present is not used, so far as observations made by a number of observers appear to confirm (O’Gorman).

FIG. 1. External features of seal and sea-lion. Note the differences in the external ear, the longer fore-limb of the sea-lion and the way the hind-limbs of the sea-lion can turn forward compared with the trailing position in the seal. The sea-lion can raise the forepart of the body into an almost erect position.. The claws of the sea-lion are small and set back from the edge of the flippers. (See also Plate 1)

The hind-limbs of a fur-seal are also long but can be directed forwards for movement on land. They are also used on land during basking as fans. When fur-seals are hauled-out for breeding the bulls have to remain on their territories for a long time and they soon dry out. Their layer of blubber and the thick undercoat of fur, which keeps them warm in water, is now a disadvantage when the sun comes out. They then use the hind flippers as huge fans, turning the body on one side so that both flippers can fan the anterior part of the body and head which has less fur on it (Pl. 1). The claws here are usually absent altogether or quite rudimentary on the first and fifth digit but well developed in the middle three which can thus be used for grooming. In the true seals the hind-limbs are permanently directed backwards and form powerful sculling organs (Fig. 2). The palmar surfaces of the feet are turned inwards to face each other. The digits are united by thin and extensive webbing and the digits themselves are strong, the outer ones (the first and fifth) being much longer and thicker than the others. By alternate sweeps from side to side in a sculling movement these flippers are able to drive the seal through the water at great speed which has been estimated at as much as 12 to 15 m.p.h. Claws are retained on all the digits but their function is obscure. On land these hind flippers trail behind and take no part in locomotion which is achieved by a ‘humping’ motion. The body is flexed and the pelvic region advanced to give a forward push to the anterior part of the body, the stomach and the chest taking the main load as the animal moves forward. The fore-limbs are not used in this motion unless obstacles have to be surmounted when they assist in dragging the body upward, helped by the thrust of the pelvis. Thus in the Pinnipedia there are two distinct means of progression both on land and in water.

In the Cetacea the principal locomotor organ is the tail, flattened dorso-ventrally into a triangular fluke, and moved up and down in a sort of ‘crawl’ action. The hind-limbs are missing and the fore-limbs are well developed into flippers. Not only are the digits joined by thick webbing but the number of phalangeal bones is greatly increased so that the flipper becomes extremely flexible in its use as a propulsive organ and also in causing change in direction. The Sirenia are rather lethargic and movement is obtained by the action of the flattened tail and by the broad, short fore flippers. The latter are peculiar in having a great power of rotation so that their direction of action can vary over a wide angle. This is of considerable advantage when used as a paddle, the animal being able to turn almost in its own length, or when erect in the water, in little more than its own breadth.

A feature associated with streamlining is the reduction of the pinna or external ear flap. In the Pinnipedia all variations are found; among the fur-seals and sea-lions it is still fairly well developed but narrow (furled) and elongate, lying back alongside the head. For this reason they are often called the ‘eared’ seals. In the true seals it is much reduced and does not extend beyond the dried hair. When the hair is wet it can just be seen, if the light is right, as a circular rim round the earhole (Pl. 1). In the walrus it is equally inconspicuous. In both Cetaceans and Sirenia it is completely absent and in the former the earhole is plugged. (Only recently has the extraordinary hearing mechanism of whales and porpoises been demonstrated by Drs Fraser and Purves.)

FIG. 2. Limb bones of seal and fur-seal: left, fore-limb skeletons; right, hind-limb skeletons. Note particularly the relatively small fore-limb of the seal (Phocidae) and the almost equal size of the ‘big’ and ‘little’ toes in the hind-limbs. In the Otariidae the ‘thumb’ and ‘big’ toe are much larger than the other digits, indicating the driving use made of both limbs. Notice too the very short thigh (femur) and upper arm (humerus) bones in both seal and fur-seal. For comparisons the hind-limbs of both are drawn to the same length and the fore-limbs to the same scale. c. x

The normal resting closed position of the nostrils has already been mentioned as common to all three groups. It remains to point out that the pinnipedes retain a ‘rhinarium’ external to the nostrils. This is the equivalent of the ‘wet nose’ of the dog and must constitute an important sense organ for the reception of chemical stimuli (scent). That it has to operate in water is no drawback because the molecules of the ‘scent’ must be dissolved before they can stimulate a receptor nerve ending, for which reason the rhinarium of land mammals is always kept moist with secreted mucus. In the pinnipedes it is possibly the sense organ which locates prey, such as shoals of fish at a distance.

The teeth of pinnipedes are greatly modified from the normal mammalian pattern. The differentiation into incisors, canines and molars is still recognisable, but they do not differ so much from each other as in other (land) mammals, consisting basically of single pegs of varying lengths. The incisors are small and degenerate as shown by the variation in their number. (It is not unusual to find two on one side and three on the other.) The canines are massive cones, used for offensive purposes in the males of some species but basically the prey-catching tooth as in land carnivores. The molars are no longer grinders or mashers but consist of a single major cusp or spike with one or more smaller cusps on each side in line with the length of the jaw. In this way there is formed a long line of pointed cusps of varying heights admirably adapted to the retention of struggling prey. The only exceptions are the walrus and the bearded seal. The walrus feeds on large bivalve molluscs, like clams and mussels, which are crushed between stub-like molars. The bearded seal is not predominantly a fish feeder, shrimps, crabs, holothurians, clams, whelks, snails and octopus forming the bulk of its food and the molars are usually much worn and not sharply pointed. In some species of seal the young do have milk teeth for a short time. Usually there is only one set of functional teeth, the milk dentition being resorbed while the foetus is still in utero. Cetacea either have no teeth and feed by a filter mechanism (whale-bone whales) or have a single row of simple cusps (the toothed whales, porpoises and dolphins), all alike, which are far more numerous than in any other mammal. This is a high specialisation for fish-eating, much greater than the pinnipede condition. The Sirenia have flattened grinders of degenerate form since the seaweeds on which they feed hardly require munching.

The skulls of most pinnipedes show traces of slow and incomplete ossification. This is particularly true of the region on a level with the eyes so that the front part can be easily detached from the hinder brainbox in even old animals. The fur-seals and seals differ considerably in the general appearance of their skulls thus lending support to the view that they are derived from different sections of the land carnivores (Fig. 3). The walrus is again an exception since there is massive ossification to provide support for the huge tusks. Cetacea have evolved quite differently since many of the cranial bones contain spaces filled with air or occasionally with oil. The Sirenia have massive skulls although the bone itself is not dense.

We now come to the respiratory modifications and it is impossible to separate these from peculiarities of the blood system since the oxygen required by the tissues is transported by the haemoglobin in the red cells of the blood. The modifications of the nostrils have already been mentioned, but there are others equally significant. Many pinnipedes have cartilaginous rings in the trachea which are incomplete on the upper side but some, and both of our British species are among them, have complete rings which thus prevent any collapse of the trachea when the seal is under pressure in diving. These rings are continued into the bronchi and bronchioles and cartilage continues to be found in the connective tissue of the lungs lying between the respiratory lobules. In addition there are, in the bronchi, valves of muscle and connective tissue which are able to form air-locks in the lungs and so prevent the residual air in the larger (and non-respiratory) tubes being forced under pressure into the respiratory alveoli. This appears to be a device to prevent nitrogen, which forms four-fifths of the air, being absorbed under pressure into the blood stream. If it were so absorbed, on return to the surface it would come out of solution in the blood under the reduced pressure to form gas bubbles in the smaller blood vessels and so cause ‘bends’ which can easily prove fatal, as it does when it occurs in man when diving.

FIG. 3. Skulls of seals and sea-lion. All the skulls are of adult males. For comparisons the grey seal skull is drawn to the same length as that of the Californian sea-lion and the common seal skull to the same scale as that of the grey seal. Note the general similarities, particularly of the dentition, but also the differences which are most marked in the region behind the articulation of the lower jaw. c. x

The lungs themselves are not abnormally large and all pinnipedes exhale before or on diving so that there is literally little or no oxygen in the lungs to provide for tissue respiration during the activities of swimming and catching prey below the surface. The thorax, which also contains the heart and great vessels besides the lungs, is more elongate than normal in mammals and the diaphragm, which separates its cavity from that of the abdomen, is set much more obliquely, its upper attachment to the body wall being set farther back than normal and the sternal support of the lower margin is shorter than usual (Fig. 4). This means that the cavity can be more completely compressed and a greater proportion of the air in the lungs exhaled than in normal land carnivores and other mammals. The small residium is driven into the nonrespiratory trachea and bronchi. When the seal returns to the surface breathing recommences and a series of deep inhalations and expirations takes place. From my own observations on a southern elephant seal the number of such breaths is roughly proportional to the length of time that the nostrils have been closed. Even when on land and hauled-out seals will continue to remain with closed nostrils for considerable periods separated by series of breathings.

FIG. 4. Side view of the pinnipede body, to show the principal organs. The right side of the body is diagram-matically opened by removing the skin, blubber and outer muscle layers. The right fore-limb has also been removed as far as the ‘elbow’. The genitalia are omitted. The oesophagus is indicated as passing above the heart and between the right and left lungs.

By way of contrast Cetaceans dive with full lungs and all the modifications are towards the prevention of collapse and the transmission of the external pressure to the air contained in the lungs; another ‘anti-bends’ device.

In the pinnipedes we are still left with the puzzle of how they obtain and maintain sufficient oxygen for their activities below the surface, and we must turn to the blood system for further information.

First it must be clear that if there is little or no air in the lungs there is no profit in circulating the blood from the lungs for there is no oxygen to pass on to the active tissues. It is therefore not altogether surprising to find that seals exhibit a phenomenon known as ‘bradycardia’. This is a reduction in the heart beat both in the number per minute and in the strength of the beat. In fact it is reduced to little more than an occasional flutter by which some blood is circulated along the carotid artery to the brain. This has been shown to arise almost immediately the seal has dived, and in this it differs from the bradycardia of Cetaceans in which the rate and strength of the heart beat is gradually reduced to a low level. This difference must be associated with the difference of lung contents, the gradual bradycardia of Cetaceans keeping pace with the gradual exhaustion of the oxygen in the lungs (Fig. 5).

To prevent the ‘used’ blood from the tissues of the body being circulated even to a very minor degree in the pinnipedes, they have evolved a powerful sphincter muscle which closes the huge venous blood vessel leading to the heart and which draws blood from the hinder part of the body, the viscera and liver. This large blood vessel (posterior vena cava) is disproportionately large (usually double and enlarged) and so can act as a reservoir for the non-circulating blood. In addition there is a large vein lying below and up the sides of the spinal cord (extradural vein) which is enormously enlarged in pinnipedes. From it only a little blood can find its way back to the heart in the front region. Elsewhere this vein is connected by special large veins both directly to the posterior vena cava (at the hinder end where it is double) and indirectly round the kidneys in huge blood sinuses (Fig. 6). All these peculiarities increase the storage capacity of the venous system when bradycardia is in action. Some idea of the size of the veins will be conveyed by saying that in the grey seal the posterior vena cava in its posterior part is ‘nearly as thick as your wrist’ and King (1964) refers to their size in the walrus ‘by the often quoted reference that they can be “pulled on like a pair of trousers”’.

FIG. 5. Bradycardia in seal, porpoise and manatee. These are representatives of the three groups of truly marine mammals, the manatee, a surface feeder on seaweeds, being the least modified for diving. The seal immediately responds to diving by reducing heart activity, having expelled all the air from the lungs, while the porpoise, retaining air in the lungs, reduces the heart-beat rate only slowly. (Redrawn from Irving, Scholander and Grinell, 1941)

But all these modifications tend to show that the blood is not the continuing source of oxygen during active diving. This is confirmed by two other facts; firstly the red blood cell count (the number of red blood cells per unit volume) is nothing out of the way, about 5–6 million (cf. human 4–5 million), and secondly that the amount of haemoglobin in a unit volume is not very high either, about 1.2 compared to the standard in man of 1.0. If this is all true, then how do seals manage to respire in their tissues during diving?

Part of the answer lies in another pigment known as myoglobin because it is present in muscle. Here we find an enormous difference from the normal and we hardly need figures to show it. Myoglobin is also coloured though not so deeply as haemoglobin and it gives the normal pink colour to muscle meat. In the Pinnipedia the muscle is almost black in colour, certainly very deep red. Those who remember the whale meat which was available after the second world war will recognise that Cetaceans too have a very high myoglobin content. To both of these groups then part of the answer is the ability to store oxygen attached to the myoglobin on the site where it is required for the respiration of the active muscle cells.

There is also evidence that these animals can run into oxygen debt, particularly in respect of metabolising the waste products, which are normally toxic if allowed to accumulate without treatment. This adaptation is not so extraordinary as it sounds, for it is known that, in humans, when slow starvation is prolonged and the organism begins to live on its muscle protein (autolysis of the muscle) a level of waste nitrogen products can build up to many times the normally lethal concentration. In these diving mammals this ability has become normal rather than a pathological occurrence. On return to the surface the repeated inhalations and exhalations rapidly restore the oxygen balance.

FIG. 6. Kidneys and posterior vena cava, showing the extra storage space in the blood sinuses. This is a view from above, the vertebral column and nerve cord being omitted. Part of the extradural vein is also omitted so as to allow the junction of the right and left branches of the posterior vena cava to be seen, with its continuation into the huge hepatic (liver) sinus. Arrows indicate the direction of the blood flow. (Redrawn and somewhat modified from Harrison and Tomlinson, 1956) X

So atypic of mammals is the environment of the pinnipedes that speculation as to the value of their principal sense organs must come to mind. Not a great deal has been found out but Harrison, working on captive common seals, has contributed some useful basic information. He finds that the eyes are well adapted for night vision or for murky water but this is associated with short-sightedness. The Weddell seal, however, avoids this myopia by having a slit pupil. Their effectiveness must therefore be strictly limited particularly in those species which live and feed through the polar winter. Auditory and touch stimuli are well received, the latter being particularly relevant to the well-developed vibrissae on the muzzle which have very large nerves running from them to the brain. Water being an incompressible medium is excellent for the transmission of sound waves, so that sensitivity to auditory stimuli is to be expected. My own impression is that grey seals are more sensitive to sounds in the higher frequency range than those in the lower. Phoca and Zalopus both have good hearing in the upper range. It is possible that they can hear sounds which, to man, are ultrasonic but this has not been proved. Recent work has shown that many marine organisms make use of these higher frequencies and sensitivity to them may well be an advantage to carnivores in search of food. Harrison also found that the common seals have an excellent sense of orientation. This is obviously of great benefit to an animal making use of a three dimensional medium such as the sea. Anyone who has done any flying will know that man, although he has the necessary sense organs, has to learn how to use and interpret the stimuli received.

In discussing the characteristics of the Pinnipedia and comparing them with those of other marine mammalia it has been necessary to draw a distinction between the true, haired or earless seals on the one hand and the fur-seals and sea-lions or eared seals on the other. This is a very profound demarcation if we add the walrus to the second group. So clearly marked are these two groups that it is now thought that they had separate origins among the land carnivora, the true seals being derived from otter-like ancestors and the fur-seals and their relatives from bear-like forms. This can be expressed as shown at the top of See here.

The Otariidae are the least modified for marine existence. They retain a prominent external ear flap, there is an obvious neck region between the head and trunk, so that the body form is not perfectly streamlined or bobbin shaped, and their hind-limbs can be turned forward and used on land as feet. The pelvic region is also very obvious when they are on land. On the other hand the claws are much reduced either in numbers or size, and the flippers of both hind- and fore-limbs large and very well developed.

Their distribution is remarkable in that they are completely missing from the north Atlantic while present in all other oceans. Moreover, of the 12 species, 9 occur in the southern waters of the Atlantic, Indian and Pacific Oceans (including the circumpolar antarctic seas), 1 in sub-tropical Pacific waters and 2 in northern waters of the Pacific. Although this suggests that the family is southern in origin, palaeozoogeographical evidence (McLaren 1960) points to a north Pacific origin and that they evolved from a littoral canoid of bear-like appearance along the north-western coasts of North America, some time before the upper middle Miocene when their first fossils appear. Weight is given to this by the presence of the other ursine derivative, the walrus, in the circumpolar seas of the arctic. There are two sub-families: the Otariinae or sea-lions (with 5 species) and the Arctocephalinae or fur-seals (with 7 species). (See Appendix A, See here.)

The Odobaenidae contain two species,* one Pacific and one Atlantic, both in arctic waters. It is probably better to consider them as sub-species of the one species: Odobenus rosmarus. Like the Otariidae they still retain hind flippers which can be used as feet, but they have become greatly modified in other respects. They are specialised feeders on shell fish; the tusks are used for digging the bivalves out of sand or mud and then the flattened molars crush them†. In the males the tusks are particularly well developed and used for fighting as well. Unlike the members of both the other families the walrus is almost hairless although the vibrissae on the muzzle are well developed as tactile organs. Although they have been hunted for many years and almost exterminated, they are now protected and some study is being made of their habits. The Atlantic form breeds on some of the Canadian islands and occasionally an immature individual is reported on the British coast.

The Phocidae have an inconspicuous external ear, concealed by dry hair but usually visible when wet. The neck is short and thick, smoothing the body outline into that of the head. The fore limbs are small and used for grooming, with well developed claws. Their use in swimming is small, usually only for changing direction or ‘treading water’. On land they are not much employed in locomotion, although in the sub-family Phocinae the digital flexure enables them to clamber over rocks and broken ice. The hind flippers are the principal locomotor organs in water but trail on land. The movement of seals on land appears clumsy, but they can move faster for short distances than would be supposed. They occur in all the oceans of the world including the Mediterranean and Baltic Seas and several inland lakes which have had connection with the sea in glacial or post-glacial times. Until recently three sub-families have been recognised: Phocinae (with 8 species), Monachinae (with 7 species) and the Cystophorinae (with 3 species). Miss King (1966) however, has shown that the characters uniting the species of Cystophorinae are only superficial and differ in fundamentals, while many other characters show that the hooded seal is really a member of the Phocinae while the two elephant seals are closer to the Monachinae. Each of the two Sub-families are divided into two Tribes thus: