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.
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).