British Seals. H. R. Hewer
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.
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.
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