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:
All the Phocinae are northern in both Atlantic and Pacific waters and their presently or recently connected seas and lakes. The Monachini or monk seals are tropical or sub-tropical while the Lobodontini are antarctic or sub-antarctic and circumpolar. The recent reallotting of the Cystophorinae species solves their equivocal position for the hooded seal as northern Atlantic and Pacific like the other Phocinae, while the elephant seals are brought more into line geographically with the southern distribution of the Lobodontini.
Despite the present wide distribution of the Phocidae the fossil evidence suggests their origin is Palaearctic from lutrine (otter-like) ancestors somewhere between early Oligocene and lower middle Miocene times.
Thus we can now say that the two seals which breed in British waters belong to the Phocinae, the two species, Phoca vitulina and Halichoerus grypus falling into the tribe Phocini.
Until recently much more had been discovered by British workers about the antarctic species of Lobodontini (and about the southern elephant seal) than about our own British species. How little was known until recently will become apparent when we consider each species separately, but something must be said about the reason for this neglect of our pinnipedes. Basically our ignorance has been due to the physical difficulties of obtaining information. Here we are dealing with animals which are amphibious. While they are on land we can watch them from the land but the slightest shift of the wind to send our scent towards them and they are away to sea where we cannot follow them. Extending this example to the period when they leave their breeding grounds on land, the problem becomes even greater. Antarctic forms, both mammals and birds, do not have a built-in fear of man and even the experiences of man’s culling them over the last century and a half have not induced such a timidity as we find in the northern Atlantic species. Consequently there has been no easy way in which naturalists and zoologists could become interested in seals or,