MAMMALS IN THE WATER.
11HE Zoo otters, which are unusually lively during the first days of spring, have conformed to the universal tendency to "extend the range of diet" by eating ship- biscuit as well as fish. They make believe that it is fish the time, biting the biscuit into fragments, then pushing these into the water with their noses, chasing them an catching them, and after the biscuit is well saturated, eat- ing it on the bank. Incidentally this shows how very prettily an otter eats its meals. It lies flat down, and holds. the " fish " neatly between its hands, "thumbs upwards," the hands being quite flat, like the two ends of a book-slide. The quickness and handiness of the otter in the water is most surprising considering the very slight difference in general form between it and allied non-amphibious mammals;. there is practically nothing which a salmon or trout can do which the otter cannot beat, except the salmon's leap up. a weir. It can even imitate that astonishing " shoot " by which a trout goes off from its feeding-place like an arrow to. the bank or weeds. It can also climb a pollard-tree, dig holes, dive in salt water, run fast on land, and run at the bottom of the water.
Compared with the aquatic powers of civilised man—the- only mammal, except a monkey, which does not swim naturally —these feats are very surprising. But the list of land animals. which are expert swimmers is very much larger than might be supposed. It also embraces many classes of animals, and the number of the aquatic or semi-aquatic members of very different families suggests that the aquatic habit was at first only accidental, and that very many creatures which do not by habit swim and dive might under other circumstances have become aquatic. Judging from our own experience, one of the most difficult " adapta- tions" of habit encountered in the amphibious life is that of keeping the eyes open under water, with no special protection. It is disagreeable in fresh water, and painful in salt water. Conceding that the regular amphibious creatures have learnt to do this gradually—otters, water-voles, water- shrews, polar bears, and seals—how are we to account for the aquatic dexterity of a creature like the land-rat P A common brown rat can see under water as well as a water-rat can. It swims and dives equally well, and can burrow into a. bank
below the water. This is less creditable engineering than the sub-aquatic work of the beaver, which buries logs and fixes the foundations of the dam under water ; but it shows that the rat is quite at home in that element. The rat has no structural adaptation of any kind to help him, and the
water-vole is to all appearance the same in structure as the land-vole. That there should be so little modification is quite contrary to the ancient and established view, that if an animal can swim and dive, it must be like a duck or a fish. When Fuller was writing of the "natural commodities" of Cardiganshire he remarked : "what plenty there was of beavers in this country in the days of Giraldus, the breed of them is now quite destroyed, and neither the fore-foot of a beaver (which is like a dog's) nor the hind-foot (which is like a goose's) can be seen therein." But the performances of the creatures, which are little or not at all changed in structure, are perhaps more interesting from the personal point of view of their human critics than those of animals like the seals, walruses, and whales, whose legs have turned into fins. Their ex- periences and difficulties in the water ought to be somewhat like our own. The surprising point is that most forms of movement in the water seem to present to them no difficulty at all. Very young otters are "taught" to swim, and so pre- aumably are the young duckbills, which lie in a sub- terranean nest for several weeks before entering the water. But the young otters at the Zoo only needed to be hauled out by their mothers when they stayed in too long. They swam like young ducks, and the teaching was by example, not instruction. When master of the art the otter swims, not with all four feet, but with the hind-feet, folding the front paws alongside its body. Mr. Trevor-Battye has noticed that the water-voles do the same. This agrees with the progress of human swimmers, who usually begin by making too mach use of the arms and too little of the legs, but discover later on that the latter are the main aids in swimming either on or below the surface. The otters are so far modified from the polecat tribe that they have webbed toes ; the water-voles have not even this advantage over their land relations. It ought to follow from this that the latter could with a little trouble become aquatic. There is a great deal of evidence to show that there is no hard-and-fast line between land mammals and water mammals so far as this distinction rests on the ability to use both elements. Stoats, for in- 'stance, are excellent swimmers, and if put to it for food, would probably learn to catch fish just as the polecat is known to catch eels. Cats, which have an intense dislike of wet, swim well, carrying the head high. Their distaste for aquatics does not extend to the larger cats. Tigers are fond of bathing, swim fast, and, in the case of the " river tigers" of the Sunderbunds, and the tigers near the coast of the Straits of Malacca, are constantly noticed in the water. Whether the trained Egyptian cats, which were used to take waterfowl in the reed-beds by the Nile, ever swam when stalking them does not appear from the ancient pictures; but the extent to which the dog voluntarily becomes aquatic entitles some breeds to be considered amphibious. A dog belonging to a waterman living near one of the Thames ferries has been known to continue swimming out in the stream for an hour without coming to land. It did this for amusement on a fine Sunday morning. Another riverside dog was taught to dive, and fetch up stones thrown in which sank to the bottom. This dog would pick out stones from the bottom of a bucket of water, selecting one which it had been shown before from a number of others. It had so far become amphibious that it could use its eyes under water. In France otter-hound puppies are introduced to their aquatic life by setting their kettle of soup in a pond or stream, so that they must go in deep to feed. Soon they become as fast swimmers on the surface as the otter itself, though the physical advantages of submarine motion give the otter the advantage when it is below the surface.
As the land-rats and water-voles can swim and run below water, there is no reason to suppose that the various tribes of mice cannot do the same. The house-mouse swims on the surface as well as the rat; but it has apparently not yet learnt to dive. All the pachyderms can swim, and very many are as much at home in the water as on land. The story that pigs cat their own throats when swimming is a myth. To prove it a whole family of pink pigs were chased into a fine muddy pond and made to swim across. They swam well, and the "contour line" of mud along their sides showed that their backs were above water as well as their heads. Elephants are almost as clever in the water as the polar bears. They can swim and walk under water without coming to the surface, keeping the trunk out of the water like a diver's tube. There is plenty of flexibility in an elephant's legs, enough to use in swimming, bat the properly aquatic hippopotamus can scarcely be said to swim. It rises and sinks at will, but it habitually walks or runs on the ground at the bottom of the river. Two South American river creatures seem quite arbitrarily aquatic,—the coypu, which might just as well be a land- rat, but is a water-rat " in the process of becoming " a beaver, and the capybara, which is a gigantic water guinea-pig. Each is quite at home in the rivers, and as the capybara is aqnatic, there seems no reason why the guinea-pigs, or the Patagonian cavies, should not learn to swim and dive, if circumstances made it useful. Even man himself becomes almost amphibious in certain regions. Temperature permit- ting, he swims as well, and dives better, than many of the animals mentioned above,—better, for instance, than any dogs. The Greek sponge-fishers and the Arab divers must have sight almost as keen below water as that of the sea-otter. They have even learnt, by practice, to control the consumption of the air-supply in their lungs. The usual time for a hippo- potamus to remain below water is five minutes. The pearl- fisher can remain below for two and a half minutes. In a tank a diver has remained under water four minutes. But temperature marks the limits of man's amphibious habits. Its effects seem less potent on other mammals in the water. The amphibious beasts of the tropics—hippos, tapirs, elephants, and manatees—need warm waters to swim in. But in temperate Europe, or even in the Arctic seas, certain animals seem indifferent to constant wet, and the intense discomfort of " wet clothes " when out of the water. A polar bear is wet, literally, to the skin. The otters, though they have an inner coat, look thoroughly drabbled when out of the water. The land-rat's coat also becomes wet through. The latter avoids water in cold weather; but the otters sit cheerfully on the bank in winter frosts, or even in wind. So do the Zoo beavers, but their lower fur is probably impervious to wet. A piece of beaver fur, with the long coat taken off, was dry at the roots after soaking for two and a half hours in a basin. If the temperature of aquatic animals were naturally low, like that of a fish, their indifference might be explained. A hibernating dormouse is as cold as death ; a tame rat, tested by a clinical thermometer, showed a temperature of 100', and a live otter can scarcely be of lower temperature than a live cat or a Cape ratel. The Zoo caution, " These animals bite," preclude any effort at taking their normal heat. But that of a rat, which takes to the water freely when the March winds are blowing, is normal, and there is no reason to suppose that that of the otter is different.
As chill to the surface tissues is always dangerous to warm- blooded creatures, in the absence of an inner layer of fat, which the whale, and in some degree the polar bear, possesses, the fur must be the non-conductor which protects them. Water, unless in movement, is not a quick conductor of heat. The fur, aided by the outer and longer hairs which keep it in place, holds the water-jacket motionless, even if it reaches to the skin ; and this " water compress" saves the animal from a chill. If the cold winds extract the warmth from it when standing wet through on land it takes to the water as the relatively warmer element.