Inefficient Cousins

Nature made a good many tries before it made mammals. Some of the experiments left no living descendants. But an accident of geology turned the continent of Australia into an evolutionary Deepfreeze where various inefficient cousins of the modern mammals have survived, protected from competition.

The New York Zoological Society (Bronx Zoo) recently imported a curious assortment of these Down Under oddities: three platypuses; two echidnas (egg-laying "hedgehogs"); two Tasmanian devils; various phalangers and kangaroos. Now the zoo can show in full series the main biological tries that raised the mammals, including man, above the birds and reptiles.

The Egg-Layers. Low beasts on the mammals' biological stepladder are the monotremes. Platypuses are the lowest of these proto-mammals. They have warm blood and fur insulation (important advances toward mammalhood), but they never quite got the hang of mammalian reproduction. Like their reptile ancestors, the females lay eggs (generally two pigeon-sized, soft-shelled ones), with the embryos already partly developed. In twelve days the young platypuses hatch and crawl to their mother's mammary pores for milk, which is exuded on her fur.

Every major zoo would like to own platypuses, but they are hard to keep alive. This time the Bronx Zoo imported them under the care of Curator David Fleay of the MacKenzie Animal Sanctuary near Melbourne. Fleay is a tall, rangy Australian who knows more about platypuses than any non-monotreme. All three of his charges survived the voyage. Chief reason : the platypusary, a Fleay invention.

In the wild state, the platypus dives at night to river bottoms, probing the mud for worms with its soft, rubbery bill. Near dawn it pops into a narrow burrow slanting up in the bank. By the time it has squeezed through 15 feet of burrow, the soil has toweled it dry. If placed in a zoo tank with adjacent nest, says Fleay, the platypus goes to bed wet, and presently dies. The Fleay platypusary consists of a modest swimming pool with a long artificial burrow made of boards and stuffed with straw.

The Pouch-Carriers. First to outgrow the reptilian egg-laying habit was the marsupial (best known marsupial: the kangaroo). The marsupials kept the pouch (in which the echidna, another monotreme, had carried its eggs) ; but marsupials bore their young alive, thus rating as genuine mammals. Little marsupials are hardly more than larvae at birth, but active enough to crawl to their mother's pouch and nipples.

The first marsupials appeared about 40 million years ago and spread to most parts of the world. In Australia they had a stroke of luck. The Down Under continent (with adjacent islands) was connected to Asia in those days by a land bridge. The marsupials reached it easily. Then, before the more efficient modern mammals had time to evolve, the land bridge sank, sheltering the marsupials for millions of years with a broad saltwater moat.

Making use of their opportunity, the marsupials developed specialized forms to fit nearly all the modes of life which will support a land animal. The big kangaroos, grazing marsupials, are equivalent to cattle or antelope. The earth-living wombat is a marsupial badger. A marsupial male, with pouch opening backward, burrows Australian earth. Marsupial squirrels (phalangers) climb Australian trees.

Carnivorous marsupials soon developed to plague the vegetarians. Most remarkable are the Tasmanian devils -- marsupial wolverines with bodies, legs and feet like a middle-sized dog's, and big, ugly heads with vast, tooth-packed jaws.

The Brighter Placentals. While the Australian marsupials differentiated in seagirt isolation, the modern "placental" mammals chased all other marsupials (except American opossums) off the earth. Placentals were brighter, more efficient, pouchless. They gestated their young internally until they reached good size. Little by little they, too, invaded all niches of life. In practically every case the placental form was superior to its marsupial equivalent.

Doomsday dawned for the marsupials when modern man, the most efficient of all placentals, landed in Australia. With him came placental slaves and hangers-on. Sheep and cattle outgrazed the kangaroos. Rabbits escaped and multiplied. House cats turned into wild cats.

A few marsupials, like opossums or the even more primitive monotremes, may find refuge in obscure modes of life. But man's inefficient cousins are a dying type, their extinction long overdue.

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