how does Ornithorhyncus produce its young?
Part III—“This Highly Interesting Novelty” Sir Joseph Banks, who had traveled with James Cook on his fi rst voyage, ventured this in : “Our greatest want here is to be acquainted with the manner in which the Duck Bill Animal [platypus] and the Porcupine Ant Eater [spiny echidna] which I think is of the same genus, breed, their internal structure is so very similar to that of Birds that I do not think it impossible that they should lay their Eggs or at least as Snakes and some Fish do Hatch Eggs in their Bellies.”
Th e French zoologist Etienne Geoff roy St-Hilaire, reading Home’s anatomical works, declared that both animals should be placed in a new animal class, the Monotremata, which means “one hole” to designate that the animal has a single opening (cloaca) through which it eliminates digestive and urinary wastes and reproductive products (eggs or sperm). Th ere were three central questions about Ornithorhyncus that emerged from the foment of the times:
. First, how can we fi t this strange beast into the classifi cation and taxonomic schemes that had worked so well in the Northern Hemisphere?
. Second, how does Ornithorhyncus produce its young? . Th ird, what relevance does this anomalous animal have for the old ideas of a perfectly created world? What is
the relevance of Ornithorhyncus to the idea of evolution, which was beginning to be whispered about?
Questions Let’s consider the fi rst question: how should we classify such an animal? Classifi cation experts like John Ray and Carl von Linneaus said that reproduction was the essential criterion for classifi cation. Linneaus set the presence of mammary glands and the suckling of the young as the defi ning characteristic for the class of animals he named “Mammalia.” He said that warm-blooded quadrupeds (four-legged beasts) with a four chambered heart and double circulation were viviparous and mammiferous.
Henri Marie Ducrotay de Blainville said mammals could be arranged by decreasing complexity from the primates down through the marsupials to the monotremes. He was the fi rst to note many resemblances between platypus and echidna and the marsupials. He said that regardless of the apparent absence of mammary glands, the monotremes belonged as mammals in their own distinct order, Ornithodelphia. France’s scientifi c leader, Georges Cuvier, pronounced they were indeed mammals but put the monotremes squarely in the order Edentata that included other toothless mammals, anteaters, and sloths.
Not everyone agreed. Although the platypus was warm-blooded, had a four chambered heart, and double circulation (two diff erent sides of the heart, one pumping to the lung and the other to the rest of the body), birds had these traits too. And it had a duck-like bill! Everard Home reported in his paper that the structure of the ear and shoulder girdle combined both mammalian and reptilian features. Th e presence of a cloaca was clearly a reptilian and avian feature. Th e absence of a well-formed uterus and the apparent absence of nipples persuaded Home that the “duck-billed mole” was related to ovoviviparous reptiles.
Lamarck said the platypus and echidna could not be mammals without mammary glands. He placed them in a separate vertebrate class called Prototheria.
. So what is the best solution for classifi cation for this unusual animal? If birds, reptiles, fi shes, and mammals are placed in separate classes, where should an animal like Ornithoryncus be classifi ed?
. What is the best logic for predicting how the young platypus is born: viviparous, oviparous, or ovoviviparous? What seems to be the most probable reproductive method and least probable method? And once produced, how will they be fed?
“An Antipodal Mystery” by Clyde Freeman Herreid Page 5
Part IV—Solving the Mystery How do platypuses reproduce? In , there was a breakthrough when Patrick Hill, a naval surgeon, wrote to the Linnean Society saying he had talked to an Aboriginal elder and “it is a fact well known to them that the animal lays two eggs about the size, shape, and colour of those of a hen; that the female sits for a considerable time on the eggs in a nest which is always found among the reeds on the surface of the water.”
More importantly, in , the German anatomist Johann Meckel reported that he had found mammary glands in the platypus! Th ey appeared primitive and opened directly onto the skin without any sign of nipples. Monotremes would represent a transitional form between reptiles and mammals. Geoff rey St-Hilaire rejected this view and said the structures described by Meckel couldn’t be mammary glands because the absence of nipples would make feeding diffi cult with a duck-bill. He stated that the monotremes belonged in their own separate mammalian order, Monotremata.
In , the Hon. Lieutenant Maule, who was stationed in Australia, reported to the Zoological Society of London that he found several nests of platypus with fragments of eggshell and in one nest he found a female and two young. Two weeks later when the female died, he reported: “on skinning her while yet warm, it was observed that milk oozed through the fur on her stomach.” No teats were visible.
Richard Owen, England’s great comparative anatomist, received two baby platypuses from Lieutenant Maule in New South Wales, and determined in that the suckling infant’s mouth was designed to take milk in the normal manner. In addition, he clearly determined that there was milk in the babies’ stomachs.
Not until was the picture clear. Th e Scottish embryologist, William Caldwell of Cambridge, arrived in Australia and gathered a group of aborigines to search the Burnett River for the elusive monotremes. He shot a platypus in the act of laying eggs: her fi rst egg had been laid and her second was still in the partially dilated mouth of the uterus. He claimed victory. Platypus was oviparous. It laid soft-shelled eggs with large yolks that were gradually absorbed by the growing young, just as in birds and reptiles! In contrast to birds, where the calcifi ed egg does not change in shape or size, the monotreme egg increases in size during its time in the uterus. Its fl exible shell is stretched as nutrients are absorbed from the uterus.
Question . Do these discoveries change your view about how to classify the platypus?
“An Antipodal Mystery” by Clyde Freeman Herreid Page 6
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Part V—The Big Picture Turning to the third question: How does the platypus fi t into the doctrine of creation? Recall that Aristotle’s view of a ladder of nature (Scala Naturae, or Great Chain of Being) suggested that species were fi xed in a position on an ascending ladder leading toward humans at the top. Th is may have made sense a couple of thousand years ago when only species of animals were known, but as new species were discovered, with more and more intermediate or hybrid characteristics, this static view of the world seemed less and less tenable. For example, in French expeditions returned from Australia with , animal specimens; , were species new to science. Trumpeted France’s scientifi c leader, Georges Cuvier, they had collected: “more new creatures than all traveling naturalists of recent times put together.” Robert Brown, who traveled extensively around Australia’s coasts, collected genera and , species of plants in , all new to science. Classifi cation schemes that had been created for Europe were completely inadequate for the Southern Hemisphere. Th e platypus was only one of a thousand new riddles, albeit the most spectacular.
Another problem was emerging: fossils were being discovered everywhere. Many were of animals no longer alive. Th is suggested that some species had gone extinct. If extinction occurred, then what had happened to the ladder of life? Are there even more missing steps?
How do scientists solve this problem? Th row out the Scala Naturae concept altogether? France’s Georges Cuvier did. He argued physical catastrophes periodically occur and destroy organisms. Th ey were replaced after each disaster by successive creations of new and more complex unrelated species. Revise it? Cuvier’s compatriot, Jean-Baptiste de Lamarck, believed there was a linear order of living organisms from simple to complex, and that organisms could move upward on the ladder via evolution—rather like an escalator. He thought extinction was impossible.
Questions . When Charles Darwin came into the picture, he had his own view of the Scala Naturae. What do
you think was his view? . Today, we have DNA evidence as well as that from the traditional fi elds of comparative anatomy
and physiology. Based upon everything that you know, draw a likely phylogenic tree showing the evolutionary relationships among birds, marsupials, monotremes, placental mammals, and reptiles.
